Field compression and uplift tests on micropiles in collapsible loess under completely-soaked and saturated conditions

In order to investigate the compression and uplift bearing capacity of micropiles in collapsible loess, a method for loess prewetting under completely soaked and saturated conditions is firstly designed. Consequently, the comparative field compression and uplift load tests on single micropile and group micropiles are respectively carried out in the two collapsible loess sites in Gansu Province. Both the site conditions and the load tests are documented comprehensively. The compression and uplift load-displacement curves of the single micropile and group micropiles in completely soaked and saturated loess generally follow a typical two-phase steep change pattern, which is quite different from those in-situ moisture content loess because they can be simplified into three distinct regions: initially linear, curvilinear transition and finally linear regions. Both in the in-situ moisture content loess and the completely soaked and saturated loess, the compression or uplift loaded single micropiles should be considered as the frictional pile foundations, and the tip resistances are only about 10% to 15% of the applied compression loads. For the micropiles in loess under in-situ moisture content, the ultimate uplift load capacities are 66% to 87% of those under compression. However, the loess under completely soaked and saturated condition will lead to a reduction of 70% in compressive bearing capacity and 50% in uplift bearing capacity for single micropile, and that for group micropiles is about 75% in compressive bearing capacity. These experimental results may provide a reference for the designers in loess in the future.