Performance of Driven Grouted Micropiles: Full-Scale Field Study

Abstract

A new installation method for micropile is introduced, called driven grouted micropiles (DGMs), in which closed-end perforated steel pipes are driven to the refusal depth using a light hammer and grout is injected incrementally into the pipe using pneumatic packers. The increased bearing capacity and ease of construction of these micropiles recommend them over conventional micropiles. A total of 102 full-scale static loading tests were analyzed to evaluate the geotechnical performance of the DGMs in both sandy and clayey soil. The load–deformation curves were analyzed, and the ultimate bearing capacity of the DGMs was extracted using micropile failure criteria. The results show that, although existing micropile design manuals do not recommend consideration of the tip resistance of micropiles, the proposed DGM method recommends consideration of end bearing. The contribution of end bearing to the bearing capacity of the DGM is approximately 30%. In order to design a DGM that meets FHWA formulas for conventional micropiles, an incremental coefficient (λ = 1.3) is proposed to apply to the theoretical micropile diameter. Another incremental coefficient (η = 1.25) is proposed to apply to FHWA recommendations for bond strength that relate to the soil type.