Characterizing Structural Bearing Capacity and Deformation Behaviors of Micropiles under Multi-Stage Static Loading

This study investigates the structural bearing capacity and deformation behaviors of micropiles considering various influence factors including the length ( L ) of micropiles, outer diameter ( d ) and wall thickness ( t ) of steel pipes, diameter ( r ) and spacing ( s ) of grouting holes, and water cement ratio ( w/c ) of grout. Multi-stage axial compressive tests were performed on 20 micropiles to obtain their deformation behavior and axial compressive loads respectively corresponding to the cracking of surrounding grout ( P cr ), yielding of steel pipes ( P y ), and global structural failure ( P ult ). Experimental results reveal that P cr and P y are respectively 0.20–0.45 and 0.85–0.95 times P ult . In general, P ult decreases with L . Increasing d produces a more significant improvement in P ult compared to increasing t . The influences of r and s on P ult are associated with the weakening of stiffness and strength of the steel pipes. Micropiles with various w/c levels perform similar P ult , whereas their axial displacement at P ult increases with w/c . Under the same L or w/c condition, increasing d leads to an obvious reduction in the axial displacement at P ult . In the existing criteria, the performance of prediction equations for P ult and allowable axial compression load ( P c ) was compared and evaluated. Furthermore, several empirical models were proposed to respectively predict P ult and P c of the micropiles considering the influences of L, d, t, r, s , and w/c and achieve well predictions.