Geomechanics 2 Report

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  • Topic: Force, Soil mechanics, Shear stress
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CIVL462 Principles of Foundation Engineering

LABORATORY REPORT

Ryan Bates 3675580

Group 13, Week 5

Date Performed: 26/03/12

Table of Contents

1. Direct Shear Test1
1.1Introduction1
1.2Test Procedure1
1.3Results and Discussion2
1.4Conclusion and Limitations4
2. Triaxial Compression Test5
2.1Introduction5
2.2Test Procedure5
2.3Results and Discussion5
2.4Conclusion and Limitations7
3. Consolidation Test8
3.1Introduction8
3.2Test Procedure8
3.3Results and Discussion8
3.4Conclusion and Limitations11
Appendix A12
Appendix B14
Appendix C16

1. Direct Shear Test

1.1Introduction

The Direct Shear Test is an investigation used by geotechnical engineers used to measure the shear strength properties of a soil. It is noted that the shear strength of a material is a term used to describe a materials ability to resist failing in the direction parallel to the applied force, and therefore an important soil property to be noted by engineers. In this lab test ‘Sydney Sand’ is used to determine the relationship between a given load and a change in cross-sectional area and also show the friction angle of the soil specimen, by testing the sample using what is commonly known as a shear box.

1.2Test Procedure

Determination of Shear Strength of a Soil – AS 1289.6.2.2 (1998)

Points to consider within the test procedure include:
The porous stones placed on the bottom and top of the soil sample not only act as a filler but help propagate a dense soil sample
The soil sample was compacted in three layers as it was placed into the shear box


1.3Results and Discussion

Given a drained soil sample of Sydney Sand of mass 139grams, at a constant density of 1930kg/m3, and the data plotted in Appendix A, the following graph has been obtained - relationship between shear force and horizontal displacement.

The dotted plots of the 5kg, 10kg, and 15kg in the above Shear force Vs Horizontal displacement graph show evidence of a peak in their curve and can easily make out where failure occurs. It is generally noted that a dense soil sample will clearly indicate a peak across its displacement whereas a loose material will continue to increase until failure. It is clear that as horizontal displacement increases (cross sectional area decreases) we can see an increase in shear force being applied to the soil sample. A dense soil sample will also have a void ratio that will increase when particles displace due to the horizontal displacement that occurs during the direct shear test. These results therefore allow us to determine the maximum value of shear stress for each case, by directly comparing results of shear force and the given area at the time, such that; 〖"max shear stress (KPa)," τ〗_n=("max horizontal force (KN)," F)/("area (" "m" ^"2" ")," A_"corrected" )

5kg load, maximum shear stress = 14.02 KPa
10kg load, maximum shear stress = 28.75 KPa
15kg load, maximum shear stress = 39.77 KPa
As expected for each increase in vertical load applied to the soil sample between the 5, 10 and 15kg masses, there is an increase in the maximum shear stress, and thus can state that the stress relationship between force and area holds true.

Thus, to analyse these results and determine whether the soil sample is dense or loose sand we need to compare the results of Shear stress Vs Normal stress, and obtain the friction angle of the material by applying Mohr’s circle theory of failure. When determining the relationship between shear stress and normal stress it is important that the results are graphed on axes of the same scale. By summarising the three tests and obtaining maximum value of shear stress we get the following data:

Weight (kg)Area (mm2)Shear Stress Max (KPa)Normal Stress (KPa)

5360014.02013.625
10360028.77827.250

15360039.77040.875

By plotting the...
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