Semester Two 2024 Mock Exam
Faculty of Engineering
EXAM CODES: CIV2242
TITLE OF PAPER: GEOMECHANICS 1
EXAM DURATION: 50 minutes writing time
READING TIME: 10 minutes (30 mins scan/upload time)
Question 1 (8 marks)
A new playground is proposed to build in Monash University and preliminary site investigations were carried out by a geotechnical company. A soil sample from the proposed area was collected and tested in the laboratory. The following results were found from the laboratory experiments.
Porosity (n) = 0.3; Specific gravity of the soil solids (Gs) = 2.7; and moisture content (w) = 10%
The density of water is 1000 kg/m3
(a) Calculate the bulk density of the soil sample (3 marks)
(b) Calculate the saturated density of the soil sample (3 marks)
(c) Determine the mass of water to be adder to 10m3 of soil for full saturation (2 marks)
Question 2 (15 marks)
The following are the results of consolidated-undrained triaxial tests on undisturbed soils retrieved from a 4-m-thick saturated clay layer in the field (γsat = 19 kN/m3, γw = 9.81 kN/m3).
In the first stage, the drainage valve was opened to allow water to drain under the applied cell pressure, so that excess pore water pressure developed during the first stage was fully dissipated. In the second stage, the drainage valve was closed and the axial load was increased.
Given the following triaxial test data at failure;
|
Test
|
Cell pressure (kPa)
|
Deviator stress (kPa)
|
Pore pressure (kPa)
|
|
1
|
50
|
180
|
20
|
|
2
|
100
|
250
|
50
|
Answer the following questions by plotting Mohr’s circle of stresses on a graph paper. Attach your graph paper along with your answer sheet.
(a) Draw and clearly label the effective stress Mohr’s Circles at the failure. (8 marks)
(b) Determine the effective shear strength parameters. (2 marks)
(c) Estimate the compressive strength of the sample if it were tested in anunconfined situation. (3 marks)
(d) Estimate the shear strength in the middle of the clay layer. (2 marks)
Question 3 (12 marks)
Figure 1 presents the ground profile of a construction site intended for a multistorey building. It has three layers, each differing in height and hydraulic conductivity. H 1=2m, H2=2.5m, H3=3m, k1= 3x10-4cm/s, k2= 2x10-2cm/s,and k3= 5x10-5cm/s.
Figure 1:Ground profile
(a) Determine the equivalent hydraulic conductivity for vertical flow. (5 marks)
(b) Determine the equivalent hydraulic conductivity for horizontal flow. (5 marks)
(c) Compute the ratio of equivalent hydraulic conductivity for horizontal flow to vertical flow. Discuss the concept of seepage and hydraulic conductivity. (2 marks)
Question 4 (15 marks)
The Metro Tunnel is the first step towards a 'metro-style' rail network for Melbourne with the 'turn up and go' train services that are the hallmark of the world's great cities such as London, New York, Hong Kong and Singapore. This project consists of twin nine-kilometre tunnels, which will deliver a new dedicated pathway through the heart of the city for Melbourne's busiest rail lines. The tunnel will be constructed under the Yarra River. The cross section shown in Figure 2 is taken through part of Melbourne along the centre line of a proposed rail tunnel. Please answer the following questions related to this cross section.
Figure 2. Cross section along the centre line of a proposed tunnel
(a) Name the oldest geological unit shown in the cross section. (2 marks)
(b) For the geological unit you have selected as the oldest, describe its material, mass and geomorphological properties. (3 marks)
(c) In order to maintain suitable gradients, the 6 m diameter TBM tunnel must pass through this section between elevations of -14 m and -20 m. List 3 geotechnical issues that you would need to consider for a tunnel at this elevation. (6 marks)
(d) For construction,a concrete batching plant will need to be constructed on the ground surface on the south side of the Yarra River (between Ch 220 m and 450 m). The batching plant will occupy an area of 50 m by 50 m and is expected to apply 200 kPa. Describe the possible ground response due to the loading applied. (4 marks)