Thematic set : COST Action TU 1202 – Impact of climate change on engineered slopes for infrastructure Quarterly

The behaviour of natural and artificial slopes is controlled by their thermo-hydro-mechanical conditions and by soil–vegetation–atmosphere interaction. Porewater pressure changes within a slope related to variable meteorological settings have been shown to be able to induce soil erosion, shrinkage–swelling and cracking, thus leading to an overall decrease of the available soil strength with depth and, ultimately, to a progressive slope collapse. In terms of numerical modelling, the stability analysis of partially saturated slopes is a complex problem and a wide range of approaches from simple limit equilibrium solutions to advanced numerical analyses have been proposed in the literature. The more advanced approaches, although more rigorous, require input data such as the soil water retention curve and the hydraulic conductivity function, which are difficult to obtain in some cases. The quantification of the effects of future climate scenarios represents an additional challenge in forecasting slope–atmosphere interaction processes. This paper presents a review of real and ideal case histories regarding the numerical analysis of natural and artificial slopes subjected to different types of climatic perturbations. The limits and benefits of the different numerical approaches adopted are discussed and some general modelling recommendations are addressed. Received 23 August 2016; revised 20 February 2017; accepted 12 April 2017 The equilibrium conditions within slopes, and hence their level of stability, depend on several factors (Terzaghi 1950), which include the boundary conditions at the ground surface. These generate exchanges of water between the topsoils and the atmosphere, through processes such as rainfall infiltration, water evaporation from the soil pores and transpiration through vegetation, which, as a whole, are referred to as ‘slope–atmosphere interaction’. Changes with time of the soil porewater pressures, and hence of the soil stress–strain conditions, are consequent to such interaction. In particular, the porewater pressure changes generate, on one hand, variations of the mobilized shear strengths and, on the other, variations of the available shear strengths, and, as such, may bring about either the onset or the progression of slope failure, with eventual final slope instability. Failure results from the slope– atmosphere interaction especially within slope portions whose stability is marginal and may be lost as a result of even small increases in porewater pressure. Otherwise, failure may occur within shallow slope portions whose stability is strictly dependent on the presence of significant suctions in the soil pores (i.e. above thewater table), which are reduced upon rainfall infiltration. In any case, in general the slope stability varies with time as a result of climate. Several observations are reported in the literature of damage to structures and infrastructure interacting with slope movements connected to the slope–atmosphere interaction, which hence represents a source of risk for society. Such risk may apply to either man-made slopes or natural slopes. For man-made soil embankments and fills, made of partially saturated compacted soils and location of either road or railway infrastructure, movements due to variations in the soil degree of saturation as a result of the soil– atmosphere interaction have been widely observed to jeopardize the serviceability and safety of the infrastructure (e.g. Fleming & Taylor 1980; Alexander 1986; Hungr et al. 1999; Guzzetti et al. 2003; Schmertmann 2006; Dijkstra & Dixon 2010; Klose et al. 2014). With regard to natural slopes, the damage to buildings and infrastructure interacting with unstable slopes is increasing, mostly as a result of the increase of structures built in areas of © 2017 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/ licenses/by/3.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc. org.uk/pub_ethics Thematic set: COST Action TU1202 – Impact of climate change on engineered slopes for infrastructure Quarterly Journal of Engineering Geology and Hydrogeology Published Online First https://doi.org/10.1144/qjegh2016-079 high landslide hazard connected to climate, and hence of structures put at risk by wrong planning schemes (Cascini et al. 2005; Cotecchia et al. 2010, 2011). The safety of railways and roads is often threatened by debris flows involving soil cover on the steep flanks of infrastructure. Such flows show the highest rates and are the most damaging landslides (Cruden & Varnes 1996; Picarelli et al. 2004; Jakob & Hungr 2005; Cascini et al. 2010b; Hungr et al. 2014), and are triggered by drops in soil suction at shallow depths due to rainfall infiltration. Furthermore, despite the fact that the highest soil–atmosphere exchange flows take place at shallow depths, recent research has demonstrated also that deep slope movements found to jeopardize the safety of roads and railways may be triggered by slope–atmosphere interaction. The deep movements may relate to the variations in seepage conditions at depth consequent to the slope–atmosphere interaction, as the whole seepage domain in the slope reacts to the phenomena taking place at the top boundary. Seasonal excursions of the piezometric heads of even 2 – 3 m have been measured from 30 m down to 50 m depth in clayey slopes (Cotecchia et al. 2014) and shown to be due to the seasonal climatic processes. Such seasonal cycles in porewater pressure have been shown to bring about yielding of deep weak clays and consequent seasonal deep movements (Cotecchia et al. 2015). Furthermore, in slopes that are the location of old landslide bodies of marginal stability, seasonal cycling of porewater pressure due to climate has been shown to trigger seasonal reactivation of sliding (Cotecchia et al. 2008). Also, seasonal reactivations of damage to buildings and infrastructure interacting with such slopes have been recorded. The work presented in this paper aims at contributing to the dissemination of knowledge about the modelling strategies that can be employed to assess the effects of slope–atmosphere interaction on the stability of engineered slopes and has been conducted within the COST programme ‘Impact of climate changes on engineered slopes for infrastructure’. The assessment of the processes activated in the soils by climate is a necessary background for the evaluation of the instability risk for slope– structure systems and has to be based on interdisciplinary analyses. These entail knowledge in various scientific fields, mainly soil hydromechanics, hydrology, meteorology, agriculture and thermodynamics. The source of the disciplinary broadness of the subject and of the complexity of the modelling is the coupling of several phenomena taking place in the slope as a result of the soil– vegetation–atmosphere interaction. Over time, the capacity of models to simulate such phenomena has increased. The paper presents a review of the possible strategies to model these phenomena, from the variation of the piezometric conditions, to the evolution of the slope stability with time, and the corresponding soil displacements across the slope. The paper reports first an outline of the phenomena to be treated in an advanced modelling, with a review of the possible mathematical formulations of the processes, according to different levels of advancement. In this way a methodological framework is portrayed as the basis for the classification of modelling strategies of different levels of accuracy. Several case histories are then considered as examples of the application of the different modelling strategies.