10.8
10.9
water level above, so that excess voids water could be drawn off more rapidly than if it had to find its way up through the clay layer. There are two principal ways of forming these drains, with paper wicks or sand.
If the surface of the fill is surcharged, with or without drains, settlement would be quicker and the surcharge could be retained until a required amount of settlement had taken place. The surcharge could then be removed and any further settlement would be prob- ably limited to an acceptable amount. There is some danger of shear failure taking place within the clay layer under a surcharge, but this method will also be considered further in this Report.
The possibility of compacting the decomposed-granite filling in layers as another means of reducing settlement is ruled out as most of the filling would be underwater. Deep compaction of loose cohesionless soil by large poker vibrators is possible but has not been tried with Hong Kong soils. In any case, it is considered too expensive for this project.
10.10 From the foregoing therefore only 4 ways of combatting settlement will be considered
further in this Report:-
(a) Mud replacement.
(b) Sand drains.
(c) Paper drains.
(d) Surcharging.
10.11 These considerations do not apply to the areas adjacent to existing land, where the underly- ing firm ground is much higher and the thickness of the clay layer, if any, is very much smaller; indeed on the seaward side of Mong Chau the seabed material is sandy. There would, therefore, be large differential settlements between the boundary regions and the remainder of the reclamation unless remedial measures are taken in the latter.
Mud Replacement
10.12 As described previously, if a 40-ft. depth of fill is placed on the 25-ft. layer of very soft clay, the clay can be expected to consolidate by about 9 ft. and the decomposed-granite fill by about 2 ft. About 2 ft. of the clay consolidation would take place during the filling period, but the remainder would take many years. Consolidation of the fill should be virtually complete within 2 or 3 years. If the 25-ft. clay layer was entirely removed and replaced with decomposed-granite filling, settlement of about 3 ft. taking 2 or 3 years could be expected in the 65-ft. thickness of fill.
10.13 If on the other hand, the clay layer is only partly removed and replaced, the settlement of the remaining clay would be less and quicker than for the full 25-ft. layer. Also settle- ment of the fill would be proportionately more than for the original 40-ft. depth thereof. For ease of comparison, it is assumed that consolidation of the fill layer would take place over 2 years irrespective of depth and Fig. 1 shows the following for different replacement depths:-
(a) The calculated combined settlement of the clay and fill after the two-year period.
(b) The total settlement after 90% of the clay consolidation has taken place.
(c) The calculated time for (b) to occur.
(d) The calculated total final settlement.
(e) The estimated unit cost of dredging and refilling.
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