Heightening and slope thickening of gravity dams with partial bonding interface： theory and practice

It is a common practice to thicken the downstream slope in gravity dam heightening， for which there are two key technical challenge： the bonding between new and old concrete and the load distribution between new and old dam blocks. This study， using the Danjiangkou Dam Heightening Project as a case study， conducted a series of research including theoretical analysis， numerical simulation， laboratory tests， and in-situ experiments， proposed the "partial bonding of new and old concrete interface" theory for gravity dam heightening and developed supporting technologies successfully applied to the engineering practice at the Danjiangkou Project. The main research findings are as follows： （1） The cracking mechanism at the interface between new and old concrete was revealed. When the thickness of new concrete on the downstream slope falls within the range affected by annual temperature variations， the combined deformation of thermal expansion/contraction due to concrete hydration heat and periodic "depressing-bulging" caused by air temperature cycles， constrained by the old concrete， induces excessive tensile and shear stresses at the interface， forming the primary cause of interface opening. （2） It was found that the heightened dam is no longer a homogeneous， continuous monolithic structure after heightening， rendering traditional material mechanics calculation methods inapplicable. Finite element equivalent calculation methods and control criteria for stress and stability were therefore proposed. （3） A comprehensive temperature control strategy of "increasing cooling intensity， extending primary cooling duration， controlling peak temperature， and reducing cooling range" was developed. Combined with surface thermal insulation and division of new concrete blocks， this strategy can effectively reduce the normal tensile stress and slope shear stress at the interface. （4） Structural enhancement measures including "strengthening interface， installing anchor bars， and cutting keyways" were proposed， significantly improving the overall joint load-bearing capacity and stress state of the dam. Application results from the Danjiangkou Dam Heightening Project show that a bonding ratio over 30% at interface between new and old concrete can ensure dam safety. Monitoring data indicate that 47% of the measuring joint meters of typical dam sections showed opening or slight opening status， corresponding to a bonding ratio of 53%， which is greater than the safety control index. The measured dam deformations agree well with the computational results， verifying that the actual operational behavior of the dam is consistent with the design state.