Abstract

The investigation of damage zones around cylindrical excava¬tions such as sealing sections in tunnels or shafts is a key issue in the field of underground waste storage. Emphasis is on the geometry and extent of the excavation-induced damage zones, on their mechanical and hydraulic characteristics and, in a broader perspective, on the stability of the cavities before and after waste emplacement. Formation damage control in production and injection wells represents a similar stability problem, suggesting that a detailed understanding of coupled hydromechanical processes in the vicinity of cylindrical cavities is just as valuable for the oil and gas industry. As part of a geoscientific research programme at the Mont Terri underground rock laboratory in the Jura Mountains of Switzerland, a horizontal microtunnel was drilled in an overconsolidated claystone formation. Prior to the excavation, the rock formation was instrumented with piezometers, inclinemeters, extensometers and stress cells for monitoring the hydromechanical response of the rock formation to the drilling process. After completion of the excavation, a geoscientific characterization campaign was initiated, comprising detailed geological mapping of structural features, laser scanning of the tunnel surface and the instrumentation of the tunnel walls with surface extensometers. Complementary investigations were conducted to estimate rock stress at the site and the geomechanical rock properties, such as elastic moduli, rock strength and anisotropy. The observations to date indicate that a distinct damage zone was created by the drilling process, comprising massive breakouts and localized spalling. Break¬outs with magnitudes of up to 15 cm were observed in one of the upper quadrants of the tunnel cross section and less marked features were mapped on the opposite lower quadrant. Long-term monitoring of pore pressure and deformation around the tunnel and progressive spalling along the tunnel wall suggest that the damage zone is still developing. The complex geometry of the damage zone is explained by the interaction of a distinct stress anisotropy and rock strength anisotropy. The initiation of excavation-induced fractures can be linked to isolated tectonic features intersecting the microtunnel. The geometry and the temporal evolution of the damage zones are similar to observations in boreholes, indicating that the basic hydromechanical mechanisms are comparable.

Introduction

Tight claystone and shale formations are being assessed as potential host or cap rocks for the disposal of hazardous wastes such as radioactive waste, toxic chemical waste, CO2, H2S and for natural gas storage (Brosse et al[1], DOE[2], IAEA[3], Katz & Lee[4], Nilsen & Olsen[5]). Their low permeability, self-sealing capacity and the long-term stability of the geological environment are favourable aspects which enable the isolation and confinement of the wastes over very long time periods. On the other hand, special geomechanical properties (low rock strength, distinct anisotropy in geomechanical properties) and unfavourable in-situ stress conditions (distinct stress anisotropy) are often faced as serious engineering problems during the construction and operation of deep underground facilities in clay-rich formations (Aristorenas[6], Martin et al[7], Steiner[8,9]). The stability problems may range from brittle failure (over¬breaks and partial tunnel collapse, local spalling and buckling phenomena at the tunnel walls) during the construction phase to time dependent-deformation during the operational period (significant convergence of tunnels and drifts) and possible dilatancy phenomena after repository closure, caused by overpressures in the backfilled tunnels (gas production of the waste). Similar stability problems have been reported in the field of oil & gas exploration when drilling horizontal boreholes in stratified shale formations (Økland & Cook[10]), sug¬gesting that the basic failure mechanisms in deep horizontal boreholes are comparable to those in the excavation damage zone around tunnels and drifts.

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