Study of Effects of Construction Methods on Performance of Drilled Shafts-Paper

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The study examines how drilling and construction practices affect the axial capacity of drilled shafts in alluvial soils at four projects in Punjab, Pakistan. It compares predicted capacities with full-scale static load-test results for shafts built using rotary bucket auger (RBA) and reverse rotary (RR) methods, under dry or fluid-supported conditions.<br /><br />Load-test results varied substantially among methods and sites, showing that soil parameters alone do not reliably determine shaft capacity. RBA shafts constructed in dry conditions generally performed better than RR shafts constructed using water: several dry-bored shafts did not reach yielding even at more than 2.8 times their design load. For shafts bored with water, predictions were sometimes too high but were more often conservative. RBA shafts constructed with bentonite also showed mixed results. When bentonite slurry was properly controlled, RR shafts using water and RBA shafts using bentonite could achieve capacities of similar order.<br /><br />Poorly controlled bentonite can reduce shaft resistance. If left in the bore too long, slurry forms a thick, stiff cake on the bore wall that may not be removed during concreting, weakening the soil–concrete interface. The study also notes that RBA generally causes less soil disturbance and offers better control of borehole verticality.<br /><br />The authors recommend strict control of slurry properties and construction timing, along with effective cleaning of the bore and pile base. They emphasize that pile load testing should be required to finalize drilled-shaft designs, and that design parameters should be calibrated against test results for the specific construction method and local ground conditions.

Keywords

drilled shafts

axial capacity

alluvial soils

static load test

rotary bucket auger

reverse rotary

bentonite slurry

soil disturbance

pile design

Punjab Pakistan