Abstract:  In order to correctly design the underpinning structure composed of piles and beams in the pile-foundation underpinning project for the super -buildings of metro‚a standard and several measures to control the deformation of underpinning structure were proposed．On the basis of experimental results of 25specimens‚it was proved that the boundary plane between the new and the old concrete had obvious destruction foreboding and excellent protending performance‚and a formula for calculating the bearing capacity of the boundary plane was established．Therefore‚the calculating method for the bearing capacity and the deformation of underpinning struc-
ture were derived．The proposed designing method has been successfully used in the underpinning work of Guangzhou Metro Lines1and2．From the fact that all measured items met the controlling indices well‚which proves the rationality of the designing method．

Key words: metro, pile and beam, underpinning structure, pile foundation, design method