QUESTION 4
NATIONAL UNIVERSITY OF SINGAPORE FACULTY OF ENGINEERING
EXAMINATION FOR (Semester 1: 2012-2013)
CE5104 - UNDERGROUND SPACE
Nov/Dec 2012 - Time allowed: 2.5 hours
INSTRUCTIONS TO CANDIDATES 1.
This examination paper contains THREE(3) questions and comprises FIVE(5) printed pages.
2.
Answer ALL THREE(3) questions.
3.
Questions DO NOT carry equal marks.
4.
This is an “OPEN BOOK” examination.
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CE5104
Question 1 [33 marks] (a) A granite rock has the following properties: unconfined compressive strength qu = 100 MPa, RQD = 60%, slightly rough joints at average 2 m spacing, and moderate ground water pressure. (i) Determine the Rock Mass Rating of the rock and the corresponding rock mass class. (ii) Propose a method of tunnelling in this granite rock. Briefly explain the principle of your proposed method. [7 marks] (b) Geological survey reveals that the mean orientations of the joint sets are 320o/50o, 300o/25o and 240o/45o. It is planned to construct a tunnel of 5.5 m width with a tunnel axis of 79o. (i) Using stereographic projection technique (Fig. Q-1), evaluate the plane(s) of the potential sliding rock block stability upon tunnel excavation. (ii) If the unit weight of the rock is 26.5 kN/m3, what would be the weight of the unstable rock block. (iii) If the rock joint surfaces are cohesionless and all have the same friction angle of 35o, estimate the factor of safety of the rock block against sliding. Would the block slide? [26 marks]
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CE5104
Fig. Q-1
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CE5104
Question 2 [33 marks] (a) Tests on a siltstone site in Singapore reveal that the following properties: RQD = 30%, three tightly healed, hard and rough joint sets under moderate water pressure. The engineer estimates the stress reduction factor SRF as 2.5 for the given tunnelling overburden and site conditions. (i) Determine the Q value of the rock. Hence determine the Hoek and Brown empirical rock strength parameters m and s before tunnel excavation. (ii) If the rock would be completely fractured after tunnel excavation, what would be the broken rock strength parameters mr and sr? [8 marks] (b) A tunnel with an equivalent radius of 6 m is to be excavated in the siltstone. The insitu stress magnitude at the tunnel elevation is 1 MPa. (i) If the rock has an unconfined compression strength qu of 20 MPa, a rock mass modulus of 2000 MPa, a Poisson’s ratio of 0.25 and a unit weight of 20 kN/m3 after tunnel excavation, plot the rock support reaction curve. You only need to show one elastic point and two plastic points. (ii) Shotcrete and rock bolt are to be installed to stabilise the excavation. The tunnel has deformed by an estimated 8 mm prior to the installation of shotcrete and rock bolts. The shotcrete is 30 mm thick, has an compressive strength of 40 MPa, a modulus of eleasticity of 20,000 MPa and a Poisson’s ratio of 0.25. The rock bolts have a length of 6 m, diameter of 25 mm, a modulus of elasticity of 207,000 MPa and to be installed at a grid spacing of 3 m by 3 m. The bolt head/deformation constant is 0.143 m/MN and the ultimate capacity of each bolt is 300 kN. Evaluate whether the combined system of shortcrete and rock bolt would stablise the tunnel excavation. [25 marks]
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CE5104
Question 3 [34 marks] As a consultant, you are required to advise a contractor on the face stability and the required face pressure to limit ground settlement for a tunnel with an excavated diameter of 6.5m. The tunnel is to be constructed by an earth pressure balance machine (EPBM). The site investigation (SI) data show that the tunnel with the axis level at 19.5m below the ground surface, will be constructed in a fluvial clay layer of the Kallang Formation. The laboratory tests have established that the design undrained shear strength of the clay is 35 kPa and that the average unit weight of the overburden above the tunnel is 17 kN/m3. To cater for the traffic at the surface, a surcharge loading of 25 kPa is required to be considered in the assessment. You can assume that there is no unsupported length, i.e. P/D = 0 and ignore the volume loss due to over-cut closure. You can use the results of the centrifuge tests on model tunnels and take the surface trough width parameter, K = 0.5. (i)
What will be your advice to the contractor on the face stability (stable or unstable tunnel face) without any face pressure? Justify your advice with calculations. [6 marks]
(ii)
The tunnel is to be constructed close to historical buildings under conservation. It is required that the immediate settlement at the ground surface (without consolidation) must be limited to 20mm. What is the minimum face pressure that should be applied at the tunnel axis level so that the maximum settlement will not exceed the limit? [8 marks]
(iii) With the volume loss achieved from (ii) above, what is the predicted settlement at 3m above the tunnel crown? [6 marks] (iv) A building is located on one side of the tunnel. The front of the building is parallel to the centre line of the tunnel and is offset by 10m from the tunnel centre. The rear of the building is also parallel to the centre line of the tunnel and is offset by 20m from the tunnel centre. Assuming a green field condition, calculate the average tensile strain of the building if the design volume loss is 3%. [8 marks] (v)
Your contractor has asked you not to impose any conditions or measures to limit consolidation settlement as he believes that consolidation settlement will be relatively uniform and will not cause any damage to the historical buildings at the ground surface. Please explain why the contractor is not entirely correct in his understanding of the impact of consolidation settlement due to EPBM tunnelling on buildings. [6 marks]
- END OF PAPER -
EXAMINATION FOR (Semester 1: 2012-2013)
CE5104 - UNDERGROUND SPACE
Nov/Dec 2012 - Time allowed: 2.5 hours
INSTRUCTIONS TO CANDIDATES 1.
This examination paper contains THREE(3) questions and comprises FIVE(5) printed pages.
2.
Answer ALL THREE(3) questions.
3.
Questions DO NOT carry equal marks.
4.
This is an “OPEN BOOK” examination.
.../2
-2-
CE5104
Question 1 [33 marks] (a) A granite rock has the following properties: unconfined compressive strength qu = 100 MPa, RQD = 60%, slightly rough joints at average 2 m spacing, and moderate ground water pressure. (i) Determine the Rock Mass Rating of the rock and the corresponding rock mass class. (ii) Propose a method of tunnelling in this granite rock. Briefly explain the principle of your proposed method. [7 marks] (b) Geological survey reveals that the mean orientations of the joint sets are 320o/50o, 300o/25o and 240o/45o. It is planned to construct a tunnel of 5.5 m width with a tunnel axis of 79o. (i) Using stereographic projection technique (Fig. Q-1), evaluate the plane(s) of the potential sliding rock block stability upon tunnel excavation. (ii) If the unit weight of the rock is 26.5 kN/m3, what would be the weight of the unstable rock block. (iii) If the rock joint surfaces are cohesionless and all have the same friction angle of 35o, estimate the factor of safety of the rock block against sliding. Would the block slide? [26 marks]
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CE5104
Fig. Q-1
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CE5104
Question 2 [33 marks] (a) Tests on a siltstone site in Singapore reveal that the following properties: RQD = 30%, three tightly healed, hard and rough joint sets under moderate water pressure. The engineer estimates the stress reduction factor SRF as 2.5 for the given tunnelling overburden and site conditions. (i) Determine the Q value of the rock. Hence determine the Hoek and Brown empirical rock strength parameters m and s before tunnel excavation. (ii) If the rock would be completely fractured after tunnel excavation, what would be the broken rock strength parameters mr and sr? [8 marks] (b) A tunnel with an equivalent radius of 6 m is to be excavated in the siltstone. The insitu stress magnitude at the tunnel elevation is 1 MPa. (i) If the rock has an unconfined compression strength qu of 20 MPa, a rock mass modulus of 2000 MPa, a Poisson’s ratio of 0.25 and a unit weight of 20 kN/m3 after tunnel excavation, plot the rock support reaction curve. You only need to show one elastic point and two plastic points. (ii) Shotcrete and rock bolt are to be installed to stabilise the excavation. The tunnel has deformed by an estimated 8 mm prior to the installation of shotcrete and rock bolts. The shotcrete is 30 mm thick, has an compressive strength of 40 MPa, a modulus of eleasticity of 20,000 MPa and a Poisson’s ratio of 0.25. The rock bolts have a length of 6 m, diameter of 25 mm, a modulus of elasticity of 207,000 MPa and to be installed at a grid spacing of 3 m by 3 m. The bolt head/deformation constant is 0.143 m/MN and the ultimate capacity of each bolt is 300 kN. Evaluate whether the combined system of shortcrete and rock bolt would stablise the tunnel excavation. [25 marks]
.../5
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CE5104
Question 3 [34 marks] As a consultant, you are required to advise a contractor on the face stability and the required face pressure to limit ground settlement for a tunnel with an excavated diameter of 6.5m. The tunnel is to be constructed by an earth pressure balance machine (EPBM). The site investigation (SI) data show that the tunnel with the axis level at 19.5m below the ground surface, will be constructed in a fluvial clay layer of the Kallang Formation. The laboratory tests have established that the design undrained shear strength of the clay is 35 kPa and that the average unit weight of the overburden above the tunnel is 17 kN/m3. To cater for the traffic at the surface, a surcharge loading of 25 kPa is required to be considered in the assessment. You can assume that there is no unsupported length, i.e. P/D = 0 and ignore the volume loss due to over-cut closure. You can use the results of the centrifuge tests on model tunnels and take the surface trough width parameter, K = 0.5. (i)
What will be your advice to the contractor on the face stability (stable or unstable tunnel face) without any face pressure? Justify your advice with calculations. [6 marks]
(ii)
The tunnel is to be constructed close to historical buildings under conservation. It is required that the immediate settlement at the ground surface (without consolidation) must be limited to 20mm. What is the minimum face pressure that should be applied at the tunnel axis level so that the maximum settlement will not exceed the limit? [8 marks]
(iii) With the volume loss achieved from (ii) above, what is the predicted settlement at 3m above the tunnel crown? [6 marks] (iv) A building is located on one side of the tunnel. The front of the building is parallel to the centre line of the tunnel and is offset by 10m from the tunnel centre. The rear of the building is also parallel to the centre line of the tunnel and is offset by 20m from the tunnel centre. Assuming a green field condition, calculate the average tensile strain of the building if the design volume loss is 3%. [8 marks] (v)
Your contractor has asked you not to impose any conditions or measures to limit consolidation settlement as he believes that consolidation settlement will be relatively uniform and will not cause any damage to the historical buildings at the ground surface. Please explain why the contractor is not entirely correct in his understanding of the impact of consolidation settlement due to EPBM tunnelling on buildings. [6 marks]
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