Tunneling

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.

(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.

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.

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.

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.

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.

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.

(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.

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.

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.

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.

(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.

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.

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.

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.

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.

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.

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.

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.

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.

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.