Mesoscale characterization of coupled hydromechanical behavior of a fractured-porous slope in response to free water-surface movement

Mesoscale Characterization of Coupled Hydromechanical Behavior of a Fractured Porous Slope in Response to Free Water-Surface Movement Y. Guglielmi a* , F. Cappa a-b , J. Rutqvist b , C-F. Tsang b and A. Thoraval c b Geosciences Azur, CNRS-UNSA-IRD-UPMC, Sophia-Antipolis - France Lawrence Berkeley National Laboratory, Earth Sciences Division, Berkeley - USA c INERIS, Ecole des Mines de Nancy, Nancy - France a * Corresponding author. Fax: (+33) 4.92.94.26.10 E-mail address: guglielmi@geoazur.unice.fr (Y. Guglielmi) Abstract To better understand the role of groundwater-level changes on rock-slope deformation and damage, a carbonate rock slope (30 m × 30 m × 15 m) was extensively instrumented for mesoscale hydraulic and mechanical measurements during water-level changes. The slope is naturally drained by a spring that can be artificially closed or opened by a water gate. In this study, a 2-hour slope-dewatering experiment was analyzed. Changes in fluid pressure and deformation were simultaneously monitored, both at discontinuities and in the intact rock, using short-base extensometers and pressure gauges as well as tiltmeters fixed at the slope surface. Field data were analyzed with different coupled hydromechanical (HM) codes (ROCMAS, FLAC 3D , and UDEC). Field data indicate that in the faults, a 40 kPa pressure fall occurs in 2 minutes and induces a 0.5 to 31 × 10 -6 m normal closure. Pressure fall is slower in the bedding-planes, lasting 120 minutes with no normal deformation. No pressure change or deformation is observed in the intact rock. The slope surface displays a complex tilt towards the interior of the slope, with magnitudes ranging from 0.6 to 15 × 10 -6 rad. Close agreement with model for both slope surface and internal measurements is obtained when a high variability in slope-element properties is introduced into the models, with normal stiffnesses of k n_faults = 10 -3 × k n_bedding-planes and permeabilities of k h_faults = 10 3 × k h_bedding-planes . A nonlinear correlation between hydraulic and mechanical discontinuity properties is proposed and related to discontinuity damage. A parametric study shows that 90% of slope deformation depends on HM effects in a few highly permeable and highly deformable discontinuities located in the basal, saturated part of the slope while the remaining 10% are related to elasto-plastic deformations in the low-permeability discontinuities induced by complex stress/strain transfers from the high-permeability zones. The periodicity and magnitude of free water-surface movements cause 10 to 20% variations in those local stress/strain accumulations related to the contrasting HM behavior for high and low-permeable elements of the slope. Finally, surface-tilt monitoring coupled with internal localized pressure/deformation measurements appears to be a promising method for characterizing the HM properties and behavior of a slope, and for detecting its progressive destabilization. Key words: Fracture; Rock slope; Mesoscale; In situ poroelastic measurements; Hydromechanics; Numerical modeling; Tiltmeter monitoring; Damage. 1. Introduction Understanding coupled groundwater and mechanical processes in complex fractured- porous rocks is essential for the safety and efficiency of subsurface and slope-stability engineering, and thus for the security and economic well-being of the general public. Although poorly documented, water is often mentioned as a triggering mechanism for failure of rocks with well-developed pre-existing fractures networks, and thus the cause for rock

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