Pile testing - Piling Engineering - page 309

of the system should be designed and installed to avoid any risk of sudden load
reduction or over stress.

9.2.2.2 Load application and measurement

The application of the load to the test pile needs to be closely controlled, and a range
of hydraulic jacks is readily available. Short, large-diameter low-pressure jacks are to
be preferred to the high-pressure type as they are more stable. The capacity of the
jack is not particularly important provided it exceeds the maximum applied load by a
generous margin, and has sufficient travel of the ram (at least 100mm). For very high
loads it may be necessary to use a cluster of jacks. These should be set in a cradle to
prevent relative movement of the jacks or uneven loading.
The jack may be actuated by a hand pump for loads up to approximately 300 tonnes.
However, for very high loadings or constant rate of penetration tests, it is preferable
to use a motorized pumping unit. Pressure gauges should be fitted to the system to
allow the operator to check the load in the system at a glance, as he may be some way
from the test pile.
The line of action of the load should be co-axial with the pile and time is well spent
setting up the jack, load cell, etc., carefully (Figure 9.6). To take up slight misalignment
or lack of parallelism a large-radius spherical seating should be inserted under the
reaction beam. Under load, spherical seatings lock and will not correct any eccentricity
of load during the test. Small-diameter ball seatings tend to lead to instability of the
loading trains and increase the likelihood of violent collapse of the train under load.
If an angular misalignment of more than a few degrees is noted, the reaction system
should be repacked to eliminate the error.
In general the pressure gauges fitted to the hydraulic jack are not suitable for measur-
ing the load applied to the test pile. Test-quality gauges may be fitted and calibrated
with the jack in a compression loading frame. Such calibrated jacks may provide
acceptable accuracy of load measurement for increasing load; however, the unloading
cycle is usually non-linear owing to friction within the hydraulic jack. A typical cali-
bration is shown in Figure 9.7; note the jack and gauge must be clearly marked and
used together.
Load measurement is preferably carried out using a load cell, and several types are
available. Commonly adopted load measuring devices, in order of accuracy, are:

1 hydraulic load capsule, maximum capacity 450 tonne;

2 load columns, maximum capacity 1000 tonne;

3 proving rings, maximum capacity 200 tonne; and

4 strain gauged load cells of various types, maximum capacity 200 tonne.

For very large loads an array (preferably at least three) of load columns or
strain-gauged load cells may be used. It is important that the load cell is calibrated
regularly and that, for those devices sensitive to temperature, suitable corrections are
determined.
Hydraulic load cell capsules are basically short, large-diameter hydraulic cylinders
(Figure 9.8). The piston is bonded to the casing with a rubber element, sealing the
oil-filled chamber. The oil pressure is measured with an accurate pressure gauge,