Vincent T. H. CHU

The concept of ground freezing involves the lowering of temperature of ground near the excavation area. Drillholes with designed spacing are installed so that a chilled brine or liquid nitrogen is introduced into the holes. Brine requires continuous circulation while liquid nitrogen is for rapid freezing and it is unrecoverable. With the addition of a chilled brine or liquid nitrogen, the groundwater is frozen into ice. Upon frozen, soils exhibit higher shear strength and the frozen zone acts as an impermeable barrier so that water could not enter the excavation zone.

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 pipes used for pipe jacking, they should possess the following characteristics:

(i) Pipes should have high concrete strength to withstand the stress induced during the jacking process.

(ii) There is tight tolerance in pipe dimension and the pipe joints are specially designed to provide trouble-free joint details. Two commonly available joints are rebated joint and butt end joint.

(iii) Pipes preferably should have smooth external concrete finishes to reduce the friction between the pipes and surrounding soil.

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.

Since in pipe jacking, the jacked pipes could hardly be jacked in the designed level and alignment and some deviation from the original one is commonly acceptable provided that the deviation are within the tolerance of the Contract. However, in order to avoid damage made to the pipe joints due to overstressing, it is necessary to estimate the stress concentrations resulting from these angular deflections.

Note: Pipe jacking is a trenchless method in which pipes are jacked underground from jacking pits and receiving pits.

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 lubricating fluid serves to reduce the jacking force in the following ways:

(i) It saturates the overcut leading to partial and complete buoyancy of the jacking pipe in the cavity. As such, the contact surface area between the jacking pipe and soils is decreased.

(ii) It stabilizes the cavity by limiting the radial effective stress acting on the jacking pipe.

(iii) The interface friction angle between the soils and the jacking pipe are reduced by the lubricants.

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.

Theoretical line and level of pipelines can hardly be achieved in pipe jacking. As such, the provision of angular deflection is made at pipe joints to accommodate such deviation. Normally, maximum allowable angular deflection at pipe joints in pipe jacking is 0.5^o.

As a rule of thumb, the stress concentration at a pipe joint is about 3 times the joint stress resulting from uniform distribution of stress. As such, for grade 50MPa precast concrete pipe, the allowable uniform joint stress is expected to be one-third of its compressive strength, i.e. 16.67MPa.

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.

Energy is a function of load and speed. Damage may occur on the jacking pipes if it is incapable of absorbing the energy. Pipe jacking requires large energy in the process. Jacking pipes are low speed energy absorbers. To cater for the low-speed-energy absorbing characteristics of jacking pipes, hydraulic jacks should be designed to provide high loads with low speeds.

Otherwise the excessive high speed generated by hydraulic jacks would cause kinetic damage to the pipes.

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 New Austrian Tunneling Method originated from rock tunnels and it requires the use of rockbolts and shotcrete swiftly after blasting. Institution of Civil Engineers has renamed it as Sprayed Concrete Lining for construction of tunneling linings by this method in soft ground.

There are some distinct differences in design philosophy between NATM and Sprayed Concrete Lining. In fact, it is not practical to mobilize inherent soil strength through deformation in soft ground. For lower strength of soils in soft ground, there should not be any delay in completing primary support and ground deformation should be reduced as much as possible.

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 the original version of the New Austrian Tunneling Method, it is a tunneling concept with a set of main principles as follows:

(i) Application of thin-spayed concrete lining

(ii) Closure at invert to form a complete ring as soon as possible

(iii) Measurement of deformation until equilibrium is attained

It is a design concept in which the ground (soil or rock) enclosing the opening becomes a load-bearing element through formation of ring-like body. It uses all available means to develop maximum self-supporting capacity to provide support of the underground opening. In essence, it makes use of geological stress of surrounding soils to stabilize the opening. Therefore, NATM is a tunneling philosophy and concept rather than excavation techniques.

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 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.