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Rehabilitation of the Braamfontein West Water Management Unit – Jan van Riebeek Park Jan van Riebeek Park Dams Top Dams: Detailed Design Report for Rehabilitation Johannesburg Road Agency & City of Johannesburg

Reference: 504630

Revision: 0

2019-06-26

Project number 504630 File Detail Design Report for the Rehabilitation of JVR Park dam_Rev0.2.docx 2019-06-26 Revision 0

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Report title Jan van Riebeek Park Dams Top Dams: Detailed Design Report for Rehabilitation

Document code 12227 Project number 504630

File path P:\Projects\504630 JRA Dams\5 DEL DES\501 Engineering\Reports\Jan Van Riebeek Park (Top Dams)\Detail Design Report for the Rehabilitation of JVR Park dam_Rev0.docx

Client Johannesburg Road Agency & City of Johannesburg

Client contact Phuti Letsaba Client reference JRA/18/107_001

Rev Date Revision details/status Author Reviewer Verifier (if required)

Approver

0 2019-06-26 Final I Bey B Collet I Bey B Collet

Current revision 0

Approval

Author signature Approver signature

Name I Bey Name B Rochecouste Collet

Title Dam Engineer & Project Leader

Title Project Director


Executive Summary The Johannesburg Roads Agency (JRA) was appointed by The Environment and Infrastructure Services Department (EISD), the implementing agent for the Water and Biodiversity project, for the rehabilitation of a number of dams within the City of Johannesburg. As part of this appointment JRA appointed Aurecon for the design of the remedial works of the Jan Van Riebeek Park Upper and Lower Dams which are located immediately upstream of the Emmarentia Dam.

Significant transformation of the upper catchment, such as loss of vegetation, floodplain and wetland encroachment and increased hard surfacing have resulted in many of the impacts now affecting the two dams. Increased flow energy and volumes of surface run-off have resulted in high sediment deposition rates into the upper of the two dams. This has resulted in the reduction of the upper dam’s storage potential and also the establishment of vegetation within the basin due to the shallow depth of water in the basin.

The increased volumes surface run-off has also resulted in the inadequacy of the existing spillway structures to safely pass the floods. The higher flood peaks passing the spillways have resulted in erosion of the spillway channels and, in the case of the lower dam, failure of the spillway overflow section.

The design of remedial works presented in this report addresses the above issues with current dam design practices.

The proposed sequence of work for the remedial work is as follows:

Desilting of the reservoir basins at the Upper and Lower Dam;

Construction of the auxiliary spillways at each dam. The auxiliary spillways are designed to pass the 1:20 yr recurrence interval flood. The auxiliary spillway is to be constructed of rectangular precast portal culverts placed side-by-side thereby providing an access way for pedestrians and maintenance vehicles to cross the spillway;

Demolishing of the existing service spillways at the right abutments and replacing with embankment.

Raising of the embankments by a nominal height to provide a level crest elevation and additional attenuation capacity; and

Construction of a sand trap upstream of the Upper Dam to allow for trapping and removal of silt before deposition in the reservoir basin.


Contents 1 Introduction .................................................................................................................................................. 1

1.1 Background .............................................................................................................................. 1

2 Principal Details of Jan van Riebeek Park Dams ..................................................................................... 3 3 Flood Hydrology .......................................................................................................................................... 5

3.1 Introduction ............................................................................................................................... 5 3.2 Catchment characteristics ........................................................................................................ 5 3.3 Design rainfall ........................................................................................................................... 6 3.4 Rational Method ....................................................................................................................... 6 3.5 SCS method ............................................................................................................................. 7 3.6 Recommended design floods ................................................................................................... 7

4 Geology and Geotechnics .......................................................................................................................... 9 4.1 Investigation overview .............................................................................................................. 9 4.2 Investigation Methodology ........................................................................................................ 9 4.3 General Geological Setting & Seismicity ................................................................................ 11 4.4 Site Description ...................................................................................................................... 11

4.4.1 Upper Dam .............................................................................................................. 11 4.4.2 Lower Dam .............................................................................................................. 12

4.5 Geological Profile ................................................................................................................... 13 4.6 Embankment Fill Material ....................................................................................................... 13

4.6.1 Upper Dam .............................................................................................................. 13 4.6.2 Lower Dam .............................................................................................................. 13

4.7 Alluvium .................................................................................................................................. 14 4.8 Groundwater ........................................................................................................................... 14 4.9 Laboratory Results ................................................................................................................. 14

4.9.1 Foundation indicators .............................................................................................. 14 4.10 Laboratory Testing.................................................................................................................. 17 4.11 Construction Materials ............................................................................................................ 20

5 Embankment Rehabilitation ..................................................................................................................... 21 5.1 Introduction ............................................................................................................................. 21 5.2 Horizontal Alignment .............................................................................................................. 21 5.3 Cross Section ......................................................................................................................... 21

5.3.1 Non-overspill crest ................................................................................................... 21 5.3.2 Embankment slopes ................................................................................................ 21 5.3.3 Embankment fill ....................................................................................................... 22 5.3.4 Upstream slope protection ...................................................................................... 22 5.3.5 Downstream slope protection .................................................................................. 22 5.3.6 Compaction ............................................................................................................. 22

5.4 Stability Analysis ..................................................................................................................... 23 5.4.1 Shear strength parameters ...................................................................................... 23 5.4.2 Slope stability results ............................................................................................... 23 5.4.3 Settlement ............................................................................................................... 25

6 Spillway Design ......................................................................................................................................... 25 6.1 Recommended Design Criteria .............................................................................................. 25 6.2 Existing Spillways ................................................................................................................... 25 6.3 Spillway Selection................................................................................................................... 26


6.4 Spillway Overflow Structures .................................................................................................. 26 6.4.1 Location ................................................................................................................... 26 6.4.2 Description ............................................................................................................... 26 6.4.3 Floods ...................................................................................................................... 27 6.4.4 Freeboard ................................................................................................................ 27 6.4.5 Horizontal alignment ................................................................................................ 27 6.4.6 Vertical alignment .................................................................................................... 27 6.4.7 Discharge capacity .................................................................................................. 27 6.4.8 Spillway bridge ........................................................................................................ 28

7 Silt Trap ...................................................................................................................................................... 29 7.1 Design criteria ......................................................................................................................... 29 7.2 Required length of silt trap ..................................................................................................... 29 7.3 Position of sand trap ............................................................................................................... 30

8 De-silting of Dam Basins .......................................................................................................................... 30 8.1 Stage 1: Desilting basin perimeter ......................................................................................... 30 8.2 Stage 2: Desilting basin centre ............................................................................................... 31

9 Dam Break Analysis .................................................................................................................................. 31 10 Construction Methodology and Estimated Construction Costs .......................................................... 31

10.1 Proposed Construction procedure .......................................................................................... 31 10.2 Estimated construction costs .................................................................................................. 32 10.3 Estimated Construction Programme ...................................................................................... 33 10.4 Lowering of basin water levels ............................................................................................... 33 10.5 Precautions and measure to ensure public safety ................................................................. 33

11 Drawings and Specifications ................................................................................................................... 34 11.1 Detail Design drawings ........................................................................................................... 34 11.2 Specifications ......................................................................................................................... 34

12 Conclusions and Recommendations ...................................................................................................... 35 13 References ................................................................................................................................................. 36

Appendices Appendix A ...................................................................................................................................................... 37

Geotechnical Investigations ................................................................................................................ 37

Appendix B ...................................................................................................................................................... 38 Embankment Slope Stability Analyses ................................................................................................ 38

Appendix C ...................................................................................................................................................... 45 Construction Cost Estimate ................................................................................................................. 45

Appendix D ...................................................................................................................................................... 46 Construction Programme..................................................................................................................... 46

Appendix E ...................................................................................................................................................... 48 Drawings .............................................................................................................................................. 48

Figures


Figure 1-1: Locality plan for Jan van Riebeek Park Top Dams .......................................................................... 2 Figure 3-1: Land-use classification ..................................................................................................................... 5 Figure 4-1: General view of the Upper Dam embankment crest from the left flank ......................................... 12 Figure 4-2: Sand bags at Upper Dam’s spillway inlet ...................................................................................... 12 Figure 4-3: General view of the Lower Dam embankment crest from the left flank showing benching of

upstream slope and large trees on the embankment. ............................................................ 13 Figure 5-1: Upper Dam embankment zones .................................................................................................... 24 Figure 5-2: Lower Dam embankment zones .................................................................................................... 24 Figure 8-1: Idealised partial section through basin showing extent of desilting during Stage 1. ..................... 31

Tables Table 2-1: Main features of Jan van Riebeek Park Dams dam .......................................................................... 3 Table 3-1: Catchment characteristics ................................................................................................................. 5 Table 3-2: Land-use distribution ......................................................................................................................... 6 Table 3-3: Recurrence interval rainfall ....................................................................................................... 6 Table 3-4: Rational Method Runoff Coefficients – future land-use ............................................................ 7 Table 3-5: Design flood peaks determined using the Rational Method – future land-use ......................... 7 Table 3-6: SCS method Curve Numbers – future land-use ....................................................................... 7 Table 3-7: Design flood peaks determined using the SCS method – future land-use ............................... 7 Table 3-8: Recommended design flood peaks ................................................................................................... 8 Table 4-1: Summary of test pit positions at Upper Dam .................................................................................... 9 Table 4-2: Summary of test pits at Lower Dam ................................................................................................ 10 Table 4-3: Summary of seepages encountered in test pits .............................................................................. 14 Table 4-4: Summary of Foundation Indicator test results for Upper Dam ........................................................ 15 Table 4-5: Summary of Foundation Indicator test results for Lower Dam ........................................................ 15 Table 4-6: Summary of laboratory test results ................................................................................................. 18 Table 4-7: Classification of Materials based properties determined from laboratory test results .................... 19 Table 5-1: Specification of embankment fill material ........................................................................................ 22 Table 5-2: Properties of Embankment Construction Materials used in Slope Stability Analysis ..................... 23 Table 5-3: Slope Stability Analysis Results for Upper Dam Embankment ....................................................... 24 Table 5-4: Slope Stability Analysis Results for Lower Dam Embankment ....................................................... 25 Table 10-1 – Estimated Construction Cost ....................................................................................................... 32

Project number 504630 File Detail Design Report for the Rehabilitation of JVR Park dam_Rev0.2.docx, 2019-06-26 Revision 0

Glossary of Terms Term Definition

APP Approved Professional Person

dia Diameter

DWA Department of Water Affairs

DWS Department of Water and Sanitation

DSER Dam Safety Evaluation Report

FSL Full Supply Level

H Horizontal

h Hour

ha Hectares

JRA Johannesburg Road Agency

km Kilometres

km2 Square kilometres

m metre

masl Metres above mean sea level

mm Millimetres

m3 Cubic metres

MAP Mean Annual Precipitation

MAR Mean Annual Runoff

m3/s Cubic metres per second

NOC Non Overspill Crest

PMF Probable Maximum Flood

Pr Eng Professional Engineer

RDD Recommended Design Discharge

RDF Recommended Design Flood

RI Recurrence Interval

RL Reduced Level

RMF Regional Maximum Flood

SANCOLD South African National Committee on Large Dams

SANRAL South African National Roads Agency Limited

SED Safety Evaluation Discharge

SEF Safety Evaluation Flood

V Vertical

yr Year

Project number 504630 File Detail Design Report for the Rehabilitation of JVR Park dam_Rev0.2.docx, 2019-06-26 Revision 0 1

1 Introduction

1.1 Background The Environment and Infrastructure Services Department (EISD) has appointed the Johannesburg Roads Agency (JRA) as the implementing agent for the Water and Biodiversity project. The JRA has appointed Aurecon South Africa (Pty) Ltd under contract JRA/18/107 for the design of rehabilitation works for two small dams located within the Jan van Riebeek Park (also known as the Johannesburg Botanical Gardens).

These dams lie at the upper reaches of the Jan Van Riebeek Park. The upper reservoir has been substantially transformed due to impoundment of silt. The deposition of silt within the upper reservoir basin is due mainly to activities and changes within the upstream catchment, such as loss of vegetation, encroachment into wetlands and floodplains, and the increase in paved areas offering no infiltration to storm rainfall. This has led to an increase in the volume and energy of surface run-off which results in high sediment loads and sediment deposition within the upper reservoir’s basin.

The spillway structures at both the upper and lower dams appear to be undersized and of insufficient capacity to discharge incoming floods. There are reports of the embankment overtopping during flood events due to insufficient spillway capacity. Uncontrolled flow over the embankment and spillway has resulted in undermining of the spillway discharge channel and even erosion of material from the downstream slopes of the embankments.

The rehabilitation works of the Upper and Lower dam at Jan Van Riebeek park will seek to address:

Increasing the size of the spillway structures to safely pass the design floods;

Removal of sediment and aquatic vegetation from the upper reservoir and lower reservoirs to extend the serviceable life of the basins;

Raising of the embankment by a nominal height to provide uniform freeboard as well as additional flood attenuation capacity;

Rehabilitation of the spillway discharge channels at both the upper and lower reservoirs by removal of damaged lining, loose foundation materials, re-sizing of the channel to accommodate the design flood and designing an appropriate liner for the channel that will prevent further undermining of the channel; and

Construction of a sand trap upstream of the Upper Dam.

The main objective of this report is to provide information regarding the design of the rehabilitation of the Upper and Lower Dams.

A locality plan for Jan van Riebeek Park Upper and Lower Dams is shown in Figure 1-1.


Figure 1-1: Locality plan for Jan van Riebeek Park Top Dams

Johannesburg Botanical Gardens

Emmarentia Dam

Lower Dam

Upper Dam


2 Principal Details of Jan van Riebeek Park Dams The Jan Van Riebeek Park Upper and Lower Dams are located immediately upstream of the Emmarentia Dam in the Johannesburg Botanical Gardens which is one of the green spaces within the Johannesburg Metropolitan. It is likely that these two top dams were constructed to act as sediment traps to reduce the rate of sedimentation of the Emmarentia Dam. The Botanical Gardens consists of wide, grassy, open spaces scattered with trees and is used by runners, picnickers and dog walkers.

The Upper and Lower Dams are located on the Westdene Spruit, with the Lower Dam being approximately 650 m upstream of the Emmarentia Dam’s basin headwaters and the Upper dam lying a further 250 m upstream.

Both the Upper and Lower Dams are earthfill embankments which are assumed to be homogenous i.e. constructed of one type of material. The Lower Dam has an embankment crest length of approximately 150 m with a crest elevation of approximately 1610 masl. The Upper Dam has a crest length of 190 m and a crest elevation of approximately 1618 masl.

The principal details of the Jan van Riebeek Park Dams Dam are presented in Table 2-1.

Table 2-1: Main features of Jan van Riebeek Park Dams dam

Main features of the dam

Description Existing details Rehabilitation details

General Information

Dam name Unnamed but referred to as Upper Dam and Lower Dam herein

Type of Dam Homogeneous earthfill embankments Zoned earthfill embankments

Owner City of Johannesburg

Classification Unclassified due to low height

Purpose Recreational

Dam characteristics and dimensions

Surface area at Full Supply Level (FSL)

~ 20 000 m2 at the Upper Dam

~ 10 000 m2 at the Lower Dam

Watercourse Westdene Spruit

Catchment Area 500 ha

Embankment Existing details Rehabilitation details

Maximum wall height ~ 3 m at the Upper Dam

~ 2.5 m at the Lower Dam

~ 3.5 m at the Upper Dam

~ 3.0 m at the Lower Dam

Embankment crest length 150 m (Lower Dam)

190 m (Upper Dam)

Embankment crest level 1610 masl (Lower Dam)

1618 masl (Upper Dam)

1610.5 masl (Lower Dam)

1618.5 masl (Upper Dam)

Crest width ± 4 m for the upper dam Crest not clearly defined at lower dam

Full Supply Level (FSL) ~ 1609.5 masl (Lower Dam)

~ 1617.3 (Upper Dam)

Upstream face slope Near vertical due to erosion 1V:2H

Downstream slope ± 1V:5H (Lower Dam)

±1V:3H (Upper Dam)

± 1V:2.5H for upper section of lower dam


1V:2.5H for upper section of Upper Dam

Upstream slope protection None Amorflex Blocks

Downstream slope protection Grass Grass

Spillway characteristics Existing details Rehabilitation details

Spillway type Both dams are fitted with an open channel spillway at the right abutments.

A 300 mm wide low concrete wall serves as the spillway at the right abutment of the Lower Dam. There is no overflow section at the Upper Dam.

The spillway discharge channels comprise of concrete slab panels placed end-over-end to create a cascaded channel with steps of approximately 100 mm height.

300 mm wide concrete overflow sills with precast concrete portal culverts acting as bridges over the spillway. 24 m wide overflow sections with a 3 m wide lower central overflow section to allow for daily flows to pass through.

Spillway length 10 m wide overflow section at Lower Dam 3 m wide open channel at Upper dam.

Upper Dam auxiliary spillway: 24 m Lower Dam auxiliary spillway: 24 m

Existing freeboard 0.5 m Upper Dam: 1.2 m Lower Dam: 1.2 m

Recommended design flood (RDF)

1:20 RI yr. flood

Flood peak 46 m3/s

Safety Evaluation Flood (SEF) 0.5 x PMF

Flood inflow peak 167 m3/s

The Upper Dam is heavily silted and water hyacinth and other aquatic plants cover most of the basin. The Lower Dam is not as severely silted. The upper reaches of the Lower Dam’s basin has significant reed growth.


3 Flood Hydrology

3.1 Introduction The guidelines used to determine and select suitable design floods are described in the SANCOLD publication “Guidelines on Safety in Relation to Floods” (SANCOLD, 1991).

The guidelines indicate that a dam with a “small” size classification and a “low” hazard rating should be designed to safely pass a recommended design flood (RDF) with a return period of between 20-50 years. The Jan van Riebeek Park Upper and Lower dams do not meet the minimum criteria to be classified as a dam with a safety risk due to their maximum heights being less than 5 m and storage volume being less than 50 000 m3. The spillways for these structures will therefore be designed to safely pass the lower bound 1:20 year RI flood without any risk of overtopping and to pass the 1:50 yr RI flood with minimal (short duration) overtopping. The embankment slope protection will be designed to withstand erosion due to higher flood peaks passing over the embankments.

The design floods were determined using the well-established Rational Method and Soil Conservation Service (SCS) methodologies. Both these methodologies are applicable to small catchments, but they differ substantially in their underlying processes and use of the catchment characteristic data.

3.2 Catchment characteristics A GIS facility was utilised to delineate the associated catchment boundaries and determine selected catchment characteristics from the Space Shuttle Radar Topography Mission (SRTM) 30-meter DEM. Details are presented in Table 3-1.

Table 3-1: Catchment characteristics

Catchment Longest flow path (km) Area (km2) Average catchment slope (m/m)

Longest Watercourse Slope 10-85 (m/m)

Riebeek Dam 4.87 5.01 0.091 0.029

The ESA (2018) 20-meter resolution land-cover, based on December 2015 to December 2016 Sentinel-2A observations, was used to identify land-use, shown in Figure 3-1. Soil types were determined using the SOTER database (Dijkshoorn, Leenaars, Huting, & Kempen, 2016).

Figure 3-1: Land-use classification


For the purposes of design flood determination, a future scenario was postulated in which 40% of the current open spaces are replaced by high-density complexes and additional streets. We based this estimate on an assessment of developable open space as per Figure 3-1 as well as the latest Google Earth image. A comparison of the present-day land-use distribution with the future postulation is shown in Table 3-2 below.

Table 3-2: Land-use distribution

Description Current-day Proportion of Catchment Area

Future Proportion of Catchment Area

Open spaces 32% 16%

Residential (38% impervious) 46% 46%

Roads/Streets 22% 24%

High-Density (72% impervious) 0% 14%

3.3 Design rainfall The recurrence interval (RI) rainfall was obtained from Smithers & Schulze, (2002) design rainfall software at Station Florida (Long 27.55, Lat 26.1) with a 26-year record and a MAP of 784 mm and the Probable Maximum Precipitation (PMP) from (HRU, 1972). The extracted recurrence interval daily rainfalls are presented in Table 3-3 and converted to 24-hour recurrence interval rainfalls using a conversion factor of 1.11 (Adamson, 1981).

Table 3-3: Recurrence interval rainfall

Recurrence interval (y) 10 20 50 100 200 PMP

Point daily rainfall (mm) 104 123 151 175 200

24-hour rainfall (mm) 116 137 168 194 222 400

3.4 Rational Method The Rational Method is represented by the following relationship:

𝑄𝑄 =𝐶𝐶𝐶𝐶𝐶𝐶3.6

Q = design flood peak (m3/s)

C= runoff coefficient (dimensionless)

I = average rainfall intensity over catchment (mm/hour)

A = effective area of catchment (km2)

3.6 = conversion factor

The Rational Method yields a design flood peak only (i.e. no hydrograph). The flood response of the catchment is expressed by two quasi-physical parameters: Runoff Coefficient (C), which is a function of average catchment slope, permeability, land-use, mean annual precipitation (MAP) and RI; and Time of Concentration (Tc), which is a function of the length of the longest watercourse and the average slope of that watercourse. By definition Tc represents the critical storm duration and Adamson's (1981) “summer rainfall region” factors were used to proportion the 24-hour design rainfall to a Tc hour design rainfall. This Study utilised the C-value guide recommended by Alexander (1990). The resulting coefficients are summarised in Table 3-4. The total area-weighted C value is 0.637.

The resulting design flood peaks based on the historically recorded rainfall are shown in Table 3-5.


Table 3-4: Rational Method Runoff Coefficients – future land-use

Description Open Spaces Residential Roads/Streets High Density PMF

Proportion of Catchment Area 18% 46% 23% 13 100%

C Coefficient 0.25 0.60 0.95 0.75 0.90

Area-weighted C Coefficient 0.04 0.276 0.228 0.105 0.90

Table 3-5: Design flood peaks determined using the Rational Method – future land-use

Recurrence interval (y) 10 20 50 100 200 PMF

Peak flow (m3/s) 41 54 82 114 131 333

3.5 SCS method The SCS methodology yields a full design flood hydrograph. The flood response of the catchment is represented by two quasi-physical parameters: Curve Number (CN), which is a function of soil group, land-use, vegetation cover, and antecedent soil moisture conditions; and Catchment Lag, which is a function of average catchment slope, length of the longest watercourse and CN. Soil groups are classified according to the Binomial Classification System for Southern Africa (Soil Group A – D), which has a strong texture and depth basis (Schmidt & Schulze, 1987). The dominant soil group for this catchment was classified as C which has a moderately high stormflow potential. The resulting Curve Numbers are summarised in Table 3-6. The total area-weighted CN value is 85.9.

Table 3-6: SCS method Curve Numbers – future land-use

Description Open Spaces Residential Roads/Streets High-Density PMF

Proportion of Catchment Area 18% 46% 23% 13% 100%

Curve Number (CN) 79 83 95 90 95

Area-Weighted CN 14.2 38.2 21.9 11.7 95

The SCS approach requires a rainfall hyetograph as input. For this Study, the 24-hour rainfall hyetograph based on the South African-derived “Storm Type III Distribution” (Schmidt & Schulze, 1987) was employed. The American SCS lag time equation was used due to the urban setup of the catchment. The SCS utility included in the HEC-HMS modelling package (U.S. Army Corps of Engineers, 2015) was used to generate the design floods. The results are summarised in Table 3-7.

Table 3-7: Design flood peaks determined using the SCS method – future land-use

Recurrence interval (y) 10 20 50 100 200 PMF

Peak flow (m3/s) 65 81 104 124 145 305

3.6 Recommended design floods The design flood peak values derived through the SCS methodology are larger than those derived via the Rational Method, while the SCS-based PMF is smaller. However, given that each of the two methodologies is subject to particular data uncertainties, we recommend final design flood values based on the average of the results of the two methodologies. The final recommended design flood peaks are presented in Table 3-8.


Table 3-8: Recommended design flood peaks

Recurrence interval (y) 10 20 50 100 200 RMF PMF

Peak flow (m3/s) 53 68 93 119 138 224 319

In order to determine the reasonableness of the recommended PMF, it was compared with the Regional Maximum Flood (RMF), determined according to Kovacs (1988) for RMF Region 5. The ratio of PMF to RMF was found to be 1.42, which is well smaller than the capping ratio of 2.0 suggested in the SANCOLD Guidelines on Safety in Relation to Floods (SANCOLD, 1991).


4 Geology and Geotechnics

4.1 Investigation overview The aim of the investigation was to determine the parameters of the existing embankment materials as well as the founding conditions as no construction records were available. These parameters are essential to the stability analysis of the embankments as well as for determining the suitability of founding conditions for the spillways and the upstream silt trap.

The site investigation was carried out on the 30th May 2019 using a tractor-loaded backhoe (TLB).

Shallow test pitting, supplemented by sampling and laboratory testing was undertaken. Representative samples were taken from the test pits and submitted to SANAS-accredited laboratory, Civilab (Pty) Ltd, for classification and geotechnical testing. Tests conducted include:

Foundation indicator tests (comprising of grading and hydrometer analyses, Atterberg limits and Linear Shrinkage);

Proctor compaction; Maximum Dry Density (MDD) and Optimum Moisture Content (OMC);

Quick direct shear tests; and

Falling head permeability tests.

4.2 Investigation Methodology A total of sixteen (16 No.) test pits were excavated using a light TLB. Test pits positioned on the embankments were excavated using hand-tools. The test pitting investigation involved the following:

Eight (8 no.) test pits were excavated over the Lower Dam embankment footprint and right abutment.

Eight (5 no.) test pits were excavated over the Upper Dam embankment footprint, right abutment.

Three (3 No.) test pits were excavated upstream of the Upper Dam’s basin where a proposed silt trap will be positioned.

Laboratory indicator tests (particle size distribution, Atterberg limits and in-situ moisture contents) were carried out on the samples collected as being representative of the range of materials available.

Further laboratory tests (quick direct shear test, proctor compaction and falling head permeability tests) were carried out on soil sampled from the test pits on the embankment as the parameters determined from these tests would be representative of material in the upgraded embankment.

Test pit positions are shown on Drawing 504630-0000-DRG-G1-0001 in Appendix E. Details of the sixteen (16 No.) test pits are summarised in Table 4-1 and Table 4-2 below.

Table 4-1: Summary of test pit positions at Upper Dam

Test Pit No. SA Lo 29 WGS 84

Depth (m) Remarks Y - Coordinate X - Coordinate

UD TP01 99 983 2 895 133 2.0 Target depth reached; test pit located on embankment crest

UD TP02 99 928 2 895 170 1.75 Terminated due to seepage – excavation unsafe; test pit located on embankment crest.

UD TP03 99 886 2 895 163 1.2 Terminated due to seepage – excavation unsafe; test pit located at embankment toe.

UD TP04 99 625 2 895 169 2.0 Target depth reached; test pit located at right bank.




UD TP05 99 825 2 895 147 2.0 Target depth reached.; test pit located along right-hand spillway channel

UD TP06 99 996 2 895 343 0.8 Terminated due to seepage; test pit located upstream of Upper Dam basin at position of proposed silt trap.



Table 4-2: Summary of test pits at Lower Dam



LD TP01 99 881 2 894 889 2.0 Target depth reached; test pit located on embankment crest

LD TP02 99 838 2 894 939 1.6 Terminated due to seepage – excavation unsafe; test pit located on embankment crest.

LD TP03 99 807 2 894 935 2.3 Target depth reached; test pit located at embankment toe, very slow seepage at the base of the test pit.

LD TP04 99 774 2 894 944 2.2 Target depth reached; test pit located on embankment crest, very slow seepage at the base of the test pit.

LD TP05 99 743 2 894 949 2.1 Target depth reached.; test pit located at edge of embankment.

LD TP06 99 724 2 894 925 1.3 Terminated due to seepage; test pit located adjacent to existing spillway discharge channel.

LD TP07 99 738 2 894 962 0.6 Terminated due to seepage; test pit located adjacent to existing spillway channel.

LD TP08 99 757 2 894 978 1.5 Target depth reached; test pit located on right bank.

As the primary purpose of the test pits was to expose the geological materials and allow sampling, there was no intention to excavate to the point of refusal. On the contrary, as the dams were functional at the time of the investigation, particular care was taken not to excavate the test pits too deep and in so doing intersect the phreatic line and create a potentially unsafe condition.

Test pit positions were positioned using a hand-held GPS apparatus. Coordinates were recorded utilising the WGS84 datum and South African grid. No detailed surveying of the test pits was carried out.

The geological profile exposed in the respective test pits was described by the engineering geologist in accordance with South African best practice, after Jennings, Brink and Williams (1973). These geological profiles are presented in Appendix A.


Representative samples were collected and submitted to a SANAS accredited laboratory, Civilab (Pty) Ltd; for classification and geotechnical testing. Tests conducted included the following;

Foundation Indicators, comprising grading analyses (sieve as well as hydrometer) and determination of the Atterberg limits,

Proctor compaction, Maximum Dry Density (MDD) and Optimum Moisture Content (OMC);

Quick direct shear tests; and

Falling head permeability tests.

Laboratory test results are included in the Geotechnical reports for the Upper and Lower Dams which in turn are included in Appendix A of this report.

4.3 General Geological Setting & Seismicity The published geological map indicates that the site is underlain by basement rocks of the Johannesburg Dome, formerly known as Halfway House granite. The specific lithologies comprise of ultramafic rocks, granites, dioritic gneiss, hornblend gneiss, biotite gneiss and hybrid mafic rocks. A fault system trending north east – south west occurs approximately 1 km south east of the site.

The greater Johannesburg area is affected by natural, and induced seismic activities related to mining in the Witwatersrand. The Jan van Riebeek Park Upper and Lower Dams are located in this area with the seismic hazard considered moderate to high. A Peak Ground Acceleration (PGA) greater than 0.1g (SANS 101610-4:2011) is associated with the area with a probability of being exceeded in a 50-year period.

The site is located close to Weinert’s N-value is about 3.3 (Weinert, 1980), which indicates a humid climate. Chemical decomposition of the underlying rock is the main mode of weathering and thick residual soil sequences generally tend to develop.

Where in-situ soils were encountered these comprised of alluvial deposits accumulating along the banks of the Braamfontein Spruit (at the position of the proposed silt trap upstream of the Upper Dam).

4.4 Site Description

4.4.1 Upper Dam The Upper Dam has a maximum height of 3.5 m and is therefore not classified as a dam with a safety risk in terms of the National Water Act. The upstream slope of the embankment has eroded due to wave action due to the absence of any slope protection. The embankment crest is approximately 4 m wide. The crest and downstream slope is grass covered. A few large trees (Figure 4-1) were noted to be growing on the downstream slope of the embankment as well as at the embankment crest at the right abutment. This is undesirable from a dam safety point of view – and it is recommended that these trees be removed regardless of whether the upgrade works are to proceed or not. The Johannesburg City Parks does maintain the grass growth on the embankment to acceptable levels.

No signs of instability were noted on the Upper Dam embankment. Sand bags at the existing spillway inlet (Figure 4-2) indicates a previous breach or overtopping at this section which means that the existing spillway lacks sufficient discharge capacity.

The Upper Dam’s basin is heavily silted and aquatic vegetation covers almost the entire extent of the basin. The area upstream of the Upper Dam basin is characterised by alluvial material and sediment at the surface.


Figure 4-1: General view of the Upper Dam embankment crest from the left flank

Figure 4-2: Sand bags at Upper Dam’s spillway inlet

4.4.2 Lower Dam The Lower Dam is an earthfill embankment with a maximum height of approximately 4 m and a crest length of approximately 150 m. The downstream face of the embankment is sloped at approximately 1V:5H and the slope is covered with thick grass cover. The upstream edge of the embankment crest has eroded due to wave action due to the absence of any upstream slope protection. A few large trees were noted along the crest and downstream slope of the embankment. The Johannesburg City Parks does maintain the grass growth on the embankment to acceptable levels.

The inlet section of the right-hand spillway channel has failed due to undercutting and erosion at the right abutment. Flow by-passes the overflow section and the water level in the basin is reduced due to failure of the overflow section.

Some erosion of the spillway channel sides has also occurred which is indicative of insufficient discharge capacity.

The right abutment is defined by wet, marshy conditions extending at least 50 m east with significant visible surface flow.


Figure 4-3: General view of the Lower Dam embankment crest from the left flank showing benching of upstream

slope and large trees on the embankment.

4.5 Geological Profile The geological profile over the two sites revealed embankment fill material underlain by alluvium, which is occasionally ferruginised. These respective geological horizons are discussed below in more detail.

The standards used for soil and rock profiling is included in Appendix A.

4.6 Embankment Fill Material

4.6.1 Upper Dam The surficial layers to the embankment fill include abundant loosely packed rubble predominantly comprising clay bricks in a matrix of loose, soft clayey sand to sandy clay. This material occurs from surface to 2 m depth (refer to test pit log UDTP01 in Appendix A1). However, the major proportion of the embankment fill comprises of medium dense clayey sand and firm silty clay. Areas of soft sandy clay with rootlets were encountered. Fine to medium, angular, scattered and significant gravel is a regular occurrence within the Upper Dam embankment fill layers.

At the test pits located on the Upper Dam embankment crest (UDTP01 and UDTP02), the embankment fill extends to depths in excess of 1.75 m. Embankment fill material is also encountered at the toe of the dam and along the spillway channel with the characteristic profile, i.e. occasional and scattered fine to medium angular gravel within a matrix of soft sandy clay.

Moisture conditions vary from slightly moist to very wet.

4.6.2 Lower Dam The embankment fill material at the Lower Dam is typically characterised by dense, clayey sand and occasionally firm to stiff, sandy clay with scattered, predominantly fine to medium, angular gravel. The moisture conditions are generally slightly moist becoming moist with depth and wet to very wet below depths at which seepage was encountered. At the embankment toe (test pit LDTP03) the embankment fill is described as moist, very soft to soft, sandy clay.


4.7 Alluvium The bulk of the alluvial soils were encountered upstream of the Upper Dam at test pits DUTP06 to UDTO08, and UDTP04 and UDTP05 located at the right bank and spillway channel of the Upper Dam. The alluvium horizon comprises of clayey sand and silty clay containing fine to medium gravel and ferricrete concretions in places. Consistencies of this alluvium horizon are typically very loose or very soft and soft to firm. Moisture conditions vary from slightly moist to very wet.

At the Lower Dam, the alluvial soils occur at the toe of the embankment, the right bank and along the spillway channel. Alluvial soils at the toe are described as soft, silty clay while at the right bank and along the spillway channel the alluvium comprises soft to firm sandy clay with fine to medium, angular to sub-rounded quartz gravel.

Of interest is the very low seepage at the boundary of the silty clayey alluvium horizons at LDTP03. Only limited seepage was noted here after the short duration that the pit was open, suggesting material with a low permeability.

4.8 Groundwater A summary of test pits where seepages were encountered is given in Table below. The locations of the test pits are shown on drawings in Appendix E.

Table 4-3: Summary of seepages encountered in test pits

Test Pit No. Seepage interception depth (m) Location of test pit

UDTP02 1.75 Crest of Upper Dam embankment

UDTP03 1.2 Toe of Upper Dam embankment

UDTP05 1.7 Along spillway channel of Upper Dam

UDTP06 0.8 At proposed silt trap position upstream of Upper Dam



LDTP02 1.5 Crest of Lower Dam embankment

LDTP03 1.2 Toe of Lower Dam embankment

LDTP04 2.0 Crest of Lower Dam embankment

LDTP06 1.0 Along spillway channel of Lower Dam

LDTP07 0.6 Spillway channel of Lower Dam

The reservoir basin water level at the Upper Dam was higher than the level at which seepage was intercepted at UDTP02 and UDTP03. The seepage at UDTP03 is assumed to be directly linked to the reservoir water level by virtue of the test pit position at the toe of the embankment. Clear seepages were observed which indicated that soil particles are not being transported.

4.9 Laboratory Results

4.9.1 Foundation indicators In order to classify and characterise the various materials, Foundation Indicator tests were conducted on representative samples. Detailed test results are presented in Appendix A3 and are summarised below in Table 4-4 and Table 4-5.


Table 4-4: Summary of Foundation Indicator test results for Upper Dam

TP No

Depth (m)

Particle Size % Atterberg Limits % GM AASHTO / USCS

Classification;

Expansion Potential

Clay Silt Sand Gravel LL PI LS

Embankment Fill Material

UD

TP01

0.0-

1.10

6 5 46 43 34 15 7 2.13 A-2-6(0) /

SP-SC;

Low

UD

TP02

1.3-

1.75

8 20 33 39 41 17 7 1.70 A-2-7(1) /

SC; Low

UD

TP03

0.0-1.0 8 10 39 43 30 12 5.5 1.96 A-2-6(0) /

SC; Low

UD

TP05

0.8-1.5 13 10 35 42 39 18 8.0 1.84 A-2-6(1) /

SC; Low

Alluvium

UD

TP04

1.1-2.0 18 23 53 6 49 24 11 0.93 A-7-6(7) /

SC;

Medium

UD

TP06

0.0-0.6 2 42 53 3 30 11 4.5 0.64 A-6(3) / CL;

Low

UD

TP07

0.5-1.7 6 34 55 5 24 8 4.5 0.80 A-4(1) / SC;

Low

UD

TP08

0.0-1.0 34 18 47 1 42 19 8.5 0.61 A-7-6(8) /

CL; Low

Table 4-5: Summary of Foundation Indicator test results for Lower Dam

TP No

Depth (m)


Classification;

Expansion Potential


Embankment Fill Material

LD

TP01

1.3-

2.0

7 34 54 5 23 11 4.5 0.71 A-6(2) / SC;

Medium

LD

TP02

0.7-1.5 9 9 45 37 51 20 7.5 1.85 A-2-7(2) /

SM; Low

LD

TP03

0.2-0.5 20 21 50 9 27 13 50.0 1.01 A-7-6(7) /

SC; Low

LD

TP04

0.5-1.2 17 34 36 13 35 20 8.0 0.91 A-6(4) / SC;

Medium

LD

TP05

0.55-

0.9

21 23 54 2 27 12 6.0 0.75 A-6(2) / SC;

Low


TP No

Depth (m)


Classification;

Expansion Potential


Alluvium

LD

TP03

0.50-

1.20

27 11 62 - 50 29 13.5 0.97 A-7-6(7) /

SC;

Medium

LD

TP06

0.90-

1.30

27 11 57 5 35 20 8.0 0.94 A-6(4) / SC;

Medium

LD

TP07

0.4-0.6 2 21 65 10 25 8 3.5 1.20 A-2-4(0) /

SC; Low

LD

TP08

0.00-

0.70

2 21 67 10 25 8 3.5 1.20 A-2-4(0) /

SC; Low

Where;

AASHTO = American Association of Sate Highway and Transport Offficials,

USCS = Unified Soil Classification System,

GM = Grading Modulus,

LL = Liquid Limit,

PI = Plasticity Index,

LS = Linear Shrinkage,

SC = Clayey sand, sandy clay mixtures,

SM = Silty sands, sand-silt mixtures, and

CL = Clay of low plasicity

The embankment fill material at the Upper Dam essentially comprises clayey sand and sandy clay mixtures with clay content which varies between 6% and 13% and a silt content between 5% and 20% while sand and gravel content varies between 33% and 46%, and 39% and 43%, respectively. There is likely to be both a lateral and vertical variation in material properties, reflecting the placement of the materials during construction. The grading modulus varies between 1,70 and 2,13 which indicates a moderately coarse material. Although there is a major granular component to the fill material, the finer matrix would influence much of the relevant soil behaviour, hence the SC (or Clayey Sand, sandy clay mixtures) classification. The values for the Plasticity Index (PI) and Linear Shrinkage values are moderate, with values between 12% and 18%, and 5.5% and 8%, respectively. The moderately plastic nature of the material, together with the clay fraction between 6% and 13% indicate material with a low potential expansiveness (after Van der Merwe, 1973).

The embankment fill material at the Lower Dam essentially comprises clayey sand and sandy clay mixtures with clay content which varies between 7% and 21% and a silt content between 9% and 34% while sand and gravel content varies between 36% and 54%, and 2% and 37%, respectively. There is likely to be both a lateral and vertical variation in material properties, reflecting the placement of the materials during construction. The grading modulus varies between 0,71 and 1,85 which indicates a low to moderately coarse material. Although there is a major granular component to the fill material, the finer matrix would influence much of the relevant soil behaviour, hence the SC (or Clayey Sand, sandy clay mixtures) classification. The values for the Plasticity Index (PI) and Linear Shrinkage values are moderate, with values between 11% and 20%, and 4.5% and 20%, respectively. The slightly higher plastic nature of the material (relative to the embankment fill in the Upper Dam), together with the clay fraction between 7% and 21% indicate material with a low to medium potential expansiveness (after Van der Merwe, 1973).


The alluvial soils at the Upper Dam contain percentages of silt ranging between 18 and 42% and sand ranging between 47 and 55%. Maximum clay percentage recorded is 34% and a lowest value of 2%. According to the Unified Soil Classification System (USCS) the material classified as SC and CL. In accordance to the method proposed by Van der Merwe (1973) this material has low and medium potential for expansion.

The alluvial soils at the Lower Dam contain percentages of silt ranging between 8 and 21% and sand ranging between 53 and 67%. Maximum clay percentage recorded is 27% and a lowest value of 2%. Gravel percentage ranges between 5 and 21%. According to the Unified Soil Classification System (USCS) the material classified as SC. In accordance to the method proposed by Van der Merwe (1973) this material has low and medium potential for expansion.

4.10 Laboratory Testing Disturbed soil samples were taken from selected horizons of test pits on embankment fill material and the alluvium material at the proposed sand trap position and submitted to the laboratory for testing. The material properties determined from quick direct shear tests, remoulded permeability and Proctor compaction tests are summarised in Table 4-6 below.

Based on the material parameters determined from the laboratory tests, the embankment fill materials at the Upper and Lower dams as well as the alluvial materials at the proposed sand trap position were classified according to whether they are Impervious, Semi-pervious or pervious to determine their suitability for use in dam construction. The results of the classification are shown in Table 4-7 where it is evident that the fill material in the Upper and Lower Dams are predominantly classified as Impervious and, in some cases, semi-pervious.

The existing embankments at the Upper and Lower Dam are therefore suitable to be incorporated into the rehabilitation of the embankments.

.


Table 4-6: Summary of laboratory test results

TP No. Depth (m)

Standard Proctor Compaction Shear Strength Parameters

Permeability, k (cm/s) Maximum Dry Density

(MDD)(kg/m3)

Optimum Moisture Content

(OMC) (%) Cohesion, c (kPa) Phi (Degrees)

Upper Dam Embankment Fill

UD TP01 0.0 – 1.0 1634 18.6

UD TP02 1.3 – 1.75 1717 19.2 49 44 4.1 x 10-7

UD TP03 0.0 – 1.0 1730 15.2 50 47 1.3 x 10-6

UD TP05 0.8 – 1.5 1767 17.7 71 38

Lower Dam Embankment Fill

LD TP02 0.7 – 1.5 1864 12.6 54 47 1.7 x 10-5

LD TP03 0.2 – 0.5 1806 13.4 45 45 1.8 x 10-7

LD TP04 0.5 – 1.2 1738 18.5 18 28

Alluvium at proposed Silt Trap

LD TP06 0.9 – 1.3 1860 14.5 56 36

LD TP08 0.0 – 0.6 1633 17.7 85 28 2.7 x 10-8


Table 4-7: Classification of Materials based properties determined from laboratory test results

Material Property Impervious* Semi Pervious* Pervious* Upper Dam Embankment Fill

Lower Dam Embankment Fill

Alluvium at proposed Silt Trap

Maximum Dry Density (100% Proctor) (kg/m3)

1350 - 1700 1600 - 1850 1700 - 2000 1634 - 1767 1738 - 1864 1633 - 1860

Impervious Semi-pervious Semi-pervious

Optimum Moisture Content (OMC) 12 - 25 10 - 15 8 - 12 15.2 – 19.2 12.6 – 18.5 14.5 – 17.7

Impervious Impervious Impervious

Cohesion (kPa) (Direct Quick Shear) 30 - 100 25 -50 25 49 - 71 18 - 54 56 – 85

Impervious Impervious Impervious

Friction Angle (Deg) (Direct Quick Shear) 20̊ - 30 ̊ 30̊ - 35 ̊ 35 ̊ 38 -44 28 - 47 28 – 36

Pervious Pervious Pervious

Permeability (cm/s) 1 x 10-7 1 x 10-5 1 x 10-3 10-7 – 10-6 10-7 – 10-5 10-8

Impervious Impervious – Semi-pervious

Impervious

* - As defined in Druyts, 2005


4.11 Construction Materials The laboratory results of the existing embankment fill material indicate a well distributed mix of soil comprising fines to gravelly and sandy fractions. The alluvial soils underlying also comprise such a mix, but the coarse fraction is predominantly sand while the fines are mostly silt.

The presence of fines would fill the voids between the coarse fractions thereby reducing permeability. While the occurrence of significant gravel and sand will increase the shear strength of the material.

Materials encountered on site, i.e. existing embankment fill material as well as the underlying alluvium, would be suitable for use in the raising of the embankments. It is assumed that the silt within the Lower Dam basin would have the same properties as the alluvium recovered from the test pits.


5 Embankment Rehabilitation

5.1 Introduction The general arrangements of the embankments are shown on Drawing No’s 504630-JVR-DRG-CC-0002 and 0003 in Appendix E.

The height of the rehabilitated Upper Dam embankment (H) from the non-overspill crest (NOC) to the river bed level at the downstream toe is 4.7 m.

The height of the rehabilitated Lower Dam embankment (H) from the non-overspill crest (NOC) to the river bed level at the downstream toe is 3.5 m.

5.2 Horizontal Alignment The existing horizontal alignments of the Upper and Lower Dam embankments were maintained. Slight adjustments were made to the alignments with the addition of defined straights and curves with dimensions and radii to facilitate setting out during construction.

The earthfill sections extend on both flanks to intersect the natural ground profile at NOC level.

5.3 Cross Section

5.3.1 Non-overspill crest The width of the NOC depends on considerations such as the following (USBR, 1987):

The nature of the embankment materials and the minimum allowable percolation distance through the embankment at FSL;

The height and importance of the structure;

The possible roadway requirements, and

The practicability of construction.

The existing embankment crest width at the Upper Dam varies between 3 to 4 m. A 4 m wide crest will be adopted over the length of the embankment. This will provide sufficient width for Johannesburg City Park maintenance vehicles to carry out maintenance along the crest and embankment slopes. Furthermore, as part of the future maintenance of the Upper Dam, it is proposed that the Dam’s basin be desilted at intervals yet to be determined. The 4 m wide crest will allow sufficient working space for machinery to carry out de-silting operations.

The crest of the existing Lower Dam embankment is not very well defined. The embankment currently slopes at approximately 1V:10H from the upstream edge of the crest to the downstream toe. The rehabilitated embankment section at the Lower Dam will consist of a 3 m wide crest which is sufficient width for pedestrian access. The Lower Dam is a popular hub with dog walkers who allow dogs to gain access to the Dam’s basin. The gentle downstream slope of the embankment (i.e. 1V:10H) allows for maintenance vehicle access. A 4 m crest width was therefore not adopted as was at the Upper Dam embankment.

5.3.2 Embankment slopes The Design Guidelines for a Category 1 earthfill dam (DWAF, 1995) suggest that an upstream slope of 1V:3H and a downstream slope of 1V:2H be adopted. The dam wall slopes may be steeper or flatter depending on the nature of fill, as well as foundation material. With the high pedestrian traffic at the Lower Dam, a flatter downstream slope of 1V:4H was adopted to facilitate easier access to pedestrians over the embankment. Furthermore, a flatter downstream slope will be able to withstand erosion due to overtopping better than a steeper slope.


The upstream slope of the existing Lower Dam embankment is near vertical due to erosion. Due to the very low height of the embankment, a 1V:2.5H slope was adopted.

5.3.3 Embankment fill The embankment will be constructed of clayey sand and silty clay with properties similar to the existing embankment fill material. As a minimum, the material used for construction of the embankment will fall within the limits specified in Table 5-1 below. The embankments at the Upper and Lower Dams will remain homogeneous embankments with the inclusion of an upstream slope protection zone.

Table 5-1: Specification of embankment fill material

Material Property Limits

Atterberg Limits

Liquid Limit (LL), %

Plasticity Index (PI), %

Linear Shrinkage (LS), %

Not less than 25%

Not less than 10%

Not less than 5%

Compaction

Optimum Moisture Content, %

Maximum Dry Density, kg/m3

Not less than 12%

1350 to 1850 kg/m3

Friction Angle, (Degrees) 20 to 30 degrees

Cohesion, kPa

Direct Quick Shear

Not less than 25 kPa

Permeability (k), cm/s Not greater than 1x10-5 cm/s

5.3.4 Upstream slope protection The upstream slope will be protected from erosion due to wave action by placing a layer of Amor flex blocks on the slopes. A layer of geotextile will be placed between the embankment fill and the Amor flex blocks to prevent the migration of soil particles from the embankment into the basin.

Small burrows on the upstream slope of the existing embankments indicate that the embankments are used by crabs. This activity can be expected to continue after the rehabilitation of the embankment. The Johannesburg City Parks is to monitor the condition of the rehabilitated upstream slope to ensure that animal burrows on the upstream slope do not lead to localised subsidence and deterioration of the slope protection.

5.3.5 Downstream slope protection The downstream slope will be protected from erosion due to rain by placing a 250 mm thick layer of topsoil which will be grassed.

5.3.6 Compaction The embankment fill will be compacted to 98% of the Proctor density.


5.4 Stability Analysis

5.4.1 Shear strength parameters The embankment sections for the Upper and Lower Dams are shown on Drawings No. 504630-DRG-CC-0010 and 0011 in Appendix E.

Quick direct shear box tests on representative soil samples from test pits located on the embankment were done by Civilab. The field densities for the embankment fill material were based on the Proctor densities and optimum moisture contents of the samples.

The laboratory determined soil parameters are summarised in Table 4-6 in Section 4.10 in Section 4. The soil parameters that were eventually adopted for the slope stability analyses of the rehabilitated embankments are summarised in Table 5-2.

Table 5-2: Properties of Embankment Construction Materials used in Slope Stability Analysis

Zone Material Field Density (kN/m3) Cohesion c’ (kPa)

Internal angle of friction φ’ (Degrees)

Upper Dam Existing Embankment Fill

Clayey sand, Silty Clay 17.1 5 35

Upper Dam New Embankment Fill

Clayey sand, Silty Clay 17 5 30

Lower Dam Existing Embankment Fill

Clayey sand, Silty Clay 18.03 5 30

Lower Dam New Embankment Fill

Clayey sand, Silty Clay 18 5 30

The field density values adopted for the stability analyses represent average values of densities obtained from the standard proctor compaction test results for the representative soil samples.

The cohesion values adopted for the stability analyses represent conservative estimates which are significantly lower than the values obtained from the quick direct shear tests.

The internal angle of friction adopted for the stability analyses represent lower bound and more conservative estimates.

Due to the very low raising of the embankment (i.e. 0.5 to 1 m), it is not expected that there will be significant build-up of pore-water pressures within the embankment during construction. A pore-pressure coefficient (B-bar) of 0.4 has been assumed for the determination of internal pore pressures for the “end-of-construction” slope stability analysis case. The B-bar value gives the pore water pressure at the base of a slice and is multiplied by the height of the slice to obtain the pore-pressure.

5.4.2 Slope stability results The slope stability analyses were carried out using the Slope/W (Geostudio, 2019) software. The analyses were based on the Morgenstern-Price method, with pore pressure conditions being defined by the phreatic surface through the embankment.

The Upper and Lower Dam embankment sections were analysed at the sections with the maximum height. Due to upstream height of the embankment being negligible as a result of sediment deposition within the basin, the upstream slopes were not subjected to an analysis of slope stability. Even with the removal of silt from within the basin, the maxim height of the rehabilitated section will be approximately 2 m at the upstream side. Only the downstream slopes of the Upper and Lower dams were assessed for slope stability.


The phreatic surface estimated for the analyses under steady-state seepage conditions was based on the reservoir water levels at FSL, as it is not expected that the phreatic surface would exceed FSL under flood conditions due to the short duration of flood conditions.

As both the Upper and Lower Dams do not have any means of lowering of the reservoir water level by means of an outlet pipe, the rapid-drawdown case was not considered.

Various conditions for the downstream embankment slopes were analysed and the desired minimum factors of safety were selected based on the following two categories:

Usual conditions – relate to the primary function of the structure and can be expected to occur frequently during the service life of the structure. A usual event is a common occurrence and the structure is expected to perform in the linearly elastic range;

Extreme conditions – refer to events which are highly improbable and can be regarded as emergency conditions. Such events may be associated with natural disasters due to earthquakes or flooding which have a frequency of occurrence which greatly exceed the life of the structure. The structure is expected to accommodate extreme loads without experiencing a catastrophic failure, although structural damage. Major rehabilitation or replacement of the structure might be necessary.

The different zones of the embankments are shown in

Figure 5-1: Upper Dam embankment zones

Figure 5-2: Lower Dam embankment zones

The results of the analyses are summarised in Table 5-3 and Table 5-4 below.

Table 5-3: Slope Stability Analysis Results for Upper Dam Embankment

Condition Category Minimum Factor of Safety

Desired* Obtained

Downstream

Reservoir Full Usual 1.50 1.96

End of Construction Usual 1.25 2.35

Reservoir full + DBE Extreme 1.00 1.82


Table 5-4: Slope Stability Analysis Results for Lower Dam Embankment

Condition Category Minimum Factor of Safety

Desired* Obtained

Downstream

Reservoir Full Usual 1.50 2.52

End of Construction Usual 1.25 2.14

Reservoir full + DBE Extreme 1.00 1.90

The critical slip circles for each analysis are shown in Appendix B.

5.4.3 Settlement The geotechnical investigations have shown that the alluvial soils beneath the existing embankment are soft to very soft and are compressible. The increase in embankment heights will be very minimal and will not cause a significant increase in foundation loads. Large settlements in the upgraded embankment are therefore not expected. To allow for long term settlement of the newly placed materials the embankments will be constructed 2% higher than the height above the general ground level to the theoretical crest level. This will be achieved by constructing the slopes 2% steeper than shown on the drawings.

6 Spillway Design

6.1 Recommended Design Criteria The guidelines used to determine and select suitable design floods are described in the SANCOLD publication “Guidelines of Safety in Relation to Floods” (SANCOLD ,1991).

Although the Upper and Lower Dams are not classified as dams with a safety risk (i.e. a dam with a maximum wall height of more than 5 m and storing more than 50 000 m3), the Guidelines were used to guide the appropriate recommended design flood.

Both the Upper and Lower Dam embankments are less than 5 m in height, which is the minimum criteria for a “small” dam size and has a “low” hazard rating. Using Table 5.4 in the “Guidelines of Safety in Relation to Floods” (SANCOLD ,1991), the recommended design flood for a dam with a “small” size and “low” hazard rating is between the 20 and 50-year return period.

Following the review of the preliminary design of the Jan van Riebeek Park dams, the Johannesburg Roads Agency recommended that the embankments be designed to accommodate the 1:50-year flood peak with no risk of failure. The 1:50 yr flood peak was therefore selected as the maximum flood peak to be accommodated by the spillway and the 1:20 yr RI flood peak was selected as the design flood.

6.2 Existing Spillways Both the Upper and Lower Dams have spillway structures located at their right abutments. The spillways consist of shallow trapezoidal concrete channels constructed of precast concrete panels laid end over end. The Upper Dam’s spillway is 1.6 m wide while the Lower Dam’s spillway is 1.8 m wide. The upper dam does not have any control structure at the entrance to the spillway. The spillway control structure at the Lower Dam consists of a 9 m long concrete wall approximately 400 mm high and 300 mm wide. This overflow structure has failed due to erosion and undercutting.

The existing spillways have a maximum discharge capacity of approximately 4 m3/s, which is less than the 1:10 year Recurrence interval food peak.


6.3 Spillway Selection Increase in spillway capacity can be achieved by either widening of the existing spillway or increasing the height of the embankment sufficiently to accommodate additional discharge Head. Increasing the embankment height to accommodate the 1:20 yr RI flood with no increase in spillway width would result in a dry freeboard (i.e. elevation difference between water level and embankment crest level) of approximately 2.3 m. For small dams this is not considered feasible. The spillway discharge capacity will therefore be increased by adding additional spillway width. Two options were considered as follows:

Option 1 - Maintaining the service spillway at the right abutment (with repairs to damaged lining and overflow section) to discharge daily flows and construct auxiliary spillways at the left abutment to discharge higher floods. The overflow crest level of the service spillway would be constructed 100 mm lower than the crest level of the auxiliary spillway to allow for smaller discharges to pass before overtopping of the auxiliary spillway crest; and

Option 2 – Construction of an auxiliary spillway with a central overflow section that is lower that the remainder of the spillway crest level. This option still allows for the daily flow to pass through the spillway due to the lowered central section and also allows for higher flood discharges without the need for an additional outlet.

Option 2 is considered the more feasible option from a construction point of view due to:

The reduced construction footprint; and

Shorter construction duration and reduced cost.

Option 2 will result in the existing service spillway at the right abutment of the Upper Dam being obsolete. Existing concrete works at the Upper Dam will therefore be demolished and replaced with earthfill embankment. The existing concrete works at the Lower Dam will be demolished and replaced with the new spillway.

There were concerns that these existing spillways are used by aquatic fauna as migratory paths between the reservoir basins and downstream areas. However, the presence of waterfalls (approximately 2 m drop) at the downstream end of the existing spillway channels would mean that these animals could not be using these structures as migratory paths.

To accommodate the possibility of migratory paths over the new spillways, the discharge channels are stepped with drops in elevation not exceeding 300 mm. The downstream edges of each step will be raised to allow for pondage of water at successive steps to allow amphibians to migrate. To prevent large exposed areas over the spillway footprint which would discourage migration, precast manhole rings will be placed at intervals and vegetated to allow fauna / amphibians the opportunity to cross the spillway discharge channel.

6.4 Spillway Overflow Structures

6.4.1 Location At the Upper Dam embankment, the spillway will be relocated to the left abutment. The existing spillway at the right abutment will be demolished and replaced with earthfill embankment.

At the lower Dam embankment, the spillway will be positioned at the right abutment over the alignment of the existing spillway.

6.4.2 Description The spillways consist of the following components (in order from upstream to downstream):

27.65 m wide overflow section comprising a 300 mm wide overflow sill. The central 5 m is 100 mm lower than the remainder of the spillway overflow section;

Concrete retaining walls either side of the spillway to retain the embankment fill material;


Ten (10 No.) 2400 x 1800 Class 75S precast portal culverts over the spillway to provide access over the spillway;

A cast in-situ concrete slab below the precast portal culverts; and

Reno mattress lined return channel with the central section lowered to allow for the discharge of daily flow.

6.4.3 Floods The spillways were designed for the following un-routed flood peak:

Design Flood 1 in 20 year RI 68 m3/s

Maximum Flood 1 in 50 year RI 93 m3/s

6.4.4 Freeboard The embankments were sized to safely discharge the 1:20 year RI flood peak without risk to overtopping. At this discharge there will be zero freeboard. The embankment will be overtopped at higher discharges.

Due to the very small sizes of the basin (i.e. fetch length less than 100 m) and due to these dams being not being classified as dams with a safety risk, no allowance was made for wave-run up due to wind action and wind set-up.

A freeboard of 1.2 m was adopted as it equates to the Head required to pass the design flood through the spillways.

6.4.5 Horizontal alignment The Upper Dam spillway is located at the left abutment and will form an integral part of the embankment. No excavation will be required in the approach channel. Retaining walls have been included either side of the spillway as the embankment will butt up on either side of the spillway.

The spillway return channels are aligned to follow the natural contours to the stream bed downstream of the embankments.

6.4.6 Vertical alignment The return channel at the Upper Dam will be excavated to ensure a gradual transition from the overflowing water to the streambed. Vertical drops in elevation are limited to a maximum of 300 mm.

The return channel at the Lower Dam will follow the alignment of the existing spillway channel to the streambed. Vertical drops in elevation are limited to a maximum of 300 mm.

6.4.7 Discharge capacity The discharge capacity of the spillways capacity is given by the following relationship:

𝑄𝑄 = 𝐶𝐶𝑑𝑑 × 𝐿𝐿 × 𝐻𝐻1.5

Where,

C = discharge coefficient of 1.44 for a broad crested spillway

H = Head above full supply level (m)

L = Spillway length (m)

The spillways will consist of 2 culverts with a spacing in between with side slopes of 1V:2H on both sides. The side slopes will be protected with stone pitching and mortar.


Using the above spillway discharge formula, the total required freeboard to discharge the 1:20-year flood is 1.2 m, which is 700 mm higher than the existing freeboard. The embankments should therefore be raised by 700 mm which raises the non-overspill crest level to 1 618.2 masl at the Upper Dam embankment and 1610.7 masl at the Lower Dam embankment.

For ease of construction and to cater for settlement allowance the Upper Dam embankment has been design for a crest level of 1618.5 masl and the Lower Dam embankment has been designed with a crest level of 1611 masl.

The maximum spillway discharge capacity is therefore 65 m3/s.

6.4.8 Spillway bridge The existing embankments are accessed by the public and Johannesburg City Park maintenance personnel and maintenance vehicles. To ensure continued access an access bridge is required over the spillway.

Two types of access bridges were investigated:

(i) A bridge with reinforced concrete deck and reinforced concrete piers; and

(ii) A bridge created using precast concrete culverts and a concrete slab on top.

A bridge with the precast concrete culverts was selected for the following reasons:

Concrete culverts are pre-cast elements and are readily available;

Easy constructible and in less construction time; and

Less formwork and complex construction method.

The access bridges have been designed to withstand a maximum load of 4 Tons to withstand loads from maintenance vehicles.

The access bridge will comprise of 3 rows of portal culverts placed over the width of the spillway to yield a bridge width of 3.6 m which provides sufficient width for maintenance vehicles.

For public safety handrails are required on both side of the spillway. Galvanised handrails are shown on the drawings. However, the galvanised handrails have recycling value and have a risk to be vandalised. The following options for handrails (post and rail), with less or not recycling value, are investigated and will be the discussed with the stakeholders:

• Recycled plastic handrails,

• Concrete columns with concrete lintels as rail, or

• PVC fencing.

The selected handrail for the access bridge will be selected subsequent to the stakeholder meeting and the drawings adjusted accordingly.


7 Silt Trap

7.1 Design criteria The design of the proposed silt-trap was based on the following parameters:

Daily flow rate of 10 Ml, thus 116 l/s;

Particle size of 100 microns;

Relative density of 1.4; and

A Front-end Loader type CAT 950 or similar will be used to remove silt from the chamber;

A ramp with a 1V:5H slope is provided into the compartment to provide access to the front-end loader. The ramp and the floor of the chamber have been designed to take the full load of the front-end loader. The walls have been given a thickness of 500 mm to withstand any impact from the front-end loader moving within the chamber.

An overflow weir is provided at the end of the chamber. The weir has a slopes upstream face to facilitate the loading of silt into the machines bucket. Water passing over the weir will return to the stream channel. A sluice gate is fitted to the downstream face of the weir in order to drain the chamber of water during cleaning.

During floods, flood water will flow above the weir safely without any risk to the safety or stability of the structure.

The silt-trap chamber is designed to accommodate a discharge of 6 m3/s.

The silt trap will be constructed on a prepared surface where the pioneer material will be compacted into the saturated ground. This will be followed by the necessary 300 mm thick transition layer and a 200 mm thick filter layer. Underdrains will also be provided under transition layer to relieve uplift pressures and reduce the structures buoyancy. The underdrains will discharge downstream of the silt trap.

7.2 Required length of silt trap To determine the required length of the chamber, the settling velocity of particles in suspension which is defined by Stoke’s law was required. Stoke’s law indicates that:

𝑉𝑉𝑉𝑉 =𝑔𝑔𝑑𝑑2(𝑃𝑃𝑃𝑃 − 𝑃𝑃𝑃𝑃)

18𝜇𝜇

Where:

• g is the acceleration of gravity

• d is the particle diameter (100 microns)

• Pp is the density of particle (1400 kg/m3)

• Pm is the density of medium (1000 kg/m3)

• μ is the viscosity of medium (0.001 kg/m-s)

It was considered that the floor of the silt trap would be made sloping for ease of operation in relation with the cleaning of the chamber and the ability to drain it, should this be required. The width of the chamber was made 6 m to accommodate the movement of the large front-end loader without being too restricted.

From the parameters mentioned above, the settling / terminal velocity for the particle was determined to be 0.00217 m/s.

Provision was made for the overflow weir to be 900 mm above the floor height to provide sufficient depth for the front end loaded to practically pick up the silt. This resulted with a flow depth of approximately 0.7 m (0.2 m above the weir height) and a flow velocity of 0.028 m/s. The minimum length required for the particle to


settle was then determined to be 9.5 m. To compensate for the sloping floor and in an attempt to have the silt settling upstream of the weir, the length of the chamber was made, for practical reason, to be 20 m.

7.3 Position of sand trap Two options are considered for the positioning of the sand trap:

(i) Immediately upstream of the low concrete bridge situated upstream of the Upper Dam; and

(ii) Further upstream of the low concrete bridge where the stream alignment is straighter.

The final position of the sand trap will be selected following the stakeholder meeting and the drawings adjusted accordingly.

8 De-silting of Dam Basins The Upper Dam’s basin is currently severely silted with less than 0.5 m of water depth over the basin. This shallow depth of water has led to the establishment of vegetation which covers most of the basin. The condition of the Lower Dam’s basin is slightly better due to the Upper impoundment preventing significant volumes of sediment from accumulating in the Lower Dam.

Removal of accumulated silt and vegetation within the basin is required to extend the useable lifespan of the impoundments.

The current water depth of less than 0.5 m is too shallow to allow for desilting by dredging. The desilting of the reservoirs will therefore be carried out in two stages, i.e.

Stage 1: Excavation of 25 m strip along the perimeter of the basins using a long-arm reach excavator; and

Stage 2: Dredging the central section of the basins by utilising the section cleared in stage 1 for positioning of the dredging barge.

A bathymetric survey will be carried out prior to dredging operations as well as after dredging to confirm the quantities of silt removed and the final storage capacities of the basins.

The anticipated procedures for each of the above stages are outlined below.

8.1 Stage 1: Desilting basin perimeter The final procedures and machinery required for the de-silting of the reservoirs will depend on the appointed Contractor. The following procedures are however anticipated:

A long-arm reach excavator is to be stationed at the edge of the reservoir basin and along the embankment crest where it will excavate silt and vegetation to the extent of its reach. Silt is to be removed to progressively greater depths towards the centre of the basin as indicated in Figure 8-1. This is to ensure that abrupt changes in water depth which can result in a safety risk to members of the public accessing the basin. Note that warning signs will be erected prohibiting public access into the basin.

Silt and vegetation removed during this stage will initially be placed a minimum distance of 120 m away from the reservoir basin to allow for drainage of water out of the silt. Silt and vegetation will then be loaded onto dump trucks to be removed from the site. A chemical analysis of the silt shows that the silt is not classified as “toxic” and is suitable to be used within the Parks precinct. It is likely that the silt will be used for landscaping purposes within the Park.

The estimated volumes of silt to be removed during the first stage of de-silting is approximately 10 000 m3 from the Lower Dam’s basin and 11 000 m3 from the Upper Dam’s basin.


Figure 8-1: Idealised partial section through basin showing extent of desilting during Stage 1.

8.2 Stage 2: Desilting basin centre Stage 1 desilting will increase the depth of water along the perimeter of the basin which will then enable a dredging barge to operate along the outer corridor. The barge will then be able to access the centre of the basin for the removal of silt and vegetation. The Stage 1 operation also increases the volume of water available for pumping operations. The following procedure is anticipated for the Stage 2 de-silting:

Removal of vegetation from the centre of the basin by using a floating barge. This vegetation will be stockpiled and removed off site.

Removal of silt by pumping using a floating barge. Silt will be removed to a depth of 1.5 m over the centre of the basin. Silt can be pumped to storage bags which will be stored a minimum of 120m away from the basin where water will be allowed to seep out of the bags and back into the environment.

Storage bags will be loaded onto dump trucks to be removed off-site to be likely used as landscaping material within the Park.

The estimated volumes of silt to be removed from the centre of the basins are 10 500 m3 for the Upper Dam’s basin and 11 000 m3 for the Lower Dam’s basin.

The total volume of silt to be removed from both the Upper and Lower Dam is approximately 42 500 m3. The total area of vegetation to be removed over the basins is approximately 42 000 m2.

9 Dam Break Analysis The Emmarentia Dam is located approximately 700 m downstream of the Lower Dam. The storage capacity of the Emmarentia Dam is significantly larger than the combined capacity of the Upper and Lower Dams and would absorb the flood caused by a dam break of either the Upper or Lower Dam. Furthermore, there is no infrastructure at risk between the Lower Dam and the Emmarentia Dam. A dam break analysis was therefore not carried out.

10 Construction Methodology and Estimated Construction Costs

10.1 Proposed Construction procedure It is proposed that construction activities commence with the with the removal of silt and vegetation from the Upper Dam’s basin followed by the Lower Dam’s basin. Commencement with the removal of silt will have the following advantages:

The removed silt can be assessed for its suitability to be used as fill material for the raising of the existing embankments;

Should the Contractor opt for removal of silt along the perimeter using an excavator stationed at the bank, the excavator track damage on the existing embankment will be of no concern as the embankment will later be raised.


Dredging operations will require that the water from the basin be used for pumping of silt. It will be recommended to carry out this dredging while the water level in the reservoir is at its maximum. Raising of the embankments will require that the water in the basin be lowered for construction of the upstream slopes, and

An excavator positioned on the embankment crest will have a greater reach positioned on the lower existing embankment than on the higher raised embankment.

Following de-silting of the reservoir basins, it is proposed that construction commence with the raising of the dams. For the purpose of this report it is assumed that both the Upper and Lower Dam will be raised simultaneously. Construction will commence towards the end of the rainy season and start of the dry season and will involve the following construction activities:

• Removal of vegetation and trees, including roots and other unsuitable material over the construction footprint of the Upper and Lower Dam.

• Lowering of water level in the Upper reservoir to accommodate construction of the spillway. Lowering can be achieved by pumping or siphoning water over the embankment into the Lower Dam’s basin.

• Excavation of existing Upper Dam embankment to founding level of new material and excavation of spillway return channel. Material to be stockpiled for re-use in embankment construction;

• Construction of the Upper Dam spillway structure and spillway return channel.

• Construction of the embankment. Embankment construction can commence once the spillway retaining walls have reached sufficient height to accommodate compaction of embankment fill behind the walls;

• Excavation of existing Lower Dam embankment to founding level of new material and excavation of spillway return channel. Material to be stockpiled for re-use in embankment construction;

• Construction of the Lower Dam spillway structure and spillway return channel.

• Construction of the embankment. Embankment construction can commence once the spillway retaining walls have reached sufficient height to accommodate compaction of embankment fill behind the walls;

• Construction of embankment upstream slope protection and gabion mattresses at the spillway return channels;

• Top-soiling of downstream slopes and embankment crests.

Construction of the sand-trap upstream of the Upper Dam can commence once the concrete teams occupied by the spillway construction are available.

10.2 Estimated construction costs The estimated construction cost is tabulated below, A detailed schedule of quantities is included in Appendix C. Rates for construction items were obtained from similar work with an allowance made for inflation to 2019.

Table 10-1 – Estimated Construction Cost

Description Amount (R)

Part 1 – Preliminary & General 1 390 685

Part 2 – Upper Dam Embankment Rehabilitation 2 815 100

Part 3 – Upper Dam Spillway 1 013 850

Part 4 – Lower Dam Embankment Rehabilitation 3 111 000


Description Amount (R)

Part 5 – Lower Dam Spillway 1 585 300

Part 6 – Sand Trap 1 297 350

Part 7 – Desilting of Basins 2 070 000

Sub Total A (Excluding VAT) 13 283 285

Part 8 - Dayworks 330 000

Sub Total B (Excluding VAT) 13 613 285

Contingencies (10% of Sub Total A) 1 328 329

Sub Total C (Excluding VAT) 14 941 614

Construction supervision and administration (7% of Sub Total C) 1 045 913

Sub Total D 15 987 526

Value Added Tax (15% of Sub Total D) 2 398 129

Total 17 339 742

10.3 Estimated Construction Programme It is understood that budget is likely to be made available for the construction of the upgrade works during 2020. It is anticipated that the construction will take approximately 6 months if the construction of the Upper and Lower Dam can occur simultaneously. It is to be expected that the construction work will take place during the dry months (i.e. April to September) to minimise the impact of rain on the Works. An estimated project programme is included in Appendix D.

10.4 Lowering of basin water levels The water levels in the Upper and Lower Dams can be lowered by means of pumping. The Lower Dam has a fixed pump located at the left bank which is used to draw water from the basin for irrigation purposes. Water pumped from both the Upper and Lower Dam can be used for irrigation of park areas.

10.5 Precautions and measure to ensure public safety Suitable access control measures will need to be put in place for local residents and to ensure public safety.

The site must be cordoned off for the duration of the construction and clear warning signs provided. Signs are to be erected at the proposed spillway bridge crossings warning people not to use them during flooding.

The embankment crests are used by the public and park maintenance personnel and will need to be closed off to the public throughout the construction period.

24-hour security should be provided by the appointed contractor. The site camp area should be located 120 m away from the reservoir basins.


11 Drawings and Specifications

11.1 Detail Design drawings The detail design drawings for the rehabilitation of the Upper and Lower Dams in the Jan van Riebeek Park are included in Appendix E.

11.2 Specifications The standard specifications for the tender documentation used for the cost estimate is SANS 1200, which can be obtained from the South African Bureau of Standards. The following sections are proposed for the Construction Works required at Jan van Riebeek Park Dams:

1200 A: General

1200 DE: Small Earth Dams;

1200 DK: Gabions and Pitching;

1200 G: Concrete (structural);

1200 GE: Precast concrete (structural);

1200 H: Handrails;

The following particular specification is proposed:

Reinforced concrete block scour protection (i.e. for AmorFlex or equivalent approved).


12 Conclusions and Recommendations This report covers the design of the rehabilitation of the Upper and Lower Dams at Jan van Riebeek Park and includes the design for the construction of a sand trap upstream of the Upper Dam.

The rehabilitation of the Upper and Lower Dams will entail the following:

Removal of large trees from the embankments and within approaches to the spillway structures;

Raising of the embankments by a nominal height to provide additional freeboard for increased flood attenuation;

Reinstatement of the upstream slopes which have eroded due to wave action. Armor Flex slope protection is provided to prevent further erosion;

Construction of new spillways with sufficient capacity to discharge the 1:20 year RI flood without any risk of the embankments overtopping. The spillway are capable of discharging the 1:50 year RI flood with minimal overtopping which does not impact on the safety of the structure due to the short duration of overtopping and the low depth of overtopping;

The central 5 m width of the spillways are 100 mm lower than the remainder of the spillway to allow for daily flow to pass through the structure;

Spillway defined with a concrete sill with upstream and downstream reno-mattress erosion protection;

An access bridge with sufficient width and load bearing capacity to accommodate maintenance vehicles has been included over both the Upper and Lower Dam spillways.

The spillway discharge channels are lined with reno-mattresses and have been stepped with a maximum elevation drop between steps not exceeding 300 mm.

The sand trap upstream of the Upper Dam has been sized to allow for maintenance vehicles to clean out accumulated silt from the chamber.

The estimated cost for the project is anticipated to be R 15 Million, which includes a 10% contingency but excludes VAT. The construction period is expected to be approximately 6 months if the construction of the Upper and Lower Dams can occur simultaneously.


13 References Adamson, P. (1981). Southern African Storm rainfall TR 102. Pretoria: Department of Water Affairs.

Dijkshoorn, J.A., Leenaars, J. G., huting, J., & Kempen, B. (2016). Soil and terrain Database for Southern Africa. https://doi.org/10.17027/isric-wdsoils.20160001.

Druyts, F. H. W. M., (2005). Testing Material for Dams, Control of Embankment Construction, Control of Concrete on Construction Sites. Department of Water Affairs and Forestry.

DWAF (1995). Design Guidelines for a Category 1 Earthfill Dam Wall. Department of Water Affairs and Forestry. May 1995.

HRU, (1972). Design Flood Determination in South Africa (Report 1/72). Johannesburg: University of Witwatersrand.

Jennings, J. E. B, Brink, A. B. A. and Williams, A. A. B., (1993). Revised Guide to Soil Profiling for Civil Engineering Purposes in Southern Africa. The Civil Engineer in SA, p 3-12. January 1973.

Kovacs Z.P., (1988). Regional Maximum Flood Peaks in Southern Africa. Technical Report TR 137.

SANCOLD 1991. SANCOLD Safety Evaluation of Dams, Report No 4, Guidelines on Safety in Relation to Floods. Published by South African National Committee on Large Dams. December 1991.

Scmidt, E. & Schulze, R. (1987a). User Manual for SCS-based design run-off estimation in Southern Africa. Pretoria: Water Research Commission.

Smithers, E. & Schulze, R. (2002). Design Rainfall and Flood Estimation in South Africa. Water Research Commission, Pretoria.

U.S. Army Corps of Engineers. (2015). Hydraulic Modelling System (HEC-HMS). Version 4.1. Institute of Water Resources, Hydraulic Engineering Centre.

USBR 1987. Design of Small Dams. A Water Resources Technical Publication. Third Edition. 1987.

Weinert, (1980). The Natural Road Construction Materials of Southern Africa. National Institute for Transport and Road Research. Pretoria.

https://doi.org/10.17027/isric-wdsoils.20160001

Project number 504630 File Detail Design Report for the Rehabilitation of JVR Park dam_Rev0.2.docx, 2019-06-26 Revision 0

Appendix A Geotechnical Investigations

Appendix B Embankment Slope Stability Analyses

Appendix C Construction Cost Estimate

JRA Dam Rehabilitation - Jan Van Riebeek Park (Top Dams)

CONTRACT No:

SUMMARY OF BILL OF QUANTITIES

JRA Dam Rehabilitation - Jan Van Riebeek Park (Top Dams):

SUMMARY OF BILL OF QUANTITIES

PART 1 PRELIMINARY AND GENERAL 1,390,685

PART 2 UPPER DAM EMBANKMENT REHABILITATION 2,815,100

PART 3 UPPER DAM SPILLWAY REHABILITATION 1,013,850

PART 4 LOWER DAM EMBANKMENT REHABILITATION 3,111,000

PART 5 LOWER DAM SPILLWAY REHABILITATION 1,585,300

PART 6 SAND TRAP 1,297,350

PART 7 DE-SILTING OF BASIN 2,070,000

13,283,285

PART 4 DAYWORKS 330,000

13,613,285

1,328,329

14,941,614

1,045,913

15,987,526

2,398,129

17,339,742TOTAL

VALUE ADDED TAX Add 15% of Subtotal C(Provisional sum based on current rate of VAT)

SUB-TOTAL A

SUB-TOTAL B

CONTINGENCIESAdd 10% of Subtotal A(Provisional sum based on estimate)SUB-TOTAL C

Construction supervision and administration (7% of subtotal C)

SUB-TOTAL D


CONTRACT No.: PART 1: PRELIMINARY AND GENERAL

ITEM PAYMENT DESCRIPTION UNIT QUANTITY RATE AMOUNT

1 PART 1: PRELIMINARY AND GENERAL

1.1SANS 1200 A

SECTION: GENERAL

8.3 FIXED-CHARGED AND VALUE-RELATED ITEMS

8.3.1 Contractual requirements Sum 1 1,148,6858.3.2 Establishment of Facilities on the Site8.3.2.1 Facilities for Engineer

c) Nameboard (1 No.) Sum 18.3.2.2 Facilities for Contractor

(PSA) All facilities scheduled in Clause 8.3.2.2 of SANS 1200A Sum 1

8.3.3 Other Fixed-charge obligations Sum 18.3.4 Removal of Site Establishment Sum 1

8.4 SCHEDULED TIME-RELATED ITEMS8.4.1 Contractual requirements Sum 1

8.4.2 Operation and Maintenance of Facilities on Site

8.4.2.1 Facilities for Engineerc) Nameboard (1 No.) Sum 1

8.4.2.2 Facilities for Contractor

(PSA) All facilities scheduled in Clause 8.4.2.2 of SANS 1200A Sum 1

8.4.3 Supervision for Duration of Construction Sum 18.4.4 Company and Head Office Cost Sum 1

8.5 SUMS STATED PROVISIONALLY BY ENGINEER

8.5(a) a) For work to be executed by the Contractor Prov. Sum

8.5(b) b) For work to be executed by a nominated subcontractor Prov. Sum

2) Handling cost and charges on (b)(1) %(PSA) c) Additional tests

1) Additional tests ordered by Engineer (grading analysis Prov. Sum 1 200,000 200,000

2) Handling cost and charges on (c)(1) % 200,000 10% 20,000

8.8 Temporary works8.8.2 Dealing with water Sum 1 22,000 22,0008.8.4 Existing Services

c) Excavate by hand m³d) Temporary protection, as per project specifications Sum

8.8.6 Special Water Control in terms of Project Specifications Sum

Compliance with OHS Act and Construction Regulations Sum

Environmental management Sum

Quality Management Plan Sum

Provision of security personnel Sum

1,390,685TOTAL CARRIED FORWARD TO SUMMARY


CONTRACT No.: PART 2: UPPER DAM EMBANKMENT REHABILITATION


2 PART 2: EMBANKMENT REHABILITATION

2.1SANS 1200 DE

SECTION: SMALL EARTH DAMS

8.3 SCHEDULED ITEMS8.3.1 Site Clearance8.3.1.1 Clear and Strip Site ha 0.5 20,000 10,000

PSDE 8.3.1.2 Remove and grub large trees, complete with stumps, of girth(a) over 1 m and up to and including 2 m No 5 2,000 10,000(b) over 2 m and up to and including 3 m No 3 11,300 33,900

8.3.1.5 Extra-over 8.3.1.1 for recovery other scheduled materiala) Fence Sum 1 10,000 10,000

8.3.2Remove Topsoil and un-suitable material to nominal depth 300 mm (or other stated depth), Stockpile and maintain

m² 2,800 30 84,000

PSDE-8.3.3 Excavations(a) Bulk excavation to level 1617.3 masl for Upper Dam) m³ 400 130 52,000

(b) Cut-off trench excavation m³ 150 150 22,500

PSDE-8.3.4 Preparation of Exposed Surfaces(b) Area to be covered by dam wall (crest and embankment m² 2,800 30 84,000

PSDE-8.3.5 Forming Embankment. (see Drawing DE - 2)(c) Unselected semi-pervious material (General Fill) m³ 100 110 11,000

(d) Topsoil from stockpile to downstream slope and crest m³ 450 110 49,500

PSDE-8.3.11 Obtaining material from commercial or off-site sources and forming of embankment (a) Selected impervious material m³ Rate Only 530 -

(c) Selected semi-pervious material (General Fill) m³ 3,300 480 1,584,000

(d) Topsoil m³ 100 500 50,000(e) Fine filter material m³ 10 530 5,300(f) Geotextile m2 820 60 49,200

PSDE 8.3.12 Concrete infilling (Grade 10 Mpa) m³ 10 2,600 26,000

PSDE 8.3.14 Grass or other vegetation cover (all inclusive) m² 2,500 100 250,000

PSDE 8.3.15 Settlement beacons, level markers and reference bench mark No 4 1,000 4,000

2.2PART SPEC AUA

REINFORCED CONCRETE BLOCK SCOUR PROTECTION

AUA 4.1 Scheduled Items

AUA 4.1.1 Reinforced concrete blocks (ARMORFLEX or similar) (bidim measured under PSDE - 8.3.11) m² 820 380 311,600

AUA 4.1.2 150 mm thick founding layer m³ 1 100 100AUA 4.1.3 Topsoil in reinforced concrete bocks m² 800 110 88,000

(ARMORFLEX or similar)AUA 4.1.4 Vegetation of reinforced concrete blocks m² 800 100 80,000

(vegetation from environment)



CONTRACT No.: PART 3: UPPER DAM SPILLWAY REHABILITATION


3 PART 3: SPILLWAY REHABILITATION

3.1SANS 1200 DE


8.3 SCHEDULED ITEMS8.3.1 Site Clearance8.3.1.1 Clear and Strip Site m² 600 5 3,000

PSDE-8.3.1.2 Remove and grub large trees, complete with stumps, of girth(a) over 1 m and up to and including 2 m No 2 2,000 4,000

8.3.2 Remove Topsoil to Nominal Depth 150 mm (or other Stated Depth), Stockpile and maintain m² 500 30 15,000

PSDE-8.3.3 Excavations(a) material unsuitable for embankment (at spillway) m³ 160 130 20,800

PSDE-8.3.4 Preparation of Exposed Surfaces(a) Area to receive gabion mattress (spillway return channel m² 310 60 18,600

(c) Areas to receive concrete m² 240 55 13,200

3.2SANS 1200 DK

GABIONS AND PITCHING

8.2 SCHEDULED ITEMS8.2.1 Surface preparation for bedding of gabions

(a) Cavities filled with approved excavated material or rock m² 10 100 1,000

(b) Cavities filled with grade 15 concrete (provisional) m² 5 350 1,750

8.2.2 Gabions(a) 0.3 m x 1 m x 2 m m³ 85 1,600 136,000

PSDK-8.2.4 Geotextile (or geomembrane)(a) Bidim Grade A4 (or similar approved) m² 300 60 18,000

PSDK 8.2.5 Pitching(a) Stone pitching (at 1V:2.5H) m² 70 500 35,000

3.3 SANS 1200 G CONCRETE (STRUCTURAL)8.2 Scheduled Formwork Items8.2.1 Rough m² 30 550 16,5008.2.2 Smooth m² 50 620 31,000

8.3 Scheduled Reinforcement ItemsPSG-8.3.2 High-Tensile welded mesh (Ref.200) kg 5,000 16 80,000

Y25 Reinforcement kg 2,000 16 32,000

8.4 Scheduled concrete itemsPSG - 8.4.2 Blinding layer (Grade 15 concrete) m³ 15 2,600 39,000

PSG-8.4.3 Strenth Concrete(a) Grade 25 m³ 80 2,600 208,000

8.4.4 Unformed Surface Finishes(a) Wood-floated finish m² 200 30 6,000

678,850TOTAL CARRIED FORWARD


CONTRACT No.: PART 3: UPPER DAM SPILLWAY REHABILITATION

678,850

3.4SANS 1200 GE

PRECAST CONCRETE (STRUCTURAL)

8.2 Scheduled Items8.2.1 Provide Structural Precast Units (Class 75S)

(a) Rectangular Portal Culvert (2.4 m span x 1.8 m high) No 10 9,000 90,000

(b) DN 1500 (250 mm high) manhole shaft sections to be staggered in spillway return channel

No 10 3,000 30,000

8.2.2 Erection of Structural Precast Units No 10 2,000 20,000

3.5 SANS 1200 H Handrails (Ball & Stanchion)8.3 Sheduled Items

8.3.7 (b) Handrail assembly complete - Two sections of 27 m each with four end closures m 54 500 27,000

3.7PART SPEC AUA



AUA 4.1.1 Reinforced concrete blocks (ARMORFLEX or similar) (bidum measured under PSDK - 8.2.4) m² 55 380 20,900





TOTAL BROUGHT FORWARD


CONTRACT No.: PART 4: LOWER DAM EMBANKMENT REHABILITATION


4PART 4: LOWER DAM EMBANKMENT REHABILITATION

4.1SANS 1200 DE


8.3 SCHEDULED ITEMS8.3.1 Site Clearance8.3.1.1 Clear and Strip Site ha 0.33 20,000 6,600

PSDE 8.3.1.2 Remove and grub large trees, complete with stumps, of girth(a) over 1 m and up to and including 2 m No 5 2,000 10,000(b) over 2 m and up to and including 3 m No 2 11,300 22,600

8.3.1.5 Extra-over 8.3.1.1 for recovery other scheduled materiala) Fence Sum 1 10,000 10,000

8.3.2Remove Topsoil and un-suitable material to nominal depth 300 mm (or other stated depth), Stockpile and maintain

m² 4,300 30 129,000

PSDE-8.3.3 Excavations(a) Bulk excavation to level 1609.2 masl for Upper Dam) m³ 500 130 65,000

(b) Cut-off trench excavation m³ 170 150 25,500

PSDE-8.3.4 Preparation of Exposed Surfaces(b) Area to be covered by dam wall (crest and embankment m² 3,200 30 96,000

PSDE-8.3.5 Forming Embankment. (see Drawing DE - 2)(c) Unselected semi-pervious material (General Fill) m³ 100 110 11,000

(d) Topsoil from stockpile to downstream slope and crest m³ 380 110 41,800

PSDE-8.3.11 Obtaining material from commercial or off-site sources and forming of embankment (a) Selected impervious material m³ Rate Only 530 -

(b) Selected semi-pervious material (General Fill) m³ 3,700 480 1,776,000

(c) Topsoil m³ 20 500 10,000(d) Fine filter material m³ 10 530 5,300(e) Geotextile m2 1,080 60 64,800

PSDE 8.3.12 Concrete infilling (Grade 10 Mpa) m³ 20 2,600 52,000

PSDE 8.3.14 Grass or other vegetation cover (all inclusive) m² 2,500 100 250,000

PSDE 8.3.15 Settlement beacons, level markers and reference bench mark No 4 1,000 4,000

4.2PART SPEC AUA

UPSTREAM SLOPE PROTECTION


AUA 4.1.1 Reinforced concrete blocks (ARMORFLEX or similar) (bidim measured under PSDE - 8.3.11) m² 1,080 380 410,400

AUA 4.1.2 150 mm thick founding layer m³ 160 100 16,000AUA 4.1.3 Topsoil in reinforced concrete bocks m² 500 110 55,000





CONTRACT No.: PART 5: LOWER DAM SPILLWAY REHABILITATION


5PART 5: LOWER DAM SPILLWAY REHABILITATION

5.1SANS 1200 DE


8.3 SCHEDULED ITEMS8.3.1 Site Clearance8.3.1.1 Clear and Strip Site m² 1,200 5 6,000


8.3.2Remove topsoil and unsuitable material to nominal depth of 300 mm (or other stated depth), Stockpile and maintain

m² 1,100 30 33,000

PSDE-8.3.3 Excavations

(a) material unsuitable for embankment (at spillway) including existing concrete and masonry m³ 160 250 40,000

PSDE-8.3.4 Preparation of Exposed Surfaces(a) Area to receive gabion mattress (spillway return channel m² 810 60 48,600

(c) Areas to receive concrete m² 200 55 11,000

5.2SANS 1200 DK





8.2.2 Gabions(a) 0.3 m x 1 m x 2 m m³ 280 1,600 448,000




8.3 Scheduled Reinforcement ItemsPSG-8.3.2 High-Tensile welded mesh (Ref.200) kg 5,000 16 80,000





1,250,300TOTAL CARRIED FORWARD


CONTRACT No.: PART 5: LOWER DAM SPILLWAY REHABILITATION

1,250,300

5.4SANS 1200 GE




(b) DN 1500 (250 mm high) manhole shaft sections to be staggered in spillway return channel

No 10 3,000 30,000




5.6PART SPEC AUA







1,585,300


TOTAL CARRIED FORWARD TO SUMMARY


CONTRACT No.: PART 5: SAND TRAP


6PART 6: LOWER DAM SPILLWAY REHABILITATION

6.1SANS 1200 DE


8.3 SCHEDULED ITEMS8.3.1 Site Clearance8.3.1.1 Clear and Strip Site m² 1,250 5 6,250


8.3.2Remove topsoil and unsuitable material to nominal depth of 300 mm (or other stated depth), Stockpile and maintain

m² 400 30 12,000

PSDE-8.3.3 Excavations

(a) material unsuitable for embankment (at spillway) including existing concrete and masonry m³ 900 250 225,000

PSDE-8.3.4 Preparation of Exposed Surfaces(a) Areas to receive concrete m² 240 70 16,800

6.2SANS 1200 DK





8.2.2 Gabions(a) 0.3 m x 1 m x 2 m m³ 20 1,600 32,000




8.3 Scheduled Reinforcement ItemsPSG-8.3.2 High-Tensile welded mesh (Ref.200) kg 50 16 800





1,101,350TOTAL CARRIED FORWARD


CONTRACT No.: PART 5: SAND TRAP

1,101,350

6.4SANS 1200 GE







6.6PART SPEC AUA







1,297,350


TOTAL CARRIED FORWARD TO SUMMARY


CONTRACT No.: PART 5: DE-SILTING OF BASINS


7 PART 7: DE-SILTING OF BASINS

SCHEDULED ITEMS

7.1Bathymetric survey at start and end of de-silting operations(a) Upper Dam Basin No 2 40,000 80,000(b) Lower Dam Basin No 2 40,000 80,000

7.2 Clear SiteRemove vegetation from basin to depth of 0.3 m below existing water surface. (a) Upper Dam Basin ha 4 25,000 105,000(b) Lower Dam Basin ha 4 25,000 105,000

Desilting of Basins to maximum depth of 1.5 m(a) Upper Dam Basin m³ 15,000 60 900,000(b) Lower Dam Basin m³ 12,500 60 750,000

7.3 Overhaul m³.km 10,000 5 50,000



CONTRACT No.: PART 8: DAYORKS


8 PART 8: DAYWORKS

8.1 Scheduled ItemsLABOUR

8.1.1 (a) Nett cost of labour Prov Sum 100,000 1 100,000

8.1.2 (b) Contractor's charges and profit associated with administration of the above item % 100,000 10% 10,000

PLANT

8.1.3 (a) Nett cost of plant (including operator, assistance, fuel, oil, maintenance, etc) Prov Sum 100,000 1 100,000

8.1.4 (b) Contractor's charges and profit associated with administration of the above item % 100,000 10% 10,000

MATERIAL8.1.5 (a) Nett cost of material Prov Sum 100,000 1 100,0008.1.6 (b) Percentage on nett cost of materials % 100,000 10% 10,000

330,000TOTAL CARRIED FORWARD TO SUMMARY

Appendix D Construction Programme

Appendix E Drawings

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Document prepared by Aurecon South Africa (Pty) Ltd Reg No 1977/003711/07 Aurecon Centre Lynnwood Bridge Office Park 4 Daventry Street Lynnwood Manor 0081 PO Box 74381 Lynnwood Ridge 0040 South Africa T F E W

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