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ESSELEN-CHLOORKOP-NORTHRAND DESKTOP STUDY – GEOLOGICAL ASSESSMENT

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ESSELEN-CHLOORKOP-NORTHRAND DESKTOP STUDY – GEOLOGICAL ASSESSMENT Powered By Docstoc
					ESSELEN-CHLOORKOP-NORTHRAND DESKTOP STUDY – GEOLOGICAL
ASSESSMENT


Executive Summary

Eskom are proposing to upgrade their power transmission lines between the Esselen and
Northrand substations from 275 kV to two 400 kV and also to construct two new 132kV power
lines between the Northrand and Chloorkop substations. Five routes are proposed between
Esselen and Northrand substations and three routes are proposed between the Northrand
and Chloorkop substations.


This report, which is limited to a desktop study, deals with the geology along the proposed
routing corridors and provides recommendations as to the best route choice based on the
geology.

The area is underlain by granite and granitic rocks of the Johannesburg – Pretoria Granite
Dome and rocks of the Transvaal Supergroup.


The dolomite bedrock profile within the Malmani Subgroup of the Transvaal Supergroup is
expected to be highly variable with pinnacles. The dolomitic residuum between the pinnacles
typically comprises clay, chert, sand, silt and wad. Due to its poor engineering characteristics
wad poses a problem for founding and it should be excavated and replaced with better
material when encountered.


The Johannesburg – Pretoria granite dome comprises granite and granitic rocks.              The
residual granite typically consists of silty sand, which is often collapsible, and gravelly sand.
Core-stones of hard rock granite are often present in the residual soil profile.


The collapsible soil should be excavated and replaced with better fill material when
encountered. Should a corestone or corestones be present they can either be removed or
the design of the founding system changed to take their presence into account.


All the proposed alternative routes between the substations run on similar geological
formations. Hence from a geological and geotechnical point of view there is not one route
which is significantly favorable over the other.



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TABLE OF CONTENTS


1.     INTRODUCTION.............................................................................................................1
     1.1.     Project Overview .....................................................................................................1
     1.2.     Scope and Objective of the Report ..........................................................................1
     1.3.     Method of investigation............................................................................................1
2.     PROJECT DESCRIPTION ..............................................................................................2
3.     GEOLOGY ......................................................................................................................2
     3.1.     Route Geology ........................................................................................................2
     3.2.     Anticipated ground conditions..................................................................................3
4.     FOUNDING CONDITIONS..............................................................................................5
     4.1.     Dolomite ..................................................................................................................5
     4.2.     Quartzite..................................................................................................................6
     4.3.     Granite ....................................................................................................................6
     4.4.     Alluvial deposits.......................................................................................................7

5.     CONCLUSIONS AND RECOMMENDATIONS................................................................7
     5.1.     Esselen to Northrand Routes...................................................................................7
     5.2.     Northrand to Chloorkop Routes ...............................................................................9
6.     REFERENCES ............................................................................................................. 10


TABLES
Table 3.1: Summary of the geology .......................................................................................3
Table 5.2: Route lengths along different geological conditions.......……………………………..8




APPENDICES

APPENDIX A

Figure 2.1: Esselen to Northrand Routes
Figure 2.2: Northrand to Chloorkop Routes

APPENDIX B

Figure 3.1: Geology along alternative routes
Figure 3.2: Dolomite on the northern section

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1. INTRODUCTION

1.1. Project Overview


The project involves the upgrading of Eskom power transmission lines between the Esselen
and Northrand Substations from 275 kV to two 400 kV and the construction of two new 132kV
power lines between the Northrand and Chloorkop substations. All the routes are situated
between Tembisa and Kempton Park.



1.2. Scope and Objective of the Report


The scope of this report was to determine the geology along the proposed routing corridor
and to provide recommendations as to the best route choice based on this information. It was
indicated that the study should be limited to a desk top type study.


The report provides the following information:


   •   The expected bedrock geology and soil cover along the proposed corridor based on a
       desk study of available data.
   •   Recommendations regarding the expected founding conditions at a depth of 1.5
       meters.
   •   Information regarding excavation potential and the presence of shallow water,
       together with the presence of active soil layers and any slope stability problems.
   •   Recommended route based on geological suitability.



1.3. Method of investigation



The desktop study involved the following:



   •   Study of published geological maps to determine the geology along the route




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   •   Reference to published literature on the characteristics of the anticipated rock and soil
       cover overlying the study area and recommended foundation solutions on such soils.
   •   A walkover survey for visual inspection of the sites to gain a better understanding of
       the physical conditions.



2. PROJECT DESCRIPTION


Two study corridors were identified by Eskom and within each of the corridors various routes
were identified:


           •   Esselen and Northrand Corridor
                   o   Route 1
                   o   Route 2
                   o   Route 3
                   o   Route 4
                   o   Route 5
           •   Northrand and Chloorkop Corridor
                   o   Route 4a
                   o   Route 4b
                   o   Route 4c


The positions of the alternative routes are indicated in Figure 2.1 of Appendix A for the
Esselen and Northrand Corridor and in Figure 2.2, Appendix A for the Northrand and
Chloorkop Corridor.



3. GEOLOGY



3.1. Route Geology


From the 1: 250 000 published geological Map (2628 East Rand) the area over which the
power lines are to be constructed, is underlain by granite and granitic rocks of the




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Johannesburg – Pretoria Granite Dome and rocks of the Transvaal Supergroup. See Figure
3.1 in Appendix B.


The geology is summarised in Table 3.1 below:




Table 3.1 Summary of the geology


                                                                           EXPECTED
                            STRATIGRAPHY
                                                                          ROCK TYPES
                      Chuniespoort        Malmani
                                                                          Dolomite, chert
                         Group           Subgroup
     Transvaal
                                                          Black Reef         Quartz,
    Supergroup
                                                           Quartzite      conglomerate,
                                                          Formation           shale
                                                                           Granite and
        Johannesburg – Pretoria Granite Dome
                                                                           granitic rock




The Esselen to Northrand corridor is underlain by rocks of both the Transvaal Supergroup
and the Johannesburg – Pretoria Granite Dome. The northern portion of the corridor (see
Figures 3.1 and 3.2 in Appendix B is underlain by rocks of the Transvaal Supergroup and the
rest of the corridor is underlain by granitic rocks of the Johannesburg – Pretoria Granite
Dome.


The Northrand to Chloorkop corridor is underlain entirely by granitic rocks of the
Johannesburg – Pretoria Granite Dome.



3.2. Anticipated ground conditions


a) Malmani Subgroup


The bedrock profile within the Malmani Subgroup is expected to be highly variable. The
dolomite typically forms pinnacles with deeply weathered slots between. The pinnacles can


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occur very close to surface or at a great depth and can be closely or widely spaced. Hence
the high variation in depth to bedrock within this Subgroup.


The dolomitic residuum typically comprises clay, chert, sand, silt and wad. The latter is a
highly erodible, highly compressible, highly water sensitive manganese rich soil. Wad has a
very high void ratio and its presence is often indicative of the potential for subsidence.


Sinkholes and dolines can form in areas underlain by dolomite and dolomitic residuum.
Sinkholes are subsidences which occur within a short space of time with little or no warning
whereas dolines are slower forming subsidences.


The thickness of the dolomitic residuum is often highly variable within a short distance and
within short distances the composition of the residuum can vary greatly.


Some dolomitic profiles are described as chert rich whereas others are described as chert
poor. Subsidence within the chert rich residuum is less common that within the chert poor
residuum where silts and wad may predominate.


The water table within the Malmani Subgroup is generally deeper than 30m.


b) Black Reef Quartzite Formation


The Black Reef Quartzite Formation comprises quartzite, quartzitic conglomerate and shale.
The quartzitic rocks are highly resistant to weathering and hence bedrock can be expected at
a shallow depth. A relatively thin layer of residual soil can be expected and this is generally
sandy in composition.


Where shale is present a slightly deeper soil profile is expected but rock should be
encountered within approximately 4m of surface. The residual soil would tend to comprise
clayey silt and silty clay. The clay may be potentially expansive in the medium range.
However shale comprises a minor component of the Black reef Quartzite Formation and may
not be encountered at all.


A “locally” high water table is expected within the Black Reef Quartzite Formation.




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c) Johannesburg to Pretoria Granite Dome


The Johannesburg – Pretoria granite dome comprises granite and granitic rocks. The
anticipated soil profile on the granite would comprise a top layer of hillwash underlain by
residual granitic soil overlying very soft rock granite. The residual granite typically consists of
silty sand and gravelly sand.     Core-stones of hard rock granite are often present in the
residual soil profile.


The silty sand, residual granite often has a collapsible grain structure. This means that the
soil is susceptible to additional settlement when exposed to water and this generally occurs
without increase of applied pressure.      Collapsible soils have colloidal coatings, normally
referred to as clay bridges, between the sand grains. Upon wetting the clay bridges are
broken down and the grain structure collapses, thus resulting in additional settlement of the
soil. In addition, the development of ferricrete is common within the residual granitic soils.


The presence of ferricrete is indicative of a fluctuating water table. The fluctuations may have
only occurred in the past but may be active at present. Ferricrete can be present as discrete
nodules within the residual granitic soil or may be in the form of hardpan. Where hardpan has
developed a perched water table is often present.


Where ferricrete is not present, a relatively deep water table is expected.



4. FOUNDING CONDITIONS


The area along all the alternative routes is generally flat with gentle slopes in some areas as
established from the walk over survey. Therefore slope stability problems are not anticipated.



4.1. Malmani Subgroup



In the sections of the route underlain by dolomite and dolomitic residuum the profile is likely to
be highly variable. The presence of wad will present particular problems because of its poor
engineering properties.    If encountered, the wad must be excavated using mechanized




                                              5
equipment (backactor) and replaced with good quality material which can be compacted to a
higher density, before placing foundations.


The chert, which is expected to consist of thin bands (in the order of 300mm thick), can be
excavated using power tools.


The excavation and compaction process should involve removal of the dolomitic residuum to
a depth of at least 1m below base of footing and replacing with 1m of G6, or better, fill
compacted in 150mm layers to at least 95% of maximum Modified ASSHTO density at
optimum moisture content (omc). This should minimize any differential settlement in the
foundation.


Formation of sinkholes in the dolomite can be prevented by grading of the site around the
pylon base to prevent ponding of water which may de-stabilise the dolomite.


Formation of dolines can be prevented by good water management in the area around the
pylons bases.




4.2. Black Reef Quartzite Formation



No founding problems are anticipated in the quartzite due to the presence of rock at shallow
depth. Problems are, also, not anticipated in the residual soil, however, the rock may need
blasting to attain the foundation level required. The residual soil, where present, can be
excavated by conventional means.




4.3. Johannesburg – Pretoria Granite Dome



The presence of collapsible soils could lead to foundation instability.       If encountered,
collapsible soils can be treated in the following manner:-




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   •   Excavation of the collapsible soil to depth below the base of the footing and to a width
       of 1.5B (where B is the foundation width).
   •   The base of the excavation should be thoroughly wetted.
   •   The removed material can then be replaced in 150mm layers compacted to at least
       95% of the maximum Modified ASSHTO density at between -1% to +2% o omc.




Corestones can create a problem of differential settlement, in a case where part of the pylon
footing is resting on a core stone and the other part is are not. Where pylon bases are to be
constructed on the granite it is recommended that a DCP test be carried prior to construction
in the middle of the proposed footing position and at the four corners. This will indicate the
presence of corestones within the footing area. Should a corestone or corestones be present
they can either be removed or the design of the founding system changed to take their
presence into account. For example the pylon could be founded on a stiff raft foundation or
removal and replacement could take place such as where collapsible soils are present.


Soil excavation and removal of corestones, where possible, can be done using a backactor.
Alternatively the corestones can be excavated using power tools or drill and blast techniques.



4.4. Alluvial deposits



Where the powerlines cross a river valley, care should be taken so that the pylons are not
founded on the alluvial deposits on the river banks. These deposits normally consist of
compressible clay which is potentially expansive and therefore unsuitable for founding.
Foundations can be placed on a competent layer below the alluvium or limited removal and
replacement can be carried out.



5. CONCLUSIONS AND RECOMMENDATIONS

5.1. Esselen to Northrand Corridor

All the alternative routes, Routes 1 to 5, between Esselen and Northrand substations run on
similar geological formations (Transvaal Supergroup and Johannesburg – Pretoria Granite
Dome). Hence from a geological and geotechnical point of view there is not one route which


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is significantly favourable over the other since they all present similar challenges. However
the dolomites present inherent risk to structures hence the shorter the route through dolomite,
the better (i.e. the more preferable). Table 5.1 below gives an indication of the extent of the
dolomite and granite along the five routes.


Table 5.1 Route lengths along different geological conditions



                                   Granite                        Dolomite
     Route          Total route
                   length (km)                     % of total                     % of total
                                  Length (km)                    Length (km)
                                                  route length                   route length
    Route 1           14.00         11.25              80           2.75              20
    Route 2           14.60         11.60              79            3                21
    Route 3           17.65         14.65              83            3                17
    Route 4           14.80         12.05              81           2.75              19
    Route 5           14.80         11.80              80            3                20


From the table above, Route 1 is the shortest, 14.00km with 2.75km of the route running
through the dolomitic area and this makes it more favorable from the geological point of view.


Route 4 and 5 have the same total lengths but Route 4 is slightly more preferable as 19% of
the route runs on dolomite compared to 20% in Route 5. The total length of Route 2 is 0.2km
shorter than that of Route 4, but Route 2 is 0.25km longer in the dolomite section. This makes
Route 4 the second preferred option.


There is little difference (0.20km) in total length between Route 2 and Route 5 and the length
through the dolomitic area is the same. This makes Route 2 the third preferred option as it is
shorter than Route 5.


Route 3 is the longest 17.65km with dolomite extending for 3km below the route making it the
least favorable.


However, as evident from table 5.1, the individual total route lengths (except for Route 3
which is 21% longer than the shortest route) and their lengths through dolomite area are very
close.   Hence, other studies should probably take precedence when determining the




                                              8
preferred route as the impacts from geological perspective are minimal between the four
alternative routes (i.e. Route 1, 2, 4 and 5).




5.2. Northrand to Chloorkop Corridor



All the alternative routes, 4a to 4c between Northrand and Chloorkop substations are located
along the same geological formation (Johannesburg – Pretoria Granite Dome) and are very
similar in length. Hence from a geological and geotechnical point of view there is no one
route which is preferred over the other since they all present similar challenges. Similarly to
the Esselen – Northrand corridor, other studies should probably take precedence when
determining the preferred route as the impacts from geological perspective are minimal
between the three alternative routes.


An experienced geotechnical engineer or engineering geologist will be required on site during
construction to assess the ground conditions before the foundations are built.




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6. REFERENCES


The following sources of information were used to produce the report:
1. 1: 250 000 scale Geological Map (2628 East Rand)
2. Brink A.B.A, Engineering Geology of Southern Africa, Volume 1. The first 2000 million
   years of geological time. Building Publications Pretoria (1979).
3. Consultation with Ms. Heather Davis (BKS Geotechnical Engineer) who also has
   specialized knowledge of dolomitic ground conditions.




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APPENDIX A




    i
APPENDIX B




    ii

				
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