PILING
AND FOUNDATIONS
By Mr. J. N. Frenkel, Dipl. C.E., A.M.I.E. Aust.
now
is
Mr. J. N. Frenkel, Dipl. C. E., Therefore, in the beginning of this cen- Oregon pine piles, the length being A. M. I. E., Aust. has given recently tury steel piles made their appearance, 40-100 ft., was considered one of the most a series of lectures on "Piling" in the
suitable. It But even before that with the develop-
is necessary to emphasize University of Hong Kong. In those
that broadly speaking there ments of the cement industry, engineers lectures he dealt with piling mainly diverted their attention to piles made of nothing inherently wrong with timber pile as a type of foundations, although concrete. Such piles quickly gained univer- as he mentioned, piling is also a part sal recognition and at present there are of the foundations.
many types in existence.
After outlining the usual problems confronting an architect or engineer he dwelt on modern aspects of the science such as principles of soll mechanics. various aspects of the technique of pile driving, and so on. Herewith is an extract from his lectures of items which might be of general interest.
4
the structure to more solid strata, which Piles are used to transfer the weight of
are to be found below the softer upper layer of soil. It is also commonly assum- ed that there are two classes of piles: friction piles and point bearing piles However, friction and point bearing do not mean types of piles, for any type can be one or the other or, more often, both.
PILING AND FOUNDATIONS
A friction pile is apparently one which carries a It is commonly accepted that the de-
load by friction of its lateral signation "Pile"
It is assumed comes from the Latin surface against the soil. word "Pila" meaning column. Such inter- therefore that such a pile did not reach a pretation from technical point of view is hard stratum, because such stratum is not quite correct, for there is an obvious not available at a practical depth. difference between the nature of the point bearing pile is apparently one which work of a column and that of a pile. did reach a hard stratum, and transfers Both are supporting loads, but as it shall the load mainly by direct pressure on be seen later, in quite a different way. such stratum.
Generally speaking, a pile is a technical It would, however, be quite amateurish device by which loads are transferred to to assume that a friction pile works only more solid strata of subsoil. It follows, by friction and a point bearing pile only naturally, that piles are driven in those by direct pressure. Comparatively long cases when the upper strata of soil are piles in deep compressible strata are arbi- not hard enough to take safely the de- trarily called friction piles, because it is signated loads resulting from the weight considered that the main source of resis
tance of such pue is friction. Such an assumption would be more justified for slim and sharp pointed types of piles in deep strata.
of the structure.
con-
Pile driving for the purpose of solidating weak grounds was used at a very early date in Europe.
Further trends of development of pile
There are four common types of piles: driving took place in the 19th Century timber, steel, precast concrete, and cast during the Industrial Revolution, which in-situ concrete. took place in Western Europe and in the United States.
ment
foundations, provided that they are de signed and built technically correctly.
A correctly designed and built timber foundation answers the following
pile conditions:
I.
Correct length of pile determined, 2. An appropriate safe load per pile
selected,
3. Adequate number of piles and spac
ing between piles determined,
4.
Piles properly prepared for driving (Creosoted, shod and supplied with iron headgear),
5.
Piles driven to an appropriate set,
6.
Heads of piles do not protrude above the level of the underground water,
7. Footings on the piles are properly
designed and built.
There are many structures all over the
timber piled foundations world which are being safely carried on built to the above requirements. Unfortunately, how- ever, in Hong Kong during the past ten totally inadequate timber piling has caused many houses to settle unevenly with accumulating damage to the superstructures.
ΟΓ years
So,
Cast-in-situ concrete piles are a com- paratively recent development. They re present a distinct and developed industry, which is highly specialised and nearly all such types are patented.
are
A cast-in-situ pile, as the name suggests Timber piles are the oldest of all sys-
is one which is cast in place, as distinct tems. Broadly speaking a timber pile from pre-cast: that is, a hole is driven It may be said that with the commence- means a trunk of a tree made suitable to or bored in the ground, and then it is
of heavy structures, foundation be driven into the ground to support a
filled with concrete. The reasons why The type of timber depends, of cast-in-situ piles generally became a
used more expensive item load.
mainly
for in the Bill of Quantities, and it was no course, on locality and supply, but foundations on dry ground, and precast longer feasible to drive piles just for soil consolidation, without considering the actual requirements for bearing capacity.
what The question naturally arose shall be the ultimate load for a certain pile in certain soils?
——
Because of high cost of actual loading test on piles, engineers had to resort to theoretical speculation. This resulted in quite a large number of piling formulae, most of them of an empirical nature. Some of the formulae were an attempt to arrive at a reasonable solution by mathe- matical methods, but it was realized that a reliable formula by such methods was virtually impossible to devise.
Thus by the end of the 19th Century there was already in existence a science dealing with foundations and also rudi- ments of the science called "Soil Mechanics''.
iL
The technique of pile driving received further impetus when it was realized that timber piles were limited in length. as well as having certain technical disad- vantages.
H
MARKS
GROUND LEVEL
WATER
TABLE
Installation procedure of "Franki” piles.
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