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How can lubricants reduce the jacking forces?

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.

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(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.

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In pipe jacking, can engineers roughly estimate the order of stress concentration in pipe joints?

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 0.5o.

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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.

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Can pipe jacking be implemented in a fast manner?

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.

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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.

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What is the difference between NATM and Sprayed Concrete Lining?

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.

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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.

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Is NATM a tunneling method or a tunneling concept?

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

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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.

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