Cl,) I cl

INTERIM REPORT' (March 1985)

BGS/ODA - ZIMBABWE OOVERNMENT

OOLLECIOR l'/ELL PROJECT

by

E P WRIGHT

K H MURRAY

R HERBERr'

R KITCHING (modelling studies)

R CARRUTHERS (geophysicist)

Overseas Hydrogeology British Geological Survey Maclean Building

Ministry of Water Resources & Development Causeway Harare

Walling ford Zimbabwe OXON OXlO 8BB UK

COLLECIOR WELL PROJECT: ZIMBABWE, 1984

1. INTRODOCTION

The collector well project is designed to maximise and optimise abstraction from extensive lCM' permeability aquifers. It is !TOre specifically designed in relation to aquifers within the overburden (regolith and weathered bed­rock) above crystalline basement rocks.

Dug wells occur in countless numbers within shallow aquifers throughout the world. Penetration belCM'the water table is typically small, rarely in excess of 2/3 metres. The well sides nay be self standing or supported by a variety of naterials, concrete, wocd, metal such as old oil drums, etc. The smaller the water inflow rate, the larger the diameter rust be made in order to provide sufficient storage for increased yields during pumping periods. Inflow rates are relatively insensitive to diameter so large increases have to be nade to provide significant improvements. Very large diameter wells are corrrnon in India, up to 15 m diameter or !TOre. In Africa, snaIl diameter wells are !TOre typical, commonly 1-2 m across.

Dug wells have certain obvious advantages in that they require little technical expertise to construct and are of low capital cost, whether in terms of labour, constructional naterials (if any) or I>.Drking tocls. The disadvantages are the commonly low yields constrained either by limited depth of penetration below the water table, low aquifer transmissivity or a combination of both factors. Dug wells are also vulnerable to surface pollution and to the effects of drought, seasonal or cyclic.

There are some circumstances when the large diameter well is !TOre appropriate, both in cost terms and efficiency in use than a deep slim borehole. Such l>.Duld be the case when the saturated overburden is only nDderately thick and over lies tight bedrock.

Crystalline basement rocks are of widesDread extent, in Africa for example, perhaps covering between 30-40% of the continental land surface. Excluding the desert regions of the Kalahari -Namib and the Sahara where [copulation density is negligible, the proportion of effective areal cover of basement rocks would be very much higher, perhaps 70-80%.

Aquifers occur in the weathered overburden and the fractured bedrock and are developed by boreholes or 'shallow' dug I-lells. Boreholes are rarely deer:ler than 70 metres and yields are unlikely to increase significantly belOH this deDth. Abstraction is nainly by handnumps in both boreholes and wells and rates are in the main range 0.1-0.35 litres/sec. Information on sustained yields of boreholes which have given higher yields on test (standard duration of one hour) is hardly ever available and it is rare to see quoted test yield values in excess of 2 litres/sec.

The aquifers of the crystalline basement are now in process of very extensive development, primarily for rural water supply. The highest sustained yields are to be expected where boreholes penetrate a Significant thickness of saturated permeable overburden and into fractured bedrock, \'lith both section" screened. Fractured bedrock tends to have high transmissivity but low storativity, contrasting with the clayey overburden of low to rroderate

transmissivity and high storativity. Exploration to identify high yielding boreholes will generally require extensive geophysical survey and costs will be high. Knowledge of the distribution of fracrure systems is limited and present evidence suggests relatively localised occurrence usually in association with tectonically controlled features. The over­burden seems likely to have more uniform characteristics in accordance with larger scale controls of bedrock type, erosional cycles, climatic patterns etc.

The collector well rrethod is designed to abstract high yields (1-4 litres/ sec) from typical extensive weathered overburden. With transmissivities in the range 10-40 m2/day which are to be anticipated from basement aquifers, such yields can be obtained from a collector well provided the aquifer has sufficient lateral extent and obtains adequate recharge in that distance. Precise data on recharge rates to regional basement aquifers is not available but could be in the range of 20 to 200 mm/a for a rrean annual rainfall c. 1000 mm. Current abstraction rates in rural areas with average population densities and boreholes fitted with handpumps are unlikely to exceed 1 mm/a. One (1) litre/sec continuous abstraction would be equivalent to a circle of radius 708 m (for 20 mm recharge) or 224 m (for 200 mn recharge) and give sarre indication of possible well site separations.

The collector well differs in a number of irrJ:x:>rtant particulars from the shallow dug well. Firstly it is designed to penetrate more deeply below the water table, of the order of 5-10 rretres or more. This circumstance creates advantages in several important aspects.

(i) A larger volurre of well storage becorres available and also a greater area of inflow into the well.

(ii) The regolith is commonly more clayey in composition and therefore of lower permeability in the upper levels; deeper penetration is likely to intersect coarser grained, less clayey (and hence more perrreable) overburden or possibly the weathered fractured bedrock.

(Hi) Deeper penetrating wells are less susceptible than shallow wells to seasonal or cyclic water level changes.

The construction of large diameter wells below the water table is constrained by depth and water inflow rates. With hand teals and low '<later influ .. J

rates, overall depths of perhaps 8-10 metres below ground level and 2/4 rretres below water level can be obtained with bricks or cement blocks (rings or segments) being emplaced laterally at the base of the well as digging proceeds. For deeper wells and "mere water inflows are higher, lateral emplacement of lining materials from below becorres progressively more difficult, even with adequate dewatering facilities, and the caisson rrethod may be the only feasible alternative. The maximum depth to which any caisson may be sunk is said to be (approximately) seven times the diameter* which would limit the depth of a 2 m caisson to 14 m and a 3 m to 21 m. Mechanised teals will generally be needed below 8/10 m, particu­larly with high water inflows. This can include a motorised winch with large bucket, a sludge pump and jack hammers for use in harder residual

* Watt, S B and Wood, W E, 1977. Hand dug wells and their construction. Edited: Intermediate Technology, London.

bedrock. Since the dewatering PUlT[l has to be used in association with manual operations, the PUlT[l must be air or hydraulically operated. Diaphragm (suction) pumps may be used if they can be located within suction distance (c. 7 rretres) but in deeper wells centrifugal PUlT[ls are more generally convenient. Bricks or solid concrete rrust be used as a lining material for caissons. Porous concrete (as presently made) is insufficiently strong for caisson construction but can be used for standard shallow dug wells (lateral emplacanent) down to 20 m. The caissons used in the present project were reinforced both vertically (BS 4461 steel rod) and horizontally (galvanised steel fence wire between each course of brickwork) in order to ensure against the stresses of caisson construction and radial drilling.

The second important difference in the collector well relates to the function of the drilled radials. These are designed to be drilled near the base of the well providing advantages in relation to increased available drawdown and most importantly in intersecting more permeable formations of the deeper regolith close to the junction with the bedrock. It is also feasible to drill inclined holes into harder fractured bedrock if so desired. The.effect of abstraction via the drilled radials is to facilitate expansion of the cone of depression in the most permeable layer, thus enabling more efficient exploitation of the high storativity of the clayey upper layers by widespread downvmrd leakage (leaky artesian process). Analysis of these effects is given later in this report.

2. PROGRAMME OF WORK

The project was planned as a joint exercise with overseas governments interested in these experimental studies. In the case of Zimbabwe, this country, via the Ministry of Water Resources Development, was independently interested in the potential of very large diameter wells (5 metres plus) with the main emphasis on well storage as compared with the present concept of smaller diameter (3 m) wells and radial drilling. The Zimbabwe Government corrpletedone 5 metre well (16 m depth) at Chibero. The well was constructed on the caisson principle down to bedrock at about 5 metres and open hole below. The saturated aquifer is mainly within bedrock and after completion of the well, performance proved extremely disappointing with maximum inflows of the order of 334 litres per day. Opportunity was taken to drill several radial boreholws which contributed virtually nothing and testify to the paucity of fractures in the local bedrock.

The conclusions which may be drawn from this first abortive attempt mainly relate to the need to ensure adequate saturated thickness of weathered overburden or the presence of significantly fractured bedrock with sufficient continuity with saturated overburden to provide adequate storage to sustain large yields. The role of the 'shallow' large diameter well is yet to be evaluated.

Following completion of the Chibero well it was decided that future wells would be constructed in accordance with the BGS/ODA concept and to include radial drilling. Co-operative project work was carried out in Zimbabwe between September 1983 and December 1984 and four sites were completed. Full details of the investigations at these four sites are given in Appendices I to v. The work was carried out in association with the Hydrological Branch of the Ministry of Water Resources Development (MWRD) with Mr L L Hindson as the Zimbabwe Government Project Co-ordinator. The British Government has provided basic equipment as set out in Table 1 below and the periodic services of a field supervising hydrogeologist and a drilling engineer.

TABLE 1 EquipJrent/Material provided by British G:lvernment for Zimbabwe Collector Well Project.

Radial Drilling Rig complete with 30 m drill pipe etc.

Sludge Ptmp

Plastic casing and screen with 'Te=afilter' mesh (2 inch diarreter)

All other equiprrent, materials and staff, professional, technical and labour, has been provided by the Zimbabwe G:lvernment and represents a substantial input. Full details are given in Appendix VI but the main items are noted below:-

Small rotary drilling rig

Air compressor (250 cfm, 100 psi) for dewatering pump and jack hammers

Small mono pump for borehole testing in 90 mm diarreter cased holes

Large mono pump for testing of LD and collector wells

Tractor for general transport of materials and to power winch for deep well excavation.

The drilling rig was constructed by the firm of Drilling and Geoservices, Lindley Grange, Watling Street, Nuneaton, Warwickshire, to the specifications laid down by BGS. The rig is hydraulically powered and designed to operate in a 3 m shaft and with a minimum capability of drilling 30 m radials (114 mm diameter) in either consolidated or unconsolidated material. Drilling can be carried out by rotary or down hole hammer technique using air, foam or water flush and either open hole or Duplex. The =rent cost of an equivalent rig with a similar range of drilling tools and spares for one years operation is of the order of £35,000.

2.1 Well Site Locations.

General locations were selected by the MWRD on the basis of demand and/or inadequacies of existing supply. The well sites were selected on a can­bination of geophysical survey and test drilling. It was only latterly that the importance of aquifer analysis by means of a pumping test became apparent. In the event, one of the four sites selected, Marikopo should have been rejected on the basis of shallow bedrock alone although even the overburden proved to be of low permeability. At this site, it would have been more cost effective to have used deep geophysical survey for location of fracture patterns to assist siting of a standard deep borehole.

2.2 Geophysical Survey.

The geophysical survey technique adopted by the Ministry of Water Development team to locate zones of deep weathering suitable for a well site was resistivity traversing. A Schlumberger array configuration was used with current electrode spacings of between 15 m and 100 m: data from three spacings 20 m, 40 m and 100 m gave information for different depths of investigation on reconnaissance traverses; six spacings of 15-60 m provided additional control for detailed surveys around the selected well sites. The results were interpreted qualitatively in tenns of apparent resistivity and apparent layer resistivity values but no quantitative analysis was attempted at the time as far as is known. Apparent layer resistivities were calculated as:

(K - K ) / (l/R - l/R ) q P q P

where Kp and Ko are the geometrical factors at consecutive array spacings; Rp and Rq are the measured apparent resistances. Layer resistivities are supposed to show better resolution than the basic apparent resistivity values which represent a weighted mean over the depth of investigation. However, as they involve additional approxirrations and are sensitive to errors in the field daga, the results need to be treated with caution. For the detailed surveys, measurerrents in orthogonal directions at each point provided an indication of lateral variations or anisotropy within the underlying formations: they also reflect the effects of near-surface inhomogeneities and errors within the field data.

It is difficult to assess the overall effectiveness of the surveys as they seem to have been undertaken on an ad hoc basis rather than as part of an integrated siting procedure. While resistivity traverses provide evidence of lateral variations within the subsurface it is not always possible to correlate these with drilling information. This is due in part to the Sffi3.ll diameter of the boreholes in cc:mparison with the volume of ground sampled by the resistivity array, and with the scale of in­homogeneities in the regolith. Another factor is that the apparent resistivity is a function of the formation thickness and its resistivity: if both these parameters are varying their effects are difficult to resolve even with data at different electrode spacings.

Some general distinctions can be ffi3.de regarding the nature of the regolith and the underlying bedrock on the basis of resistivity values at the four sites: at Marikopo School for example, where conditions were least favour­able, the resistivities were Significantly higher over the granite-gneiss only because of the superficial cover of dry sands; in contrast, the epidiorite at Hatcliffe was characterised by a clay-rich upper zone which tended to depress apparent resistivities at all separations; apparent re~;istivities for the 60 m current electrode separation at Marikopo and Murape v/ere of similar magnitude and distinctly higher t'1an those at Hatcliffe; at intermediate separations there was evidence of relatively conductive material within the regolith at all the sites, indicative of a significant clay /rroisture content and of an absence of unweathered bedrock at very shallow depths.

h'hile useful information on trends can be extracted from profile data the disproportionate contribution to the signal coming from the upperrrost layers tends to Mask the influence of depth to bedrock: the use of apparent layer resistivities reduces this problem only ffi3.rginally and curve ffi3.tching of 'sounding' data provides the rrost satisfactory analytical approach. The six curve points available from the traverse readings do not adequately define the beginning or end of the relevant section of the curves, but in view of the inhomogeneity shown by the orthogonal data sets, this is probably not the major source of uncertainty. The interpretations show general agreerrent with drilling results though the localised varia­tions deduced from the boreholes are not resolvable. There are some difficulties in equating the results because, for example, drilling may have stopped in fresh rock occurring as a remnant within the weathered zone rather than as bedrock, or unconsolidated drilling samples ffi3.y be derived from coherent weathered rock. Certainly, in those cases where

re-drilling proved the presence of boulders the geophysical data implied a deeper bedrock. Interpreted resistivities for the bulk of the regolith ranged from <20 ohm.m over the epidiorite to 40-70 ohm.m over granitic bedrock: total conductances for the regolith were typically 0.75 at Hatcliffe corrpared to 0.25 at Murape and Marikopo. Where higher resistivities were implied at Marikopo the degree of lateral variation was consistent with the presence of alternating, discontinuous bands of hard and weathered rock, while the existence of conductive clays below other sections of the grid was also indicated. Only three layers were distinguished: a superficial cover, a regolith zone and bedrock. No effects were attributable to a water table or to a zone of sandier regolith or weathered/fractured rock above the ccmpact bedrock. At Hatcliffe Windmill site, in a zone passing through and northwest of the well where the depth to bedrock was not proved, the indicated resistivity for the deepest layer beyond 15-20 m below surface was unusually low at about 100 ohm.m; corrpared with >150 ohm.m elsewhere. It is not possible to relate the geophysical results to the lithologies encountered in drilling the rakers: with only a single resistivity value encorrpassing the whole of the regolith below the sarrpling point only general suggestions can be made in terms of a sequence more or less clay-rich with some free water.

2.2.1 Future site surveys

Geophysical surveys will be most effective where: little existing informa­tion is available; where the depth to bedrock is variable and generally close to or less than the minimum for a dug well site; where variations in lithology, e.g. between granite, gneiss, epidiorite, dolerite are expected to relate to yield; where fracture systems are significant.

Reconnaissance traverses to locate more favourable areas for follow up by more detailed surveys and/or drilling should be undertaken using an ~34-3 which is quicker and requires less labour, or resistivity profiling. Preliminary resistivity depth soundings may be necessary to determine appropriate electrode spacings but at least two should be used with a view to assessing the bulk reSistivity of the regolith and the total conductance of the material overlying bedrock. In the simplest, 2-layer case this combination defines the bedrock profile. The traverses should be extended as necessary - within the limits of user requirements - and take due account of different geomorphological controls.

A limited amount of test drilling is needed at an early stage to check ~~at the conductivity variations observed can be related to the material forming the reqolith and depth to bedrock, and to determine the most favourable criteria for sitinq. Additional depth soundings can also be used for this purpose. If conductivities are inconsistent with drilling results, seismic refraction lines of about 150 m in length should be tried in different parts of the survey area.

If depths to bedrock are marginal, detailed grid surveys can be laid out in the most promising area and covered by seismic refraction or resistivity soundings. For resistivity data orthogonal arrays are necessary as a check on lateral effects and the curves should cover current electrode separations of 4-100 m. Seismic interpretation should give more accurate depths to bedrock but its usefulness has to be assessed in terms of local expertise and comparative drilling costs.

2.3 Test Drilling - Aquifer Testing.

Test drilling is designed to confirm the geophysical data interpretation and to determine aquifer characteristics by a short duration pumping test in a small diameter borehole completed at the depth to which it is planned to construct the collector well. The test holes were drilled by a small rotary (Tone) rig using either water or air-flush. The boreholes were generally lined with 90 mm plastic casing which was perforated below the water table with 1 mm wide sawn slots (0.25% open area). High well losses resulted from this poor completion and made pump testing difficult because of excessive drawdowns in the pumping well even with low rates of discharge. For future prograrrrres in which only 1/3 test holes may be drilled, improved completion would be desirable although this need not constitute more than emplacement of a slotted screen with larger open area.

The main objective of the aquifer testing is to obtain good transmissivity values for which a short duration (8 hours) borehole drawdown or recovery test is sufficient. Longer tests and the use of observation boreholes would give information on specific yield and in a limited number of cases, this procedure is to be recomrrended. A small M::Jno turbine pump capable of yields of up to 2 litres/sec was used. Slug testing was also carried out but the results in boreholes with such poor completion is not to be relied upon.

2.4 Dug Well Construction.

Fuller details of the caisson construction and requirements of tools, materials, costs, etc. are given in Anpendix VI. Hand tools etc. refer to the requirements of a team of labourers and bricklayers operating at two well sites in 'tandem' which permits efficient co-ordination of alternate bricklaying and digging. Mechanical tools - pumps, compressors, jackhammers, etc. are required for deeper operations.

2.5 Pumping Tests on the Large Diameter (LD) Well.

Follovling completion and prior to radial drilling, short term constant rate pumping tests were carried out on the LD wells. The first test (c. 60 minutes) was required to obtain the 50% and 90% recovery t~es from a small drawdown, typically <1 metre. Discharge rates were adjusted accordingly. These recovery times formed the basis for dug well analysis by the Herbert-Kitching method~ A second test of longer duration (c. 500 minutes) was designed to give indications of changing permeability with depth and also data v;hich could be used for comparative evaluation of the performance of the subsequent collector well. Time constraints limited the periods which could be allowed for equilibration prior to testing and wells were not always in equilibrium before tests commenced.

2.6 Radial Drilling.

Radial drill holes were restricted to 30 m in length which was the amount of drillpipe pro=ed in the initial purchase. Rock roller and down hole hammer drilling methods were used and on the whole the drill performed very efficiently. The actual period of drilling at each site was very short, a matter of a few days only and could have been shorter except for delays occasioned mainly by a faulty compressor. Some difficulties occurred in consequence of lost circulation and on the fourth site, the one hammer tool available failed completely. This v.Duld appear to

* Herbert, Rand Kitchinq, R 1981. Determination of aquifer narameters from large-diameter dug well pumping tests. Ground water, Vol. 19, pp. 593-599.

be an inadequacy of design of the Haleo harmer when used in horizontal node and is to be remedied in a replacerrent tool.

In a homogeneous formation, it can be demonstrated by mathematical analysis (Hantush and Papadopulos, 1962) that little improvement in efficiency (drawdown) is to be gained by drilling holes additional to four orthogon­ally. The situation could obviously be different for anisotropic and heterogeneous sequences such as those encountered and more precise guide­lines for such conditions remain to be evaluated. In the event, selection was guided by direct observations on the directions of fissures or fractures within the well walls during digging and in accordance with drawdown patterns during the initial borehole pumping tests. The radials were also sited to avoid encountering hard bedrock as indicated by the test drilling and the geophysical survey data. Attempts in this respect did not always prove successful on account of excess irregularity in the overburden sequence and not evaluated in the exploration surveys.

2.7 Pumping Test in Collector wells.

Short duration constant-rate pumping tests were carried out in the Collector Wells after final completion for periods of up to 8 hours, primarily in order to establish comparisons with large dianeter well response. It would have been preferable to have allowed the wells to equilibrate fully prior to such tests but time constraints did not always allow this. Comparisons of respcnse have therefore had to be based on the recovery time over the sane interval of well water levels on the assumption also that hydraulic conditions outside the well were comparable at the times of the different tests.

Following the constant rate test(s), a long duration pumping test (12-14 days) was carried out, designed to optimise abstraction by pumping over intermittent periods (three 2-hourly periods per day were used). These circumstances simulate practical usage or can be readily related to more extended and continuous pumping at lower rates.

3 • SUMl''lARY OF RESULTS

Full details of each site study are given in Appendices I to IV and lithological logs of test boreholes in Appendix V. The results are also surrmarised in Table 2. The plot of abstraction rates and drawdowns in the long duration tests are included in the respective Appendices but all wree are reproduced in the main report (Figures 1-3).

Of the four sites studied, the highest aquifer transmissivity (40 m2/d) and reflected in highest productivity occurs at Hatcliffe Willowtree which overlies epidiorite. Hatcliffe Windpump is slightly less and overlies a granite cupcla in the Older Gneiss complex. The well could have been dug deeper and radial drilling in other directions was warranted, both of which might have improved overall performance. Murape and Marikopo well sites are both overlying gneiSS of the Older Gneiss Complex. ~1urape may be regarded as fairly typical although overall transmissivity is on the low side. Marikopo was an unsuitable site for a collector well because of the thin overburden overlying tight bedrock. M:lre systematic geophysical survey would be needed to define locations of fractured bed­rock - if they exist in the vicinity, with a view to a deep borehole or to drill radials at inclined angles downwards.

* Hantush, M S and Papadopulos, I S, 1962. Flow of groundwater to collector wells. J. Hyd. Div., Proc. Am. Soc. Div. Eng., HY5, pp 221-244.

TABLE 2 St.ll1llkllY of Construction and Test Data on Collector Wells.

I II III IV MURAPE MARIKOPO HATCLIFFE HATCLIFFE

WILLCWI'REE WINDPUMP

1 14.3 m 11. 7 m 10.8 m 10.0 m

2 2.6 m 4.2 m 5.9 m 3.3 m

3 20.4 m 10.7 m 14.8 m >26.0 m

4 12.0 m <10.0 m 11.6 m ?

5 7.0m2/d <1.0 m2/d 40.0 m2/d 27.0 m2/d

6 (i) 495 min 2.6 days 300 min 525 mill

(H) 10.2-9.9 m 10.8-10.6 m 50% recovery 5.8-5.1 m

7 70 min 1. 7 days 100/150 min 291 min

8 1.5 l/sec 3.6 l/sec 2.6 l/sec

9 8.4 m 0.9 m 1.4 m

10 8.8 m 1.2 m 2.4 m

11 +1.8 m +2.5 m +2. 7 m

Legend on next page . . .

TABLE 2: LEGEND

RCW 1 Total depth in rretres* of collector well (excluding sump) .

2 Rest water level below ground level at tirre of long duration test.

*

3 Thickness of overburden at "1811 site.

4 Average thickness of overburden in general vicinity.

5 Transmissivity (m2/day); best probable value.

6 Large diarreter (LD) well response, either (i) (50~ Recovery or (ii) Recovery tirre, (a-b) for well water levels.

7 Comparable response, collector well, over sarre intervals.

8 Long duration test,

2 hours).

flain pumping rate (intermittent 3 tirres

9 Background drawdown ,'it end of test period. (Backqround clrawrln":n

is measured at beqinning of each day prior to commencement of first pumping cycle) .

10 Background drawdown estimated at end 100 days without rain.

11 PUDDing drawdown additional*.

All figures of depth rounded to first decimal place

Of the three successful sites, the best standards of comparison are afforded by the yields during the long duration pumping tests (Row 8) which are equivalent to rates of between 0.75-1.8 litres/sec for 12 hours continuous pumping per day with evidence to suggest that these rates could be maintained for over 100 days without the drawdown reaching the pump strainer. The small pumping drawdowns (1.8-2.7 m) for the significant rates of purrping between 1.5 and 3.6 litres/sec should also be noted.

The second feature apparent in the table of summary data is the signifi­cant advance in the performance of the collector wells as compared with that of the corresp:mding large diameter well (Rows 6 (i) and 7). The approximate doubling of efficiency in the Hatcliffe Wells corresponds with the theoretical improvement of a collector well in a homogeneous aquifer. The very much higher improvement in the case of 11urape (700%) probably relates to the response of a layered aquifer with the radials drilled in a high permeability layer near the junction of the overburden with the bedrock.

4. SUMMARY OF COSTS

A summary of costs is shown in Table 3. These essentially represent local costs but exclude amortisation on equipment other than transport. They also exclude the UK Government contribution, represented mainly by the drilling rig, supervising hydrogeologist and drilling engineer.

Costs need to be examined in the context of what may be attributed to the operations of an experimental study and in order to assess probable costs for a production development phase. The lower costs of the later sites III and IV are partly a reflection of more efficient operations. This is particularly obvious by the corresponding rates per metre 'construction' costs which mainly reflect relative labour costs due to delays in project operations rather than increased difficulty in construction. Indeed there were also significant delays in operations at Hatcliffe \~indpump due to the unavailability of a cOmoressor for dewatering.

The costs of the geophysical survey/test drilling i~e to be mainly associated with research rather than as an essential adjunct to site selection. Indeed some of the site surveys continued to be made after well digging had commenced. For future production well siting, a much reduced geophysical survey is anticipated, if required at all. Rapid EM traverses could prove sufficient in most cases - a feltl hours work, with calibration by 1/2 shallow test boreholes which would also be used for pump testing and precise location of the well site. It must be remembered that geophysical survey is mainly essential for locating fractured bedrock, particularly the more detailed tY)Je of survey such as reSistivity sounding. Collector wells are mainly planned to be emplaced within the overburden which is likely to be more regionally extensive and relatively homogeneous.

Well construction costs mainly represent labour and materials and show a significant drop in rate at the later sites. Even at these sites there was scope for improvement in increased efficiency, rrost cornrronly in relation to ancillary equipment effectiveness.

TABLE 3

Summary of Costs: Z:i.rnbal:J;ve Dollars.

I II HI IV MURAPE MARIKOPO HATCLIFFE HATCLIFFE

vlILI.CWl'REE WINDPUMP

Geophysical Survey 6,755 5,489 3,080 1,540

Test Dri11ingjBoreho1e P.T. 7,781 18,746 8,320 2,080

Well Construction 19,889 15,536 6,399 5,445

Radial Drilling, local costs 1,120 1,472 1,069 1,375

Punp Testing 704 416 1,270

Transport (Superviser/Driller) 11,542 7,400 4,381 4,381

Caravan Hire 1,725 1,000 271 271

TOTALS 49,516 49,643 23,936 16,362

Hell Depth 14.3 m 11.7 m 10.8 m 10.0 m

Ccnstruction cost per metre Basic (Row 3/depth) l391 Z$ l328 Z$ 593 Z$ 5~5 Z$

The very high cost of the Supervisers transport should be noted. This was provided either by hiring or through the Ministries own central transport section for which mileage charges presumably relate to realistic rates for depreciation and amortisation. The high component of transport costs is a common feature in Africa generally and can be reconciled and reduced only by integrating and increasing the scale and efficiency of operations.

Radial drilling costs include the hire of a crane, fuel and associated labour costs.

5. INTERPRETATION

Results of these studies require interpretation in terms of the potential of collector wells for future development. Present conclusions must be regarded as interim and it is planned that during 1985-6, additional site studies will be made in Sri Lanka, Malaysia, Malawi and also again in Zimbabwe. A second drilling rig is under course of construction which will be able to operate in a 2 metre diameter well which should reduce construction costs significantly, perhaps by as much as 50%.

A realistic assessment of costs for a production programme will depend on the scale of operations and the degree of supervision required. Con­Sidering that this type of well construction was wholly novel in Zimbabwe, there is good reason to expect that efficiency of operations would Significantly increase with time and that much of the supervision could be technical rather than profeSSional, as at present. More realistic costs for a 3 m collector well, even in a new area, could be expected to be less than Z$ 10,000 and made up as follows:

Geophysical survey Test drilling \vell construction Radial drilling

Zimbabwe $

500 1000 6000 (Appendix VI) 1000

8500*

Preliminary interpretation needs to take account of the following:

*

(i) Comparisons vlith current methods of abstraction.

(ii) Significance of the following factors:

annual recharge well storage radial drilling anisotropy and heterogeneity

includes transport costs for labour and movement of materials, rig, etc; excludes costs associated with overall supervision, drilling engineer, and amortisation on equipment.

5.1 carrparisons with Standard Boreholes.

Perfonnance: Direct and straightforward canparisons with the perfonnance of standard boreholes in basement rocks carmot be made for a variety of reasons. Boreholes are drilled deeper than collector WBll depths, typically between 50 to 70 metres. The large majority are fitted with handpumps and informa­tion is not available on long term perfonnance. Productivity data in the National Records list results of very short duration pumping tests (c. 60 minutes) and it may be reasonably assurred that in the majority of cases, drawdowns were close to the llB.Ximum, probably near the base of the borehole. Comparable handpump abstraction rates would be affected by such high draw­downs and would be lower, in addition to the normal reduction consequent upon duration of pumping.

Table 4 and Figure 4 sumnarises data on borehole records adapted fran an internal report by L L Hindson on the groundwater resources of Southern Rhodesia (c. 1960).

TABLE 4 Frequency Distribution of Borehole Yields in Granites and Gneisses in Zimbabwe.

Granites

Numbers

760 238 338 306 398

Total: 2040 Boreholes

Yields on Initial Test

0.00-0.12 litres/sec 0.12-0.38 litres/sec 0.38-0.75 litres/sec 0.75-1.20 litres/sec

>1.20 litres/sec

Average Depth to Main Supply: 30 metres

Gneisses

Nurrbers

138 61 63 51 63

'Ibtal: 375 Boreholes

Yields on Initial Test

0.00-0.12 litres/sec 0.12-0.38 litres/sec 0.38-0.75 litres/sec 0.75-1.20 litres/sec

>1.20 litres/sec

Average Depth to Main Supply: 34 metres

Cumulative %

37 49 65 80

100

Cumulative"

37 53 70 83

100

On the basis of these figures there is comparatively little difference between the yields of boreholes in the tv.\:l rock groups. The overall frequency distributions are positively skewed and more data might shcM a log normal distribution, although the highest yield groups (>1.2 litres/ sec) are exceptionally large in both cases. The rrodal values are in the range 0.(x)-().12 litres/sec and the median yield (50% of the boreholes) are less than 0.38 litres/sec. With a normal exponential reduction in specific capacity with t:i.rre, canbined with the high drawdowns which may be assurred for these test yields, the rredian rates are only capable of providing a m::xlerate handpump supply (c. 0.25 litres/sec) under normal conditions.

The sustained yields (3 x 2 hours = 6 hours daily) of the three successful collector wells are significantly in excess of these borehole yields (1.5-3.6 litres/sec). Additionally, the low pumping drawdowns (Table 2) at these ~ch higher rates of pumping will require lower energy inputs, reduce maintenance costs and optimise yields from low energy pumping plants - human and animal power and to sorre extent wind and solar.

Cost Comparisons: In a recent intensive drilling project in Zimbabwe in which sorre 370 boreholes were cornpleted, the average cost per borehole, taking all factors into account, was Z$7,333 or Z$9,621 if the 88 unsuccess­ful boreholes are discounted. This cost per borehole is very little less than the projected cost of a 3 m diarreter collector well which could produce many times the yield. Future collector well costs might also be reduced, either by a reduction in well diarreter and/or efficiency in operations, particularly for larger scale production developments. Since well construc­tion is mainly within the overburden, it may also be hoped that the success rate would be higher. Final decision on a well site need not be made until after tests on a shallow, small diameter temporary boreholes which should be fairly conclusive, and prior to expenditure of the main capital outlay of construction. This situation contrasts with the circumstances for a deep standard borehole.

5.2 Collector Well Diameter.

One main objective in excavation of the large diarreter (3 m) shaft is to allovl entl-ance of the radial drilling rig. Since it is now planned to have the facility of a drilling rig of 2 m diameter (or F, metres for a manual horizontal rig), the importance of the dug well diameter in relation to overall performance requires to be ascertained. Modelling studies are in progress to provide more precise interpretations and will be reported upon later, but some preliminary conclusions can be drawn from first prinCiples.

Since the longer t:i.rre drawdowns are proportional to the log of the reciprocal of the well radius*, there is generally little advantage to be gained by increasing well diameter, unless this increase is very large. This is indirectly the effect of radial drilling. There is however e1e significance of well storage which can be used to best advantage by cyclic pumping. The effect of cyclic pumping is to reduce the maximum drawdown

* So = 2~iiH In r: ; So is drawdown, R is dependent on geohydrological

and boundary conditions and re is the equivalent well radius, a function of the well construction.

in the purrq:>ing well and in the longer tenu to increase the overall inflow from the aquifer. 'This has been recently derronstrated in a paper by Holt and Rushton* by means of a numerical model and aquifer characteristics similar to those in basement overburden (T = 45 m2 day-I; Sy = 0.012) • Short tenu drawdawns will obviously be most sensitive to the purrq:>ing rate and frequency of cycles. Optimised abstraction in the long tenu relates to the nagnitude of daily abstraction and the munbers of purrq:>ing phases. It is less sensitive to the rate of pumping in the separate phases, i.e. the rate can be quite high.

A similar modelling study is in progress at BGS which will also examine the relationship of variations in well storage to optimised abstraction rates, taking account also of the effects of radial drilling and heterogeneity of the aquifer. It is thought that the beneficial effects of an increase in shaft diameter might perhaps be more efficiently met by an increase in radial lengths.

5.3 Radial Drilling.

Drawdowns in the collector well can be considered as being canposed of two parts: (1) a long tenu drawdown (a background level) which occurs at the start of each days purrq:>ing reg:ime and (2) an additional short term drawdown resulting from each days purrq:>ing. 'The long term dravldowns are relatively insensitive to the well radius with little effect unless dimensions are increased very significantly or in response to radial drilling.

'The equivalent well radius, re, of a collector well is a function of the well construction and an average value based on field tests has been reported by Huisnan to be around 0.7 (L + rs) where L is the length of a collector and rs is the shaft radius. For a 3 m well and 30 metre radials, this would have the effect of halving the drawdown in the collector well as canpared with the original 1.5 m radius well shaft. 'This has been the observed effect at the two well sites at Hatcliffe (Table 2) although a more precise correlation of transmissivity based upon the borehole and the ill well tests indicates an anisotropy of 0.1 (Kh = lOKv)' 'The improvement in the response t:ime at C'1urape was some 7 t:imes that of the corresponding time for the ill well, implying a much greater increase in the effective radius Lhan 0.7 (L + r s )' 'This response characteristic is thought to relate to a layered system in which the radials are being drilled in a more permeable layer close to the bedrock junction. Modelling studies are also attempting to confirm this apparent relationship and its implication on well performance, including the effects of cyclic purrq:>ing. Since recovery rates appear to be enhanced significantly in the layered case, the well storage can be manipulated differently to the homogeneous case and could achieve greater productivity. Longer t:ime drawdowns will also be more Significantly affected by this najor apparent increase in the effective well radius.

* Holt, S and Rushton, K R, 1984. An investigation into the effect of different purrq:>ing reg:imes on drawdown in large diameter wells. Hyd. ScL Journ., 29, 3, 9, pp 271-278.

5.4 Recharge.

A high degree of uncertainty exists on rates of annual recharge into baserrent aquifers and on recharge processes. Base flow analysis on large catchJrents overlying baserrent rocks tend to indicate quite rroderate annual recharge rates, such as 18 mn/a in a large basin study in Malawi. Evidence of perennial evapotranspiration from phreatophytes in dambo areas suggests a Imlch higher order of recharge, possibly of the order of 100-200 nun in regiOns of c. 8oo/1<XO nm annual rainfall with the bulk of the recharge circulating and discharging into the local dambo systems. 'Ib some degree, the low chloride content of basement water and taking account of the runoff coefficient of major river basins in this region (10% for the Limpopo, 8% for the Zambezi) implies a Imlch higher annual recharge rate than the 18 nun noted above.

A second uncertainty exists on the location of recharge. Although rr.uch of the upper sequence in the weathered overburden tends to be clayey, the evidence of water level response characteristics to rainfall tends to scggest the incidence of rapid, local recharge which is inconsistent with an apparently extensive clay cover. The feature suggests that much recharge occurs through rapid bypass mechanisms rather than by dispersed matrix flow controlled by overall soil moisture deficits in the upper profile.

Both these features, total amount of recharge and the location and process of recharge, could have important implications on the longer term drawdown responses of collector wells which would be particularly critical during seasonal or extended drought periods. It is worth noting that this recent programme was carried out during the final year of a major three year drought period when water levels would be expected to be unusually low. In fact the first significant rainfall to break the drocght occurred during the long duration testing of the fourth and final well site (Hatcliffe h'indpump). Studies on recharge rates and processes are incor­porated into the current research programme on baserrent aquifers and hopefully will enable a better understanding to be gained and more precise quantification.

6. SUMMARY AND CONCLUSIONS

(a) Four collector well sites have been constructed, three of which were successful. One site was unfavourable and in similar circumstances in the future would be rejected on the basis of preliminary exploration.

(b) The successful wells were able to produce pumping rates between 1.5-3.6 litres/sec in sustained three by two-hourly periods per day for 14 days·and by extrapolation could have continued for more than 100 days without drawdowns reaching pump strainer levels near the base of the wells. It is worth noting that these tests were carried out towards the end of a third year of a rrajor drought.

(c) Abstraction rates are thus 4 to 10 times higher than median short term yields of standard boreholes in granites and gneisses of the Basement Complex of Zimbabwe and these latter quoted yields on test are likely to be significantly higher than sustained or extended yields on production. Pumping drawdowns in boreholes on test are likely to be at a maximum, around 40-60 metres. Pumping drawdowns in the collector wells were in the range 4 to 10 metres at the Imlch higher pumping rates quoted.

(d) Construction costs for 3 m collector wells in a properly organised production programre have been estimated at Z$lO,(X» or less which is little rrore than for a standard borehole. Unit costs of water are therefore much lower.

(e) At two of the three successful well sites, the effect of the radial drilling corresponded with a theoretical increase of the well radius to 0.7 (L + rs) where L is the length of a collector and rs the shaft radius. This has had the effect of halving the drawdown in the original 3 m diameter well. In the third case, the effect of radial drilling resulted in a seven-fold improvement. The feature may relate to the radials being located in a layer of high perme­ability close to the junction of the overburden and the fractured bedrock.

(f) Although long time drawdowns will be insensitive to the well radius, excavated or effective, by virtue of these being proportionate to the log the reciprocal of this value, the use of cyclic pumping can increase the effectiveness of a large diameter well, not only in relation to ~ drawdown but also to the inflow which occurs from the aquifer into the well. l1:>delling studies are in progress to examine the sensitivity to well diameters and the increased recovery rates from radial drilling to variations of abstraction rates and phased pwnping. The results will be reported upon in due course.

(g) These large yields from the basement aquifer, if consistent and sustainable which present evidence would favour, have an obvious potential for developrrents with larger demands - small urban centres, piped distribution, small scale irrigation etc. Addi tionall y, the small drawdowns make the collector wells attractive for lower powered pumping plants - human, animal, solar or wind, with consequeni.: reduction in both capital and maintenance costs of pumping plant and higher yields for low energy inputs.

(h) There is a need for substantial additional studies to deterTIine the overall feasibility and potential of collector wells in basement aquifers. A further programre is planned in Zimbabvle, and hopefully l\1alawi using a second rig constructed to operate in a 2 m shaft. There will also be trial experiments with a manual hydraulically operated rig. Basement aquifers occur extensively in Africa and it is hoped that this report might stimulate developrrent of trial projects in one or other of these.

(i) Although present planning concerns collector wells to moderate depL~s, there is no reason why deeper levels should not eventually be tapped if economics and yield requirements are favourable. The problem of stability of caissons for deeper wells might be overcome by con­structing a 3 m shaft to water level using concrete rings bolted together and emplaced from below and a 2 m caisson constructed internally within the 3 m upper shaft and below water level.

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Appendix I

MURAPE SCHOOL

1. LOCATION

Murape School, Harava District Council, Seki Tribal Trust Land. Grid reference of well:

UR 081039 on 1:50,000 sheet SEKI 1831 Al

2. DEMAND

Domestic water supply for 600 pupils and 75 residential population (staff and families) of the primary school. A new secondary school is in course of construction. Additional demand should also allow, if possible, for numbers of people in immediate area (2 km radius) whose standard supply from shallow wells ceases during periodic dry periods. Irrigation supply for school garden would also be advantageous.

3. SITE DESCRIPTION

The selected well site is to the north of the school and in the flat valley of the Masukondora, a minor and seasonally flowing tributary of the perennial river, the Nyatsime.

The underlying bedrock belongs to the Older Gneiss Complex and exposures are rare in the general vicinity. The principal lineation identified on air photographs trends NNE-SSW which is reflected in the orientation of hill masses. Subsidiary lineations are to the ESE-WNW and NNW-SSE. In the large diameter well, shallow dipping fractures were observed striking ~~~-SSW and also vertical fractures trending 11N\'J-ESE.

4. SITE INVESTIGATIONS

4.1 Geophysics and Test Drilling.

The site plan of exploration boreholes is shown in Figure 1. Resistivity surveys were centred on the large diameter well and were carried out during the period of early construction (Figure 2).

Exploration drilling was carried out by rotary rig and mainly used air flush. Where hard rock was encountered at shallow depths in a few instances, water flush was substituted and deeper drilling demonstrated the presence of residual masses within the weathered sequence. In all, 22 boreholes were drilled of which 6 were in pairs and close to the well site so that drawdown observations could be made for a Boulton-Streltsova analysis (Table 1). In the event, shortage of time did not allow equilibration of water levels which is required for the test.

The drillers log of the first 14 boreholes, drilled by air flush, records the presence of much weathered granite throughout the sequence. However, Bh 15 samples which were examined by the hydrogeologist consisted largely of sands,

gravel and some clay, the latter being particularly abundant in the top 7 metres. In subsequent holes, which were drilled by water flush, clay featured more commonly in the log and its absence in the earlier logs may relate in some part to inadequate sampling procedures with air drilling. In the samples fran the dug well, the material from 7 to 13 metres is consolidated, and consists of very weathered 'granite' with interstitial clay (Figure 3). In surrmary, it may be assumed that the upper 3/4 metres is generally unconsolidated, sandy above to more clayey below, succeeded downwards by weathered but largely consolidated material. The first strike of water is generally about a metre or so below subsequent rest water level and confirms the general presence of a clay layer above this level. Depth to bedrock is in excess of 18 metres in the vicinity of the selected well site and decreases progressively southwards to about 9 metres at a distance of 50 m from the well (Figures 4 and 5). Static water levels on the observation boreholes are between 2/3 metres below ground level. The water table slopes to the north towards the valley (Figure 6). Observations on lithology during radial drilling demonstrated high irregularity with little apparent correlation with sequences observed in nearby boreholes.

Electrical resistivity data were collected at 48 points along 8 radial lines centred near the -site of borehole 15. The points were at 10 m intervals to a maximum distance of 70 m from the borehole. Measurements were taken using a Schlumberger array configuration for current electrode separations of 15 m, 22'; m, 30 m, 37~ m, 45 m, 52~ m and 60 m with a potential dipole of 6 m and with orthogonal array alignments of NW-SE and SW-NE. Apparent resistivity values calculated from the field data were plotted as pseudo-sections with current electrode separations, indicative of the depth of investigation, along the vertical axis. An additional parameter, the apparent layer resistivity derived from a combination of values at adjacent spacings was plotted separately for both array orientations.

Comparison of the results obtained in the two directions showed distinct differences between the data sets at about half of the points. The distribution of these points appeared to be random with no obvious pattern to suggest the presence of linear fracture zones. This is consistent with there being discontinuous bands of fresh rock within the more weathered material as indicated by the drill ing results. The sectional plot.s show both localised anorralies and nDre systerBtic variations across the grid but these are brought out more clear} y on the p},::m contour plots. 'rho overall trends are \-\i1\Tt,\i-ESE, differing S(XllC\\iilz,1;: from the strike of the contours on depth to l:;.ec1rcx::k and \"vat;?1: table: \'ih.i.ch J~oll_c\'\":3 Cl direc"tion sI i.ghtl l' north of eas·t. Apparent resistivi t,y values are lower to the north over the deeper b<e'drock and also along the first 20-30 m of the southerly line: anoIncl.lously higher values occur on DIe southeasterly 1 ine a1 "hoLli)g the dug well radial to the SSE proved only weathered granite here. ~~es.ist.iv.it.ies of >35 ohm.m at the closest electrode spacing imply an absence of moist clay in the near surface layers and with values exceeding 100 ohm.m in places a variable thickness of sands is indicated.

Plots of the apparent resistivity data in the form of sounding curves suggest that a zone of 35-70 ohm.m extends to a depth of 8-14 m over most of the grid: to the north there is less evidence of more resistive cover but it may be that it is thinner here rather than not present. The deptl1s to bedrock are not well defined and at some pcints they could be interpreted as 15-20 m. The resistivity at the widest electrode spacing is still increasing but values of >100 ohm.m are indicated for the compact granite to the north and >200 ohm.m to the south: if this difference is genuine it would imply a slight change in mineralogy or in the frequency of joints within the granite. Where the values differ with orientation there is a tendency for those measured in the SW-NE direction to be higher at the wider electrode spacings: this can be attributed

to anisotropy within the comp'l.ct granite such that the SW-NE dir,3ction is closer to the strike of steeply dipping contacts or joints, putting it at an angle to the main trends in the upper layers as expressed in the apparent resistivity contours. Hooever; 'the evidence for this is limited.

During construction of the dug well it was noted that the fissures and joints were caning in from NNW and WNW directions in conformity with the app'l.rent resistivity contours. The random set of Itihologies penetrated by the rakers implies that there is little p'l.ttern within the material of the weathered zone on a local scale: the fact that the rakers to the southwest and south-southeast p'l.ssed through more uniform sequences and did not encounter hard rock could not be predicted from the geophysical data except in that they lie close to the main trends. The occurrence of fresh granite boulders within the weathered layer is not apparent as such but the resistivity interpretations give an indication of depth to bedrock which provide some check. At borehole 12, for example, a depth of 4 m would have been inconsistent with the closest soundings which indicate values of about 15-16 m here as against <12 m on the southerly line. Results from near borehole 4, where no hard rock was found to 17 m depth, show lower resistivities for the underlying layer but the interface still lies in the 14-18 m range: as the soundings also show inhomogeneity the thickness of the weathered mantle at borehole 4 is probably close to the local maximum. As there is no data over the undisturbed well site it is not clear whether the deeper bedrock could have been predicted but the apparent resistivity contours suggest variations crossing the site.

4.2 Aquifer Testing.

The programme of aquifer testing was of limited value. The aquifer has low permeability, probably lower than typical basement aquifers, and the constraint of the low open area of the emplaced screen (0.25% open area) resulted in high pumping drawdowns and discharge rates which were difficult to maintain. Slug tests were also affected. Results are shown in Table 2. Assuming a low transmissivity about 2.5 m2/day, a standard borehole even if screened properly would be unlikely to yield much more than the minimum acceptable for hand pump abstraction (0.1-0.25 l/sec).

5. LARGE DIAMEI'ER WELL

The site selected was in the viCinity of boreholes 15 and 14 where depths to bedrock were encountered of 18.6 and 22.2 m respectively. construction commenced on 27 October 1983 and the caisson was completed at 14.25 m beloo ground level on 7 June 1984. A 2 by 1.5 m deep sump was dug beloo the caisson (to promote upward inflow) and infilled with ~!% inch stone chips. The upper 3

m of the brickwork of the caisson was solidly cemented. The annulus was gravel packed with the same size stone to within 3 m of the surface, closed off with a concrete seal and the upper collapsed zone infilled with spoil from the well (Figure 7). The well was completed with an octagonal apron of reinforced concrete and a rendered brick wall one metre high.

Unconsolidated sandy and gravelly material with some clay occurred to 8 m beloo which the formation consisted of weathered but consolidated rock, originally an augen granite gneiss. The weathered rock was difficult t excavate with picks but proved quite friable once removed from the well. Fissures occur at intervals, usually infilled with clay and residual quartz masses. Water infloos commenced below 4.5 m below ground level but the main infloos occurred beloo 12.5 metres and are generally associated with visible fractures.

5.1 Pwnp Testing.

Although main construction was completed on the 7th June, dewatering to allow digging of the basal sump vas continued until June 13th and the water levels were still well below full recovery at the time the first test corrrnenced. This circumstance made interpretation" more ambiguous.

Test I 18th June

Static water level: 3.68 m below datum (estimated) Observed water level at 1020 hrs: 10.256 m below datum Simul taneous reading of recorder tape: 10.189 m Conversion factor: +0.067 1225 hrs test ended - tape reading: 10.620 m Average discharge: 0.70 litres/sec 1100 hrs test started - tape reading: 10.171 m Duration: 85 minutes

Test II 19th June

Static water level: 3.674 m below datum (estimated) 0945 hrs test started - tape reading: 9.940 m 1730 hrs test ended - tape reading: 11.940 m Average discharge: 0.64 litres/sec Duration: 465 minutes Maximum actual drawdown: 8.266 metres Recovery time between 10.176 and 9.940 water levels (tape readings): 495 mins

NB: Recovery data to 50% was not recorded on this test due to the high initial drawdown. Duration of recovery over a fixed interval is used for comparative purposes.

6. RADIAL DRILLING

Six radial holes were drilled (Table 3) during some 6 working days between June 25th and July 2nd.

TABLE 3

Radial Drill Holes

Radial No. Length Orientation (metres) (degrees from true north)

1 30 54 2 30 143 3 25 200 4 30 235 5 30 325 6 30 22

The radials were drilled using mainly air hammer (subsidiary rock roller) and screened with Terrafilter (mesh wrapped screen). Figure 8 shows the plan of the radials and some lithological data. Radials 6 and 3 were drilled along the

strike of the shallow dipping fracture and all radials would tend to intersect the main ESE-WNW vertical fractures at steep angles •

. ,.

7. SHORT DURATION PUMPING TESTS ON COLLECTOR WELL

Two pumping tests were carried out on the Collector Well on July. The aquifer had been allowed to rest from the 3rd of levels in the well were still some way below full recovery. tests are set out below.

Test I 9th July

the 9th and lOth July but the water Details of the two

Static water level (estimated): 3.544 m below datum Observed rest water level at 1140 hrs: 6.004 m below datum Simul taneous reading of recorder tap2: 5 .941 m Conversion factor: +0.063 1240 hrs test started - tap2 reading: 1343 hrs test ended - tap2 reading: Average discharge: 2.11 litres/sec Duration: 63 minutes

Test II lOth July

5.930 m 6.833 m

Static water level (estimated): 3.809 m below datum 0930 hrs test started - tap2 reading: 6.176 m 1440 hrs test ended - tape reading: 10.176 m Average discharge: 2.04 litres/sec Duration: 310 minutes Recovery time between 10.176 and 9.940 tap2 reading levels: 70 minutes

The basis for comparison of the p2rformance of the collector well and the large diameter well has been taken as the relative recovery times between 10.176 and 9.940 tap2 reading levels which are 495 and 70 minutes resp2ctively. Conditions were not precisely the same during these tests. Prior to the LD well test (without radials) dewatering had been carried out sufficiently to allow op2rations in the well and the observed water levels were still well down at the start of the test. During radial drilling op2rations, dewatering over a larger area had almost certainly occurred but there had been substantial recovery. Measurements on the observations demonstrate that with two exceptions (bcth very close to the well) water levels were lower at the commencement of the test on the collector well than in the test on the LD well. This circumstance enhances the remarkable improvement in the collector well's p2rformance.

8 . LONG TERM TEST

A fourteen day pumping test was carried out between November 6th and November 20th. Water levels had virtually recovered from the previous pumping tests and dewatering effects of the radial drilling but a natural regression was occurring due to the absence of significant rainfall since March of that year. A single heavy rainfall event occurred on the day previous to the test commencing but appeared to have had little or no effect on the remoter wells (such as No. 7) which were indeed largely unaffected by the long term test also.

Due to the limitations of the pump column, the initial setting of the pump at 11.8 m below datum required reduction of abstraction from 1.8 l/sec in the first

two days to 1.12 Vsec for days 4 to 6. The pump setting was subsequently lowered to 14.2 m and discharge was maintained constantly at 1.53 l/sec from day 8 to day.14. Figure 9 shows a plot of the well water levels recorded at the end of each days pumping-recovery cycle. There were two periods when pumping rates were constant enough to allow a rough transmissivity value to be obtained using Jacob's rrethod. A transmissivity of about 7 m2/day was indicated. Also, it can be estimated that after 100 days of pumping at 1.53 l/sec the background level v.Duld be about 8.8 m drav..down and that an additional 2 m maximum drawda.vn w:mld occur as a result of each dailY pumping cycle. This prediction assumes there are no nearby geological boundaries and that significant recharge from rainfall will not occur during the same period. A drav..down map at the end of the pumping period is shown in Figure 10. The calculated specific yield based on the volurre of water pumped and the volume of aquifer dewatered is about 0.01 which is a reasonable figure for the clayey formations in the upper levels of the saturated overburden. Drawdowns can be expected to occur at a lower rate when water levels drop to deeper levels (coarser grained material, higher specific yield). A chemical analysis of a water sample collected on the last day of pumping is shown in Table 4.

9. COSTIN3S

The Murape and Marikopo collector well projects were the first two commenced and the work was carried out between September 1983 and August 1984 with an overlap between January and June 1984. The Murape project occupied sorre 6 team-months and Marikopo 4 team months. A summary of the basic costs reckoned on a pro-rata basis is set out in Table 5 below.

*

TABLE 5

Summary of Murape Site Project Costs

Geophysical Survey Test Drilling Well Construction* Local costs radial drilling (fuel, crane

hire etc.) Local costs well testing (fuel, staff) Personal transport for superviser and driller Caravan Hire

Zimbabwe

6,755 7,781

19,889

1,120 704

11,542 1,725

50,636

Cost of gantry (Z$lOOO) which is reusable is not included.

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1. LOCATION

Appendix II

MARIKOPO SCHOOL SITE

The site is on common land adjacent to the Marikopo School grounds which are in Derra area, Seki Tribal Trust Land. Site coordinates are:

UQ 118984 on 1:50,000 sheet Seki 1831 Al

2. DEMAND

A water supply is required for the several hundred pupils and staff of a primary and secondary school. Present main supply which is poor and intermittent is from a dug well within dolerite and sited within the school building complex.

3. SITE DESCRIPTION

The site is in the upper valley of the Madedzurgwi stream near the watershed with the main river Hunyani and occurs on rocks of the Older Gneiss Complex. Bedrock outcrops are few but scattered boulders of granite are common to the east of the well site and an outcrop of dolerite, part of an extensive sill occurs close to the school (Figure 1). The dip of the sill is not known but no dolerite was recorded in any of the boreholes drilled to the north of the projected boundary line although found in all the boreholes on the projected dolerite outcrop. The shallow water levels in the granite overburden (c. 4 m bgl) are likely to be a response to damming by the massive dolerite rock and clay overburden which occurs downs lope of the granite.

The site was not appropriate for a collector well because massive bedrock occurs at shallow depths and the overburden permeability is low. A deep borehole would have been significantly cheaper and might have had higher chances of succes. However the site was conveniently located to operate in tandem with Murape and this association greatly influenced the decision to proceed with dug well work at this site.

4. SITE INVESTIGATIONS (Figures 2A and 2B)

4.1 Geophysical Surveys.

Much resistivity surveying was carried out by a team from the MWRD under the direction of Mr L L Hindson and the resistivity profiles and apparent layer resistivity diagrams formed the basis of interpretation. Survey lines radiating from the well are shown in Figure 3.

Electrical resistivity data were available for 52 points along 7 radial lines centred on the site of the dug well. The points were at 5 m intervals to a maximum distance of 45 m from the well. Measurements were taken using a Schlumberger array configuration for current electrode separations of 15 m, 22~

m, 30 m, 37~ m, 45 m, 52~ m and 60 m with a potential dipole of 6 m and with orthogonal array alignments of NW-SE and SW-NE. Apparent resistivity values

calculated from the· field data were plotted as pseudo-sections with current electrode separations, indicative of the depth of investigation, along the vertical axis. An additional parameter, the apparent layer resistivity derived from a combination'of'values at adjacent spacings was plotted separately for both array orientations. The geophysical surveys covered a much more extensive area in the course of trying to locate a suitable site. These included traversing at 20 m, 50 m and 100 m current electrode separations with the resistivity equipment, and measurements with the EM34-3 conductivity mapping instrument. On that part of thedata relating directly to the dug well site is considered here.

EM34-3 traverses to the south of the well found apparent conductivities in the range 6-50 mS/s (equivalent to 160-25 ohm.m as resistivities). The higher values were attributable to a dolerite sill and associated clays which appear to extend southwards, while to the west there was evidence of ~ contact against more resistive, granitic material. Resistivity traverses confirmed this increase in resistivity towards the west though, as the depth of investigation was greater and the resolution reduced, the contact apparent in the near surface was less clearly defined.

The detailed survey around the well itself showed marked variations across the site. A comparison of the results obtained in the two directions indcated that most of the pcints were affected by inhomogeneity. Apparent resistivity values for the closest electrode spacing were high and erratic which suggests the results may have been distorted by high electrode resistances and variations in the superficial material. The sectional plots bring out both vertical and lateral resistivity contrasts with a 3-layered structure clearly developed to the south and west due to the presence of a conductive intermediate zone. Apparent resistivities are in the range 200-600 ohm.m at the 15 m current electrode spacing, and reach a minimum of 30-40 ohm.m in the extreme southeast of the grid at the 30-52~ m spacings: at the widest spacing the values are not invariably increasing but typically 100-150 ohm.m. Plan contour plots bring out the lateral variations across the grid more Clearly. with the 37 m current electrode separation the contours trend NW-SE overall but there are also indications of more N-S features with a marked discontinuity from 25 m to the southeast. A belt of more conductivematerial lies in an arc passing from west of the well to the southeast and this is still expressed in the plot at 60 m spacing.

Plotting the resistivity data as sounding curves emphasises the extent to which the superficial layer and anisotropy have distorted the results. This makes the curves irregular and produces gross discrepancies in shape between the two orientations. The value at the closest spacing is nearly always too high to fit a horizontally layered model while, at the widest spacings, the curves are sometimes heading in opposite directions. Resistivities of >500 ohm.m to depths of 1-3 m indicate a cover of dry sand. The intermediate layer gives values of 25-100 ohm.m covering the range from clay to weathered granite: its thickness seems to be about 6-10 m but the lack of definition and the unreliable measurements at the beginning of the curves makes this uncertain in some cases due to the range of equivalence. Even along the lines to the north and east, where shallow bedrock had been expected, the resistivities at depth are relatively low. This suggests either that the hard granitic gneiss has a varied texture or is underlain by weathered or a different, more conductive rock or that while gross mechanical changes have yet to occur, weathering processes have affected the resistivity of the rock to depths of 10-20 m. The conductance of the intermediate layer (that is the product of its thickness and conductivity) is lower to the north and east so that to maintain its thickness the resistivity has to be increased towards 100 ohm.m compared with 30-40 ohm.m where clays are

present. The resistivity of the deepest layer appears to be in range 150-500 otrrn.m though the curves do not extend far enough to define it.

Of the rakers from the dug well five were drilled through hard granite; to the north and east an alternating sequence of soft and hard granite, possibly boulders or differential weathering near the bedrock interface, was proved; to the south there was weathered granite passing into a sandy clay. The raker to the southeast did not extend far enough to penetrate the zone of ananalous resistivity - a narrow belt of higher values followed by more than usually oonductive material. Boreholes in this direction did not encounter dolerite and a thicker clay/weathered granite sequence would therefore be expected locally at about 40-50 m from the well: with the high clay content likely to restrict permeability this zone oould in fact be less productive than the more resistive, but sandier zones. The general impression from the resistivity interpretations is that the depth to bedrock is less variable than might have been first thought but that the nature of the regolith and weathered granite-gneiss above this average level does show marked differences. The depth at which the rakers were drilled was perhaps 1-2 m below the average level of bedrock and sing an upward inclination of about 50 might have produced better results, within the limitation of available drawdown.

4.2 Test Drilling.

A total of 53 boreholes were drilled at Marikopo in the oourse of the lengthy project of which 31 are shown in Table 1. Boreholes not listed in the Table include:

Bh 1-8; Bh 40: Bh 10-11: Bh 21-22: Bh 41-48:

Bh 39:

dolerite at less than 10 m Clay (above dolerite) dolerite at less than 10 m shallow boreholes drilled close to LD well for aquifer testing redrilled as 49

To summarise the results of the test drilling:-

Total

9 2 2

8 1

22

Eighteen boreholes (18) were drilled in dolerite or in clay overlying dolerite (most of which are located outside the area shown in Figure 1). Eight (8) were shallow boreholes for aquifer testing and sited close to the well. Five (49-53) were also drilled very close to the well to confirm overburden. Of the remaining holes, all of which were on Older Gneiss outcrop, 4 had to be aborted at shallow depths because of the presence of heavy clay, 13 enoountered bedrock at less than 10 metres and 5 only had an overburden thickness in exoess of 10 metres. Slug tests (and one short duration pump test) were carried out on two 'dolerite' holes and three 'granite gneiss' holes and all showed a transmissivityof less than one (1) m2 /day (Table 2). Figures 4 and 5 show variations in bedrock depths in the vicinity of the well.

5. LARGE DIAMEI'ER WELL

The well was sited at the one location where relatively thick overburden was enoountered and was completed to a depth of 11.7 metres below ground level. The upper 6 metres was composed of unconsolidated material which included

significant clay between 2 and 5 metres. Below 6 metres, the material was essentially consolidated - weathered granite with some clay infillings. The size analysis data shown in Figure 6 should note this change of condition. At 12.0 metres, massive granite bedrock was encountered.

The dug well was constructed between 20th December 1983 and 18th July 1984. There were considerable problems during caisson construction, notably skewed brickw:xk and cracking of the cutting ring. Much time was spent in carrying out remedial work including slight modifications to the vertical reinforoement structure and considerable rebuilding. Ultimately the caisson rested on solid rock at one side, the remainder of its base being bricked up from dug floor level (Figure 7).

5.1 Pumping Test.

No proper pumping test was carried out on this well on account of its poor yield, nor was it possible to wait until water levels had recovered to equilibrium. In order to make comparisons with the subsequent collector well, a period of recovery was documented between the 18th August to the 21st August, with details as shown below.

Recovery Data: Marikopo LD Well

Estimated static water level on August 18th: 5.374 m below datum Observed water level on August 18th at 1600 hrs: 10.815 m below datum 19th August, 1625 hrs: 10.725 m 21st August, 0720 hrs: 10.583 m

6. RADIAL DRILLING

On account of the extremely poor inflow into the LD well, it was decided to drill eight (8) radials at 45 to each other and wherever possible to the maximum 30m capacity of the available drill pipe (Figure 8). The rig was in the well for 13 days but the holes were completed in nine days, the remainder of the time being largely devoted to repair work on the compressor. No. 8 raker had to be stopped at 15 m due to complete failure of the downhole hammer. Final completion of each hole was with 2 inch Class 16 pvc subsurface drainage pipe (local manufacture) which is perforated by 1 mm slots. Open area is just under 2%.

Radial No.

1 2 3 4 5 6 7 8

TABLE 3

Angle (degrees from true north)

171 81

351 261 216 306 36

126

Length (m)

27 30 30 30 30 30 30 15

7. TESTS ON COLLECTOR WELL

Despite the number of radials drilled, there was little significant improvement in the well yield. Observations on water level recovery were nade between lOth and 17th October during which time a reccvery of only 1.292 m occurred. An indication of the improvement can be obtained by comparing the recovery times between water levels 10.58 and 10.80 m below datum. The elapsed time for the LD well was 2.6 days and for the collector well 1.7 days, an improvement of only 34%. On the basis of a 24 hour inflow period, at close to maximum drawdown, the well is only capable of yielding 910 litres/day or sufficient for about 40 people and therefore quite inadequate for Marikopo School's needs. The yield nay hopefully improve somewhat when water levels rise following rain.

8. LOCAL COSTS

Summarised figures in Zimbabwe Dollars are set out below.

*

9.

Geophysical Survey Test Drilling Well Construction* Local costs of radial drilling (fuel,

crane hire etc) Local costs of well testing (fuel,

staff) Personal transport for superviser and

driller Caravan hire

Zimbabwe $

5,489 18,746 15,536

1,472

7,400 1,000

Z$ 49,643

Cost of gantry ($1,000) which is reusable is not included.

WATER QUALITY

The quality of the water is good as shown in the chemical analysis of a sample taken on 19 November 1984 (Table 4).

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Appendix III

HATCLIFFE WILLCWI'REE

1. WCATION

The site is within the grounds of the Institute of Agricultural Engineering, Hatcliffe Estate, Borrowdale. Co-ordinates of the large diameter well are:-

TR 993412 on 1:50,000 sheet Dcrnboshawa 1731 Cl

2. DEMAND

Water for irrigation is required with a desired peak demand of 2 l/sec for 10 hours per day per hectare.

3. SITE DESCRIPTION

The site is in the upper valley of the Gwebi River on flatter ground leading to the thalweg. The underlying rock is epidiorite of the Basement Complex (Figure 1). The site plan is shown in Figure 2 (a and b). The small dam was constructed to serve wildlife and lies astride a minor drainage channel which traverses the site, although with negligible incision.

4. SITE INVESTIGATIONS

These have included resistivity surveys, test drilling and aquifer testing, prior to the site selection and construction of the large diameter well.

4.1 Resistivity Surveys.

The objective of the resistivity surveys was to estimate the thickness of the weathered overburden and to obtain information on probable lithology. The work was carried out by a team from the MWRD under the direction of Mr L L Hindson and resistivity profiles and apparent layer resistivity diagrams formed the basis of interpretation. The survey lines radiated from the position of BH 12 and are sh= in Figure 3.

Electrical resistivity data were collected at 77 points along 8 radial lines centred on the site of borehole 12. The points were at 5-10 m intervals to a maximum distance of 60 m fran the borehole. Measurements were taken using a Schlumberger array configuration for current electrode separations of 15 m, 22~

m, 30 m, 37~ m, 45, 52~ m and 60 m with a potential dipole of 6 m and with orthogonal array alignments of N-S and W-E. Apparent resistivity values calculated from the field data were plotted as pseudo-sections with current electrode separations, indicative of the depth of investigation along the vertical axis. An additional parameter, the apparent layer resistivity derived fran a combination of values at adjacent spacings was plotted separately for both array orientations.

A canparison of the results obtained in the two directions showed distinct differences between the data sets at about one third of the points. There was

no obvious pattern to sug"est the presence of linear features and, although discrepancies were more common along the SW-NE line of readings, no indication of discontinuity feature between boreholes 17 and 28 as implied by the loss of circulation during drilling. The sectional plots indicate that there is a systematic increase in conductivity from west to east and in resistivity with depth. The contour plots define the trends more clearly and for a current electrode spacing of 15 m are aligned SW-NE with a marked increase in resistivity in the northwest of the grid and slack gradients in the central and southeastern parts where apparent resistivities of about 20 ohm.m occurred. At the 37~ m electrode separation the values showed a slight increase except in the north, where they were noticeably lower, and in the northeast, giving a total range of 20-40 ohm.m over the grid. There is little sense of trend apart fran the increase in resistivities to the west. At 60 m the pattern is similar but with a general increase in apparent resistivity values. The north-south trend in these contours bears some relation to the basement elevations while at the 15 m separation there is clear evidence that te clay cover gives way to a drier, coarser texture towards the north and west.

Plotting the apparent resistivity data in the form of souding curves confirms that a relatively thin resistive upper layer is present to the northwest while elsewhere the cover is more conductive. The underlying zone has resistivities of 10-25 ohm.m indicating clay-rich material as might be expected from the weathering of epidiorite. The depth to bedrock is not clearly defined but curve matching gives typical values of about 10 m and a maximum of less than 15 m. There is no evidence of a layer of intermediate resistivity and values increase rapidly at the wider spacings towards a level of several hundreds of ohm. metres though measured apparent resistivities were still less than 70 ohm.m.

The relation between the gecphysical and borehole data is reasonable at a qualitative level but the resistivity results do not provide depths to bedrock to better than 20% accuracy levels. Very localised variations such as those at th2 well site tend to be averaged out. Conditions over the grid as a whole appear to be similar below the superficial layers and there is no evidence of an associated change of lithology as implied at Hatcliffe Windmill site. The lwer resistivity of the weathered zone here in canparison with the sites in granitic terrain may relate to the type of Clay mineral produced rather than to any significant increase in the proportion of clay: in both environments the presence of clays will usually obscure any indication of the depth to water in the resistivity data.

It is important to note that the geophysical plots did not pick out the thin more permeable layer immediately above the bedrock and was presumably masked by the overlying more clayey formations. This is significant since the resistivity values of the overburden are generally lower at Hatcliffe willowtree than at the Windmill site and would be inferred generally as more clayey. Despite this, transmissivity was higher at the Willowtree site.

4.2 Test Drilling.

Twenty-nine test borings were made with a small rotary rig and water flush. The borings were lined with 90 mm (occasionally 50 mm) diameter plastic casing which was perforated below the water with 1 mm sawn slots (approximately 0.25% open area). Table 1 gives canpletion details and in Appendix I lithological logs are diagrammatically shown. Figure 4 is based on size analyses of lithological samples. Clay/silt content varies from some 78 to 95% in the upper 6 metres. Sand increases in the lower 3 m approaching bedrock, with the highest proportion (36%) occurring at 7 m. No differentiation of the Clay minerals has been made

although it is hoped to do so eventually, possibly using spectral gamma ray logging for more widespread correlations. The overburden sequence grades from a dark yellowish green colour near to bedrock through buff to orange red (oxidised) in the upper layers. The top soil is a chocolate brown clay with patches of black organic material. Occasional small residual blocks of weathered epidiorite occur within the sequence which is otherwise unconsolidated.

The ground surface slopes gradually southwards with relief of less than 2 metres across the site. Overburden thickness (Table 1) varies from some 8-15 m although there may sometimes be doubt as to whether a borehole has reached bedrock or a residual mass of bedrock within the overburden sequence. Present evidence suggests a broad 'channel' of thicker overburden extending north-south (Figures 5 and 6).

4.3 Aquifer Testing.

Evaluations have been based on recovery data from slug tests and short duration pumping tests (Table 2) and on two pumping tests on BH 17 using drawdown and recovery measurements in all other boreholes. Significant 'well' losses occurred in the pumping borehole due to the small open area of the perforated casing. Slug testing was carried out using a 20 litre 'slug' of water and a manual (electrical) probe to obtain recovering levels. The short duration (c. 60 minutes) pumping tests were carried out with a Mono pump. Calculated transmissivities by these methods varied from 11 to 49 m2/day, variations which do not wholly accord with the fairly regular flow net (Figure 7). Higher values occur more frequently in the thicker overburden.

Two longer pumping tests were carried out at the site prior to making the final decision to construct the large diameter well. Observations on drawdowns were carried out and time-drawdown (selected boreholes) and distance-drawdown plots were used for analysis. Discharge remained constant, within one or two per cent.

Basic details and test results are tabulated below. Chemical analyses of samples from three boreholes are shown in Appendix II.

Test I

Pumping well: BH 17 Pumping started: 1130 hours on 25 May 1984 Pumping stopped: 1700 Duration: 330 minutes

" " "

Average discharge: 0.32 litres/sec

" "

Location of discharge: 40 m to south east of site towards dam

Test II

Pumping well: BH 17 Pumping started: 0907 hours on 26 May 1984 Pumping stopped: 0705 " " 27" " Duration: 1078 minutes Average discharge: 0.58 litres/sec Location of discharge: 40m initially and subsequently 70m by pipe towards dam Groundwater temperature: 20/210 C Groundwater conductivity: average of 108 microsiemens at 250 C with minor

variations

Time-drawdown plots for Tes·t II (Figure 8 a,b) indicate transmissivities in the range 40-70 m2/day. The departure from linearity apparent in several plots between 200 and 400 minutes could be the result of a falling pump rate (not observed in measurements) or geological heterogeneity. Transrnissivities from the distance-drawdown plots (Figure 9) occur in the narrower range of 35 to 47 m /day. Storativity in all case except one, the distance-drawdown for 1300 minutes, is in the artesian range. Flow nets for various times during the long term pumping tests have been prepared by computer technique and reproduced in Figure 10 (a ...... ). They give confirmation of the fairly uniform transmissivity across the site.

5. LARGE DIAMErER WELL: SITU}} AND CONSTRlJCrION

The well was sited at BH 29 where the overburden had a maximum thickness of 14.8 metres. Final completion was at 10.8 metres below ground level (Figure 11) selected to ensure that the horizontal rakers would generally intersect the most permeable overburden above harder bedrock. Since the well was to be used for irrigation, the top 3 metres of brickwork were not solidly cemented as for wells planned for domestic supply. The construction of the Hatcliffe Willowtree well was effected between June 8th and October 2nd 1984.

5.1 Pumping Test on LD Well.

Pumping tests were carried out on the large diameter well on October 9th and 10th. Although some drawdown and recovery data were obtained from observation wells, the information was of limited quantitative value because the duration of the tests were insufficient to overcome the effect of well storage. Papadopulos has shown that a time equivalent to 25r,}/T (equivalent to 1.4 days for T = 40 m /day) would need to elapse before the Theis or Jacob-approximation could be used for c~alysis. At the time of the tests, the available drawdown in the well was 4.8 m approximately. The test pump was a diaphragm pump which, it had been hoped, would yield 2 litres/sec but in the event could not exceed 0.8 litres/sec and gave some trouble in the second test due to icing. There had been no rainfall in the Hatcliffe area for some time before or during the tests.

Test I

Pumping started: .0910 on 9 October Pumping stopped: 1121"" " Duration: 131 minutes Discharge: 0.685 litres/sec Wlximum drawdown in pumping well: 0.476 m Recovery rates: (i) 50% 300 minutes

(ii) 90% 1234 minutes Other observation measurements: Boreholes 16 and 25

Test II

Pumping started: 0815 on 10 October Pumping stopped: 1523"" " Duration: 428 minutes Discharge: 0.80 litres/sec Wlximum drawdown in pumping well: 1.437 m Recovery rate: (i) 50% 300 minutes

(ii) 90% 1215 minutes

The comparable recovery rates following the two separate tests demonstrate that the effective transmissivity remained unchanged despite the variation in drawdown which is suggestive of artesian conditions prevailing during these periods of time (42S minutes in second test). The behaviour accords with the high clay content of the upper levels in the well and the storativity values obtained in the earlier borehole pumping tests.

The 50% recovery times were interpreted according to the Herbert-Kitching method of analysis and the results were seen to be very sensitive to anisotropy (Kv/KH ratio) and to a lesser extent to the value assumed for total saturated thickness whiCh of course varies across the site if bedrock is taken as the base. With a thickness of S.S metres (maximum value at site of well), calculated values of transmissivity are sh= in Table 3. On the basis of comparisons with borehole pumping test data, a permeability ratio in excess of .025 is indicated.

TABLE 3

Transmissivities (m2/day) from 50% Recovery Rates in LD Well (Herbert-Kitching Analysis) •

Average Minimum Maximum

(i) Kv/~ = 1 7.0 3.6 13.9

(ii) KV/~ = 0.1 (i) 22.5 12.4 40.7 Aquifer thickness reduced to 6.S m (ii) lS.2 10.3 32.1

(iii) Kv/I)l = 0.025 40.0

6. RADIAL DRILLING

Six radial holes were drilled in S days between the 15th and 26th October. The five productive holes (one aborted) were canpleted with 2" slotted extruded fibreglass pipe with mesh filter (Terrafilter). Directions of drilling were planned in general accordance with the most productive test holes and the flow net patterns (Figure 12 and Table 4). No hole was drilled down gradient. Some lithological samples were collected. Because the down-hole hammer was not functioning, all holes were drilled with rock roller bit. Because of this, hole 4 which encountered hard rock at 6 m was abandoned.

Radial No.

3 2 1 5 4 6

TABLE 4

Angle (degrees from true North)

47 lSO 237 2S0 31S 327

Length

27 27 29 30

6 25

7. PRELIMINARY CONSTANI' RATE PUMPING TESTS ON COUECTOR WELL

pumping tests were carried out on the 31st October and 1st Novenber. Al though water levels equilibrated prior to each test, there was a natural regression occurring of about 1/2 Inn/hr. There was no significant rainfall prior to or during the tests.

Test I

Static water level pre-test: 6.067 m below datum Pumping started: 1015 on 31 October Pumping stopped: 1115"" " Duration: 60 minutes Average discharge: 1.21 l/sec Maximum drawdown: 0.366 (below initial static level) 50% Recovery: 100 minutes 90% Recovery: 760 minutes Measurements in observation wells: 0630; 1500; 1730 hours

Test II

Static water level pre-test: 6.109 m below datum Pumping started: 0800 on November 1st Pumping stopped: 1640" " " Duration: 520 minutes Average discharge: 1.71 l/sec Maximum drawdown: 1.823 m (adjusted) 50% Recovery: 151 minutes 90% Recovery: not obtained Measuranents in observation wells: 0730; 1330; 1620 hours

A fully quantitative comparison of the collector well and the original dug well is not possible since the durations of all tests were insufficient to allow precise analysis. However one good comparison can be afforded by the recovery rate from comparable drawdowns resulting from short duration pumping tests. In the earlier tests with maximum drawdowns of 0.476 and 1.437 m, 50% recovery was obtained in 300 minutes. In the latter tests, 50% recovery was obtained in 100 or 150 minutes, demonstrating an increase of immediate efficiency between 2 and 3 times. Using an aquifer transmissivity of 40 m2/day and a storativity of .01, the increase is comparable to an increase in the well radius from 1.5 metres to 12 metres or 41% of the average length of drilled radials.

7. LONG TERM INTERMITTENT PUMPING TEST (12 days)

This was conducted in the period between 24 November 1984 (commencing 0600 hrs) to 1758 hrs on Decemter 5th. Figure 13 shows the pumping rates, duration of pumping and well water levels. The top of the pump strainer was set at 9.3 m below ground level and only a short distance above the radial holes. The discharge was piped 193 m away from the well into a boundary ditch. The initial pumping rate had to be kept lower than desired due to problems in the discharge main which were subsequently overcome and the rate raised to 4 l/sec. Additional minor reductions had to be subsequently made with stabilisation occurring at about 3.6 litres/sec. The amount pumped daily at this rate was in excess of peak demand of 2 litres/sec/ha for 10 hours daily. It is significant

that this test was carried out prior to any significant rainfall occurring during a previous three year period of intermittent drought.

The intermittent pumping rates were sufficiently constant to allow analysis of the observed drawdawn rates by relatively simple methods. Fig. 14 shows the semi-log plot of drawdowns at the end of each 24 hour cycle (pumping and overnight recovery) versus log of time after start of test. The long term average discharge is 80 m3/day and Jacobs analysis applied to the slope gives a T value of 50 m3/day. It is interesting to note that even after 100 days of pumping, providing no change in boundary conditions occurs, the long term drawdawn will not exceed 1.2 m. The short term additional drawdown as a result of intermittent pumping is about 2.5 m. The total drawdown after 100 days will be about 3.7 m or 9.8 m below ground level which is still above the pump strainer.

8 • WCAL COSTS

A fuller discussion on costs is given in the main report but the basic local costs of the exploration and large diameter well construction are given in Table 5.

TABLE 5

Exploration and Construction of Large Diameter Well Costs (Zimbabwe Dollars).

References

Geophysical Survey Test Drilling Well Construction Local costs radial drilling (fuel, crane

hire etc.) Local costs well testing (fuel, staff) Personal transport for superviser and driller Caravan hire

Zimbabwe $

3,080 8,320 6,399

1,069 416

4,381· 271

23,936

Herbert, R and Kitching, R, 1981. Determination of aquifer parameters from large-diameter dug well pumping tests. Ground Water, Vol. 19, pp. 593-599.

Papadopulos, I S and Cooper, H H, 1967. Drawdown in a well of large diameter. Water Resources Research, Vol. 3, pp. 241-244.

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Approx. Dissolved Salines deduced from above

Alkalinity to Methyl Orange (as CaC0

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Appendix IV

HATCLIFFE WINDPUMP

1. LOCATION

The site is within the grounds of the Institute of Agricultural Engineering, Hatcliffe Estate, Borrawdale. Co-ordinates of the colletor well are:-

TR 982403 on the 1:50,000 sheet Domboshawa 1731 Cl.

2. DEMAND

Water supply is required for irrigation with peak yields of 2 litres/sec/hectare for 10 hours-day.

3. SITE DESCRIPTION

The site is in the upper valley of the Gwebi River on flatter ground leading to the thalweg. It occurs some 1500 metres to the south west of the Willowtree site described in Site Repcrt Ill. Although the published geological map (Figure 1) indicates that the underlying bedrock should be epidiorite, exploration drilling has demonstrated the overburden to be wholly of granite origin. Residual epidiorite masses were encountered when drilling the borehole fitted with a Windpump which is located some 50 m to the north, and the contact must lie between the collector well and the Windpump borehole (Site plan, Figure 2) •

4. SITE INVESTIGATIONS

Resistivity survey lines were centred on borehole 7 (Figure 3). Five exploration boreholes were drilled at this site to varying depths between 10 and 26 m without encountering bedrock (Table 1). The upper 6/7 metres in each hole were made up of clays with some sands or sandy clays, brown or buff in colour. The formations below are described in the drillers log as primarily sandy with some admixed clay. The sands are fine to medium grained in higher levels, becoming coarser with depth and locally including gravel. The sand etc. is mainly composed of quartz or feldspar grains which become fresher with depth.

Lithological analysis from the dug well (sample interval: one metre) is shown in Figure 4. In the top 3 metres, clay was not differentiated from silt. In the bottom 7 metres, silt (on a particle size basis) strongly predominated generally between 10-20% as compared with only 1-5%, and at 10 m, 87% is compcsed of silt-sized material. The sand compcnent is ill-sorted from fine to coarse grained; gravel in the smaller ranges. In the descriptions of the samples from the boreholes, the reference is invariably to clay and not silt. Whether predominance of silt in the size analyses carried out at the Water Development Soils Labcratory relates to the method of analysis rather than actual mineral type is not known.

Preliminary aquifer testing was carried out by a combination of slug testing and recovery analysis from short duration pumping tests (one hours duration with discharges around 0.75 l/sec). Ccmparable results from the two rrethods were obtained in Bh 7.

4.1 Geophysical Survey Interpretation.

Electrical resistivity data were collected at 43 points along 8 radial lines centred on the site of borehole 7. The points were at 10 m intervals to a !lB.ximum distnce of 60 m from the borehole. Measurements were taken using a Schlumberger array configuration for current electrode separations of 15 m, 22', m, 30m, 37~ m, 45 m, 52 m and 60 m with a potential dipole of 6 m and with orthogonal array alignments of N-S and W-E. Apparent resistivity values calculated from the field data were plotted as psuedo-sections with current electrode separations, indicative of the depth of investigation, along the vertical axis. An additional parameter, the apparent layer resistivity derived from a combination of values at adjacent spacings were plotted separately for both array orientations.

A comparison of the results obtained in the two directions showed distinct differences between the data sets at abcut half of the points. There was no obvious pattern to suggest the presence of relief fractures within the regolith but discrepancies were more common to the south and east of the grid. Lateral variations across the site can be seen in the sectional plots with an anomalous area around the dug well. The contour plot for a current electrode spacing of 15 m shows a strong SSW-NNE alignment with the decrease in resistivity from northwest to southeast following approxilffitely the topographic slope: a steeper gradient occurs to the southeast of the well. This reduction in values from 50-70 ohm.m indicates a change to more conductive, clay-rich material in the upper layers to the southeast and a higher soil moisture content. At the wider electrode spacings the influence of these variations is still seen although the change is more subdued as the resistivities are increasing more rapidly in the southeast: a subsidiary high is developed over boreholes 13-15 but highest values are found west of the water tank.

Plotting the apparent resistivity data in the form of sounding curves gives a clearer indication of the changes with depth and the distinctive nature of the results in the southeast of the grid is emphasised. The conductive, upper layers extend to depths of 13 m: there is no definite evidence of an intermediate zone as the apparent resistivityvalues increase towards undefined higher levels of perhaps several hundreds of ohm.metres. The other type of sounding curve shows a resistive uppermost layer 2-3 m thick though this is a layer with a resistivity of 30-40 ohm.m which appears to continue to little more than 10 m depth before values increase again towards 70-100 ohm.m.

The interpretation of these results in relation to the outcrop and drilling information is difficult because the grid did not extend to borehole 6 or to the rock outcrop and because the well site and the boreholes around it fall within a transition zone between two areas differentiated by their geophysical response. Comparisons with the data from Hatcliffe Willowtree and Murape imply that the ground to the east and south of the well is underlain by epidiorite, with an area of granitic terrain or its weathering products to the northwest. Extrapolation of the contours southwards towards borehole 6 is consistent with this too lying close to the transition zone. The occurrence of boulders of epidiorite beside the water tank may be misleading and the contact with epidiorite in this direction would be placed north of the grid. The observed variations in resistivity near the well imply lateral effects between boreholes 13-15 and borehole 7: this was confirmed by the differences in transmissivity.

Although the precis nature of these effects could not be predicted there is qualitative agreement with the well data in that radial number 3, which penetrated into the conductive zone, gave the poorest results and a clay-rich sequence; radial number 4 also penetrated a significant section of clays in a direction sub parallel with the resistivity contours of the transition zone; radials 1 and 2 to the southwest were within the zone of higher resistivities as defined by the results at 15 m current electrode spacing, and proved a sandier sequence.

The increase in resistivities below 10-15 m depth occurs within the weathered rock according to the drilling results and it is attributed to a decrease in porosity. Higher values at depth implied for the epidiorite to the southeast may reflect a narrower weathering front and shallower bedrock in canparison with the granitic rock. Paradoxically, the higher apparent resistivity values found to the north may be associated with a thicker sequence of weathered material but this is only one possibility.

The correspondence between the geophysical survey and test drilling is variable. Higher resistivities below 10/15 metres would be suggestive of bedrock but the change is transitional and the boreholes which penetrated more deeply (6 and 7) could have been within weathered residual bedrock which would not necessarily be apparent in the rotary drilled samples. The well site is close to the boundary of granite and epidiorite and samples from the deeper levels in the well indicate residual but unconsolidated granitic material. The geophysical survey results are anomalous therefore in indicating epidiorite to the south and granite to the northwest towards the existing borehole site. The thick sequence of unconsolidated overburden may be attributed to deep weathering close to the contact of the two rock types and perhaps also in association with the marked lineation which traverses the site and can be seen on air photographs. The more clayey sequence to the east and south is also apparent on the profiles.

4.2 Long Term Borehole Pumping Test.

A pumping test was carried out on the windmill Site on Borehole 7 on the 28th and 29th May 1984. Details are as follows:-

Pumping started: 1700 hours on 28 1-1ay Pumping stopped: 1145 hours on 29 May Duration: 1125 minutes Average discharge: 0.56 litres/sec Discharge tEmperature: 20. 50 C steady

pH: 6.65-6.85 HCO: 51 ppm (at 17.55) conductivity: 180 microsiemens at 250 C, in early test reducing to

125 microsiemens immediately before pumping stopped.

At the time of the test, digging of the large diameter well had commenced and had reached about 0.5 m below water level. The presence of the dug well could have affected the drawdowns in the other observation wells. Drawdown data were obtained on Bh 13 and 15 but the water level in Bh 14 was unaffected. Time-drawdown plots are shown in Figure 6. Bh 15 showed an apparent boundary commencing to take effect at 80 minutes and subsequently equilibrium at 240 minutes, suggestive either of a second boundary (recharge; ?the dug well) or a temporary blockage of the borehole. The limited draw:1own data on the dug well shows that its levels were being strongly affected at that time. The transmissivity values calculated from the drawdown plots are significantly higher than those obtained from the willowtree Site but in view of the

circumstances which include the presence of the dug well (ShallON depth) and the absence of any readings in Bh 14, the results must be somewhat suspect.

5 • LARGE DIAMEI'ER WELL

The well was sited close to Bh 7 which gave the highest transmissivity on test. Construction was carried out over approximately 21 working weeks between April 17th and November 15th, with lengthy delays reSUlting from the absence of an adequate dewatering pump. Problems in construction related mainly to the periodic inrush of unoonsolidated clay-sand material and the consequential tendency for the caisson to become skew. InflONs were reduced by emplaCEment of straw bales against the bcttan of the cutting ring. Both bricklayers and the diggers found it difficult to canpensate for the skewness. Digging should concentrate on the ION side and extend outwards to allow the caisson to return to verticality; bricks should be laid to retain the cylindrical form and not to extend vertically in a skewed caisson. A considerable amount of stone was required for the surrounding gravel pack due to the large amount of material which had moved in laterally into the well (Figure 7). The large diameter well could have been dug to deeper levels and still remain within unconsolidated material. The difficulties enoountered in construction motivated stopping at the depth shown.

5.1 Pump Testing on Large Diameter Well.

Pump tests were carried out on the LD well immediately after construction and prior to drilling the radials. Because of limited time (shipment of rig to Sri Lanka) the tests had to be carried out before the well had attained equilibrium. Essential details of the tests are listed below.

Test I

Pumping started: 0610 hrs on 16 November Pumping stopped: 0828 hrs on 16 November Duration: 138 minutes Average discharge: 1 litre/sec Estimated rest water level: 3.52 m below well datum [3.25 m bgll Observed initial water level: 5.175 m JJelow well datum Final well water level: 5.765 Maximum true draw:lown: 2.245 metres Recovery time between well water levels [5.765 and 5.050 ml: 525 minutes Volume of inflow: 5055 litres = 0.161 l/sec Recovery time between 5.685 to 5.05 well water levels: 478 minutes Discharge conductivity: microsiemens at 250 C

Test II

Pumping started: 0550 hrs on 17 November pumping stopped: 1250 hrs on 17 November Duration: 420 minutes Average discharge: 1.1 litres/sec Estimated rest water level: 3.52 m below datum Observed initial water level: 4.490 m below datum Final water level: Maximum true draw:lown: Recovery time between 5.685 to 5.05 well water levels: 497 minutes

6. RADIAL DRILLING

Four radials were drilled over 5/6 working days between the 19th and 26th November. Drilling was carried out by rock roller bit and water flush and there were delays and some difficulty in operations due to lost circulation and collapsing fonrations. Details of radials are listed below and further infonration is on the site plan, Figure 8.

Radial No. Length (m) Position (degrees from T. North)

1 29 254 2 28 215 3 19 90 4 30 35

The radials are aligned principally at acute angles to the general ground water flow direction. Geophysical data had suggested shallow bedrock to the north west. Radial 3 might have been predicted as a poor hole in view of the negligible response to pumping of Bh 14 in the initial borehole pumping test.

7 . CONSTANT RATE PUMPING TEST ON COLLECTOR WELL

An initial constant rate pumping test was carried out in order to make quantitative comparisons with the large diameter well. Water levels were still slowly regressing although minor amounts of rain had occurred in the previous week. Details of the test are as follows.

Pumping started: 1000 hours on 8 December Pumping stopped: 1243 hours on 8 December Duration: 163 minutes Average discharge: 6 litres/sec Pump suction: 8.72 m below datum Rest water level (estimated): 3.57 m below datum Observed initial water level: 3.622 m below datum Final water level: 8.715 m below datum M3.ximum dra\\down: 5.145 m Discharge conductivity: 105 microsiemens Recovery time between 5.765 to 5.05 well water levels: 291 minutes

The comparisons of recovery time demonstrate an improvement of 80 to 90% of the collector well in comparison with the LD well which is rather less than expected. The number of radials drilled is a minimum, and hole 3 is probably non-productive. Improvements could be anticipated with additional drilling.

8 . WNG DURATION COLLEcroR WELL TEST (INTERMITTENT PUMPING)

This test of 11 days duration was affected by heavy rainfall on several days (Figure 9). The discharge was piped 193 m away onto a gently sloping vlei where additional runoff occurred. Reduction in discharge from 3 litres/sec to 2.58 was made in two days and retained for the remaining 9 days. On acoount of the effects of the rainfall, the longer term trend on abstraction is difficult to predict. Figure 10 shews a plot of drawdowns in the collector well observed

daily at the end of a 'pumping-resting' cycle. The Jaccb method was UEled to analyse the plot and assuming an average yield of 0.69 l/sec for the first four days without significant rain, a transmissivity of 27 m2/day was obtained. The ccrresponding drawdown after 100 days of pumping without rainfall would be about 2.4 metres to which must be added the 3 m additional drawdown for the abstraction period. Water levels would still remain above the pump strainer.

9. COSTINGS SUMMARY Local

Geophysical survey Test drilling Well construction Local costs radial drilling (fuel, crane

hire etc) Local costs well testing (fuel, staff) Personal transport for superviser and driller Caravan hire

Zimbabwe $

1,540 2,080 5,445

1,375 1,270 4,381

271

16,362

NB: Costs for tccling up are excluded from this sUITlll\3.ry since will be spread over longer programme. Costs of superviser and driller are met from ODA and include radial drilling materials (screen, etc.) but exclusive of fuel. The very high cost of superviser's and driller's transpcrt should be noted.

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Appendix V

LI1110lLG[CAL UX;S OF SELECTED EXPWPATION BOPEHOLES

DEPTY (M)

PROJETT= L-·D WELLS iJJU'::I O'\l: MU~AP£ SCHf]OL \·J::.LL ~iG_ : DR,LLER:

ELEVATION (H\

FILE NO_: ELEVAT!ON (M):9B. 3

Oo.n-: DRILLED: -:-YPE O~ RIG: AIR FLUSH ROTARY

I THICI<Nr::SS i LITHOLOGY

: _._--------------_: -------------_._-----! -----------! --------------------------! FRO~ I TO FRG;': TO

i _. __ . ________ . _____ ._ i ___________________ 1 ___________ ; -------------------------1

O. DO i 9.00 i 3B.30 f 89.30! S. DD ! SAI\lD AND GRAVEL 1 3.00! 3.50 1 89.30 I e8. [lO! O. SO I GRANITE

----------------_._----------------------------_._-----------------------------

:--:---·~1

~l.~--L.J

·-·--~l

I

,---------1 . ,

8::DRJCK

c.urT

SCRLE,

FiLE LilHOLOGY Loun [ON !-'IURRPE SCHOOL

-, 3

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1 2

,

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---'I S

5

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,-

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"2 , -' 25

Gl=l . .5 ~1

BRITISH GEOLOGICRL SURVEY FiGURE_ ).

~

I

\·JELL LITHf)!....OGY

PROJECT: L--D \-.)::LLS LOCfYTIcr,:: HlIqClDt~ SCH!)I)L WELL NO.: 3 DqILLER:

F:::LE NO.: ELEVATION CM):98.3 Dp'E DRILLED: 7yPE OF RIG: RIR FLUSH ROTARY

----------_._-----------------------------------------------------------------EL~~)0,TI O\!

o'D I THICf<N~SS ! I (l"!) I

LI THOLOGY D:::PTH (M) ! _________________ ) ___ . ________ ._. ________ : ___________ ! _________________________ i

" FRO;1 TO ~::~T'"1 TO I 1 _________________ I _____ . __ • ____________ , ___________ 1 ----------------------.---!

0.00) 2. OD 98.:>;[) i SS. 30 2.00 SAND-COARSE 2.00 3.50 I '3::'. :';.{\ 9',80 I : .• ;jO SAND PNa G~AvEL

il. SO RESIDUAL GRA'JITE 3. 50 4. DO 4 DO 3. 0;]

3. 00 , • , &8

':It. ,~ 0 '.'{, '.'l c'''";, :; c

~'{" . 30 PS. 30 St.. 90

S. no r), 39

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0!cOSC.c.

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PROJEC1 FiLE LOc:rr i eN

BRI iSH

le. ;:;. ~,. c. ;;. G

G. ". ~ . n. ~' !.(' ::0. G. ,0.;";. c_ (' j. G. :: .. n. G. G. :")

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L fTHOLOGY

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rU~\/r:-v ~ K L r FIGURE 2'

WELL LITHO,---OGY

PROJECT: L-D wELLS LOCPTIO~J: MUR.Cl.PE SCHrJf)L W;::LL NO.: t. DRILLER:

FILE NO. : ELEVATION·(M):98.~4

DPTE DRILLED: "'YPE OF RIG: AIR F!_USH ROTARY

DEPTI.l ~~LEVA,ION I THICKi\Ii;:SS I LITHOLOGY (M) (M) I (M:t !

I ___ ~ _____________ I ------------.-------, ------------) -----------------------_~_I

I FROM TO FROM TO I ! -.--------------.--! ------------.--------. ~ ---_·_------1-------------------------1

G.00 -:: _ 00 !

;:. DO : : C. 9D I

?·8. f, t 1 %_ t.t:

ss. F,r, >; L '7(,

2_ 00 ! SAND-COQqSr:: J (, _ 30 I ~::AND AND G::XPV!=:L

'---' ,---,-,, I , 5q~l;: _po r\~ L· ___ '

,----1.;._'_--

2 ~1 .. Li L I~ESTCNE

lOCRliO;-.J !-'L~RP:: S[:iCDL I~I"'" p:eOJe[1 FILE

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i·:). ". c. ". c. [) l:... 0.~ Cl:... ";.,";, ():.... (J

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"RC}JEC,: L-D \,E;:LL:" l"[1CAT~ON; !'"'iJ·'{Q::>.'-:: SCI-i,]DL.

F.:L_[V{FTO~

(M)

"'-ILE NO. : ~LEV.QTION (.11) :97. '38 DATE DRILLED: -YPE OF RIG: AIR FLUSH ROTARY

\ THICf<N£SS I LITHOLOGY I (M)

1 __ ~ _______________ ! _. _____ ._ .. ___ . _________ 1 ___________ ! _________________________ i

0.00 L SCl 2. 50

L 50 2. SO

! 3. 70

TO ,_._--_._--------------! -----------! -------------------------!

?-7. 98 'JS. '8 1- 50 SAND p;~m (3RAVEL SG. {,8 '3S. &8 L DO RESIDUAL GliilNITE 00 "..1. {,8 86.. 28 J.1. 20 SAND r~~m GRCjVEL

I

I

LEGE:-.lD O\!~.rzBU;::DEN

--.-. ---. -.

S iL.T

1 __

[':I~_, ~= 5 [L i5-!::~E

E~~ L;~'ir::S·fD~t

I PROJECT

77;08. c. (,. 8.8.::

~~8~;-'~~,~:;~ 0(: t~ , ~~. : . <: - :: - C,'_ :: 1- [). 8. ;- ;;. to ~c:;. : _ <;, .--,. .c:;. ~,

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SCRLE:

LITHOLOGY

PROJECT: L-D (·;;:;".LS LOCATION: MURAPE SCHOGL WELL NO.: F, DRILLER:

FlU.:: NO.: ELEVATION (M) :99.27 DqTE DRILLED: TYPE OF RIG: AUt FLUSH ROTARY

DEPTH ELEVATION I THIL:f\i\lt:SS I LITHOLOGY (M) O'I)! (M) I

1----------------- i --------.---.---------! -----------! -----------------------.-.-.--I FROM 1 TO I FROM i -'-0 I i

I-----------------!-------------------I-----------i-------------------------. I 0.00 I 1.00 i 9'3.27 i ~j8. 27 I i.I'O SAN!) AND GRAVEL

1. 00 1 2. 50 ~ SS.27 i SQ.77 I 7. 50 ~:;HND AND GRAVEL B.50 I S.OO; 90. 77 ~)O. ::7! O. 50 r;qi~i"'l T~

BR IT ~ 5:-1 GEG!...DGI CA i_ SURVt:Y

LEC;~~C

C\;~;~3LRDE:.l

n.~I·.i LL_i _i..:Jj ;:l_l .. -L::-':C'\i;.:iZ:::r:

f"';:;C.J[c.T F 1:_::

L [~ESTGNE

CORL

L-D i.'ELLS

C,. C.:-

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S i rc,\' V LJf\vt:

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J ~,:

WELL L~Ti-F):""OGY

PROJECT: L-D !.jELLS LOCATION: ~1URAPE SCHDI)L WELL NO.: 7 DRILLER:

ELEVAT:ON (M) :100. 43 !)(:;Tf~ DRILLED: "';'y'PE O~ RIG: AIR F!....USH ROTARY

DEPTH ELEW\TIm..J 1 T!-:ICi\"",~SS i LITHOLOGY (M) n·n (r<iI

1----.-------------. [ ________ . ____ .. , ____________ "-_______ ' i --- .. -----------.------------.

I FROM I TO FROM I -0 1----------------- ! ----.---.-.-.-----.-----.-- • --- --.. ---· .. ----·1 ---------------------- -,- ..

0.00 I L 50! 100. {,3 I (JfL ?:\ ::;[: ~3f:)i\lD nND GRAVr::L 1.S0 3.08 i 98.93 I ~)7.d I

, :',0 Sn~D f'::lND GRAV~:L

3.00 4. DO 37.4.3 I SS. /,_3 .. no r:LAY

4.00 9.00 %.1;.3 91.43 5. 00 :;AND AND GRfWE~ 3.00 3.70 91. 43 ~\O.?3 o. i;S GRANITE

--------------------,-----------_._ ... _ .. ,--_._---_.- ._------------_._. __ ._----_._------------

BRITISH GEO~OGICAi_

r--1-i:i;'1 '-~'~' ..

~

L_·_~I .::JiL;

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: • - I •• , •• '~

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I BR I ~ I.SH ~=.c-'-"--'-

;, "i (,' l: r- UL -.J L':l

71". c. c c. C~. c " I;~ ~ ~:~;~); 2:~:'0"

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J '2·,

25

/

DEPTH (M"J

PQOJECT: L-D t'.:~L'5

LOCAnQ:\i; t"! NAO[ SCH'1 r:"_

\~ELL ne;.: 8 D'i!LLER:

C"ILE ~JO. : ELEVATION (M) :99.43 DATE DRILLED: TYPE OF RIG: AIR FLUSH ROTA~Y

! ,H r CI<Nt;:SS !

"M) ! _________________ I __________________ ._! ___________ 1 _________________________ i

TO ! _ ..• ________ ._ . _____ ! _______ •. _~,~. ________ ! ---- ... ------; ----------------------~.-- i

G. ["If] ' •• .:' Cl ~,. CD 1 5.Ul

,:""1. :'? ':;t. {,?

94. ~_3 ,?J,. ():I

S:'i;.Jj) i::.)ND GqA'/~::'_

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fiGURE 7

I

:'"I

DEPTH lM)

t·JELL L~TH;LOGY

PROJECT: L-D WellS LOCATION: ~lURAPE SCHOOL.. [·jELL NO.: 9 DRILLER:

.-

FILE NO. : ELEVATION (M) :98.72 DATE DRILLED: TYPE OF RIG: AIR f="LUSH ROTARY

EL~Vr.)TIO:\!

(.1"1) TH I C!<N:::SS I

l---------------·--! ---.--.------ --------- ; ------------ i --------------,-----------,-

o. 00 1. 20 98. 7 ~: 97. ~,;:: l- 20 ER.:>.lD PlND CF<AVt::L 1. 20 I 17. 4O I 97. 52 Cl. 3;: ~ E. 20 , SA!\iD AND GRQ'/!:;:,_

17. 40 I 17. 50 I 8 ~ . ' '-. fil . " 0, 20 I GRHNIT:~

•

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PRO.JECf FiLE LDCRTION

I- BR I I lSH

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SCPLE:

L ITHGLOGY

l -1 ~_l 2

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GtOLOG I CRL SURVt Y I FiGURt. B' .......:...--'-----'---'

- REDRILL

i·JELi. L:::THOLOGY

PROJECT: L-O WEI_LS LOCA:- r Ot\.l: i"IUF:A PE SC: 11]']1_ ~iEL..l... NO.: .i 0 DRIU_t:.R:

FlU':: NO. : ELEVATION CM) :98.51 DATZ-: D~ILJ.ED:

TYPE Of: RI G: \.-.JATER Fe USH RDTARY

DE PTH ELC::V(;T I Cl'; : TH r C;<NE~:~S L I ':'HO _iJG~' (t.p (M) (M)

I-----------------! -·------------------1 ------------·1--------------------- ----: I FROM I TO I FROM TO : -----------------: ----_._._._._. __ .- .. -_. -., ----------.-.--! --.------.----------- -------- I

o. 00 1 20 '38. G 1 <;.,7 _ {, , . 20 S.(-='!~'.!D f':~.;;) r3R~:Vi~:'" ! 1 2.0 >. SO '37 , 1 f;S D ~ 8. {,O ~;!:l;>lD n\lD r.;;:;;:.:'i'.!:~:_

9. CO 1 O. :::0 BS_ C ! n(J_ , 1 G_ ~; 9 :::1E.SI llU::::L. G:i!=tr>JI .. 1 O. 20 ! o. 00 CS. , : t; :3 , 1 4. DD ~;PI!\lD {i,,,D i3RAVEl..

i· .. - , ;:~'";.:: <t:.CS~;~ l·_·_·~·_

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FILE LOCf.H iCN

L-D ilELLS

:v,URRPE SUICGL

BRITISH

7l~C~C~C0'C.'

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~ REDRILL

DEPTH (M)

PROJECT: L-'D i·JEl"LS LOCATI ON: \<\URAPF:: SCHO··j',_ WEU_ NO.: 11 DRILLER:

ELEVATION (M)

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Appendix VI

DUG WELL CONSTROCTION

The BGS/ODA and the Ministry of Water Resources Developnent in Zimbabwe have oc-operated on a project to evaluate the potential of large diameter hand du;r wells. The MWRD proposal was initially concerned with very large diameter wells (5 rretres plus) with the rrain emphasis on well storage. The BGS/ODA ocncept has been to ocnstruct a deeper well (greater penetration below the water table) with Sffi3.ller diameter (3 rretres) but sufficient to allow access to specialised, hydraulically powered, rig capable of drillirB radial bcre­holes. Following completion of one 5 rretre well at Chibero, it was decided that future wells would be constructed in accordance with the BGS/ODA concept and to include radial drilling.

Chibero 5 rretre Well.

This well is located at Chibero sorre 50 km to the southwest of Harare at an agricultural COllege. The total depth excavated was sone 16 m and the work tock abcut 9 nonths to complete. The well was ocnstructed by casting a 1 m deep reinforced ocncrete cylinder at ground level using made-to-rreasure steel shuttering. Excavation was carried out below the cylinder until it sank to its own depth whereupon a seocnd concrete cylinder was cast on top of the first. The process was ocntinued until the granite bedrock was reached at c. 5 m below ground level.

Considerable difficulty was experienced in getting the caisson to sink, prob­ably because of its slab-sided exterior. Techniques used to induce sinking included the injection of ocpious quantities of water down the outside of the caisson and simultaneously loading the cylinder with concrete beams supportirB bags of cement. The remainder of the shaft, all in hard rock, was excavated by use of jackhamrrers and blasting.

Three metre Caissons for Collector Wells.

The basic design was provided by Mr J Johnston, Chief Hydrological Engineer of the Ministry of Water Resources Development and consisted of a ring of concrete 0.25 m wide and 0.30 m deep with reinforced brickwork abcve. The first ring was cast on levelled ground at the well site but all others were cast below ground level into a circular trench with triangular cross section. The effect of this arrangement is to provide a cutting edge to the caisson. Subsequent excavation and simultaneous undercutting at four orthogonal points caused the cutting caisson to sink evenly. Alternative building up with brickwork and lowering by excavation was continued until a maximum depth of up to 21 m was reached. To prevent the caisson from splitting in the process, a strong continuous vertical reinforcement of eighteen 6 mm square twist high-tensile steel interlocking reinforcing rods is used and is turned outwards at right angles on final ccmpletion in order to be incorporated in the apron structure and to help prevent the caisson from rroving further. Laterally, the brickwork is strengthened by the inocrporation of two concentric rings of 8-gauge galvanised steel fencewire inserted between each ocurse of bricks below the water table.

Ingress of water directly into the caisson is allowed by omitting cement between the adjoining ends and sides of the bricks except adjacent to the vertical reinforcing rods. TI1e bricks are cemented bet:v.een the courses where the reinforcing galvanised wire is also incorporated. The brickwork pattern consists of alternating courses of a double rr:M of bricks end-to-end and a single rr:M of bricks side-to-side. The inner ed:re of the brickwork is designed to be flush with the inner wall of the caisson shoe while the outer edge of the bricl=rk is within the diarreter of the caisson shoe in order to facilitate slipping of the whole structure down the excavated shaft.

A stout gantry built of 3-inch galvanised pipe is constructed over the well site to allow simultaneous raising and lowering of bD builders buckets for excavating the shaft while the labourers entered and left the well by means of steps built into the wall at o. 3 m intervals. An al ternati ve procedure for deeper operations made use of a 45 gallon drum raised and lowered by a winch operated from a tractor. Further details of the construction procedure are given below illustrated with photographs.

CONSTRUCI'ION OF 3-METRE DIAMEIT'ER WELLS

1. Materials.

Stage I (i) Cutting ring:

2Q-gauge galvanised iron strip 0.60 m wide 11.4 m long 20-gauge galvanised iron strip 0.30 m wide 9.9 m long 36 steel stakes of 12 nm reinforcing rod each 1.2 m long 36 steel stakes of 12 nm reinforcing rod each 0.6 m long 1 roll of tying wire

(H) Cage of 12 nm reinforcing rod in tv..::> halves (for ease of transport) :

IS uprights of 6 nm square twist reinforcing rod to B.S. 4461 (6 x 0.6 m; 6 x 1.0 m; 6 x 1.5 m) bent in form of hook and L

120 rrortar blocks each 60 x 30 x 30 nm with typing wire inoorporated

(Hi) Concrete:

15 sacks of high early-setting Portland cement 1.2 m3 sand 1.2 m3 stone

$372

Stage II - Brickwork per rretre height

1,000 building bricks 5 bags high early-setting Portland cerrent 0.25 m3 sand Vertical reinforcement: 20 nm x 6 mm square twist reinforcing

rod to BS 4461 cut into IS lengths and bent in form of hook one end and eye other end

Horizontal reinforcerrent: 227 m x S-gauge galvanised steel fence wire to be laid in a pair of pexallel rails between each oourse of brickwork.

Hoop steps: 2.7 m + 3 for 3 steps bent into 'croquet hoops' and hot dip galvanised

Tying wire - 16 or so gauge - 1 roll per well

$175

Stage HI - Apron and top wall

T:i.rrber: S x 3 m planks for octagonal surround, plus 16 se=ing pegs

ReinforCEment: Steel rresh 0.25 m open square, 47 m2

Bricks: 1,000 Cement: 50 sacks of ordinary cerrent Sand: 5.S m3 Stone: 11.0 m3

$7CO

Stage N - Cover

2~inch angle iron: 5 x 3.6 m; 4 x 2 m; 4 x 1 m 1 inch dianond nesh: 10.8 x 1. 2 m Lightweight galvanised corrugated iron: 9 m3

2 inch bolts, 2 x ~ inch with nuts

$400

2. Gantry and Tools:

Stage I - Hand digging and spoil removal - practical down to 8 m below ground level when inflow srrall or negligible and strata workable by hand.

(i) Gantry: 12 m of 3-in galvanised pipe + two 900 elbows 14 m of 2-in galvanised pipe 2 m of 2-in angle iron 1 m of flat steel strip for pulley straps 2 m of 12 mm reinforcing rod for pulley hangers 4 steel bolts 6 x ~-in with nuts 2 steel bolts 3 x %-in with nuts 2 pulley sheaves for l-in rope 2 x 25 m manilla rope 1 x 25 m nylon lifeline

say £570 + manufactu...

(ii) M::mld-shaping tool: 2.0 m 12 mm steel reinforcing rod 3 mm steel plate 0.315 x 0.255 m

(iii) Wooden gauge boxes for cE!llent, sand and stone: Sufficient timber 0.2 and 0.25 m wide to make two boxes of capacity = 3 : 4, the smaller for cerrent, the larger for sand/stone; lifting rails, parallel, along the top and protruding like wheelbarrow handles helpful.

(iv) Wooden honeycomb frame for casting mortar spacing blocks: Wood strip 30 mm wide. If 10 m thick, 17.6 m thereof.

(v) Implements: 1 slump cone for concrete mix (1-4 in slump for caisson) 2 picks - long handle 2 picks - short handle 2 spades 2 shovels 4 builders buckets Safety he1rret for each man 2 mason's chisels 9-in 2 mason's hammers 2 iron floats 2 wooden floats 2 trowels 2 spirit levels 1 heavy-duty wirecutter

(v) Irnplerrents: (Cont' d) 12 m hessian, 1 m or so wide 2 vice grips Black pva paint 25 rrm paintbrush 5 kg multi-purpose grease 1 straight-edge plank more than 3.5 m long

(ii) to (v) incl. $530

Stage II - Excavation below 8 m and/or greater inflCM than can be ffi3l1aged with buckets, and strata oonsolidated rock.

1 winching tractor with bucket (reduces labour force by four men) $12,000

1 small air oompressor 20,000

1 downhole sludge pump plus hoses (20 m x 2 in for water/sludge; 20 x 1 for air supply and 20 x l~ for exhaust) 1,200

1 or 2 jackharrrners (paving breakers) to assist in reducing rock @ 867

The pump must be capable of lifting water and sludge containing much abrasive quartz to a height of 20 m. This implies a downhole impeller below 4 m. With workmen in the well the use of internal canbustion machinery or electrically propelled pumps down the well must be discounted, which leaves compressed air or hydraulics as a rroti ve power. Even then, rapid wear of pump impeller and bowl must be compensated for by adequate provision of spares.

ESTIMATE OF TIME, LABOUR AND MATERIAlS COST OF DIGGING A PAIR OF 3-METRE WELlS EACH TO A DEPI'H OF 10 m IN ZIMBABWE

TIME: (Assuming adequate transport to convey tools, materials and men to and from the sites as and when required, and little or no rainfall) •

Ordering tools and materials and having transported to sites*

M:::mlding cutting rings and allowing to =e (HESP cement)

Building and sinking caisson 1 metre at a time at the optimum rate of 1 metre per well per week

Gravel packing and construction of apron and I-metre surround (1 builder at each well)

Test pumping pre-drilling (allowing for 1 week recovery from constructional dewatering)

Radial drilling (four rakers only)

Test pumping post-drilling (allowing for 1 week recovery from drilling dewatering)

Making a total of 19 weeks or, say, 10 weeks per well

* assuming commercial fabrication of half-cages ready

COSTS: Cutting rings 2 @ $ 372 Brickwork 20 m @ $ 175 Apron and top wall 2 @ $ 700 Covers 2 @ say, $ 415 Fuel for tractor, say 300 l/week for 11 weeks Crew provisions for 3 men for 3 months @ $ 7. 84/man month

Latour (5-day working week) : 2 builders say 11 weeks @ $36/man week 2 foremen say 11 weeks @ $36/man week 1 tractor driver, say 11 weeks @ $36/man week 8 labourers say 11 weeks @ $15/man week

Hence, total cost of drilling a pair of 10 m wells =

Average ccst per 10 metre deep 3 m diameter well radia11y drilled and finished with apron, wall surround and cover = $ 5,788

$

1 week

2 weeks

10 weeks

1 week

14 weeks

2 weeks

1 week

2 weeks

5 weeks

744 3,500 1,400

830 1,683

118

8,275

792 792 396

1,320

3,300

11,575

II ,575

1. Ground is lcveLLc::d,. out(~r rif)fJ of:" stakc~s set 1JP, and cutt.L"lq (:x}qe. ]l\ould due! 'd1tJ"1 /"i.i.d of radial sbar;(;.

2. In~'J.e:c st.aJ-:e.'r7> t steel str.ip stHJtter:i.Jr."·'r i:tnd prc~Llbric.:.tt"cd

"CC in forccr.-t:...-;nt cacJe ] d.:1 d 1.n FO:3j.t.ion vlit}! n:-rcL,Tr ~;r.nc.i.rq

b.l(x:-:ks.

.1. Ve!:-tic';"11 lliqil·,,-ten::; 'i, 1 (: ~'~qu(_:r(! L',d15t :::~c:, .i.nf()rCC1~K:J)t. ;x~t. up - desiql1 recently 1'.rxLif;,cd ~.C'I h;lV(~ L:i.C:;5 :it l'ln:cc clLLFc:l:'enl.: .levels, ','lit]) decpr:"!1'" hCK1.i<S p

4 ~ Sttff (xmcrctc nrLx 3 : 4 : 4 In:-x10 witJt hi,qh (-?z.rrl.y'- .':;c:t,t.in~1

Portlwnd ccrent and !;:,lurnp of 2 inchl>S.

). CutLin~! :rirKJ rn.:.Yuld :'iLlcd d .. nd Jov(~1 h~j cd~,:'- i.~"., Y"C:~l(::i.n12::'~;; fO.t: br.ic\l,,<'(~n...r~

to be laid.

lj. J"lc.ll~i.zorl.'Lll l·(;>i,nf()n::~C:.!¥~~nt_ 'dires lx:inq laid l..xJt.v .. I(:{m 00.ch CO\'1rSE~

()f br.i,c~.kv,1()rk.

I, a.nd 10. l\53 1.'iCll~k prCXrrC!3::';CY,J, llEd1a,ni(~dl i::ln(J ~)nCllflni::',ic: ,.lids for dc·w~.1t.c'rJr:<"J

;-x'c,:'tnY'" nc'(x!ssc:uy. T"'nc i_1.1I'-dri.vc·n pu:rp h:x] to U-;c ~y:;c of j.:.~c};­

h~.mrrL;:c:; (P'::-i'\Ji.n~; bn_:;.D<.cr:3} a~; .Hi. did tu (:;<C\:i.\!<1t,:~on (yE U1!.: J;.~H~-dc.'l:­

rock, LU1<..1 lx~l.c)' ... l 8 :1'l .1..i.ft.'inq the' ~~~p:)jl },)C):".~~1r:'i~; :3(,) ~'~lO'",r tlldt-. ;u}. old tractc'r \rlas adapt-cx,! Hi Lh r:01c:::;, '"ri, ne:-) nnd bucket.

" :~'~::",!~",":' >

~>

l' . s(x)n O';jt of con:-~ tLl.l1t l.':l •

12. Drilling ri9 and console ready to bc~ put dOVlYl \'lell vlitll scr:r::U'atc p::YA>'Cr

unit and hyd1"tlu:Lic punp to rCInain on stu"facc:.

1,1 . Drillinq (1b~~ad throLKJh tl ~')icce

of DuplGX cQsin~J ins-c)rtcd throu~Jh the br_LckT.·x)rk and ~Jrdvc~l pack.

13. Drillinq riq drrl co}~solc in pJ:3ition .In tx)t_t.cm of '#011 l,'l.i th dri'1.1 ptrx: .::~nd tools :~;t.':"icked (l.!..}ttin~:-;t. U"l(~ 'ddll!3.

I\U:'kC:l' l'jt'icl:~Al()_ck i:·~·; Idithj.n Lhn !":on-rnl Ea Lurat ion zonE:, \.)1-1 i 1 C~ U-l(:;: 'i',,'C~11 is l-:x~inq

c1e\ .... 'a L_c'_n::d by tb-c.? pUlP to Uw ric;ht ot th(-) cDru?olo~

7. 'Illn ",:.Il[)IC'\.l'd vX',11-Lop \·:i Lh lcU'liC ~·jc)J)U d i(";"]-lf)W()X'e" test purp in::;L.z".J11cd. ~:;truct.ll:te on 1t;~7L is Bush pump fitLt"d LO v/t-::.-11, 1,'rlt'l: ll':.lnd.1C' n"lnDVC{1 ~