A Comparison of Open Drilled Shafts to Temporarily Cased Drilled Shafts in Clay Soil-Article

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The paper compares the axial capacity of drilled shafts with the upper 10 feet temporarily cased against open-hole shafts in expansive clay in Irving, Texas. Field load tests were conducted on three 12-inch and two 24-inch cased shafts and compared with earlier tests of open-hole shafts at the same site. The cased sections were kept saturated during installation; casing was withdrawn while maintaining a concrete head intended to push water upward and reconsolidate the surrounding clay.<br /><br />Test results showed that cased shafts achieved capacities broadly comparable to open-hole shafts. The authors argue that skin friction is not necessarily lost in temporarily cased sections. Two factors may explain the results: casing larger than the specified shaft diameter increased the contact area, and pressure from the concrete head may have remolded and reconsolidated disturbed, saturated clay during casing removal. Cased shafts showed somewhat greater deflection before reaching ultimate capacity, possibly reflecting softer soil along the cased section.<br /><br />The study challenges the practice of assigning zero skin friction to temporarily cased portions, which can lead to unnecessarily deeper, more costly shafts. It suggests that engineers may be able to credit nearly full skin friction where casing is required, provided construction is competent.<br /><br />However, the findings are preliminary: only a small number of shafts were tested, and clay sensitivity and construction conditions vary by site. The authors recommend further field and laboratory testing before establishing design factors, and caution that any skin-friction adjustment should be site-specific.

Keywords

drilled shafts

axial capacity

temporarily cased shafts

open-hole shafts

skin friction

expansive clay

field load tests

concrete head pressure

casing removal

foundation design