Response of a Drilled Shaft Foundation Constructed in a Redrilled Shaft Excavation Following Collapse-Article

Article

Pdf Summary

The study examined how excavation collapse affects the performance of drilled shaft foundations (DSFs). Two full-scale, 1.2 m diameter shafts were constructed at the Turrell Arkansas Test Site in layered clay, silt, and sand. During construction of the North shaft, the silt layer collapsed into the excavation. The hole was redrilled and the shaft completed; the South shaft served as a comparison.<br /><br />Both shafts were instrumented and tested using bidirectional load cells. The collapsed-shaft excavation was modeled with modified soil properties, and predictions were compared with measured responses.<br /><br />The North shaft showed substantially greater movement than the South shaft under similar loading, and lower unit side resistance in the silt and much of the sand. Its higher end-bearing resistance at larger movements may have resulted from being founded slightly deeper in cemented sand and from more load transferring to the base. The collapse therefore altered load distribution and reduced overall stiffness, although it did not uniformly reduce all resistance components.<br /><br />The measured response of the collapsed shaft was best approximated by reducing the silt layer’s unit weight by 10% and modeling the upper 3 m of sand as silt—representing reduced horizontal stress and partial coating of the shaft by collapsed silt. This model matched the response to about 1 cm of movement, but existing methods did not capture the continued increase in measured side resistance at larger movements.<br /><br />The authors recommend accounting for reduced soil stress after collapse and considering deeper drilling to mitigate potential losses in end bearing. They also call for additional full-scale tests and caution that results from shafts built in collapsed excavations should not be used directly to predict performance of shafts in undisturbed excavations, or vice versa.

Keywords

drilled shaft foundations

excavation collapse

load distribution

side resistance

end-bearing resistance

full-scale testing

layered clay silt sand

bidirectional load cells

soil stress reduction

shaft performance