Rate effects from pile shaft resistance measurements

To be able to properly analyse rapid load pile tests in clay soils (e.g., Statnamic) an understanding of the material's behaviour at elevated shearing rates is required, as clays demonstrate nonlinear shearing resistance as the rate of shearing increases. To determine the relationship between pile resistance under static and elevated rates of loading, rate effect parameters for viscous damping have historically been derived from pile head measurements on full scale pile tests. These parameters are then attributed to a soil type with little recognition of what aspects of a soil may give further variation to them. To study this variation a test pile was installed in glacial till and instrumented with strain gauged sister bars at various levels down the pile shaft. Direct measurements of pile shaft resistance in both rapid load and static pile tests show that derivation of average parameters from pile head readings masks the variation in rate effect along a pile shaft. The variation of rate effects along the pile shaft is apparently linked to in situ moisture content.