ABSTRACT:

The High-Pulse Poroelasticity Protocol (HPPP) is a new in-situ approach for estimation of rock fractures properties at the mesoscopic scale. The method consists of fast pressure pulse tests that simultaneously measure deformation and fluid pressure in fractures isolated in boreholes. It relies on an innovative probe that allows high-frequency measurements using fiber-optic sensors. Fully coupled, hydromechanical, numerical elastic models are then used to inverse measurements and estimate fractures hydraulic and mechanical properties (stiffness and hydraulic aperture). Applied to fractures at the Coaraze Laboratory site in southern France, the method evidenced a hyperbolic relationship between hydraulic apertures and stiffness of fractures. This result was calibrated from laboratory hydromechanical tests. It appears that the hyperbolic relationship reflects the progressive mismatching of fractures under shear displacements related to hydromechanical effects. An in-situ estimation of fracture compressive and shear strengths can then be conducted that appears less subjective and more accurate compared to other approaches based on fracture roughness analyses and/or classifications index.

1. INTRODUCTION

It is commonly admitted that complex interactions between numerous pre-existing fractures play a key role for fluid flow and mechanical deformations in fractured rocks [1]. However, the hydromechanical (HM) analysis and monitoring of any rock structure faces the major problem of determining both the properties and constitutive stress-displacement models characterizing the fractures. Several criteria have been proposed to identify the strength of a natural rock fracture [2-7]. In practice, Barton?s model is recommended [8], and, in general, used because it relies on the joint morphological parameter called the JRC (Joint Roughness Coefficient) and the JCS (Joint Compressive Strength), both being measurable on joint laboratory samples and in-situ through profiling methods, Schmidt hammer [9] and visual estimation of various parameters [10]. Those parameters are then related to joint shear strength, stiffness and aperture to fully describe joint behaviour under shear and normal deformation [11]. Indeed, the peak drained angle of friction (f?) at any given effective normal stress (s? n) is expressed as follows:

(mathematical equation available in full paper)

where fr is the residual friction angle in the case of weathered joints. When joints are unweathered, JCS equals the unconfined strength of the rock (sc) and fr equals the basic friction angle (fb). The joint normal closure is related to the joint normal stiffness (kn) that was found to approximately depend on the JCS or sc, the joint aperture (b) and the joint roughness through the JRC. Equation (2) is proposed to describe the joint initial normal stiffness (kni) which is the joint normal stiffness at zero normal stress:

(mathematical equation available in full paper)

Equation (3) gives the maximum joint closure (Vm) after on cycle loading test (i.e. without considering eventual damage of the joint asperities that can occur in the following loading cycles).

(mathematical equation available in full paper)

The Barton-Bandis? relationship (4) is then considered into geomechanical analyses:

(mathematical equation available in full paper)

or derived as shown in equations (5) and (6) to estimate the Mohr-Coulomb friction angle (fi) and instantaneous cohesion (ci):

(mathematical equation available in full paper)

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