Boreholes used for pressuremeter testing are often disturbed by drilling. For a test in a highly stressed ground, this disturbance is dominated by unloading-induced damage. In this paper, a conceptual state of weak rock around the borehole is hypothesized. A numerical approach is sought to explain the observed stress dependency of borehole modulus during pressuremeter tests in Opalinus Clay (OPA). Based on the Continuum Damage Model (CDM) approach, an elastoplastic damage formulation is developed to characterize the constitutive behavior of OPA. A stress-driven recovery function is also introduced so that the restoration of rock stiffness under recompression can be predicted. The model is calibrated with multi-stage triaxial test on OPA. It is also implemented in the finite element code Abaqus to simulate the case of borehole unloading and reloading. While further efforts are needed to refine the constitutive relationship and validate it against laboratory testing under a more representative stress/strain path, preliminary computation results using the CDM approach have captured the observed borehole modulus variation in the pressuremeter test.

1. INTRODUCTION

Pressuremeter testing measures the borehole radial displacement under a pressure-controlled inflation/deflation. Boreholes used for pressuremeter testing are often disturbed in the drilling process (Randolph and Fahey, 1984, Silvestri and Abou-Samra, 2008). For a borehole drilled in highly stressed ground, material in the near-borehole region may be damaged under the concentrated stress after unloading. The degree of damage close to the borehole wall has been confirmed by the degradation of wave velocity using borehole seismic surveys (Winkler, 1997, Balland and Renaud, 2009).

A few examples of pressuremeter tests in deep ground have shown a stiffening response of the borehole during the early stages of pressuremeter expansion (Huang et al., 1999, Zalesky et al., 2007). Several recent field pressuremeter test campaigns in Opalinus Clay (OPA) at the Mont Terri Rock Laboratory in Switzerland has allowed the borehole modulus to be determined through multiple unload-reload cycles at different pressure levels (Fig. 1). Liu et al. (2020) observed a consistent trend of the increase of measured borehole modulus with expansion pressure from two series of pressuremeter tests in OPA, particularly within the low pressure range (<5 MPa).

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