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SITE SELECTION STUDY FOR
PROPOSED NEW TRANSMISSION
SUBSTATION IN THE ESKOM
NORTH EASTERN REGION
GEOTECHNICAL REPORT
PRELIMINARY GEOTECHNICAL INVESTIGATION FOR THE
SELECTION OF A SITE FOR THE PROPOSED NEW WOLWEKRAAL
SUBSTATION
February 2010
*(27(&+1,&$/,19(67,*$7,216 64#05/+55+106'%*01.1);
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CONTENTS
1.0 INTRODUCTION
2.0 THE NATURE OF THE PROJECT
3.0 OBJECTIVES OF INVESTIGATION
4.0 LOCALITY OF STUDY AREAS
5.0 DESK STUDY
5.1 Maps Consulted
5.2 Aerial Photographic Interpretation
5.3 Review of Desk Study and Selection of Sites
5.4 Seismic Zoning
6.0 REGIONAL ENVIRONMENT
6.1 Geography
6.1.1 Topography and Drainage6.1.2 Flood Line6.1.3 Climate
6.2 Geology
6.3 Water Table
7.0 SITE INVESTIGATION
7.1
Topographical Survey7.2 Geometric Design
7.3 Exploratory Work
7.4 Laboratory Testing
8.0 GEOTECHNICAL EVALUATION
8.1 Site-I and II
8.2 Site-III
8.3 Water Table
8.4 Topography - Preliminary Substation Positioning, Geometry
(Earthworks Volumes) and Access Roads
8.5 Flood-Line Studies
9.0 CONCLUSIONS AND RECOMMENDATIONS
10.0 REFERENCES
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APPENDICES
A Flood Line Studies
B Test Pit Profiles
C Laboratory Test Results
D Contour Plans, Preliminary Geometric Designs and Sections
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TITLE :GEOTECHNICAL REPORT
PRELIMINARY GEOTECHNICAL INVESTIGATION FOR THESELECTION OF SITES FOR THE PROPOSED NEW WOLWEKRAAL
SUBSTATION
PREPARED BY: Geotechnical InvestigationsTransmission Technology
CLIENT : Land and RightsTransmission
PROJECT TEAM: F A GrovDr Johan LourensP GreybeD AngoveTyris Plant HireSoillab
REPORT NO: GR02-2010
DATE : February 2010
Approved :
F A Grov
Senior Specialist ConsultantGeotechnical Investigations
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1.0 INTRODUCTION
Eskom plans to construct a new 400/132 kV Substation near Marble Hall in the North Eastern
Region (Mpumalanga). (TheHighveld North and Lowveld Northwest Project)
A Site for the planned development is urgently required in the area near Marble Hall. The firstphase of this investigation included extensive desk studies, where a total number of three most
suitable sites were identified for the substation. For the selection of these possible sites during
the desk study, factors such as the geology, topography, farming activities, mining activities,
Eskom installations and power line servitude routes were considered. It should be noted that
options for suitable sites are limited in the Marble Hall area due to intensive irrigation farming
activities.
Information collected during this investigation is suitable for site selection purposes, and
once the final design is required, a Detailed Geotechnical Investigation will be required to
provide design parameters and confirm findings of this investigation..
2.0 THE NATURE OF PROJECT
The project comprises the construction of a Substation in the Marble Hall area (Mpumalanga).
The development would include the installation of typical equipment such as:
Electrical transformers Circuit breakers or line termination structures High-voltage switchgear Low voltage switchgear Surge & lightning protection equipment Control and metering equipment Access roads and buildings
3.0 OBJECTIVES OF INVESTIGATION
The objectives of this investigation were defined as follows:
- Conduct intensive desk studies of the area to identify a total number of three mostsuitable sites for the proposed new substation
- Obtain geotechnical information during a Preliminary Geotechnical Investigation ofeach selected site to confirm findings of the desk study for suitability evaluation of
sites
4.0 LOCALITY OF STUDY AREAS FOR PROPOSED NEW SUBSTATION
It was requested by System Planning Transmission, to find a suitable site for this substation
within a radius of 10km from the coordinate of the existing substation
S 25 00 19.7 E 29 18 9.63
Three most suitable areas were identified, namely, Site-I; Site-II and Site-III, and are located
at:
Site-I S 25 04 24 E 29 16` 57
Site-II S 25 04 12 E 29 17` 24
Site-III S 25 05 16 E 29 18` 08
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LOCALITY OF SITES FIGURE-I
5.0 DESK STUDY
5.1 Maps Consulted
An attempt was made to collect as much information as possible of the study area
demarcated by Transmission System Planning, for the proposed new substation
during the desk study. For this purpose the following maps were consulted:
i)`Topographical Maps 2429CD Marble Hall; 2529AB Groblersdal;
2529AA Siyabuswa; 2429CC Rathoke
ii) Ortho Photos Covering the area within a 10km radius from the existing
Substation
iii) Geological Maps 2528 Pretoria ; 2428 Nylstroom
iv) Mineral Map - Mineral Map of the Bushveld Complex South Africa
With Special reference to Platinum and Chrome
v) Mining - Simplified Geology, Selected Mines and Mineral Deposits South
Africa, Lesotho and Swaziland
5.2 Aerial Photographic Interpretation
In principle the following features are being studied, information being obtained and
interpreted when studying aerial photographs for geotechnical purposes:
- Reflection of the action of nature in creating the existing conditions
- Grouping of materials according to certain patterns
- Definition of various boundaries and linear features of
significance
- Field checking by visual inspection
Stereo-interpretation has a great advantage over interpretation of a single photograph,because it is better able to identify topographical and erosion features, grey tones, and
textures have greater requisite clarity contrasts. Basically, two aspects of the air photo
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image are revealed in the stereo-model of a given area, and these are surface form and
grey tone, which could be subdivided as:
a) Elements of Surface Form
- Topographic form
- Drainage form
- Erosion form
b) Elements of Grey tone and Texture of :
- vegetation
- due to land use
- soil and rock material
Geotechnical information obtained in this way was correlated with Geological Map
data.
5.3 Review of Desk Study and Selection of Sites
The final electrical design of the substations was not finalised at the time of this
investigation and it was proposed by the electrical designers to use an area of 400 x
400m for the proposed platform. Initially the desk study was performed to identify
sites of 600m x 600m. The area for suitable sites is limited due to the presence of
dolomite and the densely developed irrigation scheme in the area of study.
For the purpose of the evaluation study, three sites were identified during the desk
study, taking into account present Eskom installations, geology, established
developments and geographical constraints.
5.4 Seismic Zoning
The South African loading code, SANS 10160 1969 (Figure-IIa and IIb) , shows
that the proposed sites for this study are situated in an area where the peak groundacceleration with a 10% probability of being exceeded in a 50 year period is less than
50cm/sec. Figure-IIb also shows the zones where detailed seismic design (Zone-I)
SEISMIC HAZARD MAP OF SA FIGURE-IIa
Wolwekraal SS
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SEISMIC HAZARD ZONES OF SA FIGURE-IIb
and compliance with minimum requirements (Zone-II) are specified by the code. Theproposed Wolwekraal sites fall outside of these Zones.
Wolwekraal Sites
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More recent data produced by the Council of Geoscience place the sites within the
zone where the minimum seismic event, with a 10% probability of being exceeded in
a 50 year period, falls in the range of 0,07g to 0,09g (Figure-III). This data however,
still needs to be verified.
6.0 REGIONAL ENVIRONMENT
6.1 Geography
6.1.1 Topography and Drainage
The three sites selected for evaluation purposes are located within a 1 km radius and
are considered to be subject to the same regional topographic and drainage influences
(See appendix-A).
Site-I and II are located on the northern slopes of the same hill and Site-III is on
the ridge of the hill. The topography of the three sites forms part of the regional flat
topography, with slopes traversing Site-I (1,5%) and Site-II (1,2%) from south to
north. Site-III is traversed in no definite pattern with flat slopes of the order of2%.
Site-I and Site-II drains to the north into a vlei area. The Sites are located on portions
of the farm Loskop Noord.
6.1.2 Flood line
Flood line studies carried out favours Site-III, having the least slopes and run-offs,
with no watercourse near the site. Site-I is largely affected by the irrigation scheme of
the region, which will require major irrigation channels to be diverted and Site-II is a
risk with regards to the 1:50year flood line.(See Appendix-A)
6.1.3 Climate
N-Value
The "Weinert N-Value", that describes the climatic environment, is approximately 4
for the area. Where "N" is more than "5", disintegration is the prominent form of
weathering, and where "N" is less than "5", decomposition affects those rocks
whose minerals are liable to change chemically under atmospheric conditions.
Rainfall
This is a summer rainfall region and the average yearly rainfall is 577 mm.
. Vegetation
Sites-I and II are covered with crops and irrigation channels are traversing thesesites. Site-III is covered with medium to dense indigenous grass and bush.
6.2 Geology
All three sites are underlain by medium grained Nebo Granite of the Bushveld
Complex and Granite-Gneiss.
6.3 Water Table
A shallow perched water table is present on Site-I and Site-II
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7.0 THE INVESTIGATION
7.1 Topographical Survey
Topographical surveys of sites were carried out to enable preliminary geometric
designs, which forms an important part of the geotechnical evaluation of sites.
7.2 Geometric Design
Since the power line routes had not been fixed at the time of this investigation,
geometric designs were carried out merely to obtain an idea of volumes of earthworks
that could be expected at each site.
7.3 Exploratory Work
Site-I and II
The above sites are subject to similar constraints, re densely cultivated and traversed
by irrigation waterlines in all directions. Test pits were excavated randomly with aTLB (Cat 416) to confirm findings of the desk study. The Granite Gneiss was found
deeply weathered due to the apparent flat topography which resulted in poor
drainage of both sites.
It was found that the test pits were mostly water logged near surface and thus not
suitable for the construction of an earth platform.
Site-III
Test pits were excavated randomly to confirm findings of the desk study.
The profile showed relatively shallow weathering down to anticipated depths of theconstruction of an earth platform. It is well known that this material is of excellent
quality for the construction of the earth platform for the substation.
7.4 Laboratory Testing
Laboratory tests on soil samples were primarily conducted to determine Geotechnical
properties of the soil, confirming findings of the desk study.
The following tests were carried out:
- Grading (Mechanical and Hydrometer)
- Atterberg Limits
- Moisture Content
- Electrical Conductivity
8.0 GEOTECHNICAL EVALUATION
8.1 Site-I and II
The soil profile of the above sites is similar. Laboratory testing confirmed geotechnical
properties and findings during profiling of test pits.
Climatic factors have combined to create a mode of decomposition characterised by
chemical disintegration of the profile. Therefore, the Granite/ Gneiss have decomposed down
to depths ofat least 1,5 to 2,0 m. The Granite Gneiss, due to its composition, resisted these
climatic factors to a fair extend with depth, limiting decomposition to these shallow depths.
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Geology Influences on Construction
It is believed that the ground conditions of the above sites would have a great affect on
conventional construction methods. De-watering and diverting all irrigation channels of the
areas will be required prior to any construction taking place. Provision for flooding during
construction will also be required which might turn out to be highly costly. This could result inyears delay of the construction of the substation, since careful irrigation studies will be
required not to disturbed the integrated irrigation system of the region.
Though the material properties have been changed drastically (reworked/residual material),
most of the materials are considered suitable for civil construction.
8.2 Site-III
The granite profile show to be extremely less weathered than the profiles of Sites-I and II.
The material found on this site is considered an excellent material for the construction of a
substation platform. Inspection of the existing quarry on the property and test pits excavated
confirms the soil profile to be uniform present over area. Laboratory test results confirmed the
findings during profiling test pits.
Geology Influences on Construction
The soil profiles proved that materials from this site are of an excellent construction quality.
8.3 Water Table
There is a shallow perched water tablepresent on Site-I and Site-II, which would result in
major design measures to be taken. It is anticipated that this will result in a highly costly
exercise.
8.4 Topography - Preliminary Substation Positioning, Geometry (Earthworks
Volumes) and Access Roads
An attempt was made to position and simulate platforms for each site.
Topographical features and present soil information of each site were taken into account
during this exercise. The following volumes for earthworks were obtained during this exercise
for each site:
Site Strip m Cut m Fill m Special Measures
Site-I 25 215 23 640 18 215 De-watering import fill
Rerouting Regional IrrigationSite-II 25 215 23 250 21 130 De-watering import fill
Rerouting Regional Irrigation
Site-III 25 215 38 200 37 000 None
All three sites are accessed rather easily, with similar lengths of access roads to be
constructed.
Attached to this report are preliminary geometric designs, indicating optimised platform
positioning for each site (Appendix-C).
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8.5 Flood Line Studies
Flood line studies have revealed that Sites-I and II are not suitable for this development. Site-I
will require major construction measures to divert irrigation channels which will have an effect
on the region, Site-II has a risk of being flooded during a 1: 50 year flood.
9.0 CONCLUSIONS AND RECOMMENDATIONS
Considering the results of all the studies carried out during this investigation it is clear that
Site-III is recommended as the suitable site. Sites-I and II are not considered suitable.
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10.0 REFERENCES
10.1 Jennings Brink & Williams (1973). Revised Guide to Soil Profiling for Civil
Engineering purposes in South Africa. The civil Engineer in S.A. Jan. 1973.
10.2 Site Selection Desk Studies Dr J Lourens
10.3 Flood-line Studies Peter Greybe February 2010-03-04
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APPENDIX-A
Flood line Studies
Wolwekraal
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