Static loading test on a 45 m long pipe pile in Sandpoint, Idaho

Abstract

Design of piled foundations for bridge structures for the realignment of US95 in Sandpoint, Idaho, required a predesign static loading test on an instrumented, 406 mm diameter, closed-toe pipe pile driven to 45 m depth in soft, compressible soil. The soil conditions at the site consist of a 9 m thick sand layer on normally consolidated, compressible, postglacial alluvial deposits to depths estimated to exceed 200 m. Field explorations included soil borings and CPTu soundings advanced to a depth of 80 m. The clay at the site is brittle and strain-softening, requiring special attention and consideration in geotechnical design of structures in the area. Effective stress parameters back-calculated from the static loading test performed 48 days after driving correspond to beta coefficients of about 0.8 in the surficial 9 m thick sand layer and 0.15 at the upper boundary of the clay layer below, reducing to 0.07 in the clay layer at the pile toe, and a pile toe bearing coefficient of 6. The beta coefficients are low, which is probably due to pore pressures developing during the small shear movements during the test before the ultimate resistance of the clay was reached. The analyses of the results of the static loading test have included correction for residual load caused by fully mobilized negative skin friction down to 10 m depth and fully mobilized positive shaft resistance below 30 m depth, with approximately no transfer of load between the pile and the clay from 10 m depth through to 30 m depth.Key words: pile loading test, residual load, strain-gage instrumentation, load distribution, setup, pile modulus.