Preisig, Giona

2020-5-26, Preisig, Giona

Large (deep-seated) landslides present complex geometries, rock/soil properties, and kinematical behavior. Complex geometries are due to the presence of several sliding zones, while complex properties typically result from the dilation, compression, or fatigue of geologic materials. Kinematical behavior is often episodic, with periods of stability followed by periods of enhanced slope movements owing to shear strength reduction in response to groundwater pressure changes. These mechanisms complicate our capacity in forecasting the long-term activity and thus, the choice of a strategy for hazard management. This technical note introduces a method for predicting the long-term activity of deepseated landslides based on one-way coupled hydromechanical numerical modelling. The method is applied to analyse the longterm stability of a deep-seated compound slide in the Swiss Jura Mountains. Results indicate that, under natural groundwater pressure changes, the analysed compound slide will continue to move in an episodic fashion in response to groundwater levels in the slope, without developing velocities greater than several centimeters per year. This example demonstrates how one-way coupled hydromechanical modelling constrained by field data is a reliable tool for assessing the long-term activity of deep-seated landslides and helping the management of associated hazards.

2013, Preisig, Giona, Perrochet, Pierre

Field observations and laboratory experiments have clearly demonstrated that heavily perturbed / exploited aquifers are subject to 3D deformations, which may cause significant socio-economic impacts at regional scale. Most common examples include: (1) excessive pumping of groundwater from deep aquifers leading to land subsidence; (2) deep excavation of tunnels in permeable geological units resulting in dangerous differential consolidation, especially for dams; and (3) fluid injection into deep reservoirs causing ground uplift and microsismicity. These manifestations are due to a substantial modification of water pressures within the aquifer, leading to effective stress variations, and deformations. Moreover, such deformations modify hydrodynamic parameters, i.e., hydraulic conductivity, porosity and storage coefficient. In confined or deep aquifer systems, hydrodynamic parameters have to be considered as effective stress-dependent variables. In such environments the assumption of constant parameters can lead to significant quantitative errors. The afore-mentioned fluid-to-solid hydromechanical processes seem to be essentially governed by hydrogeological, geomechanical and structural properties of the aquifer system. In order to take into account the major processes, as well as their principal properties, regional coupled hydromechanical simulation necessarily requires simplification of the governing equations to be operational in real, large scale, hydrogeological systems.
In the present thesis, model functions relating effective stress to hydrodynamic parameters are developed from fundamental hydrogeological and physical concepts, and implemented in the groundwater flow equation. Proposed stress-dependent equations are verified by a comparison with laboratory and field data. This is carried out for (1) fractured aquifers, i.e., consolidated rocks whose porosity results principally from the presence of fractures, cracks, joints and faults, and (2) granular porous aquifers, i.e., unconsolidated rocks whose porosity results from voids between solid grains. The relation between porosity and stress is also used to elaborate a deformation model for solving aquifer vertical volume change, i.e., ground settlement / uplift. A modelling approach is proposed in order to solve fluid-to-solid hydromechanical processes at regional scale, considering detailed geological structures. In this numerical method, hydrodynamic parameters are considered as stress-dependent variables.
Exact analytical solutions solving flow in a media under stress are developed in order to verify the numerical method. The proposed approach is then applied to the analysis of real case studies. In particular, to (1) the abnormal deformation of the Zeuzier arch dam (Wallis, Switzerland) due to the drainage of an unexpected confined aquifer by the Rawyl exploratory adit; (2) the problematic of water inflow into tunnels based on the geological investigations undertaken by the Lyon-Turin railway project for the 57 km basis tunnel; as well as (3) the anthropogenic land subsidence affecting the Mexico City basin.
Quantitative studies of deep aquifer systems considering constant hydrodynamic parameters result in non-accurate volumetric discharge rates and pressure head fields. On one hand, increasing effective stress leads to decreasing hydrodynamic parameters. This results in a diminution of volumetric flow rates through a deep reservoir or in a deep excavation. Moreover, the decrease of water pressure is slowed down due to the decrease of hydraulic conductivity. This has repercussion on consolidation time. On the other hand, if - and only if - the rock is elastic, decreasing effective stress can lead to increasing hydrodynamic parameters and volumetric discharge rates.
For analytical solutions of volumetric discharge rate in deep wells or into a tunnel, the dependency of hydrodynamic parameters on effective stress can be taken into account by using a factor allowing stress consideration; whereas, in numerical analysis, such a process can be considered by implementing stress-dependent parameters in the groundwater flow and aquifer deformation models.
The proposed numerical approach for fluid-to-solid coupled hydromechanical processes, is computationally simple, based on few unknowns, and efficiently reproduces regional consolidations in geologically oriented 3D meshes. This method is critical for hydrogeological and geomechanical quantitative studies investigating the sensitivity of deep aquifers on decreasing / increasing effective stresses. In particular for regional scale projects where water pressure may be subject to substantial modifications, such as dam construction or tunnel excavation, geologic radioactive waste repositories, deep reservoir exploitation for CO₂ sequestration, geothermal energy production, as well as extraction of groundwater and / or hydrocarbons.

, Preisig, Giona, Cornaton, Fabien Joel, Perrochet, Pierre

Changes in effective stress due to water pressure variations modify the intrinsic hydrodynamic properties of aquifers and aquitards. Overexploited groundwater systems, such as basins with heavy pumping, are subject to nonrecoverable modifications. This results in loss of permeability, porosity, and specific storage due to system consolidation. This paper presents (1) the analytical development of model functions relating effective stress to hydrodynamic parameters for aquifers and aquitards constituted of unconsolidated granular sediments, and (2) a modeling approach for the analysis of aquifer systems affected by effective stress variations, taking into account the aforementioned dependency. The stress-dependent functions were fit to laboratory data, and used in the suggested modeling approach. Based on only few unknowns, this approach is computationally simple, efficiently captures the hydromechanical processes that are active in regional aquifer systems under stress, and readily provides an estimate of their consolidation.

2018, Preisig, Giona

Deep-seated landslides (DSLs) involve large-scale deformation and likely affect transportation infrastructure. Movement rates are in general very slow (less than a metre per year) with acceleration periods controlled by external factors such as the seasonal fluctuation of groundwater pressure. Acceleration response may change from season to season depending on hydrogeological conditions, changes in slope geometry and degradation of geological materials. More localized landslide types are associated with and develop within DSLs, such as rock falls, topples and debris slides. Management of hazards related to DSLs requires first the assessment of geological, hydrogeological and geomechanical processes. This is the starting point for developing a management strategy. This paper presents the characterization of a deep-seated landslide located in the Swiss Jura Mountains, Les Buges landslide, where a railway line, a power line and an aqueduct of regional importance cross the slide, as well as a highly frequented hiking trail beneath the landslide toe. Slide kinematics is governed by the geology and hydrogeology of the slope, which can be subdivided into two dominant bodies. A management strategy is subsequently discussed for this DSL. Les Buges is a good example illustrating that hazards related to deep-seated landslides must be tackled first of all by means of the observational method.

2012, Preisig, Giona, Cornaton, Fabien Joel, Perrochet, Pierre

, Preisig, Giona, Cornaton, Fabien Joel, Perrochet, Pierre

Effective stress plays an important role in aquifer dynamics, especially in those affected by high variations of water pressures. Increasing/decreasing effective stresses affect hydrogeological parameters, even in media of high stiffness, such as fractured rocks. This study presents a modelling approach of groundwater flow in fractured rocks and aquifer deformation taking into account the dependency of hydrogeological parameters on effective stress. This approach has been illustrated by modelling a fractured aquifer dynamic, the Zeuzier arch dam settlement. The calibrated model showed agreement with measured data. This simulation method could be used to study the sensitivity of aquifers to variations in effective stress due to water pressure.

2014-2, Preisig, Giona, Eberhardt, E., Gischig, V., Roche, V., Van der Baan, M., Valley, Benoît, Kaiser, P.K., Duff, D., Lowther, R.

The ability to generate deep flow in massive crystalline rocks is governed by the interconnectivity of the fracture network and its permeability, which in turn is largely dependent on the in situ stress field. The increase of stress with depth reduces fracture aperture, leading to a decrease in rock mass permeability. The frequency of natural fractures also decreases with depth, resulting in less connectivity. The permeability of crystalline rocks is typically reduced to about 1017–1015 m2 at targeted depths for enhanced geothermal systems (EGS) applications, that is, >3 km. Therefore, fluid injection methods are required to hydraulically fracture the rock and increase its permeability. In the mining sector, fluid injection methods are being investigated to increase rock fragmentation and mitigate high-stress hazards due to operations moving to unprecedented depths. Here as well, detailed understanding of permeability and its enhancement is required. This paper reports findings from a series of hydromechanically coupled distinct-element models developed in support of a hydraulic fracture experiment testing hypotheses related to enhanced permeability, increased fragmentation, and modified stress fields. Two principal injection designs are tested as follows: injection of a high flow rate through a narrow-packed interval and injection of a low flow rate across a wider packed interval. Results show that the development of connected permeability is almost exclusively orthogonal to the minimum principal stress, leading to strongly anisotropic flow. This is because of the stress transfer associated with opening of tensile fractures, which increases the confining stress acting across neighboring natural fractures. This limits the hydraulic response of fractures and the capacity to create symmetric isotropic permeability relative to the injection wellbore. These findings suggest that the development of permeability at depth can be improved by targeting a set of fluid injections through smaller packed intervals instead of a single longer injection in open boreholes.

2011, Preisig, Giona, Cornaton, Fabien Joel, Perrochet, Pierre

A model function relating effective stress to fracture permeability is developed from Hooke's law, implemented in the tensorial form of Darcy's law, and used to evaluate discharge rates and pressure distributions at regional scales. The model takes into account elastic and statistical fracture parameters, and is able to simulate real stress-dependent permeabilities from laboratory to field studies. This modeling approach gains in phenomenology in comparison to the classical ones because the permeability tensors may vary in both strength and principal directions according to effective stresses. Moreover this method allows evaluation of the fracture porosity changes, which are then translated into consolidation of the medium..

, Preisig, Giona, Dematteis, A., Torri, R, Monin, N., Milnes, Ellen, Perrochet, Pierre

Interception of aquifers by tunnel excavation results in water inflow and leads to drawdown of the water table which may induce ground settlement. In this work, analytical and numerical models are presented which specifically address these groundwater related processes in tunnel excavation. These developed models are compared and their performance as predictive tools is evaluated. Firstly, the water inflow in deep tunnels is treated. It is shown that introducing a reduction factor accounting for the effect of effective stress on hydrodynamic parameters avoids overestimation. This effect can be considered in numerical models using effective stress-dependent parameters. Then, quantification of ground settlement is addressed by a transient analytical solution. These solutions are then successfully applied to the data obtained during the excavation of the La Praz exploratory tunnel in the Western Alps (France), validating their usefulness as predictive tools.