bauchi geotechnical soil report.docx

TECTONICS ENGINEERING AND CONSULT LTD.

GEOTECHNICAL SOIL REPORT

BAUCHI STATE UNIVERSITY

SEPTEMBER 2014

GEOTECHINICAL REPORT FOR BAUCHI STATE UNIVERSITY ALONG BAUCHI-KARRI ROAD,BAUCHI.


TABLE OF CONTENTS

Cover page ...................................................................................................................................................

Table of Contents .......................................................................................................................................1

List of Tables ...............................................................................................................................................2

List of Plates .................................................................................................................................................2

List of Figures ..............................................................................................................................................3

Executive Summary......................................................................................................................................4

CHAPTER ONE INTRODUCTION

1.1 General Background .............................................................................................................................5

1.2 Scopes of work ......................................................................................................................................5

1.3 Brief Description of the Site ………………………………………………………………6

CHAPTER TWO FIELD AND LABORATORY WORKS

2.1 Surface Exploration .............................................................................................................................8

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2.2 Subsurface Exploration .......................................................................................................................8

2.2.1 Dynamic Cone Penetrometer Test ....................................................................................8

2.2.2 Test Pitting ............................................................................................................................9

2.2.3 Hand Auger Sampling .........................................................................................................9 2.2.4 Table of test pit coordinates ..........................................................................................10

2.2.5 Pictures showing exploratory activities .........................................................................14

2.2.6 Pictures showing DCP tests .............................................................................................15

2.3 Laboratory Testing...............................................................................................................................16

2.3.1 Natural Moisture Contents................................................................................................16

2.3.2 Particle size analysis ...........................................................................................................16

2.3.3 Atterberg Limits .................................................................................................................16

2.3.4 Direct Shear Test ................................................................................................................17

2.3.5 Consolidation Test …………………………………………………………17

2.3.6 Plate showing laboratory test in progress................................................................18

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CHAPTER THREE RESULTS AND DICUSSION

3.1 Natural Moisture Contents ................................................................................................................19

3.2 Particle Size Analysis ..........................................................................................................................19

3.3 Atterberg Limits ..................................................................................................................................19

3.4 Soil Classification Criteria ..................................................................................................................20

3.5 Direct Shear Test ................................................................................................................................20

3.6 Bearing Capacity .................................................................................................................................21

3.7 Consolidation test ...............................................................................................................................22

3.8 Dynamic Cone Penetration Test Results ........................................................................................23

CHAPTER FOUR SUMMARY AND CONCLUSION

4.1 General Conclusions ..........................................................................................................................24

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4.2 Recommendations .............................................................................................................................24

4.3 Limitations ..........................................................................................................................................24

REFERENCES ………………………………………………………………………….26

LIST OF TABLES

Table 2.1 Coordinates and Elevations of DCP/Test Pits ……………………………………..

Table 3.1 Summary of Test Results ………………………………………………………….

Table 3.2 Laboratory Bearing Capacity ………………………..……………………………..

Table 3.3 DCP Bearing Capacity Results ............................................................................................

LIST OF PLATES

Plate I: Showing Excavation/soil sampling at the Proposed Site …………………………….

Plate II: Laboratory Activities at Tectonics Engineering & Consults Ltd, Abuja ……………..

LIST OF FIGURES

Plate I: Showing the site’s Layout …………………………………………………………..

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APPENDICES ……………………………………………………………………………

APPENDIX A ................................................................................Natural Moisture Contents

APPENDIX B ..................................................................................Particle Size Analysis Graphs

APPENDIX C.................................................................................. Atterberg Limits Results

APPENDIX D ..................................................................................Direct Shear Graphs

APPENDIX E ................................................................................... Consolidation Test Plots

APPENDIX F...................................................................................DCP Bearing Capacity Plots

EXECUTIVE SUMMARY

The subsurface exploration and geotechnical engineering services requested for the site of Bauchi State University along Bauchi-Karri road

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have been completed. Subsurface conditions at the subject site were explored by 120 (One hundred and twenty) test pits taken to a depth of 1.5m each. The results of the sampling and the coordinates of its approximate locations are included in this report.Also,twenty points were tested for in-situ bearing capacity values with the use of the dynamic cone penetrometer (DCP). This report describes the subsurface conditions encountered in the explorations, presents the test results obtained, and provides recommendations for use in designing of structural foundations.

The soils underlying the site are classified based on Unified Soil Classification System (USCS) for the proposed area.Soils recording more than 50% of its particle passing sieve #200 is considered fine – grained soil while those below 50% are classified as coarse-grained. Based on this criteria, the soils underlying the site are predominantly coarse – grained. The group symbol of the soil is SM with group name silty sand with gravel.

The soil strenght values were determined with the use of Direct Cone Penetrometer (DCP) and the allowable bearing pressures determined by direct shear tests in the lab. The values from the DCP range from 66kn/m2 and 418kn/m2 while the allowable pressures from the direct shear tests range from 60kn/m2 and 240kn/m2. It is to be noted that areas nearer to the streams are the ones with the lower bearing values as informed by their moisture content and their particle size gradations.Based on the results above,60kn/m2 is therefore recommended as the bearing capacity of the site and can be used for design purposes.

The compressibility assessment of the soil indicates a range of coefficient compressibility range of 0.011m2/MN and 0.38m2/MN.Areas with low compressibility values fall under the category of “Very slightly compressible” materials while those above 0.35m2/MN are “Very highly compressible” soils.As a result,those areas nearer to the streams are very highly compressible and structures to be built in such areas recquire rigorous analysis and standard construction methods as primary settlement should be anticipated on the areas.

Based on the results of our findings, the following are hereby recommended:

A bearing capacity value of 60kN/m2 at 1.5m depth was established for the site and can be used for design purpose.

A dry unit weight of 18.6 kN/m3 may be used for design purpose.Page 6


The foundation types to be used depend on the complexity of the intended structures and anticipated loadings.The engineering judgement of a registered structural engineer is of importance since the strenght values and their corresponding compressibilities vary due to the size of the site.Isolated pad foundations would suffice for the areas with low compressibilities while foundation beams and raft foundation would be recquired in places with low bearing capacities and high compressiblities.Tables showing locations and their values are included in this report.

CHAPTER ONE

INTRODUCTION

1.1 General Background

Successful engineering projects often involves the use of engineering principles in the appropriate manner which in turn answers concerns such as safety and economy. Such concerns includes and is not limited to a proper understanding of site conditions on which projects are to be built. To this end, Tectonics Engineering & Consults Ltd was commisioned to investigate the proposed land belonging to Bauchi State University along Bauchi-Karre road Bauchi for site exploration and characterisation.This process involved a two-part program of tests performed on the site insitu properties ,also known as field tests and laboratory analysis .

Samples were taken at detailed positions and locations and further logged on which series of tests were performed on in the laboratory.Various field methods were employed in acquiring relevant data on the site vis-a-vis Dynamic Cone Penetration Test and Hand Auger method along with Test Pitting in collecting the soil samples.

The present assessment is conducted to present in details the findings on the underlying soil properties of the proposed Bauchi state university for the purpose of design and execution by employing relevant and reliable techniques. One hundred and twenty test pits were sampled and properly

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logged and twenty points were tested for in-situ bearing capacities determination.Each split-barrel sample was tested to determine the moisture content, Atterberg limits tests were performed on a representative soil sample to aid in soil classification. All laboratory testing was performed in general accordance with appropriate standards. The results of these laboratory tests are reported on approriate tables including their respective sample location.

The DCP test equally gave on indication of the site underlying soil as the penetrations recorded against each hammer blow was found to be low with depths of refusal ranging between 0.8m and 2.7m.

1.2 Scopes of Work

To conduct Dynamic Cone Penetrometer Test (DCP) on twenty locations of the site for in – situ bearing capacity determinations.

To obtain undisturbed/disturbed samples through accessible exploration methods by test pits and sampling.

To evaluate by conducting laboratory tests the physical properties and strength of the soils obtained for both disturbed and undisturbed samples from the test pits.

1.3 Brief Description of the Site

The proposed site is located in Bauchi state along Bauchi-karri road in the northern part of the country. The site was covered with vegetation at the time this exploration was conducted. The study area has a gentle, undulating topography characterized by relatively high ground from the northern part that slopes gently towards the southern part of the area.

The area lies within the Borno basin and it is underlain by Pleistocene sedimentary rocks of the Chad Formation, which consists of yellow, grey clay, fine to coarse-grained sand with intercalations of sandy clay, gravels and diatomites. The Chad Formation is underlain by the basement rocks at the periphery of the basin and underlain by Kerrikerri Formation and Fika Shale towards the shallow part of the basin and towards the basin centre respectively.

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The soil formation in the project area is predominantly coarse-grained with a high content of gravel,silt and clay in varying combinations resulting in medium to low strength values.

SITE LAYOUT/SATELLITE IMAGERY

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CHAPTER TWO

FIELD AND LABORATORY WORKS

2.1 Surface Exploration

The investigation involved surface exploration. The consultant’s team traversed the entire site and the surroundings noting its features and existing structures.

2.2 Subsurface Exploration

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The field crew visited the site in August, 2014 for field explorations, logging and sampling. The explorations included surface, accessible exploration methods by test pits and sampling and DCP. Various soil samples were collected. These samples were examined visual manually for possible identification and were transported to the Laboratory for further analysis.

The twenty (20) points of the DCP were found to record refusal at a minimum depth of 0.8m and maximum of 2.7m. It was therefore evident that the sites at the areas farther from the streams have low DCP values indicating high strenght while those close to the streams show low strenght values.

2.2.1 Dynamic Cone Penetrometer (DCP) Test

Dynamic Cone Penetration (DCP) Test involves driving a solid cone of 600 into the ground, using repeated blows of a hammer with a fixed mass of 8Kg falling through a distance of 575mm having maximum diameter of 20mm. The hammer strikes an anvil which is rigidly fixed to the rods which are of smaller diameters than the cone, and transmit the hammer energy to it.

As the cone was being driven into the ground, the number of blows required to drive each increment (typically 100 mm) was being recorded. The point of refusal was recorded and the test terminated at depths where hammer blows greater than 50 caused less than 100mm penetration. The blow count is inputted in to appropriate software to provide a more-or-less continuous profile of penetration resistance with depth and consequent determination of the bearing capacity usually at 0.5m depth interval.

It is to be noted that the rods are generally quite short, and as each new rod was added they are usually being turned through one or more revolutions, in order to reduce friction.

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2.2.2 Test Pits

The test pits were generally located at 120 locations within the site. The pits were excavated by hand in order that the soil could be examined in situ and samples taken. Topsoil was stripped from the pit area before the start of the work and was stockpiled separately until completion.

2.2.3 Hand Auger Sampling

Hand auger was used by two men, who press down on the cross-bar as they rotate it thus advancing the hole. Once the auger was full, or has collected sufficient material, it was brought back to the surface and the soil removed.

The hand auger samples were recommended for the borrow pit areas where the samples were collected at 1.0 – 2.0m maximum depths.

It is rule of thumb that in stiff or very stiff clays, hand-auger progress will be very slow, and the depth of boring may have to be limited to about 5 m or less. When such clays contain gravel, cobbles or boulders it will not normally be possible to advance the hole at all. In uncemented sands or gravels, it will not be possible to advance the hole below the water table, since casing cannot be used and the hole will collapse either on top of the auger (which makes it difficult to recover the auger from the hole) or when the auger is being removed. Only samples of very limited size can be obtained from the hole.

Table 2.1 presents the coordinates and elevations of the test locations

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Table 2.1 Coordinates of DCP/Test Pits

BAUCHI STATE UNIVERSITYTP

NOS NORTHINGS EASTINGSELEVATIO

N (m) REMARKTP1 10° 22' 78.1" 009°55' 92.9" 565 Not far from roadTP2 10° 22' 7.78" 009°55' 8.86" 568TP3 10° 22' 7.55" 009°55' 8.31" 567TP4 10° 22' 7.36" 009°55' 6.89" 564TP5 10° 22' 7.06" 009°55' 3.67" 563TP6 10° 22' 6.66" 009°55' 3.32" 564TP7 10° 22' 7.41" 009°55' 2.83" 568TP8 10° 22' 7.91" 009°55' 3.62" 567TP9 10° 22' 8.43" 009°55' 4.35" 569TP10 10° 22' 5.43" 009°55' 5.43 " 571TP11 10° 22' 9.46" 009°55' 6.31" 570TP12 10° 22' 9.99" 009°55' 7.62" 567TP13 10° 22' 0.45" 009°55' 7.94" 566TP14 10° 23' 0.98" 009°55' 8.79" 569TP15 10° 23' 0.84" 009°55' 9.44" 564TP16 10° 23' 1.57" 009°55' 9.64" 561TP17 10° 23' 1.46" 009°55' 8.47" 565TP18 10° 23' 1,.29" 009°55' 7.93" 565TP19 10° 23' 0.89" 009°55' 7.40" 566TP20 10° 23' 0.46" 009°55' 6.45" 568TP21 10° 23' 0.05" 009°55' 5.67" 568TP22 10° 22' 9.91" 009°55' 4.85" 569TP23 10° 22' 9.50" 009°55' 3.77" 569TP24 10° 22' 9.40 " 009°55' 2.81" 564TP25 10° 22' 9.04" 009°55' 2.37" 558TP26 10° 22' 9.92" 009°55' 2.52" 560TP27 10° 23' 0.16" 009°55' 3.12" 561TP28 10° 23' 0.68" 009°55' 3.56" 560TP29 10° 23' 1.10" 009°55' 4.18" 562TP30 10° 23' 1.30" 009°55' 4.51" 562TP31 10° 23' 1.68" 009°55' 5.08" 563TP32 10° 23' 2.00" 009°55' 5.95" 563TP33 10° 23' 2.67" 009°55' 6.67" 561TP34 10° 23' 3.01" 009°55' 7.33" 560

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TP35 10° 23' 2.43" 009°55' 7.59" 560TP36 10° 23' 2.32" 009°55' 8.19" 559TP37 10° 23' 2.32" 009°55' 8.68" 564TP38 10° 23' 2.63" 009°55' 9.04" 560TP39 10° 23' 3.20" 009°55' 9.86" 557TP40 10° 23' 0.04" 009°55' 0.04" 556TP41 10° 23' 3.96" 009°55' 0.46" 553TP42 10° 23' 4.18" 009°55' 0.02" 555TP43 10° 23' 4.29" 009°55' 9.42" 559TP44 10° 23' 4.42" 009°55' 8.67" 558TP45 10° 23' 4.27" 009°55' 7.62" 560TP46 10° 23' 4.05" 009°55' 6.47" 563TP47 10° 23' 3.95" 009°55' 6.02" 564TP48 10° 23' 3.31" 009°55' 4.92" 561TP49 10° 23' 3.21" 009°55' 3.60" 558TP50 10° 23' 3.10" 009°55' 2.92" 556TP51 10° 23' 3.08" 009°55' 2.52" 556 Close to footpathTP52 10° 23' 3.32" 009°55' 1.85" 553TP53 10° 23' 3.64" 009°55' 1.59" 552TP54 10° 23' 3.85" 009°55' 1.93" 552TP55 10° 23' 3.96" 009°55' 2.29" 553TP56 10° 23' 4.14" 009°55' 2.76" 551TP57 10° 23' 4.27" 009°55' 3.13" 553TP58 10° 23' 4.70" 009°55' 3.53" 553TP59 10° 23' 5.19" 009°55' 4.48" 552TP60 10° 23' 5.43" 009°55' 4.61" 552TP61 10° 23' 5.95" 009°55' 4.86" 552TP62 10° 23' 6.13" 009°55' 5.02" 552TP63 10° 23' 6.43" 009°55' 5.26" 551TP64 10° 23' 6.70" 009°55' 5.53" 551TP65 10° 23' 6.98" 009°55' 5.84" 551TP66 10° 23' 7.18" 009°55' 6.08" 551TP67 10° 23' 7.35" 009°55' 6.44" 549TP68 10° 23' 7.59" 009°55' 6.65" 555 Close to rockTP69 10° 23' 7.91" 009°55' 7.08" 555TP70 10° 23' 8.22" 009°55' 7.85" 558

TP71 10° 23' 6.75" 009°55' 7.85" 558 close to rock

TP72 10° 23' 7.04" 009°55' 7.70" 557 close to rockTP73 10° 23' 7.18" 009°55' 8.25" 554TP74 10° 23' 7.13" 009°55' 8.43" 553TP75 10° 23' 7.14" 009°55' 8.96" 550TP76 10° 23' 7.44" 009°55' 9.02" 551

TP77 10° 23' 7.02" 009°55' 9.64" 551 Close to streamTP78 10° 23' 7.21" 009°56' 0.24" 551TP79 10° 23' 7.44" 009°56' 0.57" 553 Road track

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TP80 10° 23' 7.77" 009°56' 0.46" 552 Road trackTP81 10° 23' 8.02" 009°56' 0.31" 551TP82 10° 23' 7.85" 009°55' 9.89" 551TP83 10° 23' 8.02" 009°55' 9.64" 550

TP84 10° 23' 7.92" 009°55' 9.20" 550 Close to track roadTP85 10° 23' 8.10" 009°55' 8.75" 551TP86 10° 23' 7.93" 009°55' 8.44" 552TP87 10° 23' 8.59" 009°55' 7.75" 552TP88 10° 23' 8.67" 009°55' 7.33" 551TP89 10° 23' 8.72" 009°55' 7.04" 551

TP90 10° 23' 9.22" 009°55' 7.83" 549 Close to rock

TP91 10° 23' 89.57" 009°55' 7.71" 547 Close to rockTP92 10° 23' 9.65" 009°55' 7.13" 548TP93 10° 23' 9.39" 009°55' 6.62" 549TP94 10° 24' 0.07" 009°55' 6.78" 547TP95 10° 24' 0.07 " 009°55' 6.39" 546TP96 10° 24' 0.66" 009°55' 6.53" 547 stream and rock

TP97 10° 24' 1.07" 009°55' 6.90" 544 Close to rockTP98 10° 24' 1.12" 009°55' 7.23" 543 Close to rockTP99 10° 24' 1.02" 009°55' 7.55" 545

TP100 10° 24' 0.39" 009°55' 7.25" 548TP101 10° 24' 0.37" 009°55' 7.94" 548

TP102 10° 24' 0.63" 009°55' 8.04" 547 Close to track roadTP103 10° 24' 1.45" 009°55' 8.77" 546TP104 10° 24' 2.20" 009°55' 8.78" 543

TP105 10° 24' 2.57" 009°55' 8.92" 542 Close to track roadTP106 10° 24' 2.67" 009°55' 9.36" 541TP107 10° 24' 9.36" 009°55' 9.77" 542TP108 10° 24' 2.19" 009°56' 0.02" 544TP109 10° 24' 1.93" 009°55' 0.39" 544TP110 10° 24' 0.09" 009°56' 0.09" 544TP111 10° 24' 1.27" 009°55' 9.71" 545TP112 10° 24' 0.84" 009°56' 0.07" 545TP113 10° 24' 0.46" 009°56' 0.45" 546TP114 10° 24' 0.04" 009°56' 0.85" 547TP115 10° 24' 9.74" 009°56' 1.08" 549TP116 10° 24' 1.33" 009°56' 1.33" 550TP117 10° 24' 9.76" 009°56' 1.87" 549TP118 10° 24' 9.24" 009°56' 0.25" 551TP119 10° 24' 9.19" 009°55' 9.31" 548TP120 10° 24' 9.04" 009°55' 8.93" 549

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BAUCHI STATE UNIVERSITYTP

NOS NORTHINGS EASTINGSELEVATION

(m) REMARKSPOINT 1 10° 23' 2.69" 009° 55' 7.34" 562

POINT 2 10° 23' 3.89" 009°55' 7.09" 562 Close to footpathPOINT 3 10° 23' 3.79" 009°55' 6.36" 563POINT 4 10° 23' 3.39 009°55' 6.19" 563POINT 5 10° 23' 4.06" 009°55' 5.86" 562POINT 6 10° 23' 3.97 009°55' 4.43" 556POINT 7 10° 23' 4.29" 009°55' 3.86" 554POINT 8 10° 23' 4.71" 009°55' 3.53" 553POINT 9 10° 23' 5.51" 009°55' 3.34" 550POINT

10 10° 23' 6.12" 009°55' 4.05 " 550POINT

11 10° 23' 6.49" 009°55' 4.58" 549POINT

12 10° 23' 6.56" 009°55' 5.31" 541POINT

13 10° 23' 6.46" 009°55' 6.12" 553POINT

14 10° 23' 6.16" 009°55' 6.61" 560POINT

15 10° 23' 6.68" 009°55' 6.99" 551POINT

16 10° 23' 5.40" 009°55' 7.13" 560POINT

17 10° 23' 5.17" 009°55' 77.6" 559POINT

18 10° 23' 4.67" 009°55' 8.38" 558 close to gullyPOINT

19 10° 23' 4.09" 009°55' 9.01" 558 close to gullyPOINT

20 10° 23' 3.00" 009°55' 7.95" 560

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Field team digging for

test pit samples.

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Hand auger used to extract

samples .

Plate I PICTURES Showing Exploratory Activities on the Site

In-situ bearing capacity

determination test in

progress.

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Engineer taking notes of number of

blows recorded.

Plate 2 PICTURES Showing Field tests in progress on the Site

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2.3 Laboratory Testings

The following tests were conducted on the collected samples for the purpose of soil classification and design:

Natural moisture content Particle size analysis Atterberg limits (Liquid limit, plastic limit) Direct shear test Consolidation

2.3.1 Natural moisture content

Water is present in most naturally occurring soils. The amount of water expressed as a proportion by mass of dry solid particles known as the moisture content has a profound effect on soil behaviour. Moisture content is regarded as a guide to classification of natural soils and is measured for most samples used for most field and laboratory tests.

2.3.2 Particle size analysis

The individual solid particles in a soil can have different sizes and shapes, and these characteristics also have a significant effect on its engineering behaviour. Therefore, geotechnical engineers often assess the distribution of particle sizes in a soil and the shapes of the particles in the soil.

Particle size analysis expresses quantitatively the proportion by mass of the various sizes of the particles present in the soil. A distribution analysis is a necessary index test for soils especially coarse-grained soils, in that it presents the relative proportions of different sizes of particles. From this, it is possible to tell whether the soil consists of predominantly gravel, sand, silt or clay sizes and to a limited extent which of the sizes range is likely to control the engineering properties.

2.3.3 Atterberg Limits

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Liquid limit, plastic limit and shrinkage limit are known as the Atterberg Limits. These are used to assess the plasticity of a fine-grained soils and its consistency at various moisture contents.

The liquid limit is the empirically established moisture content at which a soil passes from the liquid state to the plastic state. It provides a means of classifying a soil especially when the plastic limit is also known.

The plastic limit is the empirically established moisture content at which a soil becomes too dry to be plastic. It is used together with the liquid limit to determine the plasticity index which when plotted against the liquid limit on the plasticity chart provides means of classifying a cohesive soil.

Shrinkage due to drying is significant in clays but less so in silts and sands. These tests enable the shrinkage limit of clays to be determined. i.e the moisture content below which a clay ceases to shrink. They also provide ways of quantifying the amount of shrinkage likely to be experienced by clays in terms of shrinkage ratio, volumetric shrinkage and linear shrinkage.

2.3.4 Direct Shear Test

The drained shear strength (effective stress analysis) is of most importance for granular soils. The shear strength of granular soils is often measured in the direct shear apparatus, where a soil specimen is subjected to a constant vertical pressure while a horizontal force is applied to the top of the shear box so that the soil specimen is sheared in half along a horizontal shear surface. By plotting the vertical pressure versus shear stress at failure, the effective friction angle as well as effective cohesion can be obtained.

2.3.5 Consolidation Test

The consolidation test (also known as oedometer test) is the primary laboratory test used to study the settlement and expansion behaviour of

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soils. The consolidation test should only be performed on undisturbed soil specimens, or in the case of studies behaviour, on specimens compacted to anticipated field and moisture condition.

In order to evaluate the suitability of a foundation or earth structure, it is necessary to design against both bearing capacity failure and excessive settlement. For foundations on cohesive soils, the principal design criterion is typically the latter - the control of expected settlements within the limits considered tolerable for the structure. As a result, once allowable foundation displacements have been established, the estimate of total settlement over the service life of the structure is a major factor in the choice of foundation design.

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Plate 3: Laboratory Activities at Tectonics Engineering & Consults Ltd, Abuja.

CHAPTER THREE

RESULTS AND DISCUSSION

All the laboratory tests were conducted in the Soil and Material laboratory of Tectonics Engineering & Consults Ltd, Abuja. The tests were performed in accordance with relevant BS Codes of practice and are as presented below:

3.1 Natural Moisture Content

The soil specimens collected from the site were carefully preserved for the determination of the moisture contents. The test was conducted following the specifications spelt in BS 1377: Part 2: 1990, where the oven drying method was used. The range of the values obtained from the test is of the order of 3.3% - 12.8% for the test pits.

Table 3.1 shows the summary of the results while Appendix A presents the detail results.

3.2 Particle size analysis

The test was conducted in accordance with BS 1377 part 2: 1990. The detailed results are presented in Appendix B.

The gradation test results indicated that the bulk of the soils to be predominantly coarse - grained as evident from the estimate of the percentage passing sieve #200 even though a few fall in the category of fine-grained soils. These generally range between 4.4% and 68.4%. These according to USCS are largely considered coarse – grained soils.

The summary of the gradation test results are presented in Table 3.2; while the details are shown in the Appendix B.

3.3 Atterberg Limits

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These were also performed in line with the specification of BS 1377 part 2: 1990. The results are presented in Appendix C.

The liquid limit test was conducted using the Casagrande liquid limit apparatus. The majority of the results obtained for the test pit samples showed non – plastic and low to medium plasticity values in consonant with the sieve results.

3.4 Soil Classification Criteria

The soils underlying the site are classified based on Unified Soil Classification System (USCS) for the proposed building area. The former assumes soil having liquid limit greater than 50 and/or plasticity index in excess of 30 to be of high plasticity. While soil recording more than 50% of its particle passing sieve #200 is considered fine – grained soil. Based on these criteria, the soils underlying the site are predominantly coarse - grained . The group symbol of the soil is SM which is silty sand with gravel.

3.5 Direct Shear test

The direct shear test was conducted in accordance with BS 1377: Part 7: 1990. The cohesion for the test pits recorded 1.2KN/m2 to 39.2 KN/m2 while angle of repose are between 5.50 and 21.40. Detailed results of the test are presented in Appendix E. But the summary is as presented in Table 3.2 below.

3.6 Bearing Capacity

The estimate of the bearing capacity of the collected soil specimens became necessary having obtained the requisite parameters from the analysis conducted so far. In order to avoid the attainment of limit

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state of the structures due to foundation failure, efforts are made to come up with allowable bearing pressures for various trial pits to ensure that the shear stresses in the soils do not exceed the shear strength of the soils in question.

To achieve this, the following bearing capacity equations for shallow foundations for a uniform vertical loading assuming a general shear failure as developed by Terzaghi (1943) which are the most commonly used equations are as given below:

qult=Q ult

BL=c N c+γ D f Nq+

12γB N γ...............................for strip footing

qult=c N c(1+0.3BL

)+γ Df N q+0.4 γB N γ.....for square/rectangular footings

qult=1.3c N c+γ Df N q+0.3 γB N γ...................for circular footing

N c=(Nq−1)cotφ

N q=tan2(450+φ2 )e(π tan φ)

N γ=1.5(N q−1) tanφ

Where qult = ultimate bearing capacity (kN/m2)c’ = cohesion of the soil underlying the footing (kN/m2) B = width of footing (m) L = length of footing (m) γ = total unit weight of soil (kN/m3) Df= vertical distance from ground surface to bottom of footing (m)Nc, Nq, Nץ= dimensionless bearing capacity factors

qult= Qult/BL

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Where Qult= vertical concentric load causing a general shear failure of the underlying soil (kN/m2)

Allowable bearing capacity is given as:

qall=qnetF

+γ D f

Where F = factor of safety which is usually taken as 3.

The Bearing Capacity values obtained for all the Test Pits are shown in the table below:

Table 3.2: Test Pits Allowable Bearing Pressure from Laboratory Results

TP NO.

Depth (m)

C' (KN/m2) φ'(0) γb

(KN/m3) Nc Nq Nγqnet

(KN/m2) FOS qall

(KN/m2)

5 1.5 21.2 7.4 22.0 7.30 1.95 0.18 220 3 10613 1.5 6.9 21.4 22.0 16.23 7.36 3.74 395 3 16515 1.5 39.2 5.5 22.0 6.65 1.64 0.09 315 3 13817 1.5 12.5 24.8 18.6 20.43 10.44 6.54 607 3 23018 1.5 4.8 25.7 21.0 21.78 11.48 7.57 545 3 21346 1.5 1.2 21.9 21.0 16.77 7.74 4.07 306 3 13357 1.5 24.8 11.6 21.0 9.09 2.87 0.57 321 3 13860 1.5 3.8 8.6 19.1 7.76 2.17 0.27 93 3 6068 1.5 6.6 22.6 18.6 17.57 8.31 4.57 389 3 158

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76 1.5 14.6 17.7 21.0 12.87 5.11 1.97 368 3 15481 1.5 25.8 20.2 21.0 15.02 6.53 3.05 624 3 24093 1.5 18.2 9.6 21.0 8.17 2.38 0.35 226 3 107

118 1.5 24.4 7.3 19.1 7.27 1.93 0.18 233 3 106

3.7 Consolidation

The consolidation test performed was one – dimensional consolidation test and was conducted in accordance with clause 3 of BS 1377: Part 5: 1990. The consolidation test results show the soils to be highly compressible iin some areas and slightly compressible in other areas.Attached in this report are results of the oedometer tests.The values of the ratio of cc/1+e0 are found to range between 0.011 and 0.38. The detailed results of the tests are as outlined in Appendix G.

3.8 Dynamic Cone Penetrometer (DCP) Test

The trends of the Bearing Capacity obtained from the DCP generally fluctuate with depth in most cases.A value of 60kN/m2 is determined for the site.This in line with standard practice may be considered for design purpose which may prove adequate to safely support the anticipated load from the building .

Table 3.2: Bearing Capacity Determination from DCPDepth (m) 0.5 1.0 1.5 2.0 2.5

P1 21 127 198P2 92 56 66 102 137P3 46 75 145P4 108 268 355P5 27 105P6 68 74 112 159P7 65 119 255P8 70 164P9 92 150 219

P10 78 94 190

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P11 21 161P12 158 173 231 303P13 58 172 287P14 87 151 206 285P15 40 105 183P16 65 98 190P17 68 170 269P18 183 358 419P19 250 464P20 98 214 369 411

CHAPTER FOUR

SUMMARY AND CONCLUSION

4.0 General Conclusions

The geotechnical investigation for the site of Bauchi State University along Bauchi –karri road was successfully completed by Tectonics Engineering and consults and the results are detailed in this report.

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The soils underlying the site are classified based on Unified Soil Classification System (USCS) for the proposed building area; this assumes soil having liquid limit greater than 50 and/or plasticity index in excess of 30 to be of high plasticity. While soil recording more than 50% of its particle passing sieve #200 is considered fine – grained soil. Based on these criteria, the soils underlying the site are predominantly coarse – grained because they generally fall in the latter category. The group symbol of the soil is SM which is silty sand With gravel .The compressibility assessment of the soil suggests highly compressible property therefore all structures are expected to be designed to structural adequacy in line with standard engineering practices and procedures. Also, primary consolidation settlement is anticipated.

4.1 Recommendations Based on the results of our findings, the following

are hereby recommended:

A bearing capacity value of 60kN/m2 at 1.5m depth was established for the site.

A dry unit weight of 18.6kN/m3 may be used for design purpose. The foundation types to be used depend on the complexity of the

intended structures and anticipated loadings.The engineering judgement of a registered structural engineer is of importance since the strenght values and their corresponding compressibilities vary due to the size of the site.Isolated pad foundations would suffice for the areas with low compressibilities while foundation beams and raft foundation would be recquired in places with low bearing capacities and high compressiblities.Tables showing locations and their values are included in this report.

4.2 Limitations

Finally, it should be noted that the results and recommendations of this report are solely the based on the in – situ tests conducted and

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the collected soil samples within the days of the site visit and assuming that the subsurface conditions do not significantly deviate from those encountered. Geotechnical investigation involves gathering and assimilation of limited facts about the nature of subsoil in order to understand and predict their nature in a particular site and under certain conditions. Hence, results obtained are only directly relevant to the subsoil conditions at the time the investigation was conducted. As a result, any interpretation or recommendation given in this report is based on judgment and experience and not on the greater knowledge of the entire site conditions.

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REFERENCES:

1. D. P. Donald, M. R. Yeung and W. A. Kitch; (2011): Geotechnical Engineering, Principles and PracticesPublisher: Pearson, Second Edition. California.

2. C. A. Kogbe; (1989): Geology of NigeriaPublisher: Rock View (Nigeria) Limited, Second Revised Edition. Jos, Nigeria.

3. BS 1377: Parts 2,3,4,5 & 7 of 1990

4. BS 8004 of 1986 and BS 5930 of 1992.

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APPENDICES

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bauchi geotechnical soil report.docx

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