Installation of Drilled Micropiles in Weak Glacial Deposits under Pressurized Grouting

This paper investigates the problems associated with micropile installation in weak glacial deposits consisting of low plasticity silts, fine sands, and varved silts with clayey seams. The sensitivity of glacial soils to disturbance resulted in ground instability when casing withdrawal was performed under pressurized grouting. It was found that fluid pressure exerted by self-weight of grout in the borehole was sufficient to trigger instability in the glacial soils. The result was large grout penetration into the surrounding disturbed soils and upward flow of grout along the external face of the casing. Varved silts showed greater resilience against liquefaction due to their relatively cohesive nature compared to more granular materials such as silts and fine sands; however, plastic deformation may result from high grouting pressures that exceed the passive resistance. Grouting pressures locked in the hardened grout around the casing may contribute to high resistance to casing withdrawal. To avoid soil disturbance and locking up of casings, the length of grout column above the soil surface should be minimized and grouting should be performed without pressure.