An Innovative Prefabricated Pile Installation Method Utilizing Jetting and Pressure Grouting

Abstract

The majority of Florida Department of Transportation (FDOT) highway structures (e.g., bridges, signage, etc.) are founded on deep foundations, i.e., driven piles and drilled shafts. In the past, the most commonly used foundation type was driven piles. With increased urbanization, the noise and vibrations associated with pile driving has resulted in more drilled shaft usage due to its less intrusive characteristics. Unfortunately, due to horizontal soil stress relief during installation, drilled shafts suffer reduced axial and torsional resistance when compared to similar sized driven piles. Additionally, as-built drilled shaft integrity is generally more questionable than precast piles. To address these concerns, FDOT investigated post grouted drilled shaft tips, which not only increases the end bearing, but also proof tests every shaft. However, the issues of quality control (i.e., the structural integrity of the cast-in-place shaft) and the reduction in side friction due to horizontal stress relaxation still remained. Thus, the FDOT funded a research project to develop an innovative new deep foundation type which ensures both the quality control of a precast member with the lateral stress increase similar to a driven pile but with the minimum intrusion of a drilled shaft. It is referred to as a jet-grouted precast pile or jet-grout pile. The research was conducted in FDOT's new cylindrical 3.66 by 10.67 meter in-ground geotechnical test chamber, which is capable of monitoring soil stresses and deformations. The first trials involved jetting precast concrete piles with multiple sets of embedded grout delivery pipes. The early tests revealed higher horizontal versus vertical distribution of the compaction grout, as well as poor bonding between the grout and pile. Subsequently, the grout delivery system was re-designed; a new colloidal grout mix was developed along with the introduction of grout membranes on the outside of the pile. Small scale testing showed complete envelopment of the pile with excellent grout bond to the pile. Finally, a full scale (0.406m x 0.406m x 6 m) pile was jetted and subsequently grouted along its length in two stages with no tip grouting. The pile was then torque tested to 610 kN-m with 15° of rotation, resulting in a unit torsional side resistance of 77 kPa. Next, the tip of the pile was grouted and a top down compression test yielded 1,335 kN of resistance at 0.25 mm of vertical pile head movement that exceeded the resistance of a traditional similar size driven pile.