In the Rapid Load Test the load is generated in one of two ways:
(i) A large mass is accelerated vertically upward from the head of the pile using a gun propellant (or rocket fuel as it is euphemistically called). The force that pushes upward on the mass also pushes downward on the pile. The time over which the force is generated depends on the type (i.e. rate of burning) of the propellant, the geometry of the combustion chamber and the mass being accelerated.
(ii) A large mass is dropped onto springs on the head of the pile. The deceleration of the mass generates a force onto the pile. The mass, its velocity when it hits the springs and the stiffness of the springs determine the magnitude of the force and the time over which it is generated.
In both cases: a) the weight of the mass needed is approximately 5% of the test force to be applied to the pile; and b) the time over which the force is applied is made long enough for the wave length of the force to be at least twelve times the length of the pile, so that the whole pile is in compression simultaneously. “Wave equation” theory is now not relevant, greatly simplifying the analysis, which in its basic form is called the “unloading point method”.
Further details of the unloading point method can be seen by clicking here.
From this test and the unloading point method of interpretation, a graph of the slowly applied (static) pile head load v. deflection is obtained. It has been shown to be sufficiently accurate for most purposes in sand soils but to overestimate a pile’s capacity in clay soils.
Because an overestimate is unsafe, research at The University of the Witwatersrand has been done on a wide variety of soil types and degrees of saturation to improve the interpretation of the measurements made in the Rapid Test into the behaviour of the pile under static load. See “Static pile capacity from the rapid test via the unloading point method” J.S.Afr.Inst.Civ.Eng. 2021:63(2) by clicking here.
Tambew is presently unable to offer the Rapid Test, but equipment to do it is under construction.
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