Soil Mechanics and soil compaction Assessment

Questions Marks:100 Marks

QUESTION 1

Two consolidated, undrained (CU) triaxial tests was performed on a normal consolidated and saturated clay subject to two different confining pressures including 100 KPa and 200 KPa. The testing results can be found in the spreadsheet available on Blackboard, named ‘Triaxial tests_soil case study’. The pore pressure ?u, was measured during the test and at the failure it was 45 kPa. The dimensions of the two samples were listed in the table below. Based on the testing data and the information provided, please solve the following questions:

1.1 Plot the total stress-strain curves at the two different confining pressures.

[5 marks]

1.2 Plot the Mohr circles based on the testing data.

[3 marks]

1.3 What are the total major principal stresses at failure at two tests?

[2 marks]

1.4 What are the effective minor principal stresses at failure at the two tests?

[2 marks]

1.5 What are the effective major principal stresses at failure at the two tests?

[2 marks]

1.6 What is the effective friction angle of the soil, ?’, and undrained cohesion, C’?

[5 marks]

1.7 If the failure did occur along the theoretical failure plane, what is the angle between the failure plane and the horizontal direction, ??

[2 marks]

1.8 What is the effective normal stress on the failure plane at failure, ?n, in the test with 100KPa confining pressure?

[2 marks]

1.9 What is the effective shear stress on the failure plane at failure, ?, in the test with 100 KPa confining pressure?

[2 marks]

QUESTION 2

In a soil compaction test, the following data was collected;
Specific gravity of solids = 2.64

2.1 Draw an accurate graph which represents the dry density compaction curve for this soil and identify accurately from this graph the maximum dry density (MDD) and optimum moisture content (OMC) of the soil.

[11 marks]

2.2 Field compaction control tests carried out at 4 site locations recorded the following values; If field compaction in-situ moisture content may vary by 2?ove OR below the OMC value determined in Q2.1, and in-situ dry density must be at least 95%MDD as determined in Q2.1 above, determine which of the four field compaction control tests (1-4) meets specifications.

[8 marks]

2.3 Calculate and plot the line representing the Zero Air Voids for the soil in Q2.1, using a moisture content that varies from 13% to 22% in intervals of 3%. If one of the field control tests in Q2.2 were suspected of being incorrectly recorded, which one of tests 1-4 would it be any why?

[6 marks]

QUESTION 3

For an impermeable dam as shown below, 12 m deep water is on the one side of the dam. With the aid of the scale drawn in the diagram please solve the following questions. The coefficient of permeability of the soil is 0.015mm/s, and the saturated soil density is 1659 kg/m3

3.1 Calculate the water flow rate in m3 /day/m.

[4 marks]

3.2 Calculate the total head at location A, B and C.

[8 marks]

3.3 Calculate the pore water pressures at location A, B and C.

[7 marks]

3.4 Determine whether ‘piping’ occurs at location A, B or C

QUESTION 4

A 600 mm diameter circular flexible footing is located 1.5m below the surface. Soil conditions at the site consist of dense sand to great depth (water table is at ground level). The saturated unit weight of the soil is 20 kN/m3 , the bulk unit weight of the soil is 17 kN/m3, the unit weight of water is 10 kN/m3. The soil is clay and has a modulus of elasticity of 10MPa, an unit weight of 18 kN/m3 and a Poisson’s ratio of 0.5.

A direct shear testing was conducted on the clay sample and it was found the shear stresses at failure were 123 kPa and 196 kPa at normal stresses of 200 kPa and 400 kPa, respectively.

4.1 What is the Mohr Coulomb strength parameters for the clay soil?

[5 marks]

4.2 If the factor of safety is 3, determine the safe bearing capacity.

[10 marks]

4.3 If the footing is under uniform load of 400 kPa and footing supports can tolerate a maximum of 10mm differential settlement, determine if the design of the foundation is suitable.

[6 marks]

4.4 Determine the induced vertical stress at 2 m directly underneath the centre of the circular footing when the uniform load of 400 kPa is applied on the footing.

[4 marks]