Dam foundation excavation techniques in China:A review

A protective layer(PL) is commonly reserved above foundation surface to protect the underlying rock mass during dam foundation excavation. In China, the PL of dam foundation is conventionally subdivided into two or three thin layers and excavated with the shallow-hole blasting method, even by pneumatic pick method in case of soft rock mass. The aforementioned layered excavation of the PL delays the construction of the whole project. After nearly 30-year practices, several safe and effcient methods for the PL excavation of dam foundation are gradually developed. They include shallow-hole bench blasting with cushion material(SBC) at the bottom of the hole, and horizontal smooth blasting(HSB). The PL is even cancelled on the condition that horizontal pre-split technique is employed during dam foundation excavation. This paper introduces the aforementioned two PL excavation methods(shallow-hole blasting and bench blasting) and horizontal pre-split technique of dam foundation without protective layer(HPP). The basic principles of blasting method, blasting geometry, charge structure, drill-and-blast parameters of typical projects are examined. Meanwhile, the merits and limitations of each method are compared. Engineering practices in China show that HSB is basically the optimal method for dam foundation PL excavation in terms of foundation damage control and rapid construction. Some new problems for dam foundation PL excavation arising, such as strong unloading and relaxation phenomenon that encountered in the gorge region of southwest China, are needed to be addressed; and the corresponding countermeasures are discussed as well.

Numerical study on lateral wall displacement of deep excavation supported by IPS earth retention system

The objective of this study is to investigate the 3D behavior characteristics of an excavation supported by an innovative prestressed support(IPS)earth retention system.A numerical simulation was conducted in order to provide insight into the IPS system behavior by using the FLAC3D package.Prior to the parametric study,validation work was conducted by means of a comparison of the deformation between thefield test data and numerical analysis results,and strong agreement was obtained.The reasonable excavation location,lay-ered excavation thickness,and blocked excavation sequence are presented according to variable parameter analysis.In view of the pre-viousfindings,certain measurements are proposed in order to control the foundation pit deformation.The results indicate that prestress compensation has a significant effect on the IPS system behavior,while an optimized excavation sequence slightly improves its behavior.With the conclusion proposed based on the numerical results,the aim is to provide reference data for optimization design and the construction sequence.