An Experimental Investigation on Shaft Resistance of Cast In-Situ Bored Piles in Intact Rock

Pile foundations socketed into rock are used to transfer the loads from the large structures to the underlying bedrock when the surface soils are unable to provide the adequate bearing capacity. The bearing capacity of the pile foundation depends on the base resistance and the shaft resistance given by the underlying rock. The shaft resistance of the rock socketed pile is a very sensitive parameter and governed by several parameters including the mechanical properties of the surrounding rock, rock mass classification, pile diameter, roughness of rock - concrete interface, discontinuities of the rock, the radial force induced by the load applied on the pile and the rock types. The performance of rock-socketed piles at serviceability limit mainly depends on the shaft resistance induced at the rock - concrete interface. The shaft resistance should be estimated accurately for safe and economic design of the structure. This research is focused on the experimental investigation on shaft resistance of cast in situ bored piles in intact rock. The samples were prepared to get the concrete - rock interface and tests were conducted in the laboratory for different types of rock using tri-axial apparatus and unconfined compressive strength apparatus. The ultimate shaft resistance was calculated from the experimental results for different rocks commonly found in Sri Lanka. Biotite hornblende gneiss, marble rock and charnokitic hornblende gneiss are the types of rock used in this study. From the gradient of shear stress -shear strain graph, shear modulus of the interface was evaluated. It was found that these rocks have ultimate shaft resistance in the range of 3.5 - 4.5 MPa and Shear modulus to be in the range of 60 - 70 MPa. Confining stresses in the range of 0 kPa to 400 kPa do not seem to have any effect on the shaft resistance.