Climatic influence on geotechnical infrastructure: a

Environmental Geotechnics Volume 2 Issue EG3 Climatic influence on geotechnical infrastructure: a review Vardon

Environmental Geotechnics June 2015 Issue EG3 Pages 166–174 http://dx.doi.org/10.1680/envgeo.13.00055 Paper 13.00055 Received 05/06/2013; accepted 13/11/2013 Published online 01/03/2014 Keywords: embankments/environment/geotechnical engineering ICE Publishing: All rights reserved

Climatic influence on geotechnical infrastructure: a review Philip J. Vardon MEng, PhD

Assistant Professor, Section of Geo-Engineering, Delft University of Technology, Delft, the Netherlands

Geotechnical infrastructure is extensively used for transport and flood defences. Asset values are significant, with failures causing significant disruption alongside potentially significant injury, death and financial consequences. Changing climate alters the environmental load on such infrastructure, and this paper reviews the potential impacts and the current understanding of the changes in geotechnical performance. While it is seen that well-known climate change impacts (such as sea-level rise) affect the geotechnical infrastructure, other factors are also likely to have impact (such as increasing temperature causing soil drying; increasing mean rainfall causing reduction in soil suctions; increasing drought events leading to soil desiccation; and increasing intense precipitation causing soil erosion, flooding and hydro-mechanical failure).

Notation cp ET H L ra re de D g ra

specific heat capacity of air evapotranspiration rate of heat supply latent heat of vapourisation air resistance, that is, time in which 1 cm3 of air exchanges heat with 1 cm2 of surface stoma resistance vapour pressure deficit rate of change of saturated specific humidity with air temperature psychrometric constant air density

Introduction Climate change and its causes continue to be extensively discussed, with the methods and analysis determining whether anthropogenic activities are the cause taking centre stage. While of clear importance, these arguments detract from useful discussions of how to adapt or maintain our lifestyle, supported by extensive civil engineering. A practical engineering viewpoint is to analyse our understanding of future climatic scenarios to investigate whether infrastructure will be significantly affected and whether design practices will continue to be robust. Recently, there has been focus regarding the stability of natural slopes in relation to climatic changes (Briceño et al., 2007; Dehn et al., 2000; Dixon et al., 2006; Huggel et al., 2010; McInnes et al., 2007), with a number of papers in relation to temperatureinduced instability in permafrost regions due to changes in climate (Gruber and Haeberli, 2007; Harris, 2005). 166

Geotechnical infrastructure is pervasive around the world as a cost-effective method of forming topographical change for highway embankments/cuttings, flooding defences and retaining walls. Geomaterials are granular materials that interact with their surroundings (in particular by water interactions) through drying from evaporation or wetting from rain or water courses, which in turn affect internal stress states and stability. A large amount of the geotechnical infrastructure in developed countries is also old and constructed using historic construction methods (Dyer, 2004). Figure 1 presents schematically an outline of geotechnical infrastructure potential interactions with the atmosphere, including both direct interactions (such as moisture exchange) and consequences (e.g., soil desiccation or swelling due to vegetation loss). It is appropriate to subdivide such structures into those yet to be constructed and existing (World Road Association (PIARC), 2008). For new construction projects, PIARC (2008) suggests that all foreseen adverse consequences of climate change should be prevented as far as possible, including appropriate monitoring. Considering existing structures, PIARC recommends an analysis of consequences to be undertaken and expert appraisal. The maintenance cost of such pervasive infrastructure (e.g., 17 000+ km of dykes in the Netherlands and 35 000 km of estuarine/ river flood defences in England and Wales) is high. Over €800m per year is required to be invested in the Netherlands for flood protection dykes (AFPM, 2006), and the cost of failure is usually significantly more. If failure occurs at times of extreme loading, e.g., weather such as extreme rainfall or hurricanes, which could themselves

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Environmental Geotechnics Volume 2 Issue EG3

Climatic influence on geotechnical infrastructure: a review Vardon

Evapotranspiration/infiltration Precipitation, temperature, wind etc.. dependent

Hydraulic boundary condition Variable in time

Soil desiccation Localised enhanced permeability

Soil shrinkage/internal erosion Enhanced permeability Soil erosion Removing material

Seepage (variable in time)

Vegetation Growth/loss

Soil carbon Increase/decrease (vegetation dependent)

Construction methods Compaction/core material/ local materials

Vegetation (trees) Soil shrinkage/swelling

Figure 1. Geotechnical infrastructure and potential climatic interactions

trigger the failure, then damage and costs can be exacerbated, with further increases due to increasing population densities. Therefore, thorough assessment of existing infrastructure and changing environmental conditions is important for more efficient investment. Within the bounds of climatic change, there will be a number of atmospheric/climatic effects that will affect the geotechnical infrastructure. These are likely to include, for example, i) higher temperatures, ii) increasing sea levels, iii) increased frequency or intensity of extreme events (both rainfall and drought) and iv) other changes to precipitation and temperature due to elevated atmospheric CO2 (International Panel on Climate Change (IPCC), 2007b; Tubiello et al., 2008; US EPA, 2012). Such climatic effects can be split into two main categories: i) those effects that occur gradually and progressively (e.g., sea level rise or change in average temperature); and ii) those that increase in frequency/intensity (e.g., extreme weather events). In response to the first kind of effects, there are two main options: strategic withdrawal or defence. Economic judgement will be used to determine the choice of response (ICE, 2012; Tol et al., 2008). Straightforward changes in design can be made, with altered input to reflect, for example, an increase in sea level.

In response to the second type of effects, an increase in frequency/ intensity, which may lead to infrastructure collapse or damage, the response can be either proactive (i.e., protecting against these impacts) or reactive (i.e., re-building after damage). Such loading changes may be complex and may involve long-term material change (e.g., from periods of drought) and then subsequent highintensity loading (e.g., from extreme rainfall events). Design changes in response to such loadings may be compelled to be significantly more complex and take into account processes and failure mechanisms otherwise neglected. It is noted that such extreme events may occur with or without climate change; however, climate change will make such events more likely. This paper aims to examine the current state of the art in relation to the performance of geotechnical infrastructure in relation to climatic change. A substantial body of work is available on permafrost regions and in response to sea level rise; therefore, this will not be the focus of this work. The following section presents an overview of climatic predictions and scenarios with a focus on Europe, with the subsequent section reviewing the influence changes in climate are likely to have on geotechnical infrastructure. A discussion of


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the impacts, design approaches and key unknowns is then given followed, finally, by the conclusions.

In Europe, the EEA (2012) has synthesised climate change predictions. Regional differences are recognised, and it is stated that variation will also increase. Mean precipitation levels averaged across Europe are shown not to have changed significantly since 1950 (Haylock et al., 2008). However, there is a significant trend separating north (eastern) Europe and south (western) Europe, with an increase and decrease of mean precipitation of approximately 30–35 mm per decade (EEA, 2012). Projections gathered from a range of RCMs for Europe follow this general trend with a 10%–20% increase in northern Europe between 1961–1990 and 2071–2100 and a 5%–20% decrease in southern Europe in the same period (van der Linden and Mitchell, 2009; data from the ENSEMBLE project: EEA, 2012; Nikulin et al., 2011). Projections indicate it is likely that total annual precipitation will comprise more high rainfall days (with an exception of the Mediterranean and Iberian peninsulas) (IPCC, 2007a), although due to a low amount of available data no statistically significant past trends have been observed (EEA, 2012). Observed and predicted changes are split into six climate areas, four of which are shown in Figure 2 and also coastal and mountainous regions. The predicted changes are indicated by climate region in Table 1.

Climate predictions and scenarios Climate science is by its very nature uncertain. Mainly, it utilises past observations to calibrate models and then extrapolates forward. The IPCC aims to assess comprehensively and objectively the best available climate change information. IPCC (2007a) has observed a global increase of temperatures over the last 100 years of between 0×6°C and 0×74°C, reduction in ice and snow cover in both hemispheres, sea level rise of 1×8 mm average between 1961 and 2003, precipitation increase and soil drying in different areas and an increase in frequency of high-intensity rainfall in most areas. In Europe, the European Environment Agency (EEA, 2012) observed climate change impacts, including sea level rise, increased (river) floods and increase in frequency and intensity of droughts. In the USA, impacts are summarised by the National Research Council (NRC, 2008), with analysis on the transport effects, focusing on flooding potential and storm surges. General scientific consensus (IPCC, 2007a) states that the present climate change is mainly due to anthropogenic causes, i.e., net increase in greenhouse gas emissions. For engineering and mitigation, the causes of climate change are somewhat inconsequential but drive on the forward prediction process-driven models (EEA, 2012). Climate change predictions usually employ a two-stage modelling approach (EEA, 2012): i) a General Circulation Model (GCM), including the key global physical and chemical processes; and ii) a Regional Climate Model (RCM), which translate the GCM coarse solutions into local solutions. EEA (2012) generalises that while a great variation of results is obtained, confidence is held as past climate behaviour is reasonably well reproduced and the models are based on physico-chemical processes, not extrapolations. It is observed that temperature prediction has lower variation than precipitation and uncertainty increases over time. Moreover, national and international agreements and conventions, e.g., United Nations Framework Convention on Climate Change (UNFCCC, 1992) and Kyoto Protocol (UN, 1998), seem challenging to adhere to and challenging to accurately measure on a global level. Therefore, the limits set, e.g., 2°C mean global temperature rise above pre-industrial levels (UNFCCC, 2009), are unlikely to be able to constrain climate changes and do not remove uncertainty. Country-specific climate predictions exist, e.g., UK (UKCP09, 2011), the Netherlands (KNMI, 2012) and the USA (NOAA, 2013). However, this paper focused on potential impacts, not countrylevel quantification. More detailed analysis has been provided on predicted European changes to illustrate the potential changes, as a wide range of climates are included. Similar qualitative climatic trends are also predicted elsewhere, e.g., Canada (Auld and MacIver, 2007) and USA (NOAA, 2013; NRC, 2008). 168

In general terms (EEA, 2012), intense precipitation in western Europe is predicted to significantly increase with overall rainfall decreasing in southern Europe. As a consequence of climatic influence, soil moisture predictions give a reduction in summer moisture content, with significant reductions in the Mediterranean region. Soil erosion, due to both water and air, is currently

500 km 300 miles

Northern Europe

North-Western Europe Central and Eastern Europe

Mediterranean

Figure 2. Key climatic regions of Europe (EEA, 2012)



North-Western Europe

Central and Eastern Europe

Mediterranean

Coastal

Mountainous

Temperature Mean Extremes Precipitation Mean Extremes Wind Speed Sea-level rise Storm surges River flow Mean Extremes (flood/ drought) Permafrost Increased active depth (freeze/ thaw cycles)

Climate area of Europe Northern Europe

Climate feature


+ +

+ +

+ +

+ +

+ +

+ +

+ +

+ +

+ +

– +

+ +

+ +

+ + +

+ + +

+ + +

– + –

+ + +

+

+ + + – +/– +/+ ±/± ±/+

+

‘+’ indicates an increase in behaviour and ‘–‘ indicates a reduction in behaviour.

Table 1. Predicted climate changes relevant for geotechnical infrastructure (EEA, 2012)

estimated to affect over 170 million ha and is predicted to increase, but quantitative predictions are unavailable.

Climatic influence on geotechnical infrastructure The interaction of climate change and soil is significant (EEA, 2012) and includes water regulation, carbon store, vegetation, nutrient exchange, soil erosion and soil structure. Some of the impacts on soil are difficult to determine and may be affected by more than one climate change process. It is shown by Alonso et al. (2003) that for natural slopes, the deep material is governed by the (changing) water table, and upper layers are controlled by atmospheric conditions. It is shown that the hydraulic material properties, including permeability and water retention, are affected by environmental conditions and are fundamental properties for slope stability. Interfaces are important, as cyclic loading probably creates remoulded soils, with altered and reduced strengths. Extreme weather (heavy and prolonged rainfall) is indicated to reduce safety

factors and cause permanent strength reduction. Figure 1 provides an outline of climatic interactions with geotechnical infrastructure. However, it is accepted that climate-induced failure is complex and failure mechanisms are not fully understood (Briceño et al., 2007). Geotechnical failure can be directly caused by changes to stability from changes in stress conditions (e.g., high pore-water pressures), reducing the effective stress and shear strength, or those caused by hydraulic conditions (e.g., uplift, heave, internal erosion and piping) (Eurocode 7; EN, 1997). Temperature does not directly cause significant adverse effects but governs processes coupled to it: evapotranspiration, vegetation growth or loss, soil shrinkage and desiccation. The hydraulic inputs and outputs to the soil/water system are more directly significant. Table 2 presents a synthesis of the climate change features, the potential impact on geotechnical structures and the potential failure mechanisms. The surface boundary interacts with the atmosphere (as shown in Figure 1) and is an important boundary. The boundary must capture both infiltration and evapotranspiration. The majority of previous research has been undertaken at catchment or averaged level for hydrological studies (Betts et al., 1996). Infiltration is a function of rainfall and the ability for the rain to infiltrate or, if not possible, run-off and infiltrate elsewhere (Henderson and Wooding, 1964). The most common method of describing the evapotranspiration at the boundary is the Penman-Monteith equation (Monteith, 1965)

1.

ET =

D H + ρa c p (δ e) / ra L D + γ (1 + re ra )

{

}

where ET is the evapotranspiration, D is the rate of change of saturated specific humidity with air temperature, H is the rate of heat supply, ra is the air density, cp is the specific heat capacity of the air, d e is the vapour pressure deficit, ra is the air resistance, re is the stoma resistance, L is the latent heat of vapourisation and g is the psychrometric constant. Environmental conditions, including wind speed, irradiance, temperature and humidity, are included in the parameters H, D, cp, ra, re, ra and d e. This equation presumes full vegetation cover, but by modifying re a reduction could be incorporated. A maximum rate of evapotranspiration is calculated, where dry soil would not be able to provide this rate. Any changes in surface area, e.g., due to cracking, are also not included. This approach has been utilised in catchment level models (Betts et al., 1996) as well as field tests and modelling (Cui et al., 2012). An experimental programme utilising a climate chamber is presented (Cui et al., 2012), where the underlying heat-driven evaporation process from the Penman-Monteith equation is supported by the measured reduction of soil surface temperatures. Soil drying, rapid pore pressure increase and pore-water cycling An overall increase in temperature, along with possible reduced periods of precipitation (e.g., drought), can lead to long-term drying


169



Existing infrastructure Climate change feature

Increased temperature Decreased precipitation (drought)

Increased mean precipitation Intense precipitation

Freeze/thaw cycles

Potential impact on geotechnical infrastructure

Potential failure mode

Drying Soil desiccation Soil shrinkage Reduction of vegetation/soil erosion Some soil erosion/loss of soil quality Change in water table leading to instability Significant soil erosion Rapid soil wetting, highly dynamic pore pressure changes potentially Flooding Loss of soil structure

Uplift Piping, internal erosion, slope stability Piping Piping, slope stability Erosion, piping Slope stability Piping, slope stability Slope stability Piping, internal erosion, slope stability Slope stability

New infrastructure Climate change feature

Drought

Increased precipitation/ Intense precipitation

Potential impact on geotechnical infrastructure

Potential failure mode

Reduction of moisture content of fill (compaction more difficult) and mixing of fill with water is expensive Collapsing of some fill material due to wetting

Cost, serviceability failure

Slope stability, serviceability failure

Table 2. Overview of potential impacts on geotechnical infrastructure due to climate change

(e.g., by increasing H in Equation 1). If the material has a low dry density (e.g., peat), the risk of sliding may become significant, as illustrated by the Wilnis dyke failure in 2003 (Van Baars, 2004). In clay materials, desiccation behaviour has been observed (Dyer et al., 2009). The impact of clay fissuring and subsequent softening of surrounding material is well reported (Skempton et al., 1969; Stark and Eid, 1997). Dyer et al. (2009) also initially suggested possible additional hydro-mechanical failure mechanisms due to a network of cracks. Pore-water changes can affect slope stability due to the reduction of effective stress and therefore shear strength (Clarke et al., 2006; Dehn et al., 2000; Lee and Jones, 2004). Davies et al. (2008) outlined progressive collapse due to pore-pressure cycling linked to climate change, and Clarke et al. (2006) outlined initial findings regarding rainfall-induced pore-pressure cycles. Cycling of pore pressures has also been noted to cause serviceability failures, i.e., differential settlements (O’Brien, 2007) and failure due to strain softening (Kovacevic et al., 2001; Potts et al., 1997). Cyclic pore pressures have been found to have different effects depending on the soil permeability, stability decreasing with increased permeability (Nyambayo et al., 2004). This effect may be significant with increasing cycles and soil desiccation, leading to enhanced permeability. Strain softening and irreversible creep due to pore 170

pressure cycle induced shrinkages and swelling has been observed at laboratory scale by centrifuge tests (Hudacsek et al., 2009; Take and Bolton, 2011). Rapid pore pressure increases can be caused by intense rainfall or modification of the soil structure, that is, through the aforementioned cracking. Moisture can propagate preferentially through clays that crack (Baram et al., 2013) and indeed influence unsaturated zones of over 12 m in a single intense rainfall, for example, a local flooding event (Baram et al., 2012), although there is only very limited work on the effects on slope stability in relation to flow through desiccation cracks (Khandelwal et al., 2013). It is also shown that preferential flow can cause internal erosion and the development of pipes (Øygarden et al., 1997). Vegetation The effects of vegetation on swelling and shrinking soils were investigated by a series of authors in 1983 with the intention of investigating damage to buildings due to such behaviour. Ravina (1983) investigated the effects of vegetation type on soil moisture and cracking. It was found that under normal conditions, deeprooted plants could cause drying up to 2–3 m but not cracking. Cracking was observed in highly swelling soils in hot and dry weather conditions, whereas it was exacerbated in cropped soils. With cracking, soil moisture was affected at much greater depths. Driscoll (1983) studied the impact of swelling clays, noting that




the impact of trees was significant. The effects of vegetation on swelling and shrinking of soils in Australia, where a semi-arid climate exists, were investigated by Richards et al. (1983). It was seen that certain trees cause moisture deficits at depths of up to 10 m and horizontal distances of up to approximately 50 m.

carbon, affecting the soil water retention (Rawls et al., 2003) and permeability properties.

Vegetation affecting slopes can be classified into two groups: i) surface vegetation covering the embankment and ii) deeper changes due to larger vegetation (i.e., trees). The first of these can influence the evapotranspiration, and the soil carbon then influencing soil water retention and permeability. Scott et al. (2007) observed a major difference between tree- and grass-covered areas, with suctions approximately one order of magnitude higher caused by trees. Moreover, the movements observed in slopes were in opposite directions (e.g., grass-covered slopes moving downslope in the summer and upslope in the winter, with the opposing directions apparent in the tree-covered slopes). It is noted that there were different materials in the studied embankment, including a shallow ash layer. Ridely et al. (2004) described changes in pore pressure due to extreme weather (in this case a very wet winter) and related the changes in pore pressure to changes in stability based on loss of additional strength provided by suction. It is shown that suctions reduce to zero over this period even on tree-covered slopes; therefore, this strength cannot be relied upon. However, it is noted that under-drainage could be utilised to reduce excess pore pressures developing under the embankment. Summertime downslope movements influenced by trees and grass have also been observed (Lees et al., 2013) over 11 years. Both deep-seated and shallow movements are observed due to pore pressure cycles and in particular where trees are located at the embankment toe. Smethurst et al. (2006, 2012) measured the pore-water pressure response in a vegetated cutting and showed wide differences (up to 500 kPa) of pore pressure between dry and wet summers. They also concluded that the permeability of the surface layer is altered, allowing pore pressures to quickly change when rewet. Due to climate change, pore pressure cycles are likely to become more extreme, potentially extending over more than one year and potentially increasing differential settlements and strain softening. However, winter strengths have been predicted to increase due to the presence of winter suctions by way of numerical modelling study, where rain events have higher intensity but are less persistent and temperatures increase significantly (Rouainia et al., 2009). The model reproduced qualitatively shrinkage/swelling cycles at the slope toe, which could cause strength degradation, but observed a significant suction increase. The approach was validated with limited comparison to Smethurst et al. (2006) although required a large increase in surface permeability. By examining embankment dams, Vaughan et al. (2002) showed that vegetation growth may cause initial cracking and increased permeability, but suggested that for slopes including a drainage layer, stability would not be greatly affected. Vegetation may also be directly affected by climate (e.g., droughts). The impact on stability may be due to erosion or loss of soil

Soil erosion Soil erosion can take place internally (increasing permeability and reducing strength) or externally (removing material and potentially inducing instability). Gunn et al. (2009) observed ‘voiding’ in an embankment providing preferential pathways and potential for cracking. Such ‘voiding’ increases permeability, leading to a reduction in the safety provided by low permeability embankments observed by Ridely et al. (2004). The role of plant root systems in reducing soil erosion is difficult to quantify (Reubens et al., 2007) but has been shown to provide mechanical reinforcement to the surface (Greenwood et al., 2004). Vegetation can also enhance surface permeability, allowing infiltration (Greenway, 1987), which may reduce surface erosion by reducing runoff. Loss of root strength and erosion may take place when vegetation is removed (Van Beek et al., 2008), whereas pore pressure recharge can happen more quickly. Wave impact and overtopping have been investigated in a large-scale laboratory test (Piontkowitz, 2010; Piontkowitz et al., 2009), concluding that only repeated loading events will cause damage and areas with well-established vegetation cover are more protected.

Discussion There are relatively few geotechnical failures; however, the cost of failure, in terms of transport service disruption or flooding, is high. Therefore, the risk profile must be understood so that maintenance or mitigation measures can then provide a cost-effective solution (Molarius et al., 2012; PIARC, 2008). Furthermore, past data are generally limited and cannot be specifically attributed to climate change (EEA, 2012); therefore, looking at changes in vulnerability is advantageous. In terms of potential impacts, EEA (2012) states that transport infrastructure is likely to be most affected, especially rail in the UK, central and eastern Europe, France and Scandinavia. Uncertainty is generally high, however, as impacts are a consequence of uncertain inputs and phenomena. The Extreme Weather Impacts on European Networks of Transport (EWENT) project looked at transport impacts due to extreme weather events (Molarius et al., 2012). The current damages due to extreme weather on geotechnical infrastructure are only considered by way of embankment damage due to flooding in extreme precipitation (>30 mm/day) and vulnerability is assessed by utilising qualitative survey data. For general risk indicators, this is relevant, but process-driven analysis would be valuable to give further insight. The stability of geotechnical structures is calculated by a number of relatively simple techniques to calculate resistance to a number of static failure mechanisms (e.g., Eurocode 7; Dixon et al., 2006; EN, 1997), although provision is made that ‘environmental conditions shall be assessed’. However, it has been identified by Alonso et al. (2003) that the hydraulic evolution of the soil


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should also become an implicit part of geotechnical analysis for climatically influenced slopes.

Seventh Framework Programme (FP7/2007-2013) under REA grant agreement no. PCIG12-GA-2012-333177.

While individual processes and understanding their potential impact are of clear significance, for general risk-based approaches, large-scale risk analysis of slope networks is required to analyse appropriately the consequence to climate change. Initial research in this area is presented by Collison et al. (2000) and Schimdt and Dikau (2004), who analyse future and historical climate data and slope failure models using a Geographic Information System (GIS) approach. Collison et al. (2000) used one-dimensional moisture transfer linked to empirical slope stability models. Schimdt and Dikau (2004) stated that while generalisations in climate impacts on slope stability can be made, specific geological and geotechnical conditions govern the stability of a specific slope. Initial analysis of natural landslide risk in relation to climate has been made (SafeLand, 2012a), utilising continuum soil infiltration, including evapotranspiration, to analyse stability (SafeLand, 2012b). With further computer power, more slope-specific data, complex processes and reliability-based models can be incorporated. Reliability-based slope stability approaches can be used to analyse uncertain behaviours and can be utilised to understand the sensitivity of safety to processes (Arnold and Hicks, 2001; Griffiths and Fenton, 2004; Hicks and Spencer, 2010). This enables engineering decisions to be made, accepting that processes are uncertain.

References

One of the most critical areas for climate driven processes affecting the stability of geotechnical infrastructure is the climate/soil boundary. Further research and data in this area, and then projections due to climate change, will allow modelling and design to be more robust. An area that has received little attention is the propagation of desiccation fractures into embankments, as observed by Dyer et al. (2009). Such behaviour, with more extreme weather patterns, may cause hydro-mechanical behaviour that could cause otherwise unanticipated failures.

Conclusions A range of potential, best-estimate, climate-change-driven processes that could affect the stability of geotechnical infrastructure have been identified. Geotechnical failures can be highly expensive in terms of damage and consequence; however, they are also rare. Therefore, changes in risk and vulnerability can be utilised. The major climatic changes that are likely to affect geotechnical infrastructure are increasing temperature (causing soil drying), increasing mean rainfall (causing reduction in soil suctions), increasing drought events (leading to soil desiccation) and increasing intense precipitation (causing soil erosion, flooding and hydro-mechanical failure).

Acknowledgements Research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s 172

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