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1. INTRODUCTION
1.1 Theory of Operation
The ability of cast-in-place piles to support a load relies on friction along the pile and on end
bearing. The load distribution along the pile can be measured by embedding strain gauges at
different depths along the pile and by comparing the measured strains at different depths with the
strains at the top of the pile, very close to the applied load, which are assumed to be equivalent to
100% of the applied load. This ratio method does not require knowledge of the concrete
modulus. Another method, to determine the actual end-bearing load taken by the pile tip is to
measure it directly by installing a pressure cell between the pile tip and the ground below.
Application of the load to the top of the pile causes pressure to be developed inside the pile tip
pressure cell and this pressure, when multiplied by the area of the pile tip pressure cell, is
directly equivalent to the end bearing load.
1.2 Pile-Tip Pressure Cell Design and Construction
The basic cell is manufactured to be close to the diameter of the pile. It is comprised of two
circular stainless steel plates welded together around their periphery, leaving a thin space
between the plates filled with de-aired hydraulic oil. This oil filled space is connected via a
pressure tube to a vibrating wire pressure sensor. End-bearing pressure applied normal to the
plate is balanced by a corresponding build-up of internal oil pressure, which is measured by the
sensor. The use of de-aired hydraulic oil guaranties that the modulus of the pile tip pressure cell
is equal to or greater than the modulus of the surrounding concrete. This ensures that the pressure
measured by the cell is characteristic of the pressure across the entire cross-section of the
borehole and that there is no error created by a certain amount of the load being transmitted
directly through the concrete around the edges of the pressure cell.
Figure 1 - Showing a Pile Tip Pressure Cell