Abstract: The mechanical behavior of the interface between the structure and soil has become one of the hotspots in geotechnical engineering. A series of direct shear tests on the loess-cement mortar interface was carried out to study the interfacial shear properties under different normal stresses and water contents of soil. The results show that: (1) the shear stress-horizontal displacement curve of the loess-cement mortar interface presents strain-softening characteristics, including the pre-peak, post-peak and residual strength regions, with the normal stress of 50 and 100 kPa and the water content of 9.2% and 13.1%. While it presents a strain- hardening state when the normal stress is 200 and 300 kPa. When the water content of the soil is 17.1% and 20.8%, the shear stress-displacement curves at the loess-cement mortar interface show the strain-hardening effect with different vertical stresses. (2) The shear strength envelope of the loess-cement mortar interface is in accordance with the Mohr-Coulomb criterion. The shear strength of the loess-sand block interface meets the Mohr-Coulomb criterion. As the water content increases, the interface cohesive strength decreases significantly, while the frictional strength increases slightly, and the overall shear strength decreases. When the soil moisture content increased from 9.2% to 20.8%, the interface cohesion decreased from 41.5 to 3.6 kPa, and the interface friction coefficient increased from 0.50 to 0.58. The nonlinear model is used to fit the shear stress-displacement curve of the interface. The results show that the nonlinear model can better reflects the shear behavior of the loess-mortar block interface.

Key words: direct shear test, interface properties, nonlinear constitutive model, loess, cement mortar