Strain Compatibility and Safe Use of High-Strength Steel Reinforcement in Confined Micropiles

With the current demand for micropiles as small diameter, high capacity deep foundation elements, limitations on the usable yield stress for the reinforcing steel impacts project foundation costs and construction schedules. This paper presents current and historical evidence supporting the improper application of strain compatibility theory to specialty foundation systems, and outlines design protocols and conditions in which high strength steel reinforcement (e.g., F_y=669 MPa to F_u=1034 MPa) may be safely used. The focus will be on the uncased load transfer zone (bond length) of micropiles. Historical research in structural mechanics of confined concrete columns has documented the significant increases in grout ductility due to confinement, from ϵ_c of 0.003 to 0.05. Current research specifically focused on micropiles indicated clearly that ϵ_c exceeds 0.006 with confinement similar to that of very dense soils, and exceeds 0.014 for confinement similar to that of medium strength rock. These experimental data fully corroborate structural mechanics data on confined columns and support the use of full yield stress of high strength reinforcing steels in working axial resistance calculations for the micropile bond length.