Estimating soil/pile set-up

Abstract

Abstract Soil/pile set-up is time-dependent increase in pile capacity, and can contribute significantly to long-term pile capacity. If it were possible to incorporate the effects of set-up during design, it may be possible to reduce pile lengths, reduce pile sections (use smaller-diameter or thinner-wall pipe piles, or smaller-section H-piles), or reduce the size of driving equipment (use smaller hammers and/or cranes). Any one, or a combination, of these reductions should result in cost savings. Accordingly, this research investigated, through a thorough review of the literature and the state of the practice, if it is possible and practical to estimate set-up during design, using information obtained during a relative routine subsurface exploration program. A literature search was conducted; a references list is provided. Set-up is predominately associated with an increase in shaft resistance. The complete mechanisms contributing to set-up are not well understood, but the majority of set-up is likely related primarily to dissipation of excess porewater pressures within, and subsequent remolding and reconsolidation of soil, which is displaced and disturbed during pile driving. After excess porewater pressures have dissipated, aging may account for additional set-up. A number of empirical relationships have been proposed to estimate or predict set-up, and have demonstrated reasonable success in a number of studies. Empirical relationships are limited in widespread application by the relationships having been based on combined (shaft and toe) resistance determinations, inter-dependence of back-calculated or assumed variables, and the complexity of the mechanisms contributing to set-up. A number of exploration-phase field tests offering potential value in predicting set-up have been identified: SPTTorque test, SPT-Uplift test, piezocone testing, dilatometer testing, and vane shear testing. Of these, the SPTTorque test appears to offer the most-favorable combination of applicability of results, ease and simplicity of performing the test, and cost. It is recommended that SPT-Torque testing be performed on a number of sites where satisfactory set-up data is available to determine if a meaningful relationship exists between the ?set-up? which develops on a split-barrel sampler, and the set-up which develops on a driven pile, in various soil strata. If a meaningful relationship is determined, SPT-Torque tests could be added to routine subsurface exploration programs to measure sampler/soil set-up in various soil strata, and predict production pile set-up.