516 Ground investigations, contracts and pile testing
While it is essential for the toe level and final set of every pile to be recorded, BSEN
12699 does not mandate a full record of sets during driving. There are, however, advantages
in providing a log of the blow count against penetration over the full depth for every pile
driven. If, for example, piles are to be driven to end bearing on a hard stratum it may be
sufficient to record the sets in blows for each 25 mm of penetration after the pile has reached
the hard stratum. On the other hand, where piles are supported by shaft friction, say in a
stratum of firm to stiff clay, or in a granular soil overlain by weak soils, it is essential
to record for every pile the level at which the bearing stratum is encountered and hence to
check that the required length of shaft to be supported is obtained. For this purpose, the blows
required for each 500 mm or each 250 mm of penetration must be recorded over the full
depth of driving of each pile, until the final metre or so when the sets are recorded in blows
for each 25 mm. Sometimes final sets are recorded as penetration depths for 10 to 25 blows
of the hammer. The advantage of recording the full driving log for piles of every category is
that if troubles arise, such as pile breakage, the records of each pile can be scrutinized, and
any which show peculiarities can be singled out for special examination or testing.
At the preliminary piling stage the driving records are compared with the ground investiga-
tion data, the static design criteria and with the results of loading tests, and suitable criteria
regarding final sets for terminating driving are established. If the methods of Chapter 4 have
been used for calculating the penetration depth of friction piles, the depth into the bearing
stratum should, theoretically, be the only criterion, and final sets should be irrelevant.
However, because of natural variations in soil properties piles with identical lengths in the
bearing stratum will not necessarily have identical ultimate loads. By driving to a minimum
depth into the bearing stratum and to a constant final set (or to within a specified range
of set) the variations in the soil properties can be accommodated.
A minimum penetration is necessary because random compact layers in the soil may
result in localized areas of high driving resistance. The driving records within these layers
should be compared with the ground investigation data, so that suitable termination levels can
be established. The establishment of criteria for controlling the termination of piles driven
into layered soils is described in Section 4.5.
It is advisable to conduct re-driving tests on preliminary piles, and on random working
piles. These tests are a check on the effects of heave and on possible weakening in resistance
due to pore pressure changes. Re-driving can commence within a few hours in the case of
granular soils, after 12 hours for silts, and after 24 hours or more for clays. If the re-driving
shows a reduction in resistance after about 20 blows, driving should continue until the
original final set is regained.
Diagrams of the driving and re-driving tests should be made for the preliminary piles, and
compared with the borehole records and with in-situ and laboratory test data.
The temporary compression at various intervals of pile driving is irrelevant if working
loads have been obtained by the methods described in Chapter 4. However, if dynamic
formulae are adopted the temporary compression values must be taken at intervals after the
pile enters the bearing stratum. The values are obtained by securing a sheet of graph paper
to the pile by adhesive tape. A straight-edge is held horizontally close to the pile and using
the straight-edge a pencil line is drawn across the paper during the impact of the hammer
(Figure 11.5a). The pattern of the pencil line is shown in Figure 11.5b from which the
temporary compression is measured.
Other items to be recorded include any obstructions to driving or damage to the pile
and deviations in alignment which might indicate breakage below the ground surface.
Methods of checking the alignment of steel tubular and H-piles are described in Section 2.2.4.