A Self-Learning Manual - Institution of Engineers Mauritius

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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. 346
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 9. What are the differences in design between normal precast concrete pipes and pipes used for pipe jacking? (DP5) 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. 10. How could ground freezing stop ingress of groundwater in excavation? (DT5) 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. 11. What is the difference between “cut-and-cover” method and “cover-and-cut” method in tunnel construction? (DT4) “Cut-and-cover” method involves the construction of open cut at the first place, followed by the construction of tunnels under open excavation. Upon completion of the structure, backfilling and reinstatement would be subsequently carried out. “Cover-and-cut” method involves firstly the construction of cover followed by the second stage in which construction activities are carried out under the cover. As such, the disturbance to the public owing to constructional activities could be reduced to a minimum. 347
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A Self-Learning Manual - Institution of Engineers Mauritius

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