seen from Figure 1, the original coast- line is completely hidden by the man- made deposits which provide the re- clamation areas.
Several geological descriptions have been given of the Hong Kong granite. (1, 2, 3). The chief points of inter- est are concerned mainly with the re- sults of the severe weathering which has occurred as this has caused serious deterioration in its bearing properties not only where it has changed completely to a soil, but al- so within the rock due to decomposi- tion in the joints and fissures.
a
Ruxton and Berry (3) have treated this subject of weathering in some detail and a diagram illustrating their conclusions on the order of trans- formation from solid rock to a sandy clay is reproduced in Figure 2.
The difficulty in interpreting the level of the top of sound bedrock
60
CASTLE PEX
APPROX. ORIGINAL COASTLINE
PROBABLE RECLAMATION AREAS
BOUNDARY
NATHAN.
ROAD
ST.
CHO
from a series of probings is very evi- dent from a study of this figure. Moreover a number of the more re- cent exposures of the rock profile sug- gest that zones of more resistant rock may occur within the decomposed granite giving rise to non-uniform gradation in the size of the boulders with increasing depth.
The insitu decomposed granite is the principal product of weathering of the granite bedrock. It represents the decomposition of the rock, in place, and the original joints and vein structure often remain as a feature. even when almost all the mineral part has become completely altered. While at the surface of a complete profile the rock has become a clay and at the base a very compact sand, the main mass of material between gener- ally appears to be a multi-coloured gravelly, sandy silt with some clay,
HUNG
KOWLOON
BAY
STAR FERRY
VICTORIA
HARBOUR
Kowloon Peninsular
VICTORIA
HARBOUR
SAI
YING PUN
QUEENX
FIG. 1. Approximate and Victoria
CITY
STAR FERRY
WAI
adDUIT BR.
BOUNDARY
GAP RD.
Victoria, Hong Kong Island
CHAI
areas of reclamation and original coast line in Kowloon
becoming more sandy with increasing depth. It is the silt content, although not predominant quantitatively, that largely controls its behaviour.
It must be emphasised that local variations in grading and density often occur between and within sites but generally speaking both density and grain size increase with depth. The volume of voids to solids may be as high as 1:1 near the top of a profile and with increasing depth it may re- duce to 2:1 near the bedrock.
Its overall thickness can be 100 ft. and more, while borings have located its presence to at least 250 ft. below sea level in one or two places, per- haps in deeply weathered joints. All the sand and gravel sized particles are very angular and often interlocked, so that the material has a higher strength than one would expect for a silt.
Also boulders or core-stones, of the parent granite rock, sometimes sever- al feet in size, occur in the otherwise relatively fine-grained material. They may be found at any position includ- ing at the surface, where soft ground could conceivably develop beneath them.
Variable Conditions
The colluvial decomposed granite, an important soil on Hong Kong side, represents the scree and wash- down material which has been trans- ported under gravity following wea thering of the rock at higher levels on the steep slopes. It contains practic- ally all the constituents of the insitu decomposed granite, including the boulders, as this is the main source of supply; but it is without the inter- locking structure of the original solid rock.
The haphazard manner in which it has been formed gives rise to variable and unpredictable conditions. While this colluvial material must naturally always overlay the insitu soil it may be above or below the river and marine deposits.
The marine and river deposits do not appear to be dissimilar to those found elsewhere in the world, except that extensive beds of gravel are un- likely to occur. They include: soft recent clays; silts; sands. generally with angular grains from the decom- posed granite: and occasionally peat.
Although extensive reclamation work is in progress at the present time where it is possible to observe the extent and types of materials being used for raising the level of the ground, it must not be forgotten that reclamation has been carried out for many years and there is now very little information available of much of the earlier work. Generally speak- ing it is safe to say that the principal materials are the decomposed granite and marine sands.
Invariably such materials would be in a loose state owing to the prohibi- tive cost of compaction. Excavated rock and coral have also been used. Not all the fill is however granular, and garbage tips, new and old, have
Far East Architect & Builder September, 1965