10. RECLAMATION
Settlement
10.1
Site investigations indicate that between Tsing Chau and Mong Chau over much of the area in front of the existing breakwater there is a 25-ft. layer of very soft clay below the seabed. The level of the top of this layer is about 25 ft. C.D. and if as is proposed, the area is reclaimed to a level of about +15 ft. C.D., then this layer would have some 40 ft. of filling material above it.
10.2 Laboratory tests for permeability, compressibility, consolidation coefficient, and shear strength carried out on this seabed material, place it at the softer end of the range of characteristics found to be typical of marine clays in Hong Kong waters. Seabed condi- tions behind the existing breakwater in the unreclaimed Terminal area are very similar to those in front although the seabed level is 5 to 10 ft. higher.
10.3
Calculations show that the proposed reclamation would cause this very soft layer of clay to settle about 9 ft. and that 90% of this settlement would in the normal course of events, take 70 years to achieve. It is also calculated that 10-20% of the settlement, i.e. between 1 and 2 ft., would take place during the filling period. The decomposed-granite fill itself would also consolidate and although it is not possible to calculate this settlement precisely, it is estimated that it would be about 5% of the filling thickness in the first 2 or 3 years. This would amount to 2 ft. of settlement in a 40-ft. thickness of filling.
10.4 Although the methods available for calculating settlement in the clay are conservative in that they tend to over-estimate the time required for consolidation to take place, they do indicate that unless some remedial measures are taken, considerable settlement could be expected to take place for some years after completion of the reclamation. Settlements of the order indicated above would hinder the early effective use of the reclamation as a container terminal so that remedial measures have to be considered if the Terminal is to be established in a minimum of time. Settlement caused by consolidation of the soil underlying the very soft clay would be small in comparison with that in the layers above it and thus it will not be taken into account. Methods of combatting the settlement problem can be considered under 3 main headings:-
10.5
10.6
10.7
(a) soil improvement.
(b) soil replacement.
(c) settlement acceleration.
Because of its impermeability, there is no practicable method known to improve the very soft clay by injecting it with cement or a chemical. Stabilisation of decomposed- granite fill is possible by injecting cement grout or silica gel, but since the major seat of the settlement is in the clay, it is not proposed to pursue this method further.
Rock fill if used instead of decomposed granite would have negligible consolidation, but apart from cost considerations, it is unlikely that it would be available in sufficient quantities for rapid reclamation and so cannot be considered as a full solution to the problem. As another possibility the clay layer could be removed, in whole or in part, by dredging or displacing as a mud wave and replacing by fill. It must be realised however that the increased settlement in the greater thickness of fill must be balanced against the reduced settlement in the smaller thickness of the clay layer. This method will however be considered further in this Report.
Consolidation of the very soft clay could be accelerated by drainage and/or by surcharging the fill. Drainage to accelerate consolidation would be effected by forming vertical drains of pervious material within the clay layer. These would be connected to the ground-
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