Geophysical Monitoring of Underground Constructions and its Theoretical Basis

Abstract

To avoid damage of rock mass, their collapse into underground space, danger for equipment and risks for human life, during construction and exploitation of large engineering projects (e.g. underground spaces, tunnels, machine halls, etc.) in-situ geotechnical and geophysical monitoring is carried out. Geophysical monitoring is based on observations of the elastic shear – and longitudinal wave velocities ( Vs and Vp , respectively) and Acoustic Emission (AE). The behaviors of the elastic velocities and parameters of AE during rock deformation depend on the types of the future failure that, in turn, are defined by the structure and properties of the medium and characteristics of stress state s 3 /s 1 and hydrostatic pressure. These velocity variations are defined by difference in effective parameters of forming microfractures, whose geometry is distinguished at different modes of stress state. At that character of interaction between microfractures determines the types of the macrofailure. In this study, we discuss the behaviors of longitudinal wave velocities and acoustic emission during loading of large rock blocks and underground opening orienting measurements along maximum ( s 1 ) and minimum ( s 3 ) stresses. It is shown that velocity variations along the maximum stress is more informative at elastic phase of rock deformations (velocity increases), whereas velocity variations along axis of the minimum stress is more informative at the stage of nonlinear rock deformation (velocity begins to decrease during microfractures occurrence). These regularities are well observed at unloading rock mass in the Zhinvali tailrace tunnel where geophysical monitoring assisted in the construction. This knowledge could be used in planning and monitoring the stability of underground structures.