Engineering geological characterisation of the rock masses at Tannur Dam site, South Jordan Ali El-Naqa Mustafa Al Kuisi Abstract This paper highlights the geomechanical characterisation of the rock masses exposed at the dam abutments and reservoir area at the Tannur Dam site, South Jordan. The right abutment rock masses are characterised by closely to widely spaced joints. The rock-mass qualities were assigned using the rock-mass rating (RMR) and Q-tunnelling index. Both systems assigned a poor quality for foundation rocks because of the presence of weak rocks. The rock masses constituting the dam abutments exhibit fair quality. The results of packer tests indicate that the hydraulic conductivity of the rock masses of Fuheis–Hummar–Shueib (FHS) and Wadi es Sir (A7) formations range from 10 to 150 Lugeon units (LU). TheFHS was characterised by lower LU values compared with A7; this reflects the fracturing characteristics of A7. However, the A7 should be grouted especially the right abutment. However, the FHS needs less grouting because the spacing between joints seems to be tight. The estimated shear strength envelopes relevant to the rock masses of both abutments as well as the foundation rocks were quite similar and, therefore, present similar shear strength characteristics. The shear strength for jointed rock masses showed curvilinear failure planes with average cohesion values of 0.67 and 0.64 MPa and friction angles of 36.5 and 35.5° for dam abutments and the foundation area, respectively. Keywords Discontinuities rock mass Jor- dan Packer tests Rock-mass rating Introduction Jordan can be described as a semi-arid region, which suffers from water shortage and limited water supply. In the last few years the water demands have increased be- cause of the high rate of population growth, together with the higher needs of industry in the country. To satisfy the water needs in the future, the decision-makers in the Ministry of Water and Irrigation have selected a number of sites in the southern desert areas that may be suitable for the construction of storage dams to be used primarily for the domestic recharge of groundwater, and for irrigation. This study focused on the geomechanical characterisation of rock masses, which affect the stability of the dam abutments, and to control seepages that may occur through the highly fractured rock masses. The geotechni- cal investigations were aimed at confirming the data re- lating to suitability of foundations for constructing the rolled compacted concrete dam. This type of dam was proposed by Mott Macdonald Company, who advised the Ministry of Water and Irrigation and supervised the con- struction of such a dam at the southern part of the Dead Sea. This is Jordan’s first ever roller compacted concrete dam and will have a storage capacity of about 17 mil- lion m 3 and costs £20 million. The Tannur Dam will be used to irrigate 1.186-ha in southern Ghors. This work reports on the geomechanical and hydrogeo- logical problems based on the local geological model of the dam site and reservoir area. Geological and geomechanical surveys were carried out to evaluate the rock-mass char- acterisation of the dam site and reservoir area with the use of engineering geomechanical classification systems. Furthermore, the available data from bore cores obtained from previous feasibility studies were also re-elaborated and interpreted. Geological framework The Tannur Dam site is located in a steep valley of Wadi Al- Hasa at Jebel Tannur, approximately 5 km downstream from the bridge of the King’s Highway, which is about 50 km south of Karak (Fig. 1). The dam area was investigated by Received: 3 December 2001 / Accepted: 26 March 2002 Published online: 4 June 2002 ª Springer-Verlag 2002 A. El-Naqa (&) Hashemite University, Institute of Lands, Water and Environment, Department of Water Management and Environment, Zarqa, 13115, P.O. Box 150459, Jordan E-mail: [email protected]Tel.: +962-5-3826600 ext. 4231 Fax: +962-5-3826823 M. Al Kuisi Specialized Engineering Service Company, P.O. Box: 430616, Amman 11143, Jordan Original article DOI 10.1007/s00254-002-0589-9 Environmental Geology (2002) 42:817–826 817
Transcript
Engineering geologicalcharacterisation of the rock massesat Tannur Dam site, South JordanAli El-Naqa Æ Mustafa Al Kuisi
Abstract This paper highlights the geomechanicalcharacterisation of the rock masses exposed at thedam abutments and reservoir area at the TannurDam site, South Jordan. The right abutment rockmasses are characterised by closely to widely spacedjoints. The rock-mass qualities were assigned usingthe rock-mass rating (RMR) and Q-tunnelling index.Both systems assigned a poor quality for foundationrocks because of the presence of weak rocks. Therock masses constituting the dam abutments exhibitfair quality. The results of packer tests indicate thatthe hydraulic conductivity of the rock masses ofFuheis–Hummar–Shueib (FHS) and Wadi es Sir(A7) formations range from 10 to 150 Lugeon units(LU). TheFHS was characterised by lower LU valuescompared with A7; this reflects the fracturingcharacteristics of A7. However, the A7 should begrouted especially the right abutment. However, theFHS needs less grouting because the spacingbetween joints seems to be tight. The estimatedshear strength envelopes relevant to the rock massesof both abutments as well as the foundation rockswere quite similar and, therefore, present similarshear strength characteristics. The shear strength forjointed rock masses showed curvilinear failureplanes with average cohesion values of 0.67 and0.64 MPa and friction angles of 36.5 and 35.5� fordam abutments and the foundation area,respectively.
Keywords Discontinuities rock mass Æ Jor-dan Æ Packer tests Æ Rock-mass rating
Introduction
Jordan can be described as a semi-arid region, whichsuffers from water shortage and limited water supply. Inthe last few years the water demands have increased be-cause of the high rate of population growth, together withthe higher needs of industry in the country. To satisfy thewater needs in the future, the decision-makers in theMinistry of Water and Irrigation have selected a number ofsites in the southern desert areas that may be suitable forthe construction of storage dams to be used primarily forthe domestic recharge of groundwater, and for irrigation.This study focused on the geomechanical characterisationof rock masses, which affect the stability of the damabutments, and to control seepages that may occurthrough the highly fractured rock masses. The geotechni-cal investigations were aimed at confirming the data re-lating to suitability of foundations for constructing therolled compacted concrete dam. This type of dam wasproposed by Mott Macdonald Company, who advised theMinistry of Water and Irrigation and supervised the con-struction of such a dam at the southern part of the DeadSea. This is Jordan’s first ever roller compacted concretedam and will have a storage capacity of about 17 mil-lion m3 and costs £20 million. The Tannur Dam will beused to irrigate 1.186-ha in southern Ghors.This work reports on the geomechanical and hydrogeo-logical problems based on the local geological model of thedam site and reservoir area. Geological and geomechanicalsurveys were carried out to evaluate the rock-mass char-acterisation of the dam site and reservoir area with the useof engineering geomechanical classification systems.Furthermore, the available data from bore cores obtainedfrom previous feasibility studies were also re-elaboratedand interpreted.
Geological framework
The Tannur Dam site is located in a steep valley of Wadi Al-Hasa at Jebel Tannur, approximately 5 km downstreamfrom the bridge of the King’s Highway, which is about 50 kmsouth of Karak (Fig. 1). The dam area was investigated by
Received: 3 December 2001 / Accepted: 26 March 2002Published online: 4 June 2002ª Springer-Verlag 2002
A. El-Naqa (&)Hashemite University,Institute of Lands, Water and Environment,Department of Water Management and Environment,Zarqa, 13115, P.O. Box 150459, JordanE-mail: [email protected].: +962-5-3826600 ext. 4231Fax: +962-5-3826823
M. Al KuisiSpecialized Engineering Service Company,P.O. Box: 430616, Amman 11143, Jordan
Original article
DOI 10.1007/s00254-002-0589-9 Environmental Geology (2002) 42:817–826 817
local and international consulting engineering firms(McDonald and Partners 1969a; Howard Humphreys andPartners 1995; Mott MacDonald 1999). A generalisedgeological map of the dam site and reservoir area is shown inFig. 2. On this map, the outcrops of different geologic bed-rock units and the different types of surficial deposits areshown. The outcropping rocks on the dam site and reservoirarea belong to the Ajlun Group of Upper Cretaceous age. Theoldest rocks in the dam site area are the FHS formation(A3/A6) and represent the dam foundation rocks. Thisformation outcrops at the left abutment and reservoir area,and consists of thin to moderately thick bedded (1–50 cmthick) limestone, marlstone, marl and clayey marl, withgypsum bands. The limestone is moderately hard andmoderately weak. These strata were encountered in all
exploratory boreholes along the dam axis and in the aditsconstructed on the dam axis. The sequence contains bandsof marl and fossiliferous limestone up to 0.5 m thick TheWadi Es Sir limestone (A7) overlies the limestone sequenceat the top of the FHS and consists of massive, thickly andthinly bedded limestone with nodules of chert interbeddedwith marly limestone and dolomitic limestone.The colluvial deposits are frequently encounteredthroughout the dam area, primarily on hillsides and hill-tops outside of wadis. The colluvium comprises a hetero-geneous mixture of angular to surrounded particles fromboulders to sand-sized particles with some silt, silty clayand marl. Mostly, thick unstable landslide materials and/or colluvium cover the dam site and the reservoir area. Thebed of Wadi Al-Hasa downstream of the siq in the Nau’r
Fig. 1Location map of the study area
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limestone is comprised of wadi gravel up to 70 m widewith a maximum thickness of 15 m on the dam axis. Thewadi gravel is comprised of cobbles and boulders with asilty sand matrix.The major structural feature of the area is the Wadi AlHasa fault, which strikes WNW in the region of the damand lies 500 m to the north-east of the dam site. Wadi AlHasa fault strikes generally E–W to form the Dead Seafault, which is about 23 km west of the dam site. To theeast it dies away into a series of minor faults to merge withthe Karak–Faiha fault (Bender 1974).
Site investigation
McDonald and Partners (1969b) carried out a preliminaryfeasibility study of the dam site. Howard Humphreys andPartners (1995) carried out detailed logging and geotech-
nical testing of the rock cores of 15 boreholes in the damabutments and the foundation area in conjunction withpacker tests. Detailed discontinuity logging of the borecores was performed to provide basic parameters forclassification of the rock mass. The depth of these bore-holes varied from 15 to 60 m. A geotechnical cross sectionwas constructed based on the exploratory boreholes drilledalong the dam axis during the site investigation (Fig. 3).Two boreholes were drilled in the right abutment, ten inthe left abutment and two in the foundation area. The rightabutment rises steeply with a slope of 30–35�; the leftabutment rises rather gently with a slope of 10�. Thebedrock constituting both abutments is FHS and A7limestones. The bedrock of the left abutments is coveredby ancient landslide materials (colluvium) with a maxi-mum thickness of 20 m and, generally, is comprised oflimestone, marl/gypsum gravels and cobbles with a sandy–clayey silt matrix.
Fig. 2Generalised geological map ofTannur Dam site area
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On the right abutment and in the right adit, steep jointsstrike E–W, N–S and NW–SE, trend orthogonally to thebedding planes and are generally closely spaced. On theleft abutment, three major sets of joints strike NNW–SSE,NW–SE and NNE–SSW, and in the left adit, steeply dip-ping joints strike, generally, N–S, and the joints are gen-erally tight.
Discontinuity data
Ten scan-line surveys were carried out in the vicinity ofthe dam (Fig. 4). A scan-line survey is a samplingtechnique used to characterise rock discontinuities, suchas joints and fractures (Piteau 1973) and the engineeringproperties of the rock masses. The geomechanical char-acteristics of different discontinuity systems present indifferent rock masses were carried out using scan-linesof 5–10 m length. These scan-lines were distributed onboth dam abutments as well as in adits in order toobtain representative geomechanical characteristics ofthe rock masses. Five scan-lines were carried out onthe right abutment and three scan-lines on the leftabutment, in addition to two scan-lines on the left andright adits.
The discontinuity parameters were collected according tothe suggested methods of the International Society of RockMechanics (ISRM 1978). Geomechanical mapping allowsthe evaluation of the following parameters: orientation,trace length, spacing, aperture and joint conditions. Theroughness is measured by using a shape tracer device;Barton and Choubey (1977) presented a selection of scaledtypical roughness profiles to estimate this parameter byvisual matching in the range of 0 (smooth planar surface)to 20 (rough undulating surface). The wall strength wasmeasured by an L-type Schmidt hammer test. The rock-quality designation (RQD), proposed by Deere (1964), is ameasure of the quality of a borehole core. When the bore-core is unavailable, RQD can be estimated from jointfrequency using a formula proposed by Priest and Hudson(1976).
Data analysisThe discontinuity data were interpreted statistically todefine the rock-mass conditions of the dam site and res-ervoir area. The first elaboration of discontinuity data wasperformed by plotting the orientation data on a Schmidtnet using the computer program RockWorks (1999). Thestereonets can give an overall view of the orientation of thepredominant joint systems and bedding directions withinthe rock mass. Three representative sets have been iden-tified from stereodiagrams of joints in the two formations:Wadi Sir (A7) and FHS (A3/A6). The dip of bedding in
Fig. 3Geotechnical cross section along the dam axis
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both formations showed a general orientation of 010/20�on the right abutment and 300/20� on the left abutment.Three major joint sets were also recognised on the rightabutment, which are oriented WNW–ESE (186/72�) andNW–SE (244/82�), and they are inclined towards the SWand NNW–SSE (264/88�) (Fig. 4). In the left abutment,three major joint sets have been also identified, the first setis oriented NW–SE (204/88�), the second set strikes NE–SW (116/80�) and the third joint set trends (254/70�). Inthe right adit, three joint sets were identified that trendWNW–ESE (189/72�); NW–SE (254/88�) and NNW–SSE(282/82�). Three joint sets strike NNW–SSE (248/77�),NNE–SSW (285/88�) and NW–SE (215/87�), and cha-racterise the left adit.The structural features of the area are intimately relatedto the tectonics and geological processes that contributed
to the Al-Hasa fault. Tables 1 and 2 show the disconti-nuity characteristics within the FHS and A7 rock masses.The A7 rock masses are characterised by joint spacingthat range between 200 and 600 mm; the condition ofjoints are very rough with separations of 1–5 mm, andsome joints are filled with soft clay and clayey marlmaterials whereas others are open. The joints in the FHSformation are characterised by joint spacings that rangefrom less than 60–200 mm depending on the beddingplanes of the layers; the joint apertures are between 1 and5 mm, are filled with clayey marl and the joints are ir-regularly rough.
Rock-mass classificationsTo summarise the geological and geotechnical data, and toprovide tools for the designer during construction, two
Fig. 4Topographic map and stereonetsof joint sampling sites
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rock-mass classifications have been used: the Bieniawskirock-mass ratio(RMR) and Barton (Q) classification sys-tems. The RMR system was based on the following sixparameters (Bieniawski 1989): uniaxial compressionstrength of the intact rock; RQD; spacing of discontinu-ities; condition of the discontinuities; groundwater con-ditions and orientation of discontinuities. Certain ratingvalues are assigned to each parameter and the summationof these values gives the RMR quality of the rock mass.The Q-tunnelling system evaluates the rock quality (Q) onthe basis of six parameters: RQD, joint set number (Jn),joint roughness number (Jr), joint alteration number (Ja),joint-water-reduction factor (Jw) and stress reductionfactor (SRF). The Q is expressed as (Barton and others1974):
Q ¼ RQD
Jn:
Jr
Ja:
Jw
SRFð1Þ
The Q expression includes three terms that describe therock-mass properties. The term RQD/Jn represents theaverage block size of the mass; Jr/Ja represents the peakstrength; and Jw/SRF represents the effective stress of therock mass.The rock-mass qualities of two formations are assessedusing the rock-mass classifications, which are dependenton the joint characteristics (Table 3). The geomechanicalquality of the rock masses of the dam abutmentsrepresented by FHS and A7 formations was classified asfair. The rock-mass quality of the foundation rocksrepresented by FHS was placed on the poor categoryaccording to RMR, and as poor quality according to Q-index (Fig. 3). Rock-mass assessment based on the RQDvalues indicates that the RQD values recorded in the A7limestones range from 48–54% and 44–50% for marlylimestones and 0–25% for marls/shales. It seems that theassessment of rock masses by geomechanical classifica-
Table 1Discontinuity characteristics of the dam abutments
ParameterJoint set
Right abutment Left abutment
Joint set 1 Joint set 1 Joint set 3 Joint set 1 Joint set 1 Joint set 3
tion systems is more reliable because these classificationstake into consideration not only the RQD and degree ofjointing, but also other parameters of the discontinuitycharacteristics, such as alteration, roughness, spacing,etc., which affect the geomechanical behaviour of therocks.Once the rock-mass quality is evaluated, the rock-massstrength can be estimated using the criterion proposed byHoek and others (1995). Rock-mass quality can be appliedto those areas that will be involved in the construction ofthe dam.
Rock-mass permeability
Howard Humphreys and Partners (1995) carried outpacker tests directly in the vertical and inclined boreholes.Packer tests were performed in three boreholes pene-trating the dam foundation and four boreholes drilledwith inclination angles ranging between 15 and 30�. Themain objective of these tests was to determine the per-meability of the rock masses of the dam foundation andits abutments. The results of permeability tests indicatethat the permeability values of FHS range from 10 to70 Lugeon units (LU) with an average value of 27 LU.However, the Lugeon values encountered in the A7 for-mation range between 20 and 149 LU, with an averagevalue of 55 LU. It is obvious that the permeability of A7 istwice the permeability of FHS; this reflects the higherintensity of fracturing in the A7 formation compared withthe FHS. Figure 5 shows the permeability distribution inthe two formations. It is evident that the permeabilityvalues of A7 are higher than the FHS; therefore, the A7formation needs much more grouting than the FHSformation.
Rock-mass strength
The rock-mass classification provides an acceptable esti-mate of rock-mass strength using the most common em-pirical failure criteria. The most widespread empiricalstrength criterion is that proposed by Hoek and Brown(1980, 1988). This criterion was used to evaluate the in-situstrength of the rock mass.The rock mass studied at the Tannur Dam can be consideredto be moderately to heavily jointed; therefore, the Hoek–Brown failure criteria (1988) can be used. The Hoek–Brownfailure criterion (Hoek and Brown 1980; Hoek 1983) wasdeveloped to estimate the strength of jointed rock masses.This approach was updated by the same authors (Hoek andBrown 1988; Hoek 1990) and modified by Hoek and others1995). The Hoek–Brown criterion is defined by the followingequation:
r1 ¼ r3 þ mbr3
rcþ s
� �a
ð2Þ
where r1 is the major principal effective stress at failure; r3 isthe minor principal effective stress or confining pressure;
Fig. 5Histogram of permeability values in a WSL rock mass and b FHS rockmass
Table 4Parameters of rock-mass strength criteria mb, s, Em, scmass, cohesion (c) and friction angle (/)
Borehole Formation GSI UCS mb S Em rcmass C /(GPa) (MPa)
mb is the value of the mi constant of the rock mass; s and a areconstants, which depend on the characteristics of the rockmass; and rc is the uniaxial compression strength of theintact rock.The shear strength parameters can be estimated using thegeological strength index (GSI) of the rock mass (Hoekand others 1995), which is derived from RMR version ofBieniawski (1989). For RMR >18 GSI =RMR89-5 and ifRMR <18 (Barton and others 1974) the Q-index should beused. Once the GSI has been estimated, the rock-masscohesion and friction angle were computed by linear in-terpolation of the curvilinear Hoek–Brown failure criterion(1980). As there is no direct correlation between the linearMohr–Coulomb criterion and the Hoek–Brown criterion, aspreadsheet for the calculation of Hoek–Brown and Mohr–Coulomb parameters was used (Hoek and others 1995).Because of the difficulty in determining the material con-stants mb and s experimentally, these parameters wereestimated using GSI. The following expressions were usedto estimate material constants:
mb ¼ ml � expGSI � 100
28
� �s ¼ exp
GSI � 100
9
� �ð3Þ
The value of mi for the limestone, obtained from Hoek andBrown (1988), was 10. The uniaxial compression strengthof the rock mass was calculated using the followingexpression:
rcmass ¼2c � cos /1� sin /
ð4Þ
Uniaxial compression strength (sc) tests were carried outon selected core samples collected from weaker rocksencountered in the dam foundations. The samples gaveuniaxial compression strength (sc) values that rangebetween 7 and 36 MPa, with a mean value of 18 MPa.The estimated values of mb and s, as well as scmass and Em,are summarised in Table 4 . The failure envelope of therock mass is represented graphically in Fig. 6.
Fig. 6Hoek–Brown Failure envelopes based on RMR anduniaxial compression strength for the a leftabutment, b right abutment and c foundation area
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Estimation of the in-situdeformation modulus
The in-situ deformation modulus of a rock mass is animportant parameter in monitoring deformation at thedam foundation. Howard Humphreys and Partners (1995)carried out unconfined compression strength tests on se-lected core samples to determine the Young’s modulus(Em). The results show a deformation modulus that rangesfrom 1.5 to 6.1 GPa, with a mean value of 3.7. The Em
measured in the UCS tests is affected by the orientation ofjointing and, therefore, results in low values. Furthermore,the triaxial tests were carried out on selected core samples,which gave more realistic values for the intact rock. Thetriaxial tests gave a deformation modulus that ranges from17.5 to 40 GPa, with an average value of 28.1 GPa (HowardHumphreys and others 1995). Wyllie (1992) found that theratio Eintact/Erock mass approaches 2.5, which gives a meanvalue of Erock mass 11.2 GPa. This value has been checkedwith the empirical formula proposed by Serafim andPereira (1983) and modified by Hoek and others (1995)and is based on the GSI value as follows:
EmðGPaÞ ¼ 10ðGSI�10Þ=40GSI � 25 ð5Þ
Based on the above relationships, the in-situ deformationmodulus of the dam foundation rock was estimated basedon the GSI value. The estimated Em value, using a GSIvalue of 50, was found to be 10 GPa. This value is lowerthan the triaxial value because the weaker rocks were notrepresented in the tests. The average value of Erock mass of10.6 GPa can be adopted as a design value.
Conclusions
The analyses of the data obtained from geomechanical andengineering geological investigations have provided usefulinformation regarding the geometrical and mechanicalproperties of the analysed discontinuities. The statisticalanalysis of joint orientations resulted in the identificationof three major joint sets that have subvertical dip angles.Hence, they are amenable to grouting by vertical bore-holes. In addition, inclined boreholes were drilled at 45� tominimise the expected leakage in the dam abutments. Theresults of packer tests indicate that the FHS and A7 for-mations are characterised by permeability values thatrange from 10 to 70 and 20 to 149, respectively. Thisemphasises the higher fracturing characteristics of the A7formation compared with the FHS formation, which wascharacterised by tight joints.The Hoek–Brown failure criterion was used to estimatein-situ rock-mass strength. The shear strength for jointedrock masses shows curvilinear failure planes with average
cohesion values of 0.67 and 0.64 MPa, and friction anglesof 36.5 and 35.5� for dam abutments and foundation rocks,respectively.The triaxial tests gave a deformation modulus that rangesfrom 17.5 to 40 GPa, with an average of 28.1 GPa. The in-situ deformation modulus (Erock mass) was estimated usinga GSI value of 50, which was determined to be 10 GPa.This value is lower than the triaxial value because theweaker rocks were not represented in the tests. However,the average value of Erock mass of 10.6 GPa has beenadopted as a design value.
Acknowledgements The authors express their sincere thanks toHoward Humphreys and Partners, the Arab Center for Engi-neering Studies and the joint venture with Soletanche BachyCompany France and the Specialized Engineering Service Com-pany for providing the geotechnical data. The authors also wishto acknowledge the constructive suggestions made by reviewersto improve this paper.
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