Axial load transfer of drilled shaft foundations with and without steel casing

Abstract

Steel casing is commonly used in drilled shaft construction to maintain the integrity of the borehole during drilling; however, little guidance regarding the effect of the casing on axial load transfer exists in the literature. To address this aspect of drilled shaft design and construction, this paper presents a study of axial load transfer of drilled shaft foundations using four, full-scale, instrumented drilled shafts: two uncased and two cased drilled shafts. Axial loading tests were performed and used to compare various performance metrics between the cased and uncased shafts, including the axial load-displacement curves, load transfer distributions and back-calculated unit shaft resistance-relative displacement relationships (t-z curves). The uncased test shafts exhibited significantly greater axial shaft resistance compared to the cased test shafts, and data from thermal integrity profiler (TIP) sensors allowed interpretation of the differences in soil-shaft contact conditions and the resulting load transfer. Although the ultimate axial resistance of the uncased test shafts could not be mobilised, sufficient data were developed to allow comparison to the cased test shafts and extrapolation to anticipated ultimate resistance conditions. The back-calculated t-z curves of the uncased test shafts were modelled and used to estimate the anticipated large deformation response. Based on observations in this study and those previously reported, the effects of permanent casing on axial load transfer are summarised to provide an up-to-date reference on the reductions expected based on construction sequencing and installation methods.