Axial Load Transfer of Drilled Shaft Foundations With and Without Steel Casing-Article

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The study examined how permanent steel casing affects the axial load transfer of drilled shaft foundations. Researchers conducted full-scale compression tests on four instrumented shafts at a site in Oregon: two uncased shafts and two permanently cased shafts. Thermal integrity profiling (TIP) helped assess shaft geometry and identify possible gaps between casing and surrounding soil.<br /><br />The uncased shafts developed substantially greater resistance than the cased shafts. Although their ultimate capacity was not reached, one uncased shaft resisted about 210% more load than a cased shaft that had reached its ultimate resistance. The cased shafts plunged at much larger displacements. Load-transfer measurements and TIP results indicated that gaps around the casing, along with the smoother soil–steel interface, reduced resistance. A small difference in auger diameter also had a marked effect: the cased shaft drilled with an auger slightly smaller than the casing performed better than the shaft drilled to approximately the casing diameter.<br /><br />Back-calculated unit shaft resistance–displacement (t–z) relationships were developed for the tested shafts. For the cased shafts, resistance ratios relative to uncased shafts were generally low—about 3–23% at larger relative movements in the tested soil layers. The study’s review of earlier field tests showed that casing effects vary with soil type and installation sequence; vibration may increase resistance in some sands, while other methods can cause substantial reductions.<br /><br />The authors conclude that design reductions for permanently cased shafts should reflect construction method, sequencing, soil conditions, and the potential for casing–soil gaps. TIP measurements may help identify such gaps and guide remedial work.

Keywords

permanent steel casing

drilled shaft foundations

axial load transfer

full-scale compression tests

thermal integrity profiling

cased shafts

uncased shafts

soil–steel interface

shaft resistance

construction method