Statistical evaluation of design methods for micropiles in Ontario soils

This paper evaluates the performance of two design methods for micropile in Ontario soils: namely the modified Federal Highway Administration method ([2000]. Micropile design and construction guidelines implementation manual. Report no. FHWA-SA-97-070, United States Department of Transportation, 376 p.) and the Bustamante and Doix method ([1985]. Une méthode pour le calcul des tirants et des micropieux injectés. Bull Liaison Lab Ponts Chauss, Ministere de l'equipement, du logement, de l'amenagement du territoire et des transports, Paris, 140, pp. 75–92). Micropiles are small-diameter, grouted piles installed with high amount of steel reinforcement. Since a higher level of uncertainty exists in micropiles compared to conventional piles, design methods tend to predict a large variation in their geotechnical ultimate capacities. The design of a micropile is extremely dependent on the load test results and the engineering judgement of designers. This research analysed a total of 47 full-scale micropile load tests conducted by Keller Foundations Ltd to determine the most suitable design method to predict the geotechnical capacity of a micropile in Ontario soils. Since most tests terminated before reaching the failure stage, the load-displacement curves were first extrapolated to obtain the failure load. The accuracy and variation of the predictions were compared for each method. From the findings, the Federal Highway Administration method, modified with a consideration of diameter expansion parameter suggested by Bustamante and Doix method, provided reasonable estimations for the geotechnical capacities in Ontario soils.