Ground and Water Studies 2 Tutorial 1

Question 1

i) Explain the purpose of “Mohr’s stress circles”. Sketch a Mohr’s stress circle diagram for the situation where a triaxial compression test on a cylindrical clay sample has a hypothetical shear plane inclined at an angle of ??0 from the horizontal. Ensure that you label all axes and key points on your sketch diagram and denote the direction of the principal stresses so that your diagram clearly indicates the purpose of having plotted the diagram.

(5 marks)

ii) A series of ‘quick’ unconsolidated undrained triaxial tests were conducted on a sample of saturated clay with the results obtained being as follows:

Test Number 1 2 3

Cell Pressure (kN/m2) 50 100 200

Vertical Stress at Failure (kN/m2) 131 186 285

Using the graph sheet on the next page (Fig. 1.1) and constructing Mohr’s stress circles, determine the shear strength parameters of the soil sample and thus describe the soil strength using geotechnical terminology.

(8 marks)

iii) Describe the full range of shear strength testing methods available for testing clays in the field and in the laboratory. Ensure that your discussions justify the use of specific test methods for specific soil types and also states the advantages and limitations of the test methods selected.

(4 marks)

iv) Using load versus deformation plots justify what you would expect to observe when carrying out laboratory shear box test on both a loose and a very dense sand.

(3 marks)

Total 20 marks

1

Fig. 1.1

2

Question 2

i) A flexible foundation of length 6m and breadth 4m is to exert a uniform pressure of 150 kN/m2 on the surface of a 10 m layer of soil with a bulk unit weight of 21kN/m3. Using Fig. 2.1, determine the immediate settlement under the centre of the foundation if the elastic soil stiffness (E) is assumed to be 3 MN/m2.

(6 marks)

ii) A flexible foundation of length 6 m and breadth 4 m is to exert a uniform pressure of 150 kN/m2 on the surface of a layer of soil of assumed infinite thickness with a bulk unit weight of 21 kN/m3. Using Fig. 2.2, determine the total stress at a depth of 6m beneath a corner of the foundation.

(5 marks)

iii) The following results were obtained from an oedometer test on a specimen of saturated clay obtained at a depth of 2m below ground level:

Applied Stress (kN/m2) 0 25 50 100 200 400 800

Void Ratio 0.980 0.945 0.916 0.875 0.828 0.779 0.723

(a) Determine the value of mv for an effective stress range from 30kN/m2 to 150kN/m2.

(6 marks)

(b) Calculate the consolidation settlement for a 6m thick layer of this clay, when the effective stress changes from 30kN/m2 to 150kN/m2.

(3 marks)

Total 20 marks

3

Fig. 2.1

4

Fig. 2.2

5

Question 3

i) Briefly explain the following terms;

(a) Pre-consolidation pressure

(b) N-C Clay

(2 marks)

ii) Explain in detail the process referred to as ‘consolidation’ using full geotechnical reference to all parameters you deem relevant and ensure that you include the reasons for time dependency of this process in different soil types. You are required to use appropriate geotechnical terminology throughout your explanation.

(6 marks)

iii) Using Fig. 3.1 determine the total stress, pore water pressure and effective stress at each strata change and at the location of the water table and hence plot a graph to illustrate their variation with depth from ground surface to a depth of 15 m below ground level. The water table is located at a depth of 4 m below ground level within a 7 m thick deposit of sandy gravel overlying 5m of clay.

(12 marks)

Total 20 marks

Fig. 3.1

6

Question 1

i) Explain the purpose of “Mohr’s stress circles”. Sketch a Mohr’s stress circle diagram for the situation where a triaxial compression test on a cylindrical clay sample has a hypothetical shear plane inclined at an angle of ??0 from the horizontal. Ensure that you label all axes and key points on your sketch diagram and denote the direction of the principal stresses so that your diagram clearly indicates the purpose of having plotted the diagram.

(5 marks)

ii) A series of ‘quick’ unconsolidated undrained triaxial tests were conducted on a sample of saturated clay with the results obtained being as follows:

Test Number 1 2 3

Cell Pressure (kN/m2) 50 100 200

Vertical Stress at Failure (kN/m2) 131 186 285

Using the graph sheet on the next page (Fig. 1.1) and constructing Mohr’s stress circles, determine the shear strength parameters of the soil sample and thus describe the soil strength using geotechnical terminology.

(8 marks)

iii) Describe the full range of shear strength testing methods available for testing clays in the field and in the laboratory. Ensure that your discussions justify the use of specific test methods for specific soil types and also states the advantages and limitations of the test methods selected.

(4 marks)

iv) Using load versus deformation plots justify what you would expect to observe when carrying out laboratory shear box test on both a loose and a very dense sand.

(3 marks)

Total 20 marks

1

Fig. 1.1

2

Question 2

i) A flexible foundation of length 6m and breadth 4m is to exert a uniform pressure of 150 kN/m2 on the surface of a 10 m layer of soil with a bulk unit weight of 21kN/m3. Using Fig. 2.1, determine the immediate settlement under the centre of the foundation if the elastic soil stiffness (E) is assumed to be 3 MN/m2.

(6 marks)

ii) A flexible foundation of length 6 m and breadth 4 m is to exert a uniform pressure of 150 kN/m2 on the surface of a layer of soil of assumed infinite thickness with a bulk unit weight of 21 kN/m3. Using Fig. 2.2, determine the total stress at a depth of 6m beneath a corner of the foundation.

(5 marks)

iii) The following results were obtained from an oedometer test on a specimen of saturated clay obtained at a depth of 2m below ground level:

Applied Stress (kN/m2) 0 25 50 100 200 400 800

Void Ratio 0.980 0.945 0.916 0.875 0.828 0.779 0.723

(a) Determine the value of mv for an effective stress range from 30kN/m2 to 150kN/m2.

(6 marks)

(b) Calculate the consolidation settlement for a 6m thick layer of this clay, when the effective stress changes from 30kN/m2 to 150kN/m2.

(3 marks)

Total 20 marks

3

Fig. 2.1

4

Fig. 2.2

5

Question 3

i) Briefly explain the following terms;

(a) Pre-consolidation pressure

(b) N-C Clay

(2 marks)

ii) Explain in detail the process referred to as ‘consolidation’ using full geotechnical reference to all parameters you deem relevant and ensure that you include the reasons for time dependency of this process in different soil types. You are required to use appropriate geotechnical terminology throughout your explanation.

(6 marks)

iii) Using Fig. 3.1 determine the total stress, pore water pressure and effective stress at each strata change and at the location of the water table and hence plot a graph to illustrate their variation with depth from ground surface to a depth of 15 m below ground level. The water table is located at a depth of 4 m below ground level within a 7 m thick deposit of sandy gravel overlying 5m of clay.

(12 marks)

Total 20 marks

Fig. 3.1

6

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