A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 6. What is the difference between compensation grouting and normal grouting? (TBM5) Compensation grouting is a technique to offset settlement induced during tunneling and underground excavation. The main idea of compensation grouting is to inject grouts into the zone between tunnel and overlying building to compensate for ground loss and stress relieve owing to underground excavation. The injection of grout changes the in-situ stress state and influences the soil deformation. Compensation grouting could be implemented by fracture grouting and compaction grouting. Normal grouting can be carried out in situations such as post-grouting of mini-piles and filling of voids without the effect of compensation of ground loss and stress relieve. 7. What are the differences between segmentally lined tunnels and Sprayed Concrete Linings (SCL) tunnels? (SLT1) The basic difference between segmentally lined tunnels and SCL tunnels is that the sprayed concrete in SCL tunnel is used for temporary linings only. Concrete linings in segmentally lined tunnels are designed with long-term load conditions with the adoption of appropriate safety factors. However, the dimension of linings in SCL tunnels is derived from a balance between safety of lining and cost consideration. The quality of precast concrete linings can be better controlled in precasting yards. As such, the quality control of precast concrete lining is obviously better than that of sprayed concrete which depends on site workmanship [39]. 8. Why is grouting normally performed in segmental linings in tunnels? (SLT2) Grouting is usually carried out under pressure in segmental linings during tunnel construction because of the following reasons [62]: (i) It helps in the uniform transfer of ground pressure to the linings. (ii) The grout serves to reduce the surface settlement. (iii) Grout fills up the annulus of tunnel linings so that it upholds the designed tunnel shapes. (iv) The presence of grout aids in limiting groundwater seepage in the tunnels. 342
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 9. What is the consideration in selecting the number of segments for segmental linings in tunneling? (SLT3) For the construction of tunnels by segmental linings, the choice of the number of segments affects the cost and durability of tunnels. With an increase in the number of segments, the number of joints also increases accordingly. This raises the potential for water ingress into the tunnels. However, if the number of segments is kept to a minimum, the speed of the erection of segments can be increased. However, it is expected that higher bending moment would be induced in the tunnel rings for smaller number of segments and extra cost is incurred in the provision of additional reinforcement. 10. What is the purpose of subdivision of tunneling faces in Sprayed Concrete Linings (SCL) method? (SLT4) In employing sprayed concrete lining methods for lining a large tunnel in soft ground, the tunneling face is normally divided into several parts because of the following reasons [62]: (i) It enables early closure of part of the invert of the tunnel. (ii) With the excavation in each part taking place at different times, it helps to reduce the area of exposure of the tunnel face so that there is better control on tunnel stability. (iii) For unit advancement in any part of the tunnel excavation, the amount of excavation and sprayed concrete is reduced. As such, this allows for early provision of primary support. 11. What are the functions of lattice girder in tunneling? Lattice girders are supporting elements in tunnels and they normally consist of steel bars laced together in triangular pattern. They are made to suit the shape of the tunnel. Owing to their small steel reinforcing area, they are not expected to contribute much to the overall support of tunnel. Instead, they are designed and provided for the following reasons: (i) They have similar spacing with rock bolts and they are intended to provide temporary support to rock which is readily to loosen and fall. (ii) Their presence provide an indication if sufficient thickness of shotcrete is applied. 343
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A Self-Learning Manual - Institution of Engineers Mauritius

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