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What are the major factors affecting ground-support interaction in tunnels?

In general, the in-situ ground stress reduces with an increase in inward radial displacement in an unloaded ground/tunnel. The major factors affecting ground-support interaction in tunnels are as follows:

(i) The relative stiffness of ground and linings plays an important role in ground-support interaction. For instance, a stiff support could support the unloaded ground at lower deformation than a flexible support.

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(ii) Delay of support to tunnels results in the ground to be deformed by a certain degree before the installation of linings. Consequently, the linings take up less loads subsequently owing to less deformation of ground generated after the action of delay of support.

(iii) As the ground is unloaded, stress is redistributed during excavation. In case the ground is delayed in support and the ground respond from elastic region to yielding region, it results in larger displacement and supporting load.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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What is the difference between support approach and reinforcement approach?

For support approach it involves the application of reaction force at the face of excavation by using heavy structures, primarily ribs and lagging.

For reinforcement approach, it involves the overall improvement of rock mass performance by techniques such as rock dowels, rock bolts and ground anchors. The target of reinforcement approach is to keep the rock and blocks from moving and loosening so that a large dead load of rock would not be exerted onto the support system. In fact, it holds the rock together and causes the ground around the opening to form a self-supporting ground arch around the opening.

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There is a trend of tunneling industry to move from support approach to reinforcement approach because it requires less amount of structural steel support.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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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.

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(ii) Their presence provide an indication if sufficient thickness of shotcrete is applied.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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What is the purpose of subdivision of tunneling faces in Sprayed Concrete Linings (SCL) method?

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:

(i) It enables early closure of part of the invert of the tunnel.

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(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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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What is the consideration in selecting the number of segments for segmental linings in tunneling?

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.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Filed under Tunneling | 1 Comment