Civil Engineering Reference
In-Depth Information
Figure 9.7
Typical calibration of hydraulic jack.
cells, particularly the electrical resistance type, should be environmentally protected.
The load cell developed by the Mining Research Development Establishment has been
found to be particularly satisfactory. Special readout units are required for the load
cells, although battery powered devices suitable for site observations are made by
several firms. The overall accuracy of the system should be better than 0.5%. The load
cells are usually temperature compensated although extreme changes of temperature
should be avoided.
9.2.2.3 Measurement of settlement
The measurement of deformation is perhaps the most difficult observation to make
reliably. The use of dial gauges calibrated in 0.01mmmay give a false sense of accuracy
to the observations. In practice, measurement to this precision is not necessary for pile
testing and it is extremely difficult to maintain this degree of accuracy over a long
period of time. The most commonly used systems of settlement measurement are now
electronic displacement transducers, although dial test indicators or optical levelling
may also be used as a supplement.
Electronic displacement transducers
, either of a potentiometer type with a sliding
voltage divider, or a linear variable differential transformer (LVDT), are the modern
replacement of dial gauges, and have the advantage of being able to be logged
continuously throughout a test, minimizing manual readings. The resolution of the
displacement transducer should be around 0.01mm.
