Many people consider the Soil Survey of England and Wales to be focused solely on agricultural research. Indeed, it was this that provided its initial impetus and funding. However, during the late 1980s and into the early 1990s new interpretations of soil data were being considered following the end of the systematic soil survey. One such avenue was the production of a range of thematic geohazard maps. To define, a geohazard is a geological/pedological (soil) phenomenon with the potential to cause harm to the built environment or human life.
With the UK having over 700 soil types, or series, raw soil information can often be problematic for a non-soil scientist to reliably determine a specific soil series’ geohazard potential. Therefore, thematic maps help to encapsulate detailed soil knowledge into an easily understandable format. This is important for geohazard mapping applications as data is required by a broad range of users, including: policy-makers, insurers, planners, asset managers, developers and in some instances the general public.
One particular geohazard considered was the volumetric shrinkage of clay soils, further referred to in this post as clay-related subsidence.The UK is particularly affected by clay-related subsidence; a result of a large proportion of the UK, especially in the South-east of England, being underlain by clay-bearing bedrock. This results in the formation of soils containing clays prone to volumetric shrinkage and swelling. Approximately 20% of England and Wales soil stock is abundant in clay.
My own PhD research is focused on the modelling of the spatial distribution of clay-related subsidence for future climate scenarios by incorporating UKCP09 climate projections into existing geohazard models, such as the Natural Perils Directory (NPD). Below is an example map from NPD describing the spatial susceptibility to clay-related subsidence for England, Wales and Scotland.
Furthermore, I am also considering the impact that specific geohazards may have upon UK infrastructure networks both now and in the future. In this post, I’d like to provide you with an insight into the impacts of clay-related subsidence and a brief explanation of the science behind the process.
To give you an idea of what I mean by the volumetric shrink/swell of clay soils, the video below, produced by staff at Cranfield University, depicts the swelling of a clay soil on wetting compared to its sandy soil equivalent.
Impacts of clay-related subsidence
From a UK perspective, the built environment is particularly susceptible to the effects of clay-related volumetric shrinkage and swelling. Here I consider the built environment to represent the homes that we live in, the infrastructure that supports our lives (e.g. roads, rail, water, gas, telecoms, etc.) and the business properties which we both frequent and are employed within.
It was after the 1975/1976 drought that an appreciation of the highly damaging nature of clay-related subsidence, particularly by the insurance industry was felt – this was due to a high number of claims made relating to clay-related subsidence damage to buildings. Since this time, annual insurance losses have run at between £300-500 million on an average year.
During my previous experience as a geotechnical engineer, I witnessed first-hand that many UK domestic and some commercial properties have extremely shallow foundations (~50cm below ground level) which means that they are potentially at risk, especially during warm periods, of differential settlement as a result of clay-related subsidence. This can lead to cracking of walls and floors and in worst-case scenarios can lead to the evacuation of properties and expensive remedial works (e.g. underpinning).
Infrastructure is also affected by clay-related subsidence. Shallow buried pipelines in particular are at risk of clay-related ground movement, which can result in the fracturing of water and gas pipes. This is especially so with cast-iron pipes, which in the UK can be up to 150 years in age. Cast iron pipes are brittle compared to modern plastic-based piping, and may also be corroded (see picture below) making them more susceptible to fracture following ground movements.
Road networks, especially the lesser road network (e.g. unclassified and some B & C roads) are also susceptible to subsidence. This can result in severe longitudinal cracking of pavements, making for poor driving conditions. I have explored the impact of clay-related subsidence impacts on roads in a previous Geoscientist article, and am due to present my research at the upcoming EGU General Assembly 2015.
Remediation of clay-related subsidence impacts include the underpinning of properties, removal of trees which draw excessive moisture from the soil and the surrounding of buried pipes with non-shrinkable backfill (e.g. sand and gravel). However, the removal of trees can result in excessive heave as clay soils take up the moisture previously taken up by the tree(s); therefore, solutions can be complex.
Policies and guidance exist to limit the susceptibility of new developments to the risk of clay-related subsidence. The National House Building Council (NHBC) recommend that in highly expansive subsoils, a minimum foundation depth of 1.0m should be adhered to. Planning Policy Guidance (PPG) 14, Development on Unstable Land (Annex 2: Subsidence and Planning) documents the broad planning and technical issues associated with developing on unstable land. Current documents however, do not take into account climate change, which is likely to result in more frequent and thus damaging clay-related subsidence events in future.
In the following sections, I give consideration to the factors which cause clay-related subsidence.
Clays represent the most abundant constituent of soils globally. Individual clay minerals represent particles which are less than 0.002mm in diameter (see picture above), making them invisible to the human eye. They have enormous surface areas, up to 150-175 square metres per gram; putting that into real-life terms, that represents the area of just over one half of a tennis court on our campus!
When viewed with a scanning electron microscope, clay minerals represent an abundance of plate-like particles, often stacked upon each other.
There are several main groups of clay minerals, including;
Not all of these clay groups are considered to be expansive and subsequently prone to volumetric shrink-swell. Two types of mineral arrangement exist, including the 1:1 and 2:1 ratios. This describes the ratio of silica tetrahedron and aluminium octahedron (see structure of clay diagram below). All 1:1 clays are non expansive as they do not allow water to expand the particles. In contrast, some 2:1 clays are expansive (i.e. smectite and vermiculite). However, not all 2:1 clays allow expansion through water bonding (i.e. Illite and Chlorite).
Smectites are the most highly expansive clay minerals in the UK. Both smectites and vermiculites allow water to be absorbed between the unit layers due to weak interlayer forces and the polar nature of water molecules, which is then held in place by hydrogen bonding. It is this adsorption of water that causes clay minerals to expand and swell. Similarly, when water is lost under drying conditions, the unit layers contract and the soil mass shrinks.
The formation of cracks in clay soil, which we are all undoubtedly familiar with (if not, see cover picture) are examples of clay-related volumetric shrinkage in action. Whereas what we see on the ground is the horizontal shrinkage of soil, often into its differentiating peds, a vertical component also exists, which results in vertical shrinkage of the ground surface. The graph below shows the ground displacement of London Clay near an Oak tree, whereby the maximum movement is in the summer months when the ground is driest. Contrastingly, we can see the heave of the ground surface (i.e. +ve displacement) in the winter months as winter rainfall wets the soil once again. It is this cyclical process which results in damage to the built environment.
The role of soil moisture
As well as requiring a mineralogical composition prone to shrink/swell activity, the volumetric change of clay soils is determined by a soil’s moisture content, regulated by the external climate. The tank model video below, produced by the British Geological Survey, gives an indication of how increasing moisture levels can lead to a swelling of clay soil.
In civil engineering, a clay soils plasticity index is measured to routinely assess its potential for volumetric change. This is often undertaken for new developments during the intrusive ground investigation stage; the geohazard mapping that is discussed in this article is predominantly for scoping-out potential issues prior to planning applications or for infrastructure asset management purposes. An intrusive site investigation is generally recommended prior to development.
Anyway I digress. The plasticity index is a function of the difference between the plastic and liquid limit. The British Standards Institution define the liquid limit as the ‘moisture content at which a soil passes from the liquid state to the plastic state‘. The plastic limit is defined as ‘the moisture content at which a soil becomes too dry to be plastic‘. The chart below, known as the Casagrande chart is used to plot the plasticity index. Generally, soils which behave plastically over ranging moisture contents and that high liquid limits have a greater tendency to shrink and swell. To note, a number of analytical methods also exist for measuring the direct volumetric shrinkage of a clay soil.
Within my research, the potential soil moisture deficit (SMD) of a soil is used as a determinant, alongside the soils clay mineralogy, to categorise the soils susceptibility to shrink and swell. The UK has a highly variable PSMD, which is measured in millimetres (mm) and ranges between 0mm (e.g. Scottish Highlands) to >225mm (e.g. East Anglia) in a typical year. Furthermore, trees and similar large vegetation can have a profound effect on soil moisture levels. For example, some trees can uptake and transpire the equivalent of 800mm soil moisture from the soil in any one year. Peter Biddle has undertaken a comprehensive review of the effect of trees on soil moisture and subsequent subsidence effects to the built environment. As a result, trees are often blamed for specific damages caused to structures resulting from clay-related subsidence.
PSMD is calculated using the equation below;
[Potential Soil Moisture Deficit (PSMD) = Rainfall – Potential Evapotranspiration]
It is easiest to imagine the calculation of PSMD through the analogy of a bucket of water. Essentially, when the bucket is full (i.e. the ground is saturated and at field capacity) the PSMD is 0 mm. If we add more water to the bucket (i.e. rainfall) the bucket will simply overflow as no more water can be stored in the soil, creating a soil moisture surplus (SMS) or runoff; as a result, the quantification of soil moisture is important for flood assessment. However, if the bucket is exposed to levels of potential evapotranspiration which subsequently exceeds rainfall then the water level will eventually decrease, causing a soil moisture deficit. In a typical year, a PSMD normally develops in the spring, and reaches its maximum during September/October. Winter rainfall then typically brings the PSMD back to 0 by January/February (although PSMD is reset to 0 in February), however, PSMDs can prolong into the following year if a particularly dry year. It was this issue that made the 1975-76 drought the worst on record, and where between October-March only 303 mm of rainfall fell on England and Wales.
In summary, it is the cyclical nature of PSMD which governs the magnitude and frequency of a clay-susceptible soil to undergo shrinkage and swelling. History tells us that large-scale drought events have the potential to cause significant damage to the built environment. Furthermore, the recent UKCP09 climate projections suggest that the future climate is likely to further exacerbate this geohazard. Research is currently ongoing to fuse UKCP09 projections with the NPD model. Ultimately, this fusion will provide a risk management tool to guide those looking to future-proof their stock of built environment assets.
Cover Picture: A cracking clay soil in Northamptonshire (Photo: O. Pritchard)