Civil Engineering Reference
In-Depth Information
should be at least four pile diameters from the test pile. It is noted that a compromise
must frequently be made between a long beam prone to vibration and temperature-
induced deflections, and a shorter beam with foundations in the zone of influence of
the test pile or kentledge.
The reference beam should be a heavy steel section or a rectangular hollow section
tube clamped at one end. The other end is free to slide, but should be restrained against
lateral and vertical movement (see Figure 9.13). Such a system will allow longitudinal
movement of the beam caused by changes in temperature. The beam should be shaded
from direct sunlight, and in hot climates it may be necessary to carry out the test at
night when temperature fluctuations are less marked.
To check for gross errors in the observations or disturbance to the datum beams, it
is good practice to carry out precise level checks of the test pile and datum beams at
intervals throughout the test. If tension piles are being used to provide the reaction,
these should also be levelled to check for undue movements.
The accuracy of any
optical levelling system
is poorer than that of displacement
transducers or dial gauges by a factor of at least 10. However the absolute accuracy
of the system may be of a similar order, particularly in situations where it is difficult
to establish a stable reference beam. Using optical methods it is difficult to arrange a
constant rate of penetration test or to check the cessation of movement under a load
Figure 9.13
Details of datum beam support. (Building Research Establishment: reproduced by
permission of the Controller, HMSO. Crown copyright.)
