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
marine fill is lower.
8. Why are observed settlements in reclamation normally larger than
calculated? (R6)
Settlement in reclamation area occurs as a result of primary consolidation
(i.e. by dissipation of excess pore water pressure) and secondary
compression which involves creep of soils. Creep of soils occurs by
viscous squeezing out of absorbed water in double layers of clay particles
and rearrangement of clay particles under loading.
In the calculation of settlement in reclamation, there are generally two
methods available. The first method assumes that creep occurs during the
process of primary consolidation, which appears to be logically correct. On
the other hand, the second method assumes that creep of soils occurs
after primary consolidation and hence at the end of consolidation period
the calculation settlement is equal to primary consolidation only without
any consideration of creep effects. This method of settlement calculation is
well adopted by most consulting firms and hence leads to underestimation
of total settlement.
9. Should silt curtain be designed to touch seabed?
Silt curtains are impermeable vertical barriers extending from the seawater
surface to their designed depths. The curtains are held in a vertical position
by the carrier float on their top and a curtain weight at their bottom. A
tension cable is designed at the carrier float to resist stresses incurred by
currents. Moreover, the silt curtains are anchored to the seabed to hold
them in the designed configuration.
In essence, the depth of silt curtains should not be so long and touch the
seabed because the bottom segment of the silt curtains would be trapped
inside the newly accumulated sediment, thus resulting in the sinking of the
curtain. It is difficult to remove these sunken curtains. Moreover, reversal
tidal and current actions may cause movement of bottom region of curtains
which stir up the settled suspensions and create additional turbidity.
10. Vibrocompaction is carried out to loose sand after reclamation by
filling sand. How to determine the in-situ density of filled sand? (R7)
It is difficult to obtain undisturbed samples of sand to measure the in-situ
density. The relative density of in-situ sand can be determined by
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