Impact of Water-Level Variations on Slope Stability

Sammanfattning: Waterfront-soil slopes are exposed to water-level fluctuations originating from either natural sources, e.g. extreme weather and tides, or from human activities such as watercourse regulation for irrigation, freshwater provision, hydropower production etc. Slope failures and bank erosion is potentially getting trees and other vegetation released along with bank landslides. When floating debris is reaching hydropower stations, there will be immediate risks of adverse loading on constructions, and clogging of spillways; issues directly connected to as well energy production as dam safety. The stability of a soil slope is governed by slope geometries, stress conditions, and soil properties. External water loading, pore-pressure changes, and hydrodynamic impact from water flow are factors being either influencing, or completely governing the actual soil properties. As a part of this study, knowledge concerning water-level fluctuations has been reviewed; sources, geotechnical effects on slopes, and approaches used for modelling, have been focused. It has been found a predominance of research focused on coastal erosion, quantification of sediment production, bio-environmentally issues connected to flooding, and effects on embankment dams subjected to rapid drawdown. Though, also water-level rise has been shown to significantly influence slope stability. There seems to be a need for further investigations concerning effects of rapidly increased water pressures, loss of negative pore pressures, retrogressive failure development, and long-term effects of recurring rise-drawdown cycling.Transient water flow within soil structures affects pore-pressure conditions, strength, and deformation behavior of the soil. This in turn does potentially lead to soil-materialmigration, i.e. erosion. This process is typically considered in the context of embankment dams. Despite the effects of transient water flow, the use of simple limitequilibrium methods for slope analysis is still widely spread. Though, improved accessibility of high computer capacity allows for more and more advanced analyses to be carried out. In addition, optimized designs and constructions are increasingly demanded, meaning less conservative design approaches being desired. This is not at least linked to economic as environmental aspects. One non-conservative view of slope-stability analysis regards consideration of negative pore pressures in unsaturated soils. In this study, three different approaches used for hydro-mechanical coupling in FEM-modelling of slope stability, were evaluated. A fictive slope consisting of a wellgraded postglacial till was exposed to a series of water-level fluctuation cycles. Modelling based on classical theories of dry/fully saturated soil conditions, was put against two more advanced approaches with unsaturated-soil behavior considered. In the classical modelling, computations of pore-pressures and deformations were run separately, whereas the advanced approaches did allow for computations of porepressures and deformation to be fully coupled. The evaluation was carried out by comparing results concerning stability, vertical displacements, pore pressures, flow, and model-parameter influence. It was found that the more advanced approaches used did capture variations of pore pressures and flow to a higher degree than did the classical, more simple approach. Classical modelling resulted in smaller vertical displacements and smoother porepressure and flow developments. Flow patterns, changes of soil density governed by suction fluctuations, and changes of hydraulic conductivity, are all factors governing as well water-transport (e.g. dissipation of excess pore pressures) as soil-material transport (e.g. susceptibility to internal erosion to be initiated and/or continued). Therefore, the results obtained underline the strengths of sophisticated modelling.