Abstract: To explore the influence of slurry thickness and roughness on the mechanical properties of pile-soil interface, the distribution of protrusion size and roughness on the side of pile is obtained by statistical analysis based on the relationship between diameter and depth of borehole. The surface roughness of the concrete pile is simulated by the surface smooth and trapezoidal groove concrete respectively. A series of large-scale direct shear tests of concrete-sand interface is conducted with different slurry thickness and roughness. The results show that for the rough interface without slurry, the shear stress-tangential displacement curve is softened. For the rough interface with slurry at thicknesses of 5 mm and 10 mm, the shear stress-tangential displacement curve is hardened. The shear modulus G0.02 at the horizontal shear strain value of 0.02 decreases as the slurry thickness increases. For smooth interface, the peak shear strength is decreased exponentially as slurry thickness increases. For rough interface, the peak shear strength and the peak friction angle are decreased in an approximately linear way as slurry thickness increases. The interface strength becomes damped as result of the increasing initial thickness of the slurry and the thickness of the mixed layer after the test. Furthermore, a critical roughness on the peak intensity of contact surface under no slurry condition is found. When the roughness of concrete slab is less than the critical roughness Icr, the peak shear strength and the peak fiction angle of the interface will increase with the increase of roughness. When the interface roughness I is larger than Icr, both of them will decline with the increase of roughness. However, the existence of mud changes the relationship between roughness and peak shear strength.

Key words: pile-slurry-soil interface, large-scale direct shear tests, critical roughness, peak shear strength, peak friction angle