Civil Engineering Reference
In-Depth Information
lengths of shaft, or over relatively short rock sockets. They also rely on the excavation
remaining open long enough to lower the reinforcing cage and to fi ll it with concrete.
This may require temporary casing or the use of bentonite or other drilling mud. In
order to choose the method of construction, and to calculate the capacity of the pile,
the properties of the various strata all the way down the length of the pile must be
known. Also, whereas triangulated driven pile systems are generally analysed as if the
soil around the piles did not exist, vertical bored piles rely on the ground for support.
When horizontal loads are applied to the foundation, it tries to sway and the soil around
the piles resists the movement. Thus accurate knowledge of the properties of the upper
levels of the soil is required to analyse correctly this soil/structure interaction.
The site investigation should start with a desk study based on geological maps, and
on previous site investigations or foundation construction in the vicinity. It should
also look at any archaeological history that may be relevant, so that the presence of
man-made objects may be predicted. The study of aerial photographs, particularly
stereoscopic pairs when available, can be very revealing. For instance if valleys between
adjacent hills are seen disappearing under the sediments of the plain, one may expect
to meet the complications caused by these buried valleys when piling. When a bridge
designer interprets the results of a site investigation, he is tempted to extrapolate
the information obtained from isolated boreholes to the remainder of the site. The
knowledge obtained from the desk study helps dispel such complacency.
The investigation should then continue with a fi rst stage campaign of boreholes at,
say 100 m centres, to establish the general succession of strata, to plan which type of
testing will be required, and to allow a specifi cation for the second stage investigation
to be written. The second stage investigation should consist of at least one borehole at
each pier position. If the pile caps are large, say longer than 10 m, and if the strata are
known to be sloping, faulted or discontinuous, more boreholes per pier position will
be justifi ed. It is very rare to see an over-specifi ed site investigation, and it is also rare
that additional knowledge of the sub-soil will not result in cost savings that far exceed
the cost of the investigation.
There are, of course, some sites where the sub-soil is very uniform over large
distances, when all that is needed is confi rmation that there are no unexpected changes.
In these circumstances, the above warnings are less relevant.
However, the experience of one section of the investigation for the STAR railway
viaduct foundations will act as a useful antidote to the complacency or lack of
imagination that appears to affl ict designers when dealing with the sub-soil. These
foundations consisted of a single 1.8 m diameter pile per pier, as described in
7.15.3
below. For a particular length of the viaduct, the ground consisted of soft strata
overlying limestone. Although it is well known that the limestone in Kuala Lumpur
may be heavily eroded, a borehole sunk at each pile position was interpreted as regular
rock head profi le shown in Figure 7.7 (a).
When the fi rst pile was sunk, it reached bedrock at a much greater depth than
expected. Additional boreholes showed that the pile was located at the edge of a near-
vertical 20 m deep cliff face in the limestone. Thereafter, three additional boreholes
were sunk in a triangular pattern at each pile position, demonstrating that the rock
head profi le, far from being regular was tortuous in the extreme, with the construction
of each pile becoming a major engineering feat, Figure 7.7 (b).
