Laboratory Study of Bonding and Wall Smear in Rock Socketed Piles

This paper describes the results of laboratory direct shear tests conducted on concrete/rock interfaces under a range of conditions appropriate to bored piles in weak siltstone. In particular, the effects of bonding and wall smear are investigated. The tests show that cement bonding can occur across the concrete/rock interface in fine grained siltstone if the rock surface is clean of smear. Bonding is found to affect the peak resistance obtained and the ductility of the response. However, the tests also show that even a very thin smear zone can prevent bonding, but otherwise has very little effect on the frictional resistance of the interface. Relatively thick smear zones are found to reduce interface shear strength significantly. The likely consequences of these observations on pile performance and construction procedures are briefly discussed.