An Investigation of Grouted Pile Capacity Using Instrumented Pile Load Tests-Paper

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The study investigates how base and shaft grouting affect the axial capacity and load-transfer behavior of bored piles. It analyzes four instrumented pile load tests from Egypt’s Foundations Database and uses a PLAXIS finite-element model to examine changes in soil stresses caused by compaction grouting.<br /><br />In a representative test, a 1.2 m diameter, 26.3 m long pile carried 11,600 kN at only 9.13 mm of head settlement. Shaft friction carried most of the load, while the base contributed about 22%. The measured capacity was roughly three times estimates from conventional static design methods. However, the tests generally produced too little displacement to establish ultimate capacity reliably.<br /><br />The clearest pattern emerged in the grouted shaft. Grout initially provides high unit side friction, which increases with relative displacement; the estimated rate is 112 kPa/mm in sand and 457 kPa/mm in rock. This grout resistance is expected to dominate up to about 2 mm of relative displacement. Beyond that threshold, the grout may fail brittly, and ordinary pile–soil friction becomes dominant. Compaction grouting also increases lateral soil pressure, enhancing this later-stage friction. The numerical analysis supports a substantial increase in the soil’s horizontal-to-vertical stress ratio; the authors propose using 0.75 times the passive earth-pressure coefficient in the model.<br /><br />Base-resistance measurements did not show a consistent relationship with tip displacement, and the ultimate resistance of the grouted shaft was not reached in the tests. The authors propose a displacement-based model for shaft friction but emphasize that more field data are needed to validate it and refine its parameters. Overall, the findings indicate that grouting can substantially increase pile capacity, while also highlighting the difficulty of predicting ultimate capacity and the need for better-established design methods.

Keywords

bored piles

base grouting

shaft grouting

axial capacity

load-transfer behavior

compaction grouting

finite-element analysis

pile load tests

shaft friction

soil stresses