also become part of the reclaimed
areas.
Site Investigation
The object of a site investigation is to provide sufficient reliable informa- tion, in readily understandable terms, for the designer and contractor to analyse their respective problems of design and construction in the most suitable and economical forms. For these purposes the following informa- tion is required:-
the
(a) A coherent picture of the ground
below the site including ground water table.
All types of investigation are limited to detailed surveys below isolated points on the site and sufficient of these surveys are re- quired which, when correlated. will provide adequate informa- tion on what has to be inferred between and around the 'survey points'. The number of these 'survey points' must always be arbitrarily decided and may have to be increased where the initial results are inconsistent. The in- vestigation of uniform conditions can be less intensive than where the conditions are complex.
The depth of the investigation should include all the ground likely to be significantly affected by the new project. One widely accepted rule is to explore to a depth equal to one and a half times the least width of the build- ing. In a few cases this rule may be relaxed, as for example when bedrock is encountered within this depth, which is known from previous experience to be sound and adequate.
In Hong Kong, due to the deep weathering and presence of boulders it is essential to drill at least 20 ft., and preferably 30 ft., into sound rock in order to establish that it is bedrock.
Descriptions of soils which convey information on their be- haviour have the most meaning
and this is helped by correlating the strata to the local geological formations. The boring opera- tor's log can provide only a basis for the interpretation of the ground conditions and they often require intelligent modification because of the inherent limita- tions of the techniques used for exploring the ground.
(b) Representative figures for the re- levant properties of all strata likely to be significantly affected, corresponding to the insitu' con- dition.
These figures may be obtained by tests in the natural ground and laboratory tests on soil and rock samples. The reliability of the laboratory results depends not only upon properly conduct- ed testing techniques but even more so on the production of re- presentative samples with the minimum of disturbance. The boring operations have an im- portant influence on the value of both the insitu testing and sam- pling. Many soils by virtue of their uncemented, weak or brittle nature require suitable equipment and experienced operators to ob- tain satisfactory samples of
them.
This is particularly true for the decomposed granite and soft clays. The samples for testing must also be of a sufficiently large size not to be affected by the larger particles which are often present in the decomposed granite.
Finally owing to the natural variations in the ground suffici- ent tests are required to obtain the representative figures for de- sign.
It is the authors' opinion that the local ground conditions are such that exploratory 'soft ground' borings ex- tended, where necessary, by core drill- ing is the only reliable means for obtaining the information described above at sites for high buildings. The amount of this work at any site will
depend upon the information avail- able on local conditions to the requir ed depth.
The method of driving a rod or tube and measuring the resistance. known as an indirect method of in- vestigation, suffers from the serious disadvantage that the soils cannot be identified, also it usually cannot be taken to the full depth affected by the foundation, This can often occur where piling is being considered.
Such techniques are also limited by the empirical methods which have to be employed for evaluating the re- sults, while the large differences between these, for the pile driving formulae which are often used, are notorious. Full scale pile loading tests provide the most reliable means for assessing the carrying capacity of piles.
Construction Aspects
All too often the contractor has to base his tender for the work on un- certainties due to lack of information on the ground conditions and the level of ground water. This can re- duce his offer to a speculation in which the building owner participates often to his detriment. It is particul- arly so when difficulties arise due to unexpected soil conditions which de- lay the completion of the work with the result that the developer is un- able to realise his investment at the earliest possible occasion.
Before deciding upon the choice of foundation for a high building it is preferable to consider the influence of the ground conditions on the method of construction because this reflects the cost and time for comple- tion and focuses the problems onto a sadly neglected aspect. Usually the deeper the foundation the greater the cost and so the contractor is very much in need of knowing his commit- ment ahead of his programme.
In certain cases where the stress- ing of the soils from the super-struc- ture is carried to depth, excavations have to be carried out which can be
FIG. 2.
Field
occurrence
Residual debris
Character
Percentage of solid rock
structureless sand clay or usually 0%
clayey sand
Zone
I
Residual debris
with core stones
core
stones sub-ordinate, rounded and free
less than 50%
II
Core stones
with residual debris
Partially wea- thered rock
core stones dominant
rectangular and locked
50-90%
III
minor residual debris along major structural planes, but may be considerably iron-stained
Far East Architect & Builder September, 1965
greater than
90%
IV
NELUVIATED & COMPACTED
יו
114
Zones of a mature profile of weathering on granite (after Ruxton and Berry)
61
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