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
Level One (Core FAQs)
Part II: Excavation
1. What is the difference between free earth support method and fixed
earth support method? (D1)
For free earth support method, the soils at the lower part of piling is
incapable of inducing effective restraint so that it would not result in
negative bending moments. In essence, the passive pressures in front of
the sheet piles are insufficient to prevent lateral deflection and rotations at
the lower end of piling. No passive resistance is developed on the backside
of the piling below the line of excavation.
For fixed earth support method, the piling is driven deep enough so that the
soil under the line of excavation provides the required restraint against
deformations and rotations. In short, the lower end of piling is essentially
fixed.
Fig. Free earth support method and fixed earth support method
2. In braced excavation, why is Rankine’s theory of lateral earth
pressure not applicable? (D2)
In braced excavation, sheetpiling is constructed at the first place, followed
by the installation of struts as excavation proceeds. Following the
installation of first row of struts, the depth of excavation is small so that
there is no major yielding of soils. However, as further excavation takes
place, soils yield before the installation of n th row of struts. The first row of
struts prevents yielding near the ground surface. As such, deformation of
wall increases with depth with the smallest at the ground level. Owing to
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