J.05_Geo-Technical_Report_Tshekedi_Crescent.pdf

Soils Engineers Botswana (Pty) Ltd

Geotechnical Investigation Report, Proposed Double storey buildings on Plot 2483, Tshikedi

Crescent, Gaborone

Prepared for: Prepared by:

AMA Projects Botswana (Pty) Ltd Soils Engineers (Pty) Ltd P O Box 1127 P O Box 203008 Mogoditshane Gaborone se_1905_rpt Date of Issue:Apr-19

Geotechnical Investigation Report, Proposed Double storey building on Plot 2483, Tshikedi Crescent, Table of Contents

Page

INTRODUCTION 3

Preamble Database Objectives

FACTUAL REPORT 3

Programme of Work Field work Site description Site geology Hydrology Observations Laboratory and Field Test Results

INTERPRETIVE REPORT 4

Geotechnical appraisal Conclusions Design Solutions Construction Problems Additional Investigations

CONSTRUCTION MONITORING 8

Excavation Inspection Control Testing

REFERENCES

Appendix 1 – Sketch Plan of Site

Appendix 2 – Test Pit Profiles and DCP’s

Appendix 3 - Laboratory Results

Appendix 4 – Photos

Introduction

Preamble

Soil Engineers Botswana (Pty) Ltd. was appointed by AMA Projects Botswana to undertake a site investigation on Plot 2483, Tshikedi Crescent, Gaborone. The proposed structures are double storey masonry buildings together with the associated access road and parking areas.

Database

The Client provided Soils Engineers Botswana (Pty) Ltd with a sketch plan showing the location of the proposed building and trial pits were located as outlined in the sketch plan.

Objectives

The objectives of the investigation were to complete a geotechnical survey of the site giving:-

The soil/rock profiles below the site, to 3.00m or refusal.

The nature and extent of near surface and outcropping rock strata.

Comments on the presence of groundwater.

Detailed foundation recommendations for the proposed structure.

Comment on any perceived geotechnical problems, which may affect either the design or construction of the structure.

Factual Report

Programme of Work

Field Work

The following works were undertaken on the 7th March 2019.

4 pits were excavated by machine to 3.00m or refusal and profiled using the revised

Jennings, Brink and Williams’s method, (1990).

4 DCP’s were undertaken for 1.0m to provide information on the Allowable Bearing

Pressure of the material.

4 samples were undertaken for Indicator testing to determine the nature of the material in the area falling within the likely zone of the foundations.

2 samples were undertaken for CBR testing to determine the compaction properties of the material on site.

1 sample was undertaken for chemical test.

1 undisturbed sample was undertaken for collapse potential test.

1 undisturbed sample was undertaken for Shear box.

On completion the pits were photographed and backfilled.

Site Description

The site is generally flat with no rock outcrops. There are existing single storey buildings on the plot. Vegetation has been cleared.

Site Geology

The area is underlain by Archaean aged Gaborone granites. They commonly form low lying outcrops and when covered by soil often have a ferricrete layer between the uppermost soil horizon and the residual granite. Depth of weathering is variable.

Hydrology

There are no natural watercourses on site and no seepage or water table was encountered.

Observations

Trial pits were placed as outlined in the sketch plan. See sketch plan in Appendix 1. The graphical profiles of the pits and DCP results are given in Appendix 2.

The characteristics of the soil type occurring within the site are summarised in Table 1a and b below. Table 1b indicates the irregular nature of the depth to refusal.

Table 1a

Origin Thickness

(m)

Type Consistency - Allowable

Bearing Pressures*/#

(kPa)

Topsoil 0.15-0.28

Ave 0.21

Silty sand <75-75* <50-50#

Fine colluvium 0.0-1.37

Ave 0.54

Silty sand 75-150* 50-100#

Pedogenic

0.0-0.52

Ave 0.32

Ferricrete gravel 75-400* 75-400#

Residual soil 0.0-1.35

Ave 0.77

Highly to completely weathered ferrugeneous granite

75-400* 75-400#

Termination/Refusal 0.95-3.00, average 1.84m

* DCP/Field observation Allowable Bearing Pressure.

# Allowable Bearing Pressure from SABS 0161 1980.

Table 1b: Depth to refusal

TP No Depth to refusal (m) Type

1 0.95 Ferricrete

2 3.00 Residual granite

3 1.35 Residual granite

4 2.05 Residual granite

Laboratory and Field Test Results

All laboratory testing and field-testing data results are attached as Appendix 3.

Interpretive Report

Geological Appraisal

For more accurate identification and classification purposes Indicator Testing (Particle Size distribution and Atterberg Limits) were undertaken on a disturbed sample from the material below the likely founding horizon.

These are summarised in Table 2 below.

Table 2

TP

No

Material Depth

(m) fines sand

GM PI Activity Type

1 Pedogenic 0.65-0.95 8 29 2.30 NP Low A.1.a(0)

2 Fine colluvium 0.28-1.65 33 64 0.98 6.7 Low A.2.4(0)

3 Residual 0.66-1.35 15 35 2.10 NP Low A.1.b(0)

4 Residual 1.02-2.05 11 33 2.20 NP Low A.1.b(0)

CBR testing was undertaken and the results are summarised in Table 3 below:

Table 3 Summary of Laboratory Testing (CBR)

TP No Material Depth (m) M.D.D O.M.C CBR Value @

90/93/95/98%

Classification

2 Fine colluvium 0.28-1.65 1952 7.7 19/21/23/29 G7/SG15

4 Residual 1.02-2.05 1978 7.9 35/43/47/50 G6/SG15

CBR testing of the material on site indicates Good to Reasonable compaction properties classified as G6/SG15-G7/SG15 according to TRH14.

Summary of Soil Profiles

Topsoil

The uppermost horizon is Topsoil of silty sand varying in thickness from 0.15-0.28m averaging approximately 0.21m across the site. The consistency of the horizon is very loose to loose.

Fine colluvium

Below the topsoil in TP1, 2 and 4 is fine colluvium of silty sand varying in thickness from 0.0-

1.37m averaging approximately 0.54m across the site, classified as A.2.4(0). The consistency of the horizon is loose to medium dense.

The fine content of the sample is classified as having Low Plasticity according to Figure 31 of

BS5930. Plotting of the Weighted Plasticity Index against Clay Content using van der Merve’s method predicts Low Swell Potential.

Pedogenic

Below the fine colluvium is ferricrete gravel in matrix of sand (not fully penetrated at TP1) varying in thickness from 0.0-0.52, averaging approximately 0.32m classified as A.1.a(0).

The consistency of the pedogenic horizon is generally loose to medium dense becoming very dense with depth.

Residual

Below the ferricrete (TP3-4) and fine colluvium (TP2) is the residual granite excavating as silty/clayey gravel, which was penetrated for 0.0 to 1.35m averaging approximately 0.77m.

The consistency of the granite gravel is generally loose to medium dense becoming very dense at pit base.

Allowable Bearing Pressure

The consistency of the topsoil within the profile is loose to very loose with an Allowable

Bearing Pressure of <75-75kPa. The consistency of the fine colluvium within the profile is loose to medium dense with an Allowable Bearing Pressure of 75 to 150kPa. The consistency of both the pedogenic and residual material is generally loose to medium dense becoming very dense with depth or at pit base with Allowable Bearing Pressures of 75-400kPa.

Chemical Test

The soil profile on site is not normally considered to contain pyrite or any other sulphate bearing minerals. Testing was therefore guided by BRE (2005) Section 5.1.1. A soil sample was undertaken at TP1 and returned results as shown in Table 4 below.

Table 4: Chemical Test Results

TP No Depth pH Sulphate

SO4

2-

(mg/kg)

Chloride

Cl- (mg/kg)

Interpretation

1 0.65-0.95 7.05 31.4 28.12 Slightly Alkalinic

According to BRE Special Digest 1 (2005) Table C1 results indicate that the material is classified as AC-1s and considered not aggressive to concrete and steel.

Collapse Potential The Fine colluvium has pinhole structures, which is indicative of collapse potential. An undisturbed sample was undertaken at TP2 (0.70-1.00m) for collapse potential determination and returned a result of 12.72% at 200kPa, which according to Jennings and Knight is classified as ’’Severe Trouble’’.

Shear Box test

The objective of the shear box test is to determine the effective shear strength parameters of the soil, the cohesion (C’) and the angle of internal friction (ɸ’).these value can be used for calculating the stability of slopes. Samples were collected to test for the shear box test and results are summarised in Table 5 below and given in full in Appendix 3:

Angle of Internal Friction

Angle of internal friction for a given soil is the angle on the graph (Mohr's Circle) of the shear stress and normal effective stresses at which shear failure occurs. Angle of internal friction can be estimated using the following Relationship between , and standard penetration number for sands, (from Meyerhof 1956, Foundation Engineering Handbook). See Table 5 below.

Table 5: Summary of Shear Box (Natural angle of repose results)

TP No Depth(m) Angle of Internal friction(ɸ’) Cohesion (C’)

2 0.70-1.00 23.6 20.2

Biotic Activity

There are no signs of ongoing biotic activity.

Conclusions

The following conclusions are drawn from our investigation.

It is concluded that the plot has an irregular soil profile consisting of four horizons as follows: Topsoil, Fine colluvium, Pedogenic and Residual. It is further concluded that the depth to refusal varies from 0.95-3.00m bngl.

It is concluded that the fine’s content of the fine colluvium has Low Plasticity and

Low Swell Potential.

It is concluded that the fine colluvium has Reasonable compaction properties classified as G7.

It is concluded that the fine colluvium has collapse potential originating from pinhole structures, classified as “Severe Trouble” and is not suitable as a founding horizon without some improvements.

It is concluded that the pedogenic and residual horizons which occur from 0.20-1.65m averaging approximately 0.75m are suitable as a founding horizon.

It is concluded that these horizon are generally loose to medium dense with Allowable

Bearing Pressures of 75-150kPa becoming very dense with depth or at pit base

(Allowable Bearing Pressure of 400kPa). It is therefore concluded that irregular refusal depths are the best competent horizons.

It is concluded that the residual material has Good compaction properties classified as

G6.

It is concluded that using the SAICE Code of Practice 1995 the site is classified at a founding depth of 0.90m b.n.g.l as “C1”.

Recommendations

Design Solutions

Based on the information obtained during this investigation and the design information submitted the following recommendations are offered:-

Structures

It is recommended that all vegetation and roots should be removed from the foundation excavations and footprint of the building.

It is recommended that the uppermost horizon be removed down to 250mm b.n.g.l.

and the excavated material be sent to stockpile for landscaping.

It is recommended that foundation excavations be taken down to refusal depths from

0.95-3.00m into the ferricrete or residual granite. The excavated fine colluvium should be stockpiled for landscaping and the ferricrete/residual granite be stockpile for use as fill.

It is recommended that Irregularities in the ferricrete should be filled with mass concrete.

It is recommended that, if required, a suitable inert fill, i.e. G6 (or material from stockpile) be placed in layers not exceeding 150mm compacted to 95% Mod.

AASHTO at OMC+2 to -1% to the required founding level of approximately 0.90m b.n.g.l.

It is recommended that lightly reinforced strip foundations be used on this site placed at approximately 0.90m b.n.g.l. on a surface of a 50mm thick blinding, with a minimum width of 700mm and thickness of 300mm.

It is recommended that imported inert material i.e. G7, (or material from stockpile) be used under the floor slab compacted to 93% Mod. AASHTO at OMC +2 to -1% in layers not exceeding 150mm.

It is recommended that the maximum Bearing Pressure be 150kNm2.

Construction Problems

No construction problems are anticipated.

Construction Monitoring

Excavation Inspection

It is strongly recommended that a competent person inspect foundations before the foundations are cast to ensure that the material at the base of the excavation is uniform.

Control Testing

Regular testing of the concrete used in the construction process is recommended.

Reviewed and approved by:

9-Apr-19

A. Golding

B.Sc. Hons Geology, Pr. Nat. Sci., M.S.A.I.E.G.

For and on behalf of:

Soils Engineers Botswana (Pty) Ltd

References

Guidelines for Soil and Rock logging Geotechnology Workshop (1990) SAIEG

SABS 0161 1980 Rev. 1984 Foundation Design

Code of Practice-Foundations and Superstructures for single storey Residential

Buildings of masonry construction. SAICE 1995

Jennings, J and Knight, K. (1975). A guide to construction on or with materials exhibiting additional settlement due to “collapse” of grain structure. Proceedings of the

Sixth Regional Conference on Soil Mechanics and Foundation Engineering. Durban.

BS5930: 1981 Code of Practice for Site Investigations

Appendix 1

Sketch Plan of Site

Proposed Founding Level

Resultant void be filled with G6 material placed in layers not exceeding 150mm

Termination

Depth 3.00m

Refusal Depth

1.35m

Refusal Depth

2.05m

Refusal Depth

0.95m

Appendix 2

Test Pit Profile and DCP Results

Starting Depth: 0.00 Starting Depth: 1.00 Starting Depth: 0.00

No of Penetration Depth Pen/blow CBR No of Penetration Depth Pen/blow CBR No of Penetration Depth Pen/blow CBR blow s mm m mm blow s mm m mm blow s mm m mm

0 110 0.11 0 0 110 1.11 0 0 110 0.11 0

5 425 0.43 63 5 180 1.18 14 5 300 0.30 38

10 535 0.54 22 10 220 1.22 8 10 305 0.31 1

15 620 0.62 17 15 265 1.27 9 15 410 0.41 21

20 670 0.67 10 20 310 1.31 9 20 445 0.45 7

25 685 0.69 3 25 350 1.35 8 25 470 0.47 5

30 690 0.69 1 30 395 1.40 9 30 485 0.49 3

35 690 0.69 0 35 440 1.44 9 35 500 0.50 3

40 40 470 1.47 6 40 515 0.52 3

45 45 510 1.51 8 45 525 0.53 2

50 50 550 1.55 8 50 545 0.55 4

55 55 580 1.58 6 55 555 0.56 2

60 60 610 1.61 6 60 555 0.56 0

65 65 630 1.63 4 65

70 70 655 1.66 5 70

75 75 675 1.68 4 75

80 80 690 1.69 3 80

85 85 695 1.70 1 85

90 90 710 1.71 3 90

95 95 720 1.72 2 95

100 100 735 1.74 3 100

110 110 740 1.74 1 110

120 120 740 1.74 0 120

130 130 130

140 140 140

150 150 150

160 160 160

170 170 170

180 180 180

190 190 190

200 200 200 v.loose >75 loose 30-75 medium dense 12.5-30 dense 12.5-5.0 v.dense 5.0-2.0

Checked by:

DYNAMIC CONE PENETROMETER

Plot 2483, Tshikedi CrescentClient: Project:AMA Projects

Pit No 1 Pit No 3Pit No 2

Date 07-Feb-19 07-Feb-19Date07-Feb-19Date

Staff Housing at Letlhakane

Starting Depth: 0.00 Starting Depth: 0.00 Starting Depth: 0.00

No of Penetration Depth Pen/blow CBR No of Penetration Depth Pen/blow CBR No of Penetration Depth Pen/blow CBR blow s mm m mm blow s mm m mm blow s mm m mm

0 150 0.15 0 0 0 0.00 0 0 0 0.00 0

5 290 0.29 28 5 5

10 348 0.35 12 10 10

15 385 0.39 7 15 15

20 410 0.41 5 20 20

25 445 0.45 7 25 25

30 473 0.47 6 30 30

35 520 0.52 9 35 35

40 570 0.57 10 40 40

45 610 0.61 8 45 45

50 630 0.63 4 50 50

55 640 0.64 2 55 55

60 650 0.65 2 60 60

65 650 0.65 0 65 65

70 70 70

75 75 75

80 80 80

85 85 85

90 90 90

95 95 95

100 100 100

110 110 110

120 120 120

130 130 130

140 140 140

150 150 150

160 160 160

170 170 170

180 180 180

190 190 190

200 200 200 v.loose >75 loose 30-75 medium dense 12.5-30 dense 12.5-5.0 v.dense 5.0-2.0

Checked by:

Date 07-Feb-19 Date 07-Feb-19 Date 07-Feb-19

Client: AMA Projects Project: Proposed Construction of Police Station and

DYNAMIC CONE PENETROMETER

Pit No 4 Pit No Pit No

Appendix 3

Laboratory Results

Appendix 4

Photographs

TP1

TP2

TP3

TP4

Site Rehabilitation