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Why is grouting normally performed in segmental linings in tunnels?

Grouting is usually carried out under pressure in segmental linings during tunnel construction because of the following reasons:

(i) It helps in the uniform transfer of ground pressure to the linings.

(ii) The grout serves to reduce the surface settlement.

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(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.

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 differences between segmentally lined tunnels and Sprayed Concrete Linings (SCL) tunnels?

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.

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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.

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 compensation grouting and normal grouting?

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.

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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.

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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Can shotcrete be adopted as permanent linings in tunnels?

Shotcrete can be employed as permanent linings in tunnels. However, it is only expected that they would not carry loads in eth same way as structural linings. The use of shotcrete is limited to temporary works in tunnels with fair good ground conditions which is self-supporting. Its presence is used for avoiding rock falls or erosion problems.

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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.

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How can shotcrete stabilize tunnels?

When shotcrete is sprayed on a rough ground surface, it fills small openings and cracks. It serves as initial support and also immediate support after excavation. It decreases the possibility of relative movement of rock bodies or soil particles and, therefore, controls the loosening of the exposed ground surrounding the tunnel.

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Shotcrete lining could take up significant loads though it forms a flexible support system. In fact, shotcrete lining is expected to undergo large deformations which enable the intrinsic strength and self-supporting properties of the ground to be mobilized as well to re-distribute stresses between the lining and ground. During the deformation of lining, stresses within the shotcrete lining are relocated to the surrounding ground. As such, this mechanism of load transfer in turn generates subgrade reaction of the ground which gives support to the shotcrete lining.

Friction between shotcrete and the ground also reduces the differential movement of the ground. Even though shotcrete may not form a complete ring, the frictional forces between shotcrete and the ground could provide support to the ground.

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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 | 0 Comments