Mesoscale characterization of coupled hydromechanical behavior of a fractured-porous slope in response to free water-surface movement
Résumé
To better understand the role of groundwater-level changes on rock-slope deformation and damage, a carbonate rock slope (30 m x 30 m x 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-h 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, FLAC3D, and UDEC). Field data indicate that, in the faults, a 40 kPa pressure fall occurs in 2 min and induces a 0.5-31 x 10-6 m normal closure. Pressure fall is slower in the bedding-planes, lasting 120 min, 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 x 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 kn_faults = 10-3 x kn_bedding-planes and permeabilities of kh_faults = 103 x kh_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 % is 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 10-20 % variations in those local stress/strain accumulations related to the contrasting HM behavior for high- and low-permeability 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.