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
Bentonite slurries are commonly used in silty, sandy and granular soils but
it is not recommended to use in clayey soils because it may cause swelling
in clays by absorption of water and this results in increased contact
between the jacking pipe and soils. Hence, non-aqueous lubricants should
be considered for cohesive soils.
16. Swelling of clay develops significant stresses on pipe and its
effect is even magnified by the use of lubricant. How can swelling
inhibitors help to resolve this problem?
Lubricants are introduced in pipe jacking to reduce the jacking loads.
However, for pipe jacking in clay the swelling of clay is accelerated by
water-based lubricant as the free water in the lubricant is readily absorbed
by clay. To address the problem, swelling inhibitors are added to lubricants
which enhance a reduction of free swelling for clays. In essence, the ability
of swelling inhibitors to reduce free swelling of clay is achieved by the
alternation of clay properties and formation of barrier on cavity surface.
17. Why is sharp-edges shield normally used in pipe jacking with
manual excavation? (EP5)
The shield connected to lead pipe is normally sharp-edged in design
because:
(i) It helps to reduce the resistance when the shield enters into soils;
(ii) It reduces the amount of soil dropping into the shield.
Sometimes the shield is equipped with jacks so that it allows tilting of the
shield and adjustment could be made to the direction of pipe jacking.
Fig. Sharp-edged shield
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