Pile Design and Construction Practice, Fifth edition

524 Ground investigations, contracts and pile testing
BS EN 1536 advises that the supports or anchorages of a reaction system for pile loading
tests should observe minimum clearances from the test pile (diameter D) as follows:
(a) Kentledge supports: clearance � 3 � D
(b) Bond lengths of vertical anchorages: clearance � 3 � D and 3 m
(c) Bond lengths of inclined anchorages: clearance � 5 � D and � 5 m
BS EN 12699 for tests on displacement piles and EC7-1 are silent on support spacing.
The combined weight of the kentledge and reaction girders, or the calculated resistance
capacity of tension piles or cables, must be greater than the maximum jacking force
required. In the case of kentledge loading the combined weight should be about 20% greater
than this force. Cable anchorages or tension piles should have an ample safety factor against
uplift. The former can be tested by stressing the anchors after grouting them in. If there is
any doubt about the uplift capacity of tension piles a test should be made to check the design
assumptions. Increased capacity of tension piles in clays can be obtained by under-reaming
them (see Section 6.2).
The reaction girders and load-spreading members should be so arranged that eccentric
loads caused by any lateral movement of the pile head will not cause dangerous sidesway, or
buckling of the girders. Connections should be bolted so that they will not become dislodged
if there is a sudden rebound of load due to the failure of the pile shaft or of the jack.
Similarly the kentledge stack should not be arranged in such a way that it may topple over.
Restraint by a pair of anchors from a single pile to each end of the reaction girder is not
a good practice as it can cause dangerous sidesway of a deep girder. The piles or anchor
cables should be placed in pairs at each end of the girders, as shown in Figures 11.8 and
11.9. Permanent piles can be used as anchorages for ML tests on working piles but it is
unwise to use end-bearing piles for this purpose when the shaft friction will be low and the
pile may be lifted off its seating. When using tension piles special threaded anchor bars
extending above the pile head should be cast into the piles for attachment to the reaction
girders. It is inadvisable to weld such bars to the projecting reinforcing bars because of the
difficulty in forming satisfactory welds to resist the high tensile forces involved.
The hydraulic jack should have a nominal capacity which exceeds by 20% or more the
maximum test load to be applied to the pile. This is necessary in order to avoid heavy manual
pumping effort when nearing maximum load and to minimize the risks of any leakage
of oil through the seals. The jacking force on the pile head should be measured by a load
cell or pressure capsule since the pressure gauge fitted to most jacks is not sufficiently accurate,
particularly when working towards the maximum capacity of the jack. However, the
jack should have a pressure gauge mounted on the pumping unit which is calibrated to read
in terms of the force on the ram. This gauge is necessary since the load cell mounted on the
pile head may not be visible from the pumping position. For high-capacity piling tests, much
heavy manual effort is saved by providing a mechanical pumping unit, and where CRP tests
are being made a load pacer is a useful addition. The ram of the jack should have a long
travel where piles are being loaded near to the failure condition. This avoids the necessity of
releasing oil pressure and repacking with steel plates above the ram as the pile is pushed into
the ground. Equipment is available for monitoring and restoring jacking loads at intervals of
only a few seconds (11.20) .
The reaction girders, anchorages and jacking arrangements for a 5800 tonne static load
test in Taipei are shown in Figure 11.10.