Why are intermediate jacks designed in some pipe jacking projects?
When the process of pipe jacking stops, building up of resistance is very fast in some soil. For instance, increase in jacking force of 20%-40% is required for a stoppage of pipe jacking for just several hours. Hence, it is recommended that pipe jacking should be carried out in a continuous operation.
For a long pipeline, the frictional forces established between the jacking pipes and soil is high. Sometimes, such resisting forces may be so high that they can hardly be overcome by the jacks in jacking pits. Moreover, even if the jacks can overcome the high frictional forces induced during jacking, high loads are experienced in jacking pipes during driving. Jacking pipe’s material e.g. concrete may not have sufficient strength to resist these stresses and hence pipe strength is another factor that governs the need for consideration of using intermediate jacks.
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.