Full-Scale Load Testing of 75–90-m-Long Post-Grouted Drilled Shafts in Suzhou Stiff Clay

Abstract This article evaluates the effects of post-grouting on axial behavior of long drilled shafts in stiff clay. Static axial compression and tension load tests were conducted on four ungrouted and six post-grouted shafts to investigate the effects of post-grouting on axial capacity and load-transfer mechanism. After post-grouting, axial capacity increased more than 100 %. Prior to failure, the relationship between the normalized axial load Q/Qmax and shaft head displacement Δ/D was nonlinear for ungrouted shafts. Compared with cases presented in the literature, much greater shaft displacements were required to mobilize shaft tip resistance and skin resistance, indicating a softer load-displacement response. The contribution of tip resistance to the overall load capacity was small for both ungrouted and post-grouted shafts because of their length. Shaft load capacity was significantly underestimated by the standard penetration test (SPT)-based method and α-method and was more reasonably estimated by the β-method. The cone penetration test (CPT) direct method yielded generally reliable predictions of skin resistance mobilized in the upper stiff clay soils; however, this method significantly underestimated skin resistance in the underlying very stiff to hard clay layers. Considering uncertainties in current design methods, results of this study suggest that field load tests are needed prior to construction to obtain site-specific design parameters.