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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.
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.