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
3. What are the rationales of Observational Method in geotechnical
works? (M2)
The idea of Observational Method was first discussed by Peck in the
Rankine Lecture in 1969. The Observational Method is commonly adopted
in geotechnical works in construction phase, though it is also feasible in
design stage.
In essence, in the conforming design by engineers during planning stage,
the design is usually based on over-conservative approach or most
unfavourable conditions owing to a lack of precise and actual site
information. During subsequent construction, with precise site information
and condition available the Observational Method is adopted in which the
original design is revised based on most probable conditions with
instrumentation monitoring. If the monitoring results show that performance
of the revised design approaches the limit of acceptable level of risk, then it
shall be reverted to planned modification which is based on most
unfavourable conditions and hence the level of risk is lowered back to the
original design. Otherwise, the revised design shall continue and this
results in cost reduction without comprising safety of works.
However, care should be taken in implementing Observational Method
when rapid deterioration of the site may occur so that there is insufficient
time for carrying out the planned modification. For instance, rapid
deterioration can result from development of high pore water pressure in
heavy rainfall or burst watermain.
4. How does reinforcement function in embankment built on soft
clay?
The factor of safety for the embankment constructing on soft clay is
minimum at the end of construction. After that consolidation takes place,
thereby increasing the shear strength of foundation soils. Reinforcement is
normally introduced to maintain the stability owing to the following reasons:
(i) The reinforcement at the clay surface, which is capable of carrying
tensile forces, generates shear stresses to resist the lateral deformation
of clay and improves its bearing capacity.
(ii) The reinforcement could also hold in equilibrium the lateral trust
developed by the fill above so that it reduces the stresses which tends
to cause failure of clay foundation.
(iii) The reinforcement has a tendency to drive the failure mechanism
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