September_1965 — Page 26

major civil engineering works, and due regard must be given to a wide number of problems that can arise under such circumstances.

So often architects and engineers specify what they require without re- gard to the difficulties of construction leaving these to be resolved on site as and when they arise.

Broadly speaking, foundations are shallow or deep. In the former case excavation is the usual approach, but when this becomes costly and dif- ficult recourse is often made to piling in one form or another.

There are two main methods of constructing piles, either by driving a preformed unit into the ground; or by boring a hole which is then backfilled with concrete. There are numerous combinations of these two essential features of driving or boring the pile, but generally the piling method that should be adopted is that which is most suited to the ground condition at the site under consideration.

The important thing in constructing the pile is to ensure that it can be relied upon as the greater part of the construction is undertaken out of sight; it is this aspect that brings the ground conditions into great impor- tance with piling and conversely brings any type of piling which is carried out with disregard to ground conditions into disrepute. There are many methods of piling, some of which are proprietary, and none of which are of universal use in the sense that they are the optimum method of piling, irrespective of the ground and site conditions.

If piling is to be used, then it re- mains for the architect and engineer to make sure that he is using the cor- rect pile to suit the ground conditions.

Choice of Foundation

Economy is usually paramount but the architect and engineer, in their deliberations, must consider the pro- ject as a whole. Although one type of foundation may appear to be cheaper, if it takes longer to construct then the overall cost of the founda- dation in relation to the whole invest- ment may mean that it is more expensive.

Since the foundation is usually that part of the structure which calls for a great deal of personal judgment and experience in design and construction, it is essential that those responsible have the earliest opportunity to con- sider all the relevant facts before they make their choice. A decision taken on assumptions, or the application of the wrong method, often leads to greater difficulties during the period of construction, generally resulting in delays. Once a method is started it is not always easy to make a radical change.

some

The separate types of foundations that are usually considered are briefly reviewed here together with general remarks on their use:- (a) Strip footings. These are rela- tively narrow in width and con-

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form usually with the plan of the structure. Generally being shal- low, of simple design and cons- truction they do not provide any difficult problems. (b) Isolated footings. These are

(c)

separate foundations, generally at a shallow depth, designed to carry specific point loads, usually from columns. Where the carry- ing capacity is adequate for this type, the amount of excavation in the unsuitable soils is reduced to a minimum, although often the closeness of the pits makes a complete ‘over-site' `excavation a more practical proposition. Rafts. Generally these cover the whole building area and are de- signed to reduce the bearing pressures to a minimum. The technique is favoured where there are local weak zones within an otherwise uniform and satis- factory stratum in order to minimise differential settlements.

For taller buildings, weaker soil, or to reduce settlements, a useful device in conjunction with rafts is the basement. The rea- son being that the weight of soil removed for the basement offsets part of the overall building load and so reduces the pressure on the ground.

The special case of bouyant rafts offers a solution when loads have to be carried on soft cohe- sive soils which exist to great depth, in particular on sites where the depth of such soils is greater than can be dealt with either economically or practically by piling.

(d) Pier foundations. These are isolated foundations carrying a large load at depth on rock or on a hard bearing stratum. Con- struction methods include 'open and pneumatic caissons. (e) Piles. These are a method of

transferring the building load into the ground at depth to gain the advantage of support from stronger and less compressible soils without having to resort to excavation.

The mechanism of the be- haviour of piles is only partially understood, but generally piles in cohesive soil, that is clays for example, carry most of the load in friction, and in other soils and rock, in end-bearing on the toe. There are many cases of such loads being carried in both fric- tion and end-bearing.

Both the driven and bored type have individual features which usually indicate that one is to be preferred to the other depending on site and soil con- ditions.

There are machines now avail- able for boring holes of over 1 metre in diameter through all kinds of soils so today there are no known soil conditions which can prevent piling being under- taken in one form or the other.

The present state of knowledge on soil mechanics and the large fund of recorded experience provides us gen- erally with at least one established method of approach, and often there are more, for computing the probable behaviour of the ground under the various conditions which have been described. The foremost problem al- ways to be solved for foundation design is the conditions under which a stable support can be provided for the building loads.

Main Requirement

It is evident from the earlier con- siderations that there are two main requirements to be satisfied, namely an adequate factor of safety against overall shear failure, and a range of settlements within the capacity of the building.

Current practice for assessing the

of foundations stability

be may studied in a number of publications. An interesting paper on this subject, including a number of examples of different types of foundations, with descriptions of the ground conditions in each case, has been presented by Skempton (4).

More often than not the loading conditions from the structure on the underlying soils or rock, together with the soil profile decide the type of foundation to be used.

When a suitable bearing stratum is present near the surface, shallow Under foundations may be used. favourable circumstances strip ог isolated footings may be sufficient. Otherwise it may be necessary to re- sort to the use of a raft, particularly where there are small local variations

in the ground. As the depth to the suitable bearing stratum increases, the cost of excavation, possibly involving control of the ground water, may mean that piling is a less expensive alternative.

Even when rock is exposed at the surface. its capacity to carry heavy loads must not be taken for granted. Such rock can be fissured and can be weak, and as has been emphasised earlier in this paper, the stressing of soils and rocks from the structure can be to depth. Due regard must be given on sites, where rock is apparent, that it is of satisfactory quality to the necessary depth and is not in actual fact large boulders or varied in decomposition, either of which are common occurrences in Hong Kong. In many cases, such as in the re- clamation areas and where there are marine and river deposits, piles are used for high buildings which obtain their support from the insitu decom- posed granite. It is most important to identify the presence of this for- mation by means of exploratory bor- ings to a sufficient depth to design the piles and to distinguish it from any colluvial material which may be superimposed on top of it.

The insitu decomposed granite is commonly exposed at or near the

surface in Kowloon. Where this is the more compact variety as may occur in

Far East Architect & Builder September, 1965