nus confidential ce5104 national university of singapore
NUS CONFIDENTIAL CE5104 NATIONAL UNIVERSITY OF SINGAPORE
CE5104 – UNDERGROUND SPACE (Semester I: AY2014/2015)
Time Allowed: 2.5 Hours
______________________________________________________________________ INSTRUCTIONS TO CANDIDATES 1. Please write your student number only. Do not write your name. 2. This assessment paper contains THREE(3) questions and comprises FIVE(5) printed pages. 3. Answer ALL questions. All questions DO NOT carry equal marks. 4. Please start each question on a new page. 5. Linear MM graph paper is provided. 6. This is an “OPEN BOOK” assessment.
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Question 1 [30 marks] (a)
A rock has the following properties: (i) unconfined compressive strength 3 MPa, (ii) rock quality designation 30%, and (iii) average joint spacing 200 mm with average opening 3 mm. If it is judged that the rock mass is subject to moderate water pressure, determine the rock mass rating of the rock. What are the rock classification, estimated standup time and minimum rock mass strength in terms of cohesion and friction angle? [6 marks]
(b)
Geological survey revealed that the mean joint orientations are 150/40, 080/67 and 000/30. The unit weight of the rock is 27 kN/m3. If the tunnel axis orientation is 000/00, determine the mode of failure of the rock and the volume of the falling/sliding block using the stereonet shown in Figure Q1. [16 marks]
(c)
Ignoring the rock joint cohesion and taking the minimum rock joint friction angle obtained from (a) above, determine the factor of safety of the block upon tunnel excavation. If the desired minimum safety factor of the block is 1.5, what would be the required bolt force to achieve the desired margin of safety? You may take that the bolts can be installed at 90o to the weakness plane. [8 marks]
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Figure Q1
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Question 2 [36 marks] (a)
A siltstone rock has rock quality designation of 50%, 3 joint sets with smooth and slightly altered joint wall contact, experiencing large inflow of water and favourable medium stress conditions. (i) Determine the Q value of the rock mass. (ii) Hence determine the Hoek and Brown empirical strength parameters, m and s, of the siltstone rock mass. [7 marks]
(b)
Upon tunnel excavation, the rock is completely fractured and broken. If the unconfined compressive strength of the rock is 6 MPa, the modulus of elasticity of the rock mass is 1 GPa, the Poison’s ratio is 0.25, the unit weight of the broken rock is 20 kN/m3, the tunnel diameter is 10 m and the insitu overburden pressure is 0.7 MPa, plot the rock support reaction curve for the roof excavation. [20 marks]
(c)
Is a 50-mm thick shotcrete sufficient to support the roof excavation? You may take the compressive strength of the shotcrete as 40 MPa, modulus of elasticity of shotcrete as 20.7 GPa, Poisson ratio as 0.25 and roof deformation before placement of shotcrete as 5 mm. [9 marks]
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Question 3 [34 marks] A tunnel of 6.5m excavated diameter is to be built under a road by a tunnel boring machine (TBM) at a depth of 23m from the surface to its axis level. The ground water table is at the ground surface. a)
If the tunnel is to be driven in the soft marine clay with an undrained shear strength of cu = 40 kPa and a unit weight of 16 kN/m3, what will be the expected volume loss if a face pressure of 100 kPa is maintained? You can assume that there is no unsupported length, i.e. P/D = 0 for the TBM in using the results of the centrifuge tests on model tunnels and ignore the volume loss due to over-cut. A surcharge of 20 kPa shall be considered for the traffic loading. [10 marks]
b)
If the tunnel is to be driven in the fluvial sand of the Kallang Formation (loose sand with SPT < 10) with a unit weight of 17 kN/m3, calculate (i) the minimum face pressure at the tunnel crown to avoid collapse; and (ii) the target face pressure at the tunnel crown to limit the volume loss to 1%. You can assume that the piezometric head inside the TBM excavation chamber will be maintained to balance the ground water pressure and that the effects of TBM tunnelling will increase the ground water o pressure by 30 kPa. The shear strength of the fluvial sand is c’ = 0 kPa and ’ = 32 . A partial factor of safety of 1.2 is to be considered for ultimate limit state calculation. A pressure variation of 20 kPa is to be considered to limit the volume loss to 1%? [10 marks]
c)
With the volume loss of 1%, what is the expected sub-surface settlement at 2m above the tunnel crown? You can assume that the trough width parameter, K = 0.5. [5 marks]
d)
Explain the forms that consolidation settlement profiles will take due to TBM tunnelling in soft clay and their impacts on building damage. [5 marks]
e)
Explain how the number of radial joints in a ring will affect the bending moments in the segments and how this can be considered in the design of the tunnel lining segments. [4 marks]
- END OF PAPER -
CE5104 – UNDERGROUND SPACE (Semester I: AY2014/2015)
Time Allowed: 2.5 Hours
______________________________________________________________________ INSTRUCTIONS TO CANDIDATES 1. Please write your student number only. Do not write your name. 2. This assessment paper contains THREE(3) questions and comprises FIVE(5) printed pages. 3. Answer ALL questions. All questions DO NOT carry equal marks. 4. Please start each question on a new page. 5. Linear MM graph paper is provided. 6. This is an “OPEN BOOK” assessment.
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CE5104
Question 1 [30 marks] (a)
A rock has the following properties: (i) unconfined compressive strength 3 MPa, (ii) rock quality designation 30%, and (iii) average joint spacing 200 mm with average opening 3 mm. If it is judged that the rock mass is subject to moderate water pressure, determine the rock mass rating of the rock. What are the rock classification, estimated standup time and minimum rock mass strength in terms of cohesion and friction angle? [6 marks]
(b)
Geological survey revealed that the mean joint orientations are 150/40, 080/67 and 000/30. The unit weight of the rock is 27 kN/m3. If the tunnel axis orientation is 000/00, determine the mode of failure of the rock and the volume of the falling/sliding block using the stereonet shown in Figure Q1. [16 marks]
(c)
Ignoring the rock joint cohesion and taking the minimum rock joint friction angle obtained from (a) above, determine the factor of safety of the block upon tunnel excavation. If the desired minimum safety factor of the block is 1.5, what would be the required bolt force to achieve the desired margin of safety? You may take that the bolts can be installed at 90o to the weakness plane. [8 marks]
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Figure Q1
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Question 2 [36 marks] (a)
A siltstone rock has rock quality designation of 50%, 3 joint sets with smooth and slightly altered joint wall contact, experiencing large inflow of water and favourable medium stress conditions. (i) Determine the Q value of the rock mass. (ii) Hence determine the Hoek and Brown empirical strength parameters, m and s, of the siltstone rock mass. [7 marks]
(b)
Upon tunnel excavation, the rock is completely fractured and broken. If the unconfined compressive strength of the rock is 6 MPa, the modulus of elasticity of the rock mass is 1 GPa, the Poison’s ratio is 0.25, the unit weight of the broken rock is 20 kN/m3, the tunnel diameter is 10 m and the insitu overburden pressure is 0.7 MPa, plot the rock support reaction curve for the roof excavation. [20 marks]
(c)
Is a 50-mm thick shotcrete sufficient to support the roof excavation? You may take the compressive strength of the shotcrete as 40 MPa, modulus of elasticity of shotcrete as 20.7 GPa, Poisson ratio as 0.25 and roof deformation before placement of shotcrete as 5 mm. [9 marks]
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CE5104
Question 3 [34 marks] A tunnel of 6.5m excavated diameter is to be built under a road by a tunnel boring machine (TBM) at a depth of 23m from the surface to its axis level. The ground water table is at the ground surface. a)
If the tunnel is to be driven in the soft marine clay with an undrained shear strength of cu = 40 kPa and a unit weight of 16 kN/m3, what will be the expected volume loss if a face pressure of 100 kPa is maintained? You can assume that there is no unsupported length, i.e. P/D = 0 for the TBM in using the results of the centrifuge tests on model tunnels and ignore the volume loss due to over-cut. A surcharge of 20 kPa shall be considered for the traffic loading. [10 marks]
b)
If the tunnel is to be driven in the fluvial sand of the Kallang Formation (loose sand with SPT < 10) with a unit weight of 17 kN/m3, calculate (i) the minimum face pressure at the tunnel crown to avoid collapse; and (ii) the target face pressure at the tunnel crown to limit the volume loss to 1%. You can assume that the piezometric head inside the TBM excavation chamber will be maintained to balance the ground water pressure and that the effects of TBM tunnelling will increase the ground water o pressure by 30 kPa. The shear strength of the fluvial sand is c’ = 0 kPa and ’ = 32 . A partial factor of safety of 1.2 is to be considered for ultimate limit state calculation. A pressure variation of 20 kPa is to be considered to limit the volume loss to 1%? [10 marks]
c)
With the volume loss of 1%, what is the expected sub-surface settlement at 2m above the tunnel crown? You can assume that the trough width parameter, K = 0.5. [5 marks]
d)
Explain the forms that consolidation settlement profiles will take due to TBM tunnelling in soft clay and their impacts on building damage. [5 marks]
e)
Explain how the number of radial joints in a ring will affect the bending moments in the segments and how this can be considered in the design of the tunnel lining segments. [4 marks]
- END OF PAPER -
