Shear strength and interface friction characteristics of expandable foam grout

The deterioration of buried pipes in urban areas leads to the formation of underground cavities, which may induce road subsidence. To fill these cavities, this study investigates the shear strength characteristics of expandable foam grout (EFG) and the interface between EFG and two types of sands. A series of direct shear tests is conducted using EFG specimens prepared under different conditions of curing time and unit weights of sand. After three days of curing, the shear stress curves of EFG and EFG–sand interface obtained at three normal stresses indicate brittle behavior and ductile behavior, respectively. Friction angles of the EFG increase with curing time, while those of the EFG–sand interface remain almost constant. The interface friction angles between the EFG and sands are smaller than or equal to the internal friction angles of the sands. Therefore, the shear strength of the EFG–surrounding soil interface should be given more attention than the high shear strength of the EFG, to fill underground cavities.