Please two assignments in Geotechnical engineering.
GEOTECHNICAL ENGINEERING A:
Object of Experiment:
To determine the undrained shear strength of a soil using the triaxial compression test.
Theory/Apparatus:
The apparatus consists of a cell, which is filled with water under pressure; the specimen is loaded vertically, via a proving ring to measure load.
Triaxial Cell:
The vertical load on the specimen is increased until failure occurs, the vertical strain being recorded at the same time using a dial gauge. The test is repeated on different specimens from the same soil, using different values of cell pressure.
s1 = major principal stress
s3 = minor principal stress
Therefore, P/A = (s1-s3) =Deviator stress
The deviator stress is the load on the specimen, P, divided by the cross sectional area of the specimen. However, as the sample is compressed during the test, the cross sectional area will increase. Therefore, in calculating the deviator stress an allowance for the change in area must be considered.
For the calculation of deviator stress, it is assumed that the volume of the specimen remains constant and that the sample will deform as a cylinder, e.g.
Method:
1. Extrude the sample from the tube and trim to size - soil sample of 38mm diameter and 76mm long.
2. Sleeve the sample with the rubber membrane.
3. Put the sample on the pedestal at the bottom of the cell and seal with the rubber ring. Place the loading cap on top of the sample and seal with rubber ring, before securing top drainage tube.
4. Mount the cell over the sample and fill as per the Flooding Triaxial Cell checklist.
5. Set-up the test with the Clisp Studio assistant, and complete the Pressurising Triaxial Cell checklist before running the test stages.
6. When test stages are complete, end the test via Clip Studio and complete the Draining Triaxial Cell checklist.
Results and Calculations:
• Calculate the moisture content of the soil as per Appendix A.
• Calculate the results as follows:
(i) For each sample tested:
- Find the failure strain (either the final value or the 20% value); this is denoted ε.
- σ3 is the cell pressure.
• Area at failure is given by A=Vo/(Lo-X) or A= Ao/(1 - ε)
• (σ1-σ3), the Deviator Stress, is given by (Proving ring divisions at failure × constant) ÷ (Area at failure)
(ii) Plot the Mohr circle for each of the samples.
(iii) Determine the apparent cohesion (cu) and the angle of shearing resistance (φu) of the soil by using the best common tangent method
Conclusion:
• Comment on the values (cu and φu) for the sample you tested.
• Comment on the failure modes of your samples.
• Comment on the use of this test and your findings.
GEOTECHNICAL ENGINEERING B:
Object of Experiment:
To determine the angle of shear resistance and volumetric displacement of a sand using the direct shear box test.
Theory:
The shear strength of a soil may be defined as the maximum shear stress that can be applied to that soil in any direction. When this maximum has been reached the soil yields and is regarded to have failed. It should be appreciated that the shear strength of a soil is derived from the frictional resistance, F, generated from inter-particle forces, N; the pore water has no shear strength. The shear strength is a function of the total normal stress to that plane, and is given by:
τf = C + σn tan Φ
In the direct shear box the sample is caused to shear along the plane dividing the upper and lower pieces by applying a horizontal load to the upper piece while the lower piece is held in position. The load is generally applied via a proving ring, hence the load causing the sample to shear can be read directly and the shear stress, t, is the load causing shear divided by the plan area of the box.
The test is repeated several times on different specimens of the same sample using different normal loads. The results are then be plotted, to give the shear strength envelope, form which a value of Φ may be obtained.
Apparatus:
The apparatus, as shown above, comprises a square box construction in two separate pieces, an upper piece and a lower piece. The vertical normal load is applied directly through the upper pressure plate and it is divided by the plan area of the box to give the normal stress σ.
The volumetric behavior of the soil is also determined during the test by measuring the amount of horizontal displacement and vertical displacement using dial gauges.
Method:
Assemble the empty shear box as shown, without the upper and loading platens. The two halves of the box should be screwed together with the screws marked by a cross cut into the heads; the two screws marked ‘L’ should be in position but clear of the joint between the two halves. Ensure that the apparatus is moving freely on its runners. Fill the box with loose sand and level it off about 1mm below the top of the box. Place the top platen, on the sand and the loading platen on the top platen. Put the ball bearing in place and the hanger on the ball bearing. Place a weight on the hanger.
Adjust the apparatus to take up any slack, and then zero the proving ring dial gauge.
REMOVE THE TWO SCREWS HOLDING THE UPPER AND LOWER HALVES OF THE BOX TOGETHER, then screw in those marked ‘L’ until resistance is just felt: give each one a further half turn to ensure that the two halves are slightly separated so that the normal stress is being applied to the sand only, then remove them.
Switch on the motor. Record the maximum reading on the proving ring dial gauge, then switch off and slacken off the apparatus.
Dismantle the box entirely and pour all the sand back into the container.
Repeat the test three more times, increasing the mass on the hanger. Record the results on the attached sheet.
Results and Calculations:
• Calculate the values of effective normal stress (σ’) for each loading condition.
• Plot τ against horizontal movement for each loading condition.
• Determine values for shear stress at failure (τf)
• Plot a graph of τf against σ’. Draw the straight line of best fit through the origin and the points, and calculate the value of the angle of shearing resistance of the soil from the co-ordinates of any convenient point on the line.
• Plot vertical displacement against horizontal displacement.
Conclusion:
• Comment on the value of φ for the sand you tested.
• Explain the main points to the vertical vs horizontal displacement plot.
• Comment on the use of this test and your findings.