Study on uplift mechanism of grouted implantation steel pipe pile by direct shear and model tests

The construction of steel pipe piles combined with side grouting is regarded as an effective way to improve the uplift bearing capacity of piles for transmission towers in rugged mountainous areas. This new piling technology involves the injection of grout into the annular space between the steel pipe pile and the side boundary of the pre-dug hole, which aims to improve the properties of soil-pile interface. However, to date, there are few studies on the soil-pile interface property and uplift failure model of this new kind of steel pipe piles with side grouting. In this study, the shear properties of the grouting interface between soil and steel plate were investigated, and then compared with the shear tests of undisturbed plastic clay and gravel clayey soil. It is indicated that the shear strength of the grouting interface between soil and steel plate is less than that of corresponding undisturbed soils. In addition, the uplift loading test about two grouted implantation steel pipe piles (GISPP) instrumented with Fiber Bragg Grating (FBG) sensors in plastic clay and gravel clayey soil was carried out, which demonstrates that the failure surface of GISPP mainly occurs in the interface of soil and grouting body. The grouting body can be well bonded to the steel pipe pile after uplift, but the ground surface around the GISPP presents a conical failure model. Finally, a new method considering the conical failure model was proposed to calculate the ultimate uplift capacity of GISPP, which was validated to be efficient and reliable compared with previous calculation methods.