According to the large scale simple shear tests and direct shear tests of the contact face between clay and concrete
slabs, the stress-strain curves or stress-displacement curves and shear strength envelope curves were obtained. The
phenomenon that the contact area is decreased and uneven stress distribution exist in the process of both kinds of tests. The
stress-strain curves of the simple shear tests presented the character of strain hardening, and the stress-displacement curves of
the direct shear tests presented the character of softening slightly. The cohesion that obtained by the simple shear tests was
harder than the cohesion that obtained by the direct shear tests, but the friction angle of the simple shear tests was smaller than
that of the direct shear tests. When the normal soil pressure that distributes on the surface of the structure is low, the failure of
the interface will occur on the soil-structure contact surface, and when the normal soil pressure that distributes on the surface of
the structure is high, the failure of the interface will occur in soil nearby the soil-structure contact surface. Therefore, different
calculating parameters should be used according conditions during engineering design calculates.