Field Testing of Jet-Grouted Piles and Drilled Shafts

A field study of deep foundations supporting high mast lighting and signage was undertaken in typical Florida soils. Three drilled shafts (48 in x12 ft and two 48 in x18 ft) and two jet-grouted piles (28 in x18 ft) were constructed in Keystone Heights, and subject to axial as well as combined lateral with torsional loading. In the case of combined loading, a heavy-duty mast arm equivalent to Florida Department of Transportation's (FDOT’s) E7-T6 type was placed on the top of each foundation and loaded with a crane. In situ as well as laboratory data were collected for the sand, silt, and clay layers to measure properties in order to predict capacities (axial and torsional) based on FDOT (ϖ) and Federal Highway Administration (FHWA) (β and rational) methods. In the case of the axially loaded drilled shafts, FHWA’s rational approach agreed the closest to measured results, followed by FHWA’s β method (<20% difference); however, for combined loading (lateral and torque), FHWA’s β method was the closest (<12% difference), followed by rational approach (<40% difference). It was noted under combined loading that the short shaft underwent lateral and rotational failure, and one of the longer shafts had rotation failure in combination with nonlinear lateral response. Both of the jet-grouted piles were side grouted, followed by tip grouting to have lateral dimensions similar to the drilled shafts. The piles could not be failed axially with top-down testing due to pullout failure of the 40-ft reaction shafts. Under combined loading, the jet-grouted piles had 3 times the capacity of the drilled shafts (<30 of rotation) when loading was stopped (capacity of mast arm). A Statnamic axial capacity of 1000 kip was obtained for one of the piles, of which 450 kip was skin friction. Both torsional and axial skin friction of the piles were predicted based on pressuremeter testing, as well as observed grout pressures during construction. A comparison with similar capacity drilled shafts showed that the jet-grouted piles were 20% to 40% more economical.