Integrating Advanced 3D Groundwater Modelling into Slope Stability Assessment

The stability of a slope is governed by a combination of factors, where the hydromechanical properties of the soil are the most prominent ones. The groundwater conditions in such assessments are however, by Swedish practice, generally simplified to a two-dimensional (2D) linear interpolation between measured data, although the three-dimensional (3D) conditions may vary greatly at a site. This can lead to critical areas to be overlooked, especially for sites with variable topography, complex soil stratification or varying soil depth.

This study thereby investigates the integration of a 3D groundwater model into 3D LEM slope stability analysis to account for spatial variations. The groundwater model is generated as a finite difference model via the open-source software MODFLOW and the LEM analysis is performed with PLAXIS 3D LE using the General Limit Equilibrium (GLE) with half-sine function. The PLAXIS 3D LE applies the two-directional 3D-method and the Cuckoo search method, which allow for asymmetrical failure mechanisms and does not require any predefined search area by the user, in contrast to e.g. SCOOPS3D.

The study was applied to a geological site, Skälsbo, located along the Göta River valley. The site consists of thick deposits with soft sensitive clays with eroded slopes facing Göta River. Thorough geotechnical investigations have been performed at the site as a part of the Göta River Commission work to reduce landslide risks along the river.

The results show that the advanced 3D groundwater model can be successfully imported into 3D LEM for a simple, yet computational efficient, uncoupled hydromechanical analysis of the slope stability at regional scale. Comparisons of results from dry 2D analysis shows comparative results between LEM and corresponding finite element analysis. The method has thereby great potential in incorporating future climate scenarios and their effect on regional stability, to detect both migration of critical stability areas and changes in its distribution over time. The method also shows that the user can seamlessly generate 2D models from the regional model for further assessment. The strength of using an advanced groundwater model, such as MODFLOW, is that both historical and future groundwater scenarios can be accounted for and thereby bring a robustness to the stability evaluation. This approach accounts for the complex groundwater situation, to ultimately better predict and optimize the need and extent of mitigation measures for cost- and environmental purposes.