SITE INVESTIGATIONS
Sounding Surveys
4.1
4.2
4.3
To assist in the preparation of the Container Committee's report, a series of echo soundings were taken in the Kwai Chung area and in the sea approaches to it. These have been more recently augmented by hand soundings taken on a 100-ft. × 50-ft. grid in the actual area reserved for the terminal and in the area behind the existing breakwater. The seabed contours as shown on Drawing 3 are based on these soundings which have been used in making an assessment of the quantity of fill required to reclaim these areas and of the volume of dredging needed.
In addition to the foregoing, echo soundings have been taken in the shallower parts of the approach channel off the south-east corner of Tsing Yi Island to augment the depths of water indicated on the Admiralty Charts. These confirmed ample water for 40-ft.-draught ships.
In assessing the change in level of the seabed over the years in the vicinity of Tsing Chau, Admiralty Charts and soundings taken in 1954, 1966 and 1968 were compared.
Marine Borings
4.4
4.5
In all, 14 borings had been made for the Container Committee's report, but to enable alternative quay-wall alignments to be investigated a further 47 bores were sunk. These were required for determining:-
(a) The depth to which seawall foundations should be taken.
(b) The length of piles required.
(c) The amount of settlement the reclamation would be likely to undergo and the time
it would take.
(d) The suitability of the dredged material for reclamation purposes.
(e) The likelihood of scouring of the seabed taking place and the depth to which this
might go.
This work was carried out by two boring barges, one equipped for taking disturbed samples and carrying out standard penetration tests, while the other had facilities for taking undisturbed samples and for making vane tests. Samples taken in the boring operations were analysed and tested in the Public Works Department Laboratory. In all, 61 disturbed samples and 12 undisturbed samples were taken while 39 penetration and 10 vane tests were made. Selected results of these are contained in Appendix 1.
Geophysical Survey
4.6
4.7
In determining the optimum alignment of quaywall it became obvious during the site investigations that the level of rock beneath the seabed was a critical factor especially in the vicinity of Mong Chau. It was also obvious that it was essential to determine whether or not the dredging for the berths and the approaches to them was likely to encounter rock.
With this in mind a geophysical survey of the area and approaches was commissioned. This survey was carried out by the "sparker" system which employs a similar technique to a standard echo sounder but the impulses produced by the equipment penetrate the seabed and are reflected by the different subsurface strata. Instead of the single line representing the seabed produced by the echo sounder, a pattern is recorded which can be interpreted with the aid of correlating boreholes to indentify the various strata. A plan of the lines surveyed and the interpretations for the critical sections are shown on Drawings 9 and 10.
Current Survey
4.8
Two types of current survey were carried out in the Kwai Chung area:— (a) Float tests.
(b) Current-meter readings.
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