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Grout holes are present in precast jacking pipes for the following reasons:
(i) They serve as the locations for injection of bentonite or other lubricant. Lubricant is used for both granular soils and cohesive soils to trim down the frictional resistance. For cohesive soils, the soils cannot get onto the pipes by the presence of lubricant and the shearing plane lies within the lubricant as suggested by R. N. Craig (1983). On the other hand, for granular soils, the lubricant mixes with soils with a significantly reduced friction. With the use of lubricant, longer pipe lengths can be jacked without the use of intermediate jacking station.
(iii) They are used as lifting holes when placing the precast jacking pipes into rails inside the jacking pits.
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.