ABSTRACT A surface instability apparatus was used to produce spalling in a laboratory setting, and damage in the rock was monitored by acoustic emission (AE) and digital image correlation (DIC) techniques. Lateral displacement served as the feedback signal to control the post-peak response with a closed-loop, servo-hydraulic load frame. A clustering analysis using the concept of a hierarchical dendrogram applied to low signal to noise ratio events provided "super AE" locations that matched the crack trajectories. DIC was used to investigate incremental displacement fields during surface spalling. Real-time images were successfully captured under high stress levels through modification of the device. Displacement fields were computed in the early stage of loading and around peak stress. Young’s modulus was reasonably estimated with the axial strain measured by DIC. Fracture from spalling phenomena was revealed by contours of incremental horizontal displacement around peak stress. A concentration of deformation leading to fracture was identified, as was a region of relaxation behind the damage zone.
1. INTRODUCTION In highly stressed rock, axial splitting may develop in an area adjacent to a free surface. Fracture usually appears in a form of numerous microcracks parallel to the opening (Fig.1), and a spalling phenomenon is the consequence of linkage and clustering of microcracks that splits the rock into parallel slabs. In underground excavations, rock bursts have been attributed to this type of failure [1]. Laboratory uniaxial compression tests also frequently show axial splitting in rock specimens [c.f. 2, 3]. However, the cylindrical shape of the specimen in a typical uniaxial compression test limits observation and measurements of surface spalling. In this study, axial splitting tests were performed in a device called the surface instability apparatus or SIA (U.S. patent 5,024,103), which overcomes limitations in uniaxial compression when investigating axial splitting and spalling phenomena. The SIA allows the control of axial splitting near the only free surface within a twodimensional deformation state with accurate measurements of force and displacement, as well as the possibility to observe microcracking [4]. In this study, the SIA was modified to accommodate both acoustic emission (AE) and digital image correlation (DIC) techniques to monitor damage development and incremental displacement fields during initiation and propagation of the failure process from axial splitting.
2. EXPERIMENTAL PROCEDURE The concept of the SIA is to simulate a plane-strain condition with zero minor principal stress, i.e. a free surface. The SIA consists of six major parts: two rigid side walls, one back wall, one front wall with an open window, and top and bottom plates (Fig. 2a). The two side walls are bolted into the front and back walls to create a rigid frame. The machined right-orthogonal prismatic rock specimen is wedged between two rigid side walls. The secondary principal stress is developed passively in the Z-direction due to the Poisson’s effect, as the specimen is loaded axially (vertically) in the Y-direction (Fig. 2b). The front wall does not contact the specimen and guarantees a free face, while the rear wall ensures that the lateral deformation and spalling take place in the exposed front face.