Coupling material characteristics with water–cement ratio for elastic wave based monitoring of underground structure

Abstract

Elastic wave based technology (e.g., acoustic emission or micro seismic sensors) has been widely used to monitor the stability of underground structures or rock failure. To achieve successful measurement, it is important to select an appropriate coupling material for the installation of sensors that are embedded in holes bored into the wall of a structure. The characteristics of the coupling material are related to its wave propagation ability (i.e., wave velocity and damping ratio), setting ability in the borehole (i.e., bleeding), and its workability (i.e., viscosity) in relation to being able to fill the holes tightly with the sensors. In this study, micro cement, Portland cement, and gypsum were the materials considered to couple with sensors in the borehole. Laboratory tests were performed to evaluate the characteristics of the coupling material according to the water-cement ratio (w/c, range 0.5–2.0). The test results show that the propagation ability and setting ability tend to decrease with increasing w/c ratio; on the other hand, workability benefits from an increasing w/c ratio. In addition, the characteristics of the coupling materials were analyzed to suggest a material with an optimized w/c ratio appropriate for a target test-bed site. The most suitable coupling recipe was found to be micro cement with a w/c ratio of 1.0. This study provides information about coupling material characteristics that should be useful for determining a suitable coupling recipe for field applications.