Determining the True Distributions of Load in Instrumented Piles

Tests on piles—driven or bored—sometimes include instrumentation for determining the load distribution. The instrumentation can range from a telltale or two through a sophisticated array of strain gages, and from tests with the limited purpose of separating shaft and toe resistances through tests for the purpose of detailing the shaft resistance distribution along the pile. Most of the time, the measurements are analyzed from the assumption that the "zero readings", which are the readings taken at "zero" time, i.e. at the outset of the test, also have registered "zero" load. This assumption is more than a little off. It neglects the existence of locked-in loads—residual load—in the pile and is one of the sources of the myth of the so-called "critical depth". Neglect of the residual load distribution is also the main foundation behind conclusions of instrumented tests that suggest shaft resistance to be smaller when the pile is loaded in tension as opposed to when it is in loaded in compression. This paper presents examples of measured distributions of residual load and true resistance, and indicates how to determine the distribution of residual load from measurements when the residual load distribution is not measured directly.