Civil Engineering Reference
In-Depth Information
Fig. 10.4
Installation of a Franki pile.
The
Franki pile
is another example. A steel tube is erected vertically over the place where the pile is to
be driven, and about a metre depth of gravel is placed at the end of the tube. A drop hammer, 1500 to
4000 kg mass, compacts the aggregate into a solid plug which then penetrates the soil and takes the steel
tube down with it. When the required set has been achieved the tube is raised slightly and the aggregate
broken out. Dry concrete is now added and hammered until a bulb is formed. Reinforcement is placed in
position and more dry concrete is placed and rammed until the pile top comes up to ground level. The
sequence of operations is illustrated in Fig.
10.4.
10.3.4 Large diameter bored piles
The driven or bored and cast-in-place piles discussed previously generally have maximum diameters in
the order of 0.6 m and are capable of working loads round about 2 MN. With modern buildings column
loads in the order of 20 MN are not uncommon. A column carrying such a load would need about ten
conventional piles, placed in a group and capped by a concrete slab, probably about 25 m
2
in area.
A consequence of this problem has been the increasing use of the large diameter bored pile. This pile
has a minimum shaft diameter of 0.75 m and may be under-reamed to give a larger bearing area if neces-
sary. Such a pile is capable of working loads in the order of 25 MN and, if taken down through the soft
to the hard material, will minimise settlement problems so that only one such pile is required to support
each column of the building. Large diameter bored piles have been installed in depths down to 60 m.
10.4 Pile load testing
The only really reliable means of determining a pile's load capacity is through a pile load test. These tests
are expensive, particularly if the ground is variable and a large number of piles must therefore be tested,
but they do provide reliable data by which the design of further piles can be based. In the tests, full-scale
piles are used and these are installed in the same manner as those placed for the permanent work.
During pile testing, a load is applied to the top of the pile and the settlement of the pile is recorded
against force or time, depending on the test. Tests can be categorised as either
static load
or
dynamic
load
tests. In addition, soil test results can be used to aid the determination of the pile load capacity.
10.4.1 Static load tests
Maintained load test (MLT)
In this test, a load is applied to the pile in discrete increments, usually equal to 25% of the designated
working load for the pile, and the resulting pile settlement is monitored. Subsequent load increments are
only applied when the rate of induced settlement drops below a specified criteria.
