Bending failure mechanisms and kinematics of micropile threaded connections

Abstract

Micropiles are used in a broad array of applications that impose substantial bending loads, including slope stabilization, foundations for integral abutment bridges, offshore structures and wind turbine towers, among others. For these applications, the flexural capacity of micropile threaded joints must be quantified. This study details a comprehensive testing program designed to assess the relative influence of thread details and casing geometry on the failure mode (jump-out versus rupture), and flexural capacity of micropile threaded joints. Four-point bending tests on grout-filled steel micropile specimens were performed to assess the flexural weakness and failure mechanisms at threaded joints. A total of 31 tests were performed on casings with nominal outer diameters of 178, 194, 244, and 346 mm and threaded joints of varying lengths and thread shapes. Displacements were measured with string potentiometers and detailed strain distributions around threaded connections were assessed via digital image correlation (DIC) techniques. The DIC measurements illustrated that the extent of plastic deformation and failure was strongly influenced by the development of hoop strains around the box-end of the connections. It was found that casing diameter, threaded length, thread shape and wall-taper of the casing govern the overall connection behavior and associated failure mode and flexural strength.