Excavation unloading destruction phenomena in rock dam foundations

Excavation unloading destruction (EUD) is a common phenomenon in the large-scale construction sites in Southwest China, where intense horizontal tectonic stresses are present in the rock mass. The paper discusses the Xiaowan hydropower dam where stresses of 23-35 MPa were measured in the strata in the valley sides and 44-57 MPa in the floor of the valley. It describes the characteristics of EUD, discussing the laminar, arched and imbricate fractures which occurred when the near surface strata were excavated for the dam construction. The EUD fractures were largely restricted to the upper 6 m below the excavation and were particularly pronounced in the top 2 m. It is concluded that the typical Mohr Coulomb formula is not applicable where a rock mass near its critical stress is rapidly unloaded. A better understanding of EUD is important for the prediction of rock bursts etc. and hence safety in large engineering constructions in highly stressed strata.