Group Efficiencies of Grout-Tipped Drilled Shafts and Jet-Grouted Piles

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Group Efficiencies of Grout-Tipped Drilled Shafts and Jet-Grouted Piles

Author: McVay, Michael | Size: 11.03 MB | Format: PDF | Quality: Original preprint | Publisher: University of Florida, Gainesville | Year: 2011 | pages: 206

In current practice, driven piles/drilled shafts are constructed in a group at 3D center to center (c/c) spacing to minimize group interference (i.e., group efficiency factor ~ 1) as well as reduce the cost of the reinforced concrete cap. Presently, group efficiency factors for a number of new foundation types, e.g., post-grouted drilled shafts and jet-grouted piles, are unknown. This research looked into the group interaction of post-grouted drilled shafts and jet-grouted piles at typical 3D spacing. Two sets of group tests for grout-tipped drilled shafts and jet-grouted piles were performed in the Florida Department of Transportation (FDOT) test chamber in medium dense sands. The experimental top down group testing revealed that the jet-grouted piles behaved as a block at 3D spacing, whereas postgrouted drilled shafts acted independently of one another, i.e., no group interaction. A group interaction factor, as well as an analytical approach for predicting load versus displacement for single or group of jet-grouted piles, is suggested. In the case of the grout-tipped drilled shafts, the grout pressure always resulted in an upward flow of grout alongside the shaft which increased both the side and tip areas. The increased areas and grout tip preload were the three factors identified with the increased shaft and group resistance. Due to uncertainty in estimating areas, a conservative design approach for assessing single and group capacities of post-grouted drilled shafts at 5% displacement is proposed. Finally, the use of stage grouting to assess the increase in shaft capacity (skin and tip) is suggested.

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