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GEOSTRATEGIESCONSULTING ENGINEERS

ENVIRONMENTAL SCIENTISTS

ENGINEERING & LAND SURVEYORS

P. O. Box 227, Maraisburg, 1700Construction House, 1164 Minnie Postma Street, Florida Ext. 11

Tel. +27 11 674 1325 : Fax. +27 11 674 4513 : Email. [email protected]

GEOTECHNICAL INVESTIGATIONfor

Electrical Sub- Station, Mthata

Client: Eskom Date: May 2009 Job No:09057



C O N T E N T S

1 INTRODUCTION 1PreambleData BaseObjectives

2 FACTUAL REPORT 2Programme of WorkSite DescriptionSite GeologyHydrologyObservationsLaboratory and Field Test ResultsGeotechnical mapping of site

3 INTERPRETIVE REPORT 12Discussion of ResultsDesign Solutions

Construction ProblemsAdditional Investigations

4 CONSTRUCTION MONITORING 21Excavation InspectionControl Testing

APPENDICES

1. GENERAL “GOOGLE” SITE PLAN

2. TRIAL HOLE PROFILES3. BOREHOLE PROFILES4. CO-ORDINATES OF BH’S & TP’S5. LABORATORY TESTS6. GEOPHYSICAL TEST DATA7. GENERAL PHOTOGRAPHS8. BOREHOLE PHOTOGRAPHS9. SLAKING TEST PHOTOGRAPHS10. MAIN SITE PLAN11. ROCK CONTOUR DRAWING12. GEOLOGICAL CROSS-SECTIONS



GEOSTRATEGIES 1

Eskom Sub-Station, Mthatha Report No. 09057 Z:\GEOSTRATEGIES\Projects 2009\Eskom\Mthatha Sub Station\Steve's Final Review mc.wpd May 2009

1 INTRODUCTION

1.1 Preamble

On the 19

th

January 2009, a proposal reference F:\GEOSTRATEGIES\Projects 2009|Eskom|Mthatha Sub Station \ Tender Docs was submitted to Eskom Tender Department for ageotechnical investigation for the proposed Eskom Sub-Station, Mthatha, TenderJC 100 909 210.

On the 20th February 2009, Geostrategies signed a contact document to undertake therequired geotechnical investigation while a programme scheduling the various phases ofthe investigation was issued to Eskom on the 23 February. All Health and Safety issueswere concluded by the 17th of March.

1.2 Database

The following information was given to Geostrategies prior to the commencement of theinvestigation:

1.2.1 A drawing titled “ Mthatha - 2m Step “ showing cross sections through the twoproposed soil terraces to be constructed at the site. It should be noted that uponcompletion of the field work, and based on the results of the laboratory testing, athird terrace option was proposed by Eskom.

1.2.2 A drawing titled “ Mthatha - Layout “ showing the plan layout of the proposedsub-station..

1.2.3 A “google aerial photo “ with the outline of the site and an adjacent water pipeline

.1.2.4 A document titled “ Specification and Scope for a Geotechnical Investigation -

Proposed New Mthatha Sub - Station “ Document Geospec Mthatha 11/11/08.

Geostrategies were also advised that the proposed development would comprise theconstruction of a 400/132kV Electrical Sub- Station on a site located approximately 10kms to the south east of the town of Mthatha, in an area known as Orange Grove.

The development would include the installation of typical equipment, with a maximumbearing pressure of the order of 150 kPa, as listed below :

Electrical transformersCircuit breakers and line termination structuresHigh-voltage switchgearLow-voltage switchgearSurge and lightning protection equipmentControl and metering equipmentAccess roads and buildings

1.3 Objectives

At the time of tender, the scope of the work was generally as described in your tenderdocuments as modified below:-



GEOSTRATEGIES 2


1.3.1 The resistivity survey was to be based on a site with plan dimensions ofapproximately 340m x 360m, and would comprise:-

1.3.1.1 four continuous vertical electrical soundings sections, each 381m long to be

collected using a 3m electrode separation

1.3.1.2 three CVES lines with a 1m electrode separation, each 63m long

1.3.2 A 23 ton excavator would be supplied to site for a period of three days (thisexcludes time spent for HSE induction) during which time around 20 trial pits willbe excavated to refusal or maximum practical reach of the machine. Each holewill be profiled by an engineering geologist, sampled as necessary and thenloosely backfilled. Should additional time be required on site to complete thenecessary field work, Eskom will be advised before the extra work is undertaken.

1.3.3 Allowance was made to drill four rotary core boreholes, with a combined length

of 70m

1.3.4 The type and quantity of soil samples taken would be determined by the sitegeologist, depending on the soil conditions encountered, but would be based onthe requirements of the tender.

1.3.5 Selected soil samples would be tested at our laboratory to determine their majorengineering properties, again the number and type of tests would conform ingeneral to the requirements of the tender

1.3.6 A report would be prepared which would address the deliverables specified inpara 5.3 of appendix 1 of the tender documents.

2 FACTUAL REPORT

2.1 Programme of Work

2.1.1 Literary Review

A literary review was conducted in order to obtain data from previousinvestigations carried out by Geostrategies and other consultants in the area.The 1:250 000 geological map, “No 3128 Umtata” was consulted in order todetermine the regional geology in the vicinity of the site.

2.1.2 HSE Requirements

With regards to health and safety issues, Geostrategies appointed outsideconsultants, Comprac Gauteng, to deal with the various issues on our behalf.

Negotiations started on the 27th February 2009, and our safety file was signed offby Eskom Head-Office on the 16th March, 2009.

2.1.3 Field Work

During the initial planning of the field work, Mr Fred Grové of Eskom requested

that all trial pits and boreholes be placed on a regular rectangular grid, to enableselected geological cross-sections to be drawn once the fieldwork had beencompleted.



GEOSTRATEGIES 3


We were instructed that boreholes must only be placed in the proposed areas ofcut, and that the final depth of each borehole would be influenced by theproposed depth of cut at that position.

The layout was sent to Mr Grové, and the fieldwork only commenced once thepositions of test pits and boreholes had been accepted by him, and modifiedwhere requested to do so.

Geostrategies established its Rotary Core Drilling crew on the at site on the 18th

of March, and between the 18 th - 28th March, 5 boreholes were drilled in theareas of terrace cut. The boreholes ranged in depth between about 8 - 12m,depending on the depth of cut at each locality.

Between the 24th - 28th March, twenty one trial pits were excavated using aKomatsu PC 220, 22 ton tracked excavator on a regular grid covering the entiresite. The trial pits were excavated to depths of between 4.5 - 5.0m, or refusal on

bedrock where it was present at shallower depths.

The positions of the boreholes and trial pits are shown on the site plan given inAppendix 9. Each borehole and trial pit was profiled by an engineering geologistaccording to the Jennings, Brink and Williams system, sampled as necessary,and then loosely backfilled. The detailed trial pit profiles and their co-ordinates,appear in Appendix 2, while the borehole profiles and co-ordinates appear inAppendix 3.

Between the 18th - 21st March, ten electrical resistivity traverses were carried outat the site, by Engineering & Exploration Geophysical Services cc.

An ABEM Lund resistivity meter was used to collect the resistivity data, toproduce a resistivity image or cross section of the ground with depth.

Of the ten resistivity traverses, 5 were carried out with a 3m electrodeseparation, and the remaining 5 with a 1m electrode separation.

All geophysical data is contained in Appendix 6. The positions of the resistivitytraverses are shown in Figure 1, with the traverses themselves given in Figures2 - 4 of the enclosed documents.

The co-ordinates of the boreholes and trail pits are given in Appendix 4. .

2.1.4 Office and Laboratory Work

From the recovered soil samples, the following tests were conducted:-

CBR and Road Indicator 18 testsFoundation Indicator 29 testsOedometer Swell Tests ( 10 kPa) 4 testsBulk Density & Void Ratio 9 testsSoil Chemical Aggresivity Tests 6 testsSoil Conductivity Tests 20 testsUCS Tests on core samples 4 tests

Jar Slake Tests 6 tests

The individual test results are shown in Appendix 5 of this report.



GEOSTRATEGIES 4


2.2 Site Description

The site is situated approximately 10kms to the south east of the main town of Mthatha,in the Orange Grove area.

It is a “greenfield” site, currently covered in natural veld grass and indigenousvegetation. The area had previously been cultivated under maize crops, with theremnants of ploughed terraces remaining.

The remains of a wire fence demarcate a large rectangular area, within which thesmaller site area has been positioned.

The site has been positioned on the western slope of a large spur. A rural villagesettlement has developed along the lower southern extremities of the spur, upslope of ariver flowing through the area. Refer to attached “google” photo, appendix 5.

Access to the site is via a gravel road sign posted “Orange Grove”, leading off the oldMthatha - East London tar road.

A small water dam is present in a localized valley to the immediate north west of thesite.

A water pipeline is present running parallel to the gravel road, to the south west of thesite.

The site is almost square in shape, with an approximate area of 14 Hectares, andslopes down towards the south at a fairly constant gradient of approximately 6%.

The photographs which appear in Appendix 5 indicate the general layout of the site.

2.3 Site Geology

From the available literature as well as the observations during the site investigation, it isevident that the site is underlain by Mudstones and Sandstones belonging to theAdelaide Subgroup, Beaufort Group, of the Karoo Sequence.

Typically these Karoo Rocks can be overlain by younger dolerite intrusive sills anddykes, belonging to the Drakensburg Group, although none were intersected in any ofthe trial pits or boreholes.

Relatively deep weathering was evident at the site, with the mudstones reduced toclayey silt and sandy silt, residual soils. Where present, the sandstone rock hadweathered to a silty sand.

The residual soils have subsequently been covered in a thin transported hillwash cover.

A continuos, weakly cemented ferricrete horizon is present at the base of the hillwash.This pedogenic horizon is an indication of a fluctuating moisture content within the nearsurface sub-soils over time.

2.4 Hydrology

The mean annual rainfall in this area is between 725 - 875mm, most of which occurs inthe period between November and March.



GEOSTRATEGIES 5


With the exception of a slight inflow in TPC4, no groundwater seepage was encounteredin any of the trial pits, although evidence of pedogenisis ( ferricrete development) wasrecorded in the majority of the trial pits indicating periodic fluctuation of a perched watertable.

No preferential drainage paths were noted and it is assumed that stormwater drainagewill be in the form of fairly strong sheetwash towards the south..

Due to the fairly steep gradient at the site, it is recommended that a suitable cut-off drainbe placed upstream of the proposed terrace.

The depth of the permanent watertable is not known.

2.5 Observations

While the nature and stratigraphic relationships of the various soil and rock elements that

were encountered in the trial pits and boreholes were relatively uniform, considerablevariations were noted in both depth and thicknesses. Table 1 summarises the thicknessesand depths of the various horisons:-

Table 1: Summary of soil profile

Horison AvgThickness

(m)

Thickness“range”

(m)

Avgdepth to top

( m )

Depth totop “range”

(m )

Material

Topsoil + Hillwash 0.50 0.3 - 1.0 0.00 0.0 Mainly Silty Sands &Sandy Silts

Ferricrete Horizon 0.80 0.4 - 1.3 0.50 0.3 - 1.0 Gravelly silty sands

Residual MudstoneSoils

3.00 0.6 - 8.0 1.25 1.2 - 1.9 Clayey Silts &Sandy Silts

Mudstone Bedrock na na 4.00 1.3 - 9.2 Mainly soft rockMudstone

The various major soil horisons are described below:-

2.5.1 Transported Soil (Topsoil & Hillwash)

This material comprises a grey brown and reddish brown soil of variable

composition, ranging between silty sand, sandy silts and clayey sands. Thehillwash horizon was generally of medium dense / f irm consistency and ranged inthickness in thickness between 0.3 - 1.0m, with an average of 0.5m.

The hillwash was covered by a loose silty sand topsoil with roots.

2.5.2 Ferricrete

A consistent, weakly developed ferricrete horizon was present across the site. Thishorizon compromised cemented and ferruginised, gravelly silty sands, of dense tovery dense consistency. It had a thickness ranging between 0.4 - 1.3m, with anaverage thickness of 0.8m.



GEOSTRATEGIES 6


2.5.3 Residual Mudstones

This horizon was continuously developed across the site, and generally comprisedan olive brown, mottled dark yellow brown, orange or reddish brown, clayey silt,

frequently becoming a sandy silt with increased depth. The clayey silts were micro-shattered and slickensided, indicating that they are potentially active ( expansive )materials.

In certain areas, the upper horizon was slightly reworked, resulting in a mixedprofile containing transported soils which have worked their way down into theunderlying residual strata, due to natural weathering processes.

In other areas, the lower residual mudstone soils had been partially calcretised,and contained scattered calcrete nodules, generally of medium gravel size.

The residual mudstone horizon was extremely variable in thickness, ranging from

0.6m in BH I2, to 8m in BH 3, while its consistency ranged from firm, becoming stiffwith increased depth.

2.5.4 Bedrock

Mudstone bedrock was intersected at depth, in all the five boreholes drilled at thesite. In BH 1 however, the mudstone was overlain by a thin sandstone sequence.

The bedrock was generally weathered and highly fractured, and of soft rockconsistency. SPT refusal was seen to occur in materials of very soft rock or betterconsistency.

With depth the degree of weathering reduced with the consistency increasing tosoft to medium hard rock.

The depth to the bedrock was highly variable ranging form 1.3m in TP I2,increasing to 9.2m in BH 3. Results from the 5 boreholes drilled at the site, wouldindicate an average bedrock depth of approximately 4m.

Table 2 below presents the results of UCS tests carried out on borehole cores :

Table 2 : UCS Tests on Borehole Core Samples

BH

.No

Depth Material UCS

(MPa)

Rock Classification

1 11.70 - 11.85 Mudstone Bedrock 54.5 Hard Rock

2 6.80 - 6.95 Mudstone Bedrock 35.7 Hard Rock

3 9.40 - 9.55 Mudstone Bedrock 0.6 Very stiff soils

5 9.30 - 9.45 Mudstone Bedrock 124.7 Very Hard Rock



GEOSTRATEGIES 7


2.6 Laboratory Test Results

Summaries of the various laboratory test results aregiven below :

Table 3: Summary of Foundation Indicators

TPNo.

Depth(m) Material PI PI(ws) Clay%

GM Activity

C6 1.80 - 2.50 Clayey Silt - ResidualMudstone

33 27 18 0.61 High

C6 3.80 - 4.40 Clayey Silt - ResidualMudstone

23 19 18 0.54 Medium

C8 0.90 - 1.30 Gravelly Silty Sand -Ferricrete

17 7 8 1.59 Low

C8 2.20 - 2.80 Sandy Silt - ResidualMudstone 21 15 12 0.84 Medium

C8 4.00 - 4.40 Sandy Silt - ResidualMudstone

20 15 22 0.73 Medium

E2 0.40 - 0.80 Gravelly Silty Sand -Ferricrete

27 15 8 1.21 Medium

E3 2.50 - 3.40 Clayey Silt - ResidualMudstone

21 20 21 0.23 Medium

E3 3.90 - 4.80 Sandy Silt - ResidualMudstone

31 18 12 1.25 Medium


19 17 25 0.44 Medium

E4 4.00 - 4.90 Sandy Silt - ResidualMudstone

23 20 16 0.37 Medium

E6 0.30 - 0.50 Clayey Silt - Hillwash 21 18 19 0.35 Medium


24 19 13 0.59 Medium


23 21 23 0.33 Medium


26 21 18 0.52 Medium

G4 1.60 - 2.50 Clayey Silt - ResidualMudstone

32 31 27 0.16 Very High


21 21 21 0.13 Medium


22 22 22 0.10 Medium


30 29 27 0.18 High

I4 1.50 - 2.00 Sandy Silt - ResidualMudstone

26 24 5 0.38 High

I4 3.30 - 4.50 Clayey Silt - ResidualMudstone

28 27 17 0.14 High



GEOSTRATEGIES 8

Table 3: Summary of Foundation Indicators

TPNo.

Depth(m) Material PI PI(ws) Clay%

GM Activity


I6 0.20 - 0.50 Sandy Silt - Hillwash np np 9 0.70 Very Low

I6 0.80 - 1.20 Gravelly Silty Sand -Ferricrete

26 11 12 1.63 Medium


22 21 22 0.21 Medium


33 33 29 0.12 Very High

I8 0.20 - 0.40 Sandy Clay -Hillwash

26 18 16 0.90 Medium

I8 1.00 - 1.80 Clayey Silt - Partially

Calcretised ResidualMudstone

28 25 22 0.34 High

I8 2-80 - 3.40 Clayey Silt - ResidualMudstone

28 27 18 0.13 High


25 25 21 0.11 High

K4 0.70 - 1.20 Gravelly Silty Sand -Ferricrete

25 12 11 1.50 Medium

Table 4: Oedometer Swell Tests

TPNo

Depth(m)

Material Dry Density(kg/m3)

MoistureContent

(%)

Swell (%)@ 10kPa

Initialvoidsratio

A4 1.5 - 2.5 Clayey silt - Residual Mudstone 1787 17.65 2.40 0.50

C2 3.0 - 4.2 Clayey silt - Residual Mudstone 1301 32.14 0.84 1.02


E4 2.0 - 4.5 Clayey silt - Residual Mudstone 1648 13.21 2.02 0.63

Table 5: Bulk Density and Void Ratio Tests

TPNo

Depth (m) Material Dry Density(kg/m3)

MoistureContent

(%)

SpecificGravity

Initialvoidsratio

A4 0.40 - 0.70 Clayey silty sand - Hillwash 1724 4.0 2.579 0.50

A4 0.80 - 1.40 Gravelly silty sand - Ferricrete 1778 11.0 2.636 0.48

A4 1.50 - 2.50 Clayey silt - Residual Mudstone 1908 17.0 2.688 0.65

C4 0.30 - 1.00 Silty sand - Hillwash 1664 8.0 2.559 0.54

C4 1.20 - 1.80 Gravelly silty sand - Ferricrete 1626 13.0 2.533 0.56


E4 2.00 - 4.50 Clayey silt - Residual Mudstone 1581 20.0 2.658 0.68



GEOSTRATEGIES 9

Table 5: Bulk Density and Void Ratio Tests

TPNo

Depth (m) Material Dry Density(kg/m3)

MoistureContent

(%)

SpecificGravity

Initialvoidsratio


G3 2.10 - 4.30 Sandy silt - Residual Mudstone 1428 18.0 2.574 0.80

I4 2.20 - 4.20 Clayey silt - Residual Mudstone 1612 16.0 2.68 0.66

Table 6: Summary of Road & CBR Tests

TPNo.

Depth(m)

Material Swell90%

PI PI(ws)

LS GM CBR @93%

TRH14Class

E4 2.00 - 4.50 Residual Mudstone - cs + ss 1.38 23 15 11 1.26 0 G10

C4 1.20 - 1.80 Ferricrete 1.17 15 8 7 1.51 1 G10

G4 0.30 - 1.00 Hillwash 0.84 17 8 8 1.72 3 G10

A4 1.50 - 2.50 Residual Mudstone - cs 1.16 15 13 7 0.82 0 G10

C2 3.00 - 4.20 Residual Mudstone - cs 1.18 21 19 11 0.62 0 G10

I4 0.30 - 1.00 Ferricrete 1.12 18 8 9 1.84 1 G10

K4 1.50 - 3.50 Residual Mudstone - cs 1.17 12 9 12 1.03 1 G10

C4 0.30 - 1.00 Hillwash + Ferricrete 0.79 14 12 7 0.82 8 G9

C2 0.70 - 1.60 Ferricrete 0.94 23 13 11 1.53 5 G10

G3 2.10 - 4.30 Residual Mudstone - ss 1.06 17 15 8 0.57 1 G10

A4 0.80 - 1.40 Ferricrete 0.98 25 14 13 1.55 6 G10

E2 1.70 - 4.20 Residual Mudstone - ss 1.05 27 24 13 0.75 3 G10

G3 0.70 - 1.60 Ferricrete 1.06 22 11 11 1.62 5 G10

I4 2.20 - 4.20 Residual Mudstone - cs 1.27 19 18 9 0.58 1 G10

I2 1.70 - 3.10 Soft Rock Mudstone 0.87 17 5 8 2.12 6 G10

E3 2.00 - 4.00 Residual Mudstone - cs + ss 1.24 17 13 8 0.94 1 G10

C4 2.00 - 3.50 Residual Mudstone - cs 1.27 14 12 7 0.68 1 G10

A4 0.40 - 0.70 Hillwash 1.13 9 8 4 0.71 7 G9

Table 7 : Jar Slaking Tests on Borehole Core Samples

BH.No

JarNo

Depth Rock Type Rock Hardness Slaking Description SlakingClass

1 B 8.30 - 8.40 Mudstone Soft Rock Breaks slowly and/or forms severalfractures

4

2 F 4.10 - 4.20 Mudstone Soft Rock Degrades into a pile of flakes or mud 1

2 D 5.30 - 5.40 Mudstone Soft Rock Breaks slowly and/or forms severalfractures

4

5 C 7.00 - 7.10 Mudstone Soft Rock Breaks slowly and/or forms severalfractures

4



GEOSTRATEGIES 10

Table 7 : Jar Slaking Tests on Borehole Core Samples

BH.No

JarNo

Depth Rock Type Rock Hardness Slaking Description SlakingClass


5 E 9.00 - 9.10 Mudstone Soft Rock Breaks rapidly and/or forms manychips 2

5 A 9.50 - 9.60 Mudstone Medium Hard Rock No change 6

Table 8 : pH and Conductivity Tests

TP No Depth pH Conductivityms/cm

ResistivityOhm/cm

IndicatedCorrosiveness

A4 1.5 - 2.1 7.3 0.21 4762 Moderately Corrosive

C8 0.9 - 1.3 7.6 0.16 6250 Slightly Corrosive

E3 2.5 - 3.4 7.4 0.16 6250 Slightly Corrosive

E6 0.3 - 0.5 6.6 0.13 7692 Slightly Corrosive

E8 1.6 - 2.2 7.7 0.24 4167 Moderately Corrosive

G3 2.6 - 3.9 7.4 0.12 8333 Slightly Corrosive

G8 2.2 - 3.0 8.3 0.99 1010 Very Corrosive

I2 1.7 - 3.1 8.6 0.20 5000 Moderately Corrosive

I4 1.5 - 2.0 8.2 0.50 2000 Very Corrosive

I6 0.2 - 0.5 7.6 0.07 13514 Non Corrosive

I8 1.0 - 1.8 8.4 0.14 7143 Slightly Corrosive

K4 0.7 - 1.2 8.4 0.41 2439 Moderately Corrosive

E4 1.8 - 3.2 7.8 0.07 15152 Non Corrosive

G3 2.6 - 3.9 7.3 0.05 20000 Non Corrosive

Table 9 : Corrosivity Tests on Soil Samples

TP.No

Depth FinalAggressiveness

Index

Aggressiveness Recommendation

C2 2.3 - 3.1 725 Mildly to fairly aggressive Good concrete design andconstruction essential

C4 0.3 - 1.0 977 Highly aggressive Identify dominant corrosion index -Follow Recommendations

C6 1.8 - 2.5 516 Mildly to fairly aggressive Good concrete design andconstruction essential

G2 1.5 - 2.2 755 Mildly to fairly aggressive Good concrete design andconstruction essential

G4 1.6 - 2.5 630 Mildly to fairly aggressive Good concrete design andconstruction essential



GEOSTRATEGIES 11


2.7 Geotechnical Mapping of Site.

In order to map the geotechnical characteristics of the underlying soil, theresidential site class designations method as proposed by Watermeyer and

Tromp (1992) and the Joint Structural Division of the SAICE as prescribed by theNHBRC has been applied to this site. Table 6 below indicates the variousgeotechnical characteristics and the criteria used to evaluate the soils.

TABLE 10 - Geotechnical Characteristics

TYPICAL FOUNDING MATERIAL CHARACTER OFFOUNDINGMATERIAL

EXPECTED RANGEOF TOTAL SOIL

MOVEMENTS (mm)

ASSUMEDDIFFERENTIAL

MOVEMENT (%OF TOTAL)

SITECLASS

Rock (excluding mud rocks which mayexhibit swelling to some depth)

STABLE NEGLIGIBLE - R

Fine grained soils with moderate to very

high plasticity (clays, silty clays, clayeysilts and sandy clays)

EXPANSIVE SOILS <7,5

7,5-1515-30>30

50%

50%50%50%

H

H1H2H3

Silty sands, sands, sandy and gravellysoils

COMPRESSIBLEAND POTENTIALLYCOLLAPSIBLESOILS

<5,05,0-10

>10

75%75%75%

CC1C2

Fine grained soils (clayey silts andclayey sands of low plasticity), sands,sandy and gravelly soils

COMPRESSIBLESOIL

<1010-20>20

50%50%50%

SS1S2

Contaminated soils; Controlled fill;Dolomitic areas; Landslip; Land fill;Marshy areas; Mine waste fill; Mining

subsidence; Reclaimed areas; Very softsilt/silty clays; Uncontrolled fill

VARIABLE VARIABLE P

Based on these parameters, and the soil profile encountered, this site has beenclassified as:

Class H3 Founding horizons subject to heave due to expansivesoils

2.8 Resistivity

A brief report with comments on the results of the Resistivity Survey carried out by

Engineering & Exploration Geophysical Services cc is included in Appendix 7,along with a site plan and the resistivity sections.

2.9 Commercial Borrow Materials

Brief research into locating sources of borrow materials for the construction of therequired earthwork terraces and roads at the site, identified a commercial quarrynamed “ Transkei Quarries “ which according to information supplied, is situatedapproximately 5 kms outside the main Mthatha residential and business area,alongside the main Mthatha - Kokstad freeway. Attached in Appendix 8 is ageneral information leaflet setting out the various aggregate materials available,

and their current prices.



GEOSTRATEGIES 12


The Mthatha Local Municipality was also contacted to see if they could supply uswith any relevant information on potential good quality borrow areas in the vicinityof the site.

Geostrategies was able to track down Camdekon Engineering, the contractorswho built the existing gravel access road to the site. We spoke to the actual siteagent, Ruben Lewis, who told us that three Mudstone Rock borrow areas hadbeen opened. These were positioned at the beginning, middle and end of thegravel road. He has agreed to mark the positions of these borrow pits on a“google” aerial map, which we will forward to Eskom once it is received by us.

Ruben informed us that the material taken from the borrow areas was obacceptable quality to be used in the construction of the gravel access roadpassing the site.

As we have not examined any of these previous borrow areas, we are not able to

comment of the quality of the material available. Caution should be exercisedhowever, in only using mudstone of upper soft to medium hard rock, or betterquality, as the mudstones of lower hardness, could be subject to slaking.( Refer to Table 7 ),

The area around Mthatha is intruded by various Dolerite dykes and sills, Thepossibility exists that such outcrops are present reasonably close to the site, andthat a borrow pit could be established to quarry these materials. The advantage ofusing dolerite materials, is that they do not have slaking problems, as do themudstones.

The identification and subsequent soils testing of potential borrow areas, was not

within the scope of this investigation.

3 INTERPRETIVE REPORT 3.1 Discussion of Results

3.1.1 Topsoil and Hillwash

The topsoil horizon generally comprised a silty sand, but it wascontaminated with roots and vegetation, and will need to be removedbefore any development takes place.

The underlying hillwash materials generally classified as sandy and clayeysilts, with a PI of between 21 - 26%, classifying as being of “mediumexpansiveness”. In TP I6, the hillwash layer was very sandy and nonplastic in nature, but this was not typical of hillwash encountered over themajority of the site.

A sample of hillwash tested in TP C4, had a dry density of 1664 kg/m3,and an initial void ration of 0.54.

With regards to compaction characteristics, the hillwash classified betweena G9 – G10 according to TRH 14.

Due to their expansive nature, and fairly low consistency, these soils arenot considered to be a suitable founding medium for any of the structures,



GEOSTRATEGIES 13


nor for use as fill for terraces.

3.1.2 Ferricretes

The ferricretes comprised a weakly cemented and ferruginised, gravellysilty sand, of dense to very dense consistency. Laboratory analysisindicated a PI ranging between 17 - 27% with the PI of the whole samplewithin a range of 7 - 15%. Consequently, these soils classify as being of“low to medium” potential expansiveness.

Two samples of ferricrete tested gave a dry density between 1626 - 1778kg/m3, and void ratios between 0.48 - 0.56.

Compaction tests on these soils classify them as being G10 qualityaccording to TR14.

Due to their dense to very dense consistency, these soils could possiblybe thought of as a suitable founding medium, but due to their limitedthickness and the considerable depths of potentially expansive materialsunderlying them, they are not recommended as a founding horison.

3.1.3 Residual Mudstone

The residual mudstones were plastic in nature with a PI ranging between19 - 33% and a potential activity, ranging from “medium” to “very high”.

Laboratory tests indicate a variable dry density from a low of about 1300kg/m3, to a high of about 1900 kg/m3. The average dry density is about

1550 kg/m3.

The void ratio was correspondingly variable, ranging from 0.65 to 1.0, withan average value of 0.75.

Compaction results indicate these soils to be of G10 quality, according toTRH 14.

. Due to the potentially expansive nature of these soils, they are notconsidered to be suitable as a founding medium for any of the proposedstructures.

3.1.4 Bedrock

Both the Sandstone and Mudrock bedrock at the site can be considered asa suitable founding medium for the structures.

Allowable bearing pressures of 800 Kpa can be assumed for bedrock ofsoft rock or better quality.

Results from the Jar Slaking Tests, carried out on six borehole coresamples of the mudstone bedrock, indicate a definite slaking problembeing present. ( Refer to Table 7 )

This problem is especially severe with mudstone of very soft to soft rockconsistency. It would appear from the testing, that once the rock reaches



GEOSTRATEGIES 14


the upper transition of soft rock, moving into the medium hard rock orbetter category, then its slaking potential becomes more acceptable.

It is therefore recommended that only very competent and hard mudstone

rock, excavated during terrace construction, be considered for use as ageneral fill material. ( ie of upper soft rock or better consistency )

3.2 Design Solutions

3.2.1 Cross Sections

Six typical cross-sections across the site have been drawn, and these arepresented in Appendix 6 to this report.

The cross-sections, which show the two original terrace arrangementsproposed by Eskom, (described as Eskom Terrace Option 1 and Eskom

Terrace Option 2) plus the additional terrace option proposed by Eskomafter the completion of the field work (described as Eskom Terrace Option3), have been drawn along the following grid lines :

Grid Line CGrid Line GGrid Line EGrid Line IGrid Line 2Grid Line 4

Based on the sections it can be seen that there are a variety of subsoil

profiles below the finished level of each of the three proposed terraces.On the “cut” side of the terrace there are two major conditions:-

3.2.1.1 Very Soft Rock Mudstone is Exposed at Finished Terrace Level

This condition can be seen in areas typical of BH 1, BH 2 and TPI2. At these locations the excavation for the terrace should bestopped at finished terrace level, and structures should be foundeddirectly in the exposed bedrock.

Due to the potential for slaking of the very soft rock and soft rock

mudstone, excavations should not be exposed to the atmospherefor any length of time, and blinding should be installed immediatelythe excavation has been completed, cleaned and approved by thesite resident engineer.

3.2.1.2 Residual Mudstone / Ferricrete / Hillwash is Exposed at FinishedTerrace Level

This condition can be seen in areas typical of BH 3, BH 4, BH 5, TPE2, TP E3, TP E4, and TP G4. At these locations the proposedfinished terrace level will be underlain by varying thicknesses ofactive soils, and in order to reduce the potential heave at terrace

level to acceptable levels, the terrace will need to be over-excavated to a depth dependent on:-



GEOSTRATEGIES 15


the depth of the underlying active soilsthe activity level of the underlying soilsthe quantity of heave that the civil design is able to tolerate.

The over-excavated material must be carted to spoil and replacedwith an inert soil mattress, construction and founding details ofwhich are given below in para 3.2.2.2.

From the results of the laboratory tests, the approximatepercentage ratio of each activity class were as follows ;

Medium Activity 70%High Activity 25%Very High Activity 5%

i.e. an average activity of around medium to medium/high

In addition, the tests show that the in situ moisture content of themudstones has an average value of around 18%, which suggeststhat the actual heave will be less than predicted from theconventional formulae.

Consequently it is recommended that the subsoils at the siteshould generally be regarded as being “medium active” in nature,as far as the practical interpretation of the heave potential isconcerned.

Tables 11, 12 and 13 indicate the likely heave at terrace level

assuming either, medium, high or very high soil activity, for a rangeof thicknesses of both underlying active material and inert soilmattress.

Table 11 : Calculation of anticipated heaveActive horizon below finished terrace level - 9m thick

Thickness of inertfill(m)

Thickness ofremaining active

insitu soils(m)

MediumActivity

HighActivity

Very HighActivity

0.00 9.00 50 100 200

2.00 7.00 25 50 100

4.00 5.00 10 20 40

6.00 3.00 5 10 20

7.00 2.00 3 5 10



GEOSTRATEGIES 16



Thickness of inertfill

(m)

Thickness ofremaining active

insitu soils(m)

MediumActivity

HighActivity

Very HighActivity

0 6 50 100 200

2 4 20 40 80

4 2 5 10 20


Thickness of inert

fill(m)

Thickness of

remaining activeinsitu soils(m)

Medium

Activity

High

Activity

Very High

Activity

0 4 30 60 120

2 2 15 30 60

3 1 5 10 20

On the “fill” side of the terrace (this will be only applicable to TerraceOptions 1 and 2) there is only one condition, i.e. imported fill overlyingactive in-situ soils. However, the depths of both the imported fill and the

underlying active soils will vary considerably and the procedure describedin section 3.2.1.2 will again be applicable. Table 14 indicates the likelyheave at terrace level assuming either, medium, high or very high soilactivity, for a range of thicknesses of both underlying active material andinert soil mattress/imported fill.

Table 14 : Calculation of anticipated heaveFill Section - In situ active horizons with thickness of either 9, 6 & 3m

Thickness of inertfill(m)

Thickness ofactive insitu

soils(m)

MediumActivity

HighActivity

Very HighActivity

0.00 9.00 50 100 200

2.00 9.00 25 50 100

4.00 9.00 15 30 60

6.00 9.00 5 10 20

0.00 6.00 50 100 200

2.00 6.00 25 50 100

4.00 6.00 10 20 40

6.00 6.00 5 10 20

0.00 3.00 40 80 160

2.00 3.00 20 40 80



GEOSTRATEGIES 17

Table 14 : Calculation of anticipated heaveFill Section - In situ active horizons with thickness of either 9, 6 & 3m

Thickness of inertfill

(m)

Thickness ofactive insitu

soils(m)

MediumActivity

HighActivity

Very HighActivity


4.00 3.00 10 20 40

6.00 3.00 5 10 20

It should be noted that the final design of the proposed platforms whichwill be undertaken by Eskom’s design team will have to take into accountnot only the heave predictions given above, but also the unit costs ofexcavation in soft, excavation in rock, and the costs of importing in fill.

.3.2.2 Structures

It is recommended that the following foundation solutions be adopted forall the structures at the site :

3.2.2.1 Solution 1 - Conventional Foundations

Structures be founded on the Mudstone or Sandstone Bedrock, ofsoft rock or better consistency. An allowable bearing pressure of650 Kpa can be assumed. This would be applicable on the “cut”side of the terrace in the areas of BH 1, BH 2 and TP I2. Due tothe potential for slaking of the very soft rock mudstone, excavationsshould not be exposed to the atmosphere for any length of time,and blinding should be installed immediately the excavation hasbeen completed, cleaned and approved by the site residentengineer.

In all other areas of “cut”, structures should either be founded on a soilmattress, constructed of inert, granular materials, or on drivendisplacement piles, where structures are particularly sensitive tomovement.

3.2.2.2 Solution 2 - Soil Mattress

The following guidelines are given for the construction of a soilmattress :

The depth of over-excavation must be determined by thedesign engineer as discussed in para 3.2.1.2

During excavation, all excavated materials should be spoilt,as they are generally of poor quality.

Once the bulk excavation is completed, the base of theexcavation should be ripped and re-compacted to 90% ModAASHTO.

Material of at least G7 quality is then imported, and placedin 150mm layers and compacted to 95% Mod AASHTO.



GEOSTRATEGIES 18


Foundations should then be excavated back into the fill, tothe minimum depth compatible with the provision ofservices into the building (approx 0.5m). Allowable bearingcapacities for foundations whose minimum width does not

exceed 1000mm would then be 125 kPa.

Normally loaded floor slabs should be placed on theimported compacted backfill.

3.2.2.3 Solution 3 - Driven Displacement Piles

Where structures are particularly sensitive to settlement or likely toexert substantial loads onto the foundations, the use of drivendisplacement piles should be considered. The piles will have to bedesigned by a specialist, and must take into account the heavepotential of the residual mudstone soils. This will generally require

the formation of a enlarged base and additional tensile reinforcingin the pile shaft, to overcome the heave uplift.

3.2.2.4 Solution 4 - Terrace Fill

On the “fill” side of the terrace, the fill must be constructedaccording to the following procedure:-

All topsoil containing organic material must be removed andcarted to temporary stockpile, for later use in landscaping

The exposed material, which will typically be hillwash should

be ripped to a depth of 150mm and recompacted to 90%Mod AASHTO

Material of at least G7 quality is then imported, and placedin 150mm layers and compacted to 95% Mod AASHTO.

Foundations should then be excavated back into the fill, tothe minimum depth compatible with the provision ofservices into the building (approx 0.5m). Allowable bearingcapacities for foundations whose minimum width does notexceed 1000mm would then be 125 kPa.

Normally loaded floor slabs should be placed on theimported compacted backfill.

Specific care must be taken at the boundary between “cut” and “fill”to ensure that there is always a sufficient depth of either soilmattress, or terrace fill below the proposed structures to limit theheave to acceptable levels.

3.2.3 Roads and Terraces

The results of the CBR and Road Indicator tests have been used to

classify the in-situ soils to determine their suitability for use in theconstruction of terraces and pavement layers. According to the TRH 14specification, the bulk of the materials tested classified as G10 or worse in



GEOSTRATEGIES 19


quality, with only two samples of a mixture of hillwash and ferricrete,classifying as G9 quality.

For this reason, all general fill materials will need to be imported from

either a local borrow area, or the closest commercial source. ( Refer tosection 2.9. )

The Eskom design engineer will therefore need to design the terraces withthe minimum quantity of fill, due to the fact that all fill materials will need tobe imported. This may result in the platform having a number of steps, tooptimize the design.

Due to the poor quality of the in situ soils it is recommended that uponcompletion of the construction of the works, a 150mm wearing courseshould be placed across those areas of the terrace where a soil mattresshas not been installed, and where regular vehicular access will be

required. A typical specification for such material would be a granular soilwith a PI < 6 and a CBR >15 at 93% Mod AASHTO

3.2.4 Excavation Classification

During the field work, only two of the twenty one trial pits interceptedmudrock bedrock.

In these trial pits it was apparent that the bedrock rapidly progressed tosoft rock quality, and the excavator was only able to penetrate deeper dueto the strongly bedded, and fractured nature of the bedrock.

In TP I2, the excavator was able to penetrate 2.5 m into the mudrock. Thetrial pit profiles indicate approximately 500mm of very soft rock,progressing into about 1500mm of very soft to soft rock, with refusaloccurring in the remaining 500mm of soft rock.

In TP G2 this reduced to a penetration depth of about 1.5 m. The trail pitprofile of this hole indicated about 500mm of very soft rock, with theremaining 1000mm being of soft rock consistency.

All excavations within the residual mudstone / sandstone soils can beregarded as “soft” according to SABS 1200 D: Earthworks, while the upper

1.5 - 2.0m of mudstone bedrock should be regarded as “Intermediate“.This will be in materials of very soft to soft rock consistency.

Excavations beyond these depths should be regarded as “Hard“, with thebedrock falling within the soft rock, bordering on medium hard rockcategory. In the bill of quantities, allowance should be made for blasting ofthese materials.

In practise however, it may turn out that these bedrock materials can beripped with the aid of a large, heavy, powerful bull dozer, and removedusing a powerful tracked excavator, especially due to the bedded nature ofsedimentary deposits..

In the field, the 22 ton tracked excavator experienced definite refusal at thedepths given above, but in a large open excavation as that required, such



GEOSTRATEGIES 20


refusal may well be somewhat deeper.

Preliminary calculations have been undertaken to assess the quantity of“rock” that would be excavated for each of the three terrace options

currently being considered by Eskom, the results of which are given intable 11. In preparing both these calculations, and the cross sectionsincluded elsewhere in the report, the following assumptions were made:-

Bedrock surfaces are interpolated on a linear basis between datapoints,Bedrock surfaces have been extrapolated where no data pointsexistRock quantities include all categories of rock from Very Soft Rockto Medium Hard RockThe plan area of the terrace is as shown on your drawing entitled“Mthatha Layout”

In considering these results it is also essential to recognise that theiraccuracy is also limited by the large spacing (in excess of 50m in places)between the boreholes/trial holes which represent the data points.

Table 15 : Estimated rock volumes generated from cut operations

Terrace Option Estimated Rock Volume (m2)

1 25250

2 13250

3 69300

3.2.5 Stormwater Management

The sub-station will be built in an area of generally high annual rainfall, andthe terraces should be designed to rapidly remove stormwater, especiallyduring periods of prolonged rainfall.

Should Terrace Option 3 be selected, where the terrace is generally all in“cut”, particular care will need to be taken to ensure that surface isappropriately graded to ensure that storm water is able to the drain awayfrom the terrace, and that the terrace does not act as a sump in which rain

water can collect.

Due to the strong sheetwash that could develop upslope of the site, a cutoff drain should be built at the top of the slope of the excavation.

3.2.6 Chemical Tests on the soils

.pH, Conductivity, and Corrosivity tests were carried out on a number ofrepresentative soil samples. The results of these testa are summarized inTables 8 & 9, with the actual test sheets presented in Appendix 4.

Generally the tests indicate the soils to be moderately corrosive, requiringgood concrete design and construction methods to be applied, to preventany later deterioration of concrete and steel structures.



GEOSTRATEGIES 21


It should be noted that test results from TP G8, I4, & C4, indicated thepotential of more aggressive soil conditions to that generally foundelsewhere.

3.2.7 Stability of excavations

During the field work, no water seepage was encountered in any of thetrial pits and no collapse of the sidewalls occurred in any of the holes,even though some were excavated to final depths of 5m.

In periods of heavy rainfall, the stability of the side slopes of theexcavation could decrease considerably, should the sub-soils becomesaturated.

It is our understanding from Eskom, that they do not want to artificiallysupport or clad any of the excavated faces, and would rather grass them

to provide long term protection from erosion.

Taking these consideration into account, it is recommended that all cutslopes be relatively flat with a nominal 1.5 horizontal : 1.0 vertical slope. Inorder to limit possible slaking taking place on the exposed excavationfaces, it is recommended that these face be top-soiled and hydro-seeded.This needs to be done soon after excavation by a competent contractor.

3.2.8 General

As mentioned in section 2.1.2 above all the test holes were looselybackfilled on completion of the fieldwork. It is therefore recommended that

prior to construction all trial pit locations be identified and be properly backfilled with suitable material compacted to 90% Mod AASHTO density at+2% to - 1% of optimum moisture content.

4 CONSTRUCTION MONITORING

4.1 Excavation Inspection

It is recommended that all foundation excavations be inspected by a competentperson prior to placing any concrete.

4.2 Control Testing

Regular checks on the quality and compaction of the backfill to the terracesshould be made.



GEOSTRATEGIES 22

REFERENCES

Jennings, J.E. Brink, A.B.A and Williams, A.A.B. "Revised Guide to Soil profiling for CivilEngineering Purposes in Southern Africa" - Civil Engineer in South Africa , January1973.

Joint Structural Division of the South African Institution of Civil Engineers and theInstitution of Structural Engineers. “Code of Practice for Foundation and Superstructuresfor Single Storey Residential Buildings of Masonry Construction” Johannesburg 1995.

Jennings, J and Knight, K. (1975). A guide to construction on or with materials exhibitingadditional settlement due to “collapse” of grain structure. Proceedings of the SixthRegional Conference on Soil Mechanics and Foundation Engineering. Durban.

Partridge T.C, Wood C.K, Brink A.B.A. “Priorities for urban expansion within the PWV

metropolitan region: The primacy of geotechnical constraints.” - South African

Geographical Journal. Vol 75. 1993.

South African Institute of Engineering Geologists. “Guidelines for Urban Engineering

Geological Investigations.” - SAIEG, 1998.

TRH 14, "Guidelines for Road Construction Materials" National Institute for Transport and

Road Research. Pretoria. 1985.

“A Guide to Practical Geotechnical Engineering in Southern Africa” July 1995, Franki

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