10.19 Fig. 2 shows that with 12-in. sand drains at 7-ft. triangular centres, the amount of settle- ment after the 2-year period during which consolidation of the fill should take place would be comparatively small. This means that levels would have to be made up only occasionally thereafter. This arrangement would be the most suitable therefore to bring the reclamation into use as soon as possible. The estimated cost is $4.70 per sq. ft. of reclamation.
Paper Drains
10.20 The same effect as previously described for sand drains can be achieved by inserting cardboard wicks into the soil. This method was invented by Dr. W. Kjellman of the Royal Swedish Geotechnical Institute and has been used in Europe and Japan. The wicks are 100 mm. wide x 3 mm. thick and have 10 inner longitudinal channels, each of about 3 sq. mm. cross-sectional area. The wicks are supplied in 400-metre rolls and are inserted inside a mandrel which is driven or jacked into the ground; special machines have been developed for this purpose.
10.21 The advantage of the band-shaped cross section compared with a circular one is that drainage of the soil depends more on surface area in contact with the soil than on cross- sectional area and a paper drain is reckoned to be equivalent to a sand drain of about 5 to 8 cm. diameter. The spacings of paper drains to achieve the same drainage effects as the 12-in. sand drains previously described is shown on Fig. 2.
10.22 In Japan it has been claimed that the effective cost of paper drains there is about half that of sand drains, but since special plant would have to be imported or constructed for paper drains to be used at Kwai Chung. It is proposed therefore for estimating purposes in this Report, to assume that paper drains would cost the same as sand drains at equivalent spacings as shown on Fig. 2. If the drainage method of accelerating settlement comes to be used, it is recommended that alternative tenders for sand and paper drains be called in order to establish which is more economical.
10.23 From Fig. 2 and from the same considerations as made previously for sand drains, it will be seen that paper drains at 4.5 ft. centres would be the most suitable for bringing the reclamation into use as soon as possible.
Surcharging
10.24 The rate of settlement caused by consolidation of the very soft clay layer would depend on the overburden pressure on it from the soil above. The overburden pressure caused by 40 ft. of filling material above a seabed level of −25 ft. C.D. would be about 3,350 lb./sq. ft. and this could be increased by surcharging the surface of the fill in order to increase the rate of settlement until a required amount had taken place. The surcharge could then be removed with the knowledge that a reduced amount of settlement would take place thereafter.
10.25 The easiest way of providing surcharge on the reclamation would be by placing on it additional fill which would be used later for further reclamation. The effects of 10 ft. and 20 ft. of additional fill have therefore been calculated, both with and without vertical drains. The estimated costs of the double-handling involved are $0.11 and $0.22/sq. ft. respectively but to get the material, additional early expenditure estimated at $0.50 and $1.00/sq. ft. respectively would be incurred, although it would be offset by later savings when the further reclamation was required. The rates of consolidation of typical 25-ft. layer of the very soft clay under 40 ft. of filling, with 10 ft. and 20 ft. of additional filling are shown on Fig. 3.
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