A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
6. In pipe jacking, can engineers roughly estimate the order of stress
concentration in pipe joints? (DP2)
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.
7. How can lubricants reduce the jacking forces? (DP3)
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.
8. In the design of pipe jacking, what particular areas on pipe joints
should engineers take care of? (DP4)
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.
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