Quantifying the Effect of Stress Changes on the Deformation and Cracking Behavior of Solid Rock using Acoustic Emission Techniques

In recent years, the demand on underground openings increased steadily and provoked major engineering progress in tunneling technology and modern constructional methods. In deep tunneling, tunnel boring machines (TBM) are usually applied, since they are more efficient than drilling and blasting, when excavating long tunnels. The efficiency of deep tunneling with TBM is strongly affected by the primary stress state and the principal stresses in the tunnel face. Depending on the orientation and the ratio of major to minor principal stresses, increasing stresses and stress relaxation may occur in the tunnel face. Until now, the influence of stress changes on rock properties and on the excavation process are not fully understood. Therefore, an experimental approach should provide insight into the effect of stress changes on the deformation and cracking behavior of different rock types. In order to assess the stress relaxation behavior of rocks, measurements were performed on core material recovered from its in-situ stress field at the Pyhasalmi Mine in Finland. The microcracking activity of a fresh core was measured during stress relaxation after drilling. The results of acoustic emission monitoring and p-wave velocity measurements on metavolcanic rocks revealed that stress-induced microcracking continues for several days or weeks before a new state of equilibrium is reached. Additionally to the field study, an experimental approach was adopted to investigate the influence of stress changes on laboratory rock properties. The research included the characterization of rock mechanical and acoustic properties of different rock types, using uniaxial compression tests combined with non-destructive testing methods. The results for the deformation and cracking behavior of the tested rocks resulted in five groups of rocks with a different failure behavior. Furthermore, the influence of stress changes on rock properties was assessed using uniaxial constant loading and subsequent uniaxial compression tests. Different compressive stress levels were applied to evoke stress-induced damage. The experimental results showed that a reduction of the uniaxial compressive strength is only observed in samples loaded beyond the crack damage stress. This confirms that the stress level of beginning crack coalescence and major crack formation represents the long-term laboratory strength of rocks. Petrographic analyses of fluorescent thin sections showed that a high number of microcracks was already present in some rock types. In granite samples loaded beyond the crack damage stress, load-parallel microcracks evolved. Microcracks mainly occurred in feldspar minerals and quartz but not in biotite. Instead, microcracks initiated at the boundary between biotite and adjacent minerals, as well as at preexisting cracks, grain boundaries and heterogeneities. In a massive rock mass, stress changes in the tunnel face are dependent on the ratio of rock mass strength to the stresses at the excavation boundary. In areas of increased stresses, the failure envelope of the rock mass may be reached so that cracks initiate. Dependent on the confining stresses in the face, face instabilities like spalling or rock burst may occur, or the generation of microcracks is inhibited due to an increased confinement. TBM penetration and cutter wear is thus strongly dependent on the stresses in the tunnel face.