Statistical Evaluation of Design Methods for Micropiles in Ontario Soils-Article

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The study evaluates two methods for predicting the geotechnical axial capacity of micropiles in Ontario soils: a modified Federal Highway Administration (FHWA) method and the Bustamante–Doix method. The modified FHWA approach incorporates a diameter-expansion factor to account for grout-body enlargement. Researchers assessed 47 full-scale load tests, representing several micropile types and soil conditions. Because most tests did not reach failure, load–displacement curves were extrapolated, and failure loads were estimated using the Butler–Hoy method. Predictions were compared for accuracy and variability.<br /><br />The modified FHWA method performed differently across construction types and soils. It tended to underpredict capacity for Type A micropiles, particularly in coarse soils; Type B estimates were generally closer to the method’s upper-bound predictions. For continuous grout-flush (CGF) micropiles, the upper-bound modified FHWA estimates and Bustamante–Doix estimates showed broadly similar accuracy. Bustamante–Doix slightly underpredicted capacity in sands and overpredicted it in silty and mixed soils. The authors concluded that the modified FHWA method, using the Bustamante–Doix diameter-expansion factor, provided reasonable capacity estimates for Ontario soils.<br /><br />Predictions showed substantial variability, generally greater than that reported for conventional driven piles. Type B micropiles had comparatively low variability, while other types showed less consistent results. The study’s conclusions are limited by the reliance on extrapolated rather than observed failure loads, incomplete soil information at some bond zones, and the modest number of tests in several subgroups.

Keywords

micropiles

geotechnical axial capacity

Ontario soils

modified FHWA method

Bustamante-Doix method

diameter-expansion factor

load tests

Butler-Hoy method

Type A micropiles

Type B micropiles