by Anthony Lymath
This is the sixth in an eight-part series of articles, Climate Change Adaptation in Buildings, examining the impact of climate change on the built environment, and the responses that can be made to those changes for both new-build and retro-fitting. Previous articles have covered an overview of the issues, excess heat and flooding. Forthcoming articles will look at drought and wind.
The incidence and prevalence of subsidence can be dated back to the earliest times, with famous historical examples including the Leaning Tower of Pisa and Venice’s Campanile in St Mark’s Square. But the rise in prominence of subsidence at a smaller and more local scale began in the 1970s with the introduction of insurance policy clauses, around the time of the 1976 heatwave. Since then, subsidence has been a key feature of building surveys and ground movements, whether swelling due to excess ground water or shrinkage due to drought are the cause. Although problems can be exacerbated by the proximity of tree roots to foundations, it is the ground movement that is caused by excessive wet and dry weather periods that is the focus of this guidance – although the two are to a significant extent related.
During periods of excessive and prolonged rainfall, ground water levels can rise and swell prone soils, particularly cohesive soils with a high clay content (and to a lesser extent silt), which are particularly susceptible to volumetric change. Conversely, excessive and prolonged dry periods cause shrinkage. In winter, waterlogged ground can move further by frost heave. Clay soils are found extensively across England and Wales, and the potential for volumetric change is measured through the plasticity index where an index of 20 is defined as medium shrinkage and swelling. Shrinkage and swelling tends to occur within 5m of the ground surface, and although rarely more than 150mm either horizontally or vertically, this is obviously more than enough to cause significant damage and risk to safety.
Non-cohesive soils by contrast, such as sands and gravels, can also give rise to subsidence when the fine particles are washed away during floods.
Tree-induced ground movement occurs when either there is a period of drought, in which case the tree removes moisture from the soil at a faster rate than it is replaced by natural rainfall (mature deciduous trees can remove 50,000 litres of water per year), or due to ground swelling if a mature tree is removed suddenly and hence the water depletion rate drops. Similarly, extensive pruning can have a significant effect on the amount of water uptake that the tree exhibits. The proximity of trees to a building is therefore likely to impact on the degree of risk of subsidence.
Peat can, if it is exposed to the air, decompose naturally due to oxidization, with a consequential reduction in volume. This can occur not only by exposure during removal for burning or horticultural purposes, but also by a reduction in the ground water table level.
Subsidence can also occur due to human intervention, on old in-filled sites such as excavation workings where the fill can consolidate, degrade or decompose over time.
Subsidence can also occur due to human intervention, on old in-filled sites such as excavation workings where the fill can consolidate, degrade or decompose over time. Local Authority or Environment Agency searches should be able to identify the locations of old workings at the initial planning stages of a project. In addition, subsidence can also occur due to other problems, including cracked and broken drains or water supply pipes, which should all be investigated and ruled out before concluding that problems are due to shrink-swell. Finally, shrink-swell only occurs due to changes in water content – a constantly-wet or constantly-dry soil should not exhibit such unstable tendencies.
Preliminary work to avoid (or minimise) the risk of subsidence will include Local Authority searches, desk studies and ground investigations, and risk assessments. Advice from an experienced consulting engineer should be sought. Once suitable information has been gathered and assessed, building siting and orientation can be considered, in consideration of the location and proximity of natural hazards, such as water courses and mature vegetation, together with the soil type, depth and bearing capacity.
If the survey results indicate that the ground has a high risk of susceptibility to movement, and if the choice of location for the development is limited, then appropriate structural foundation options should be considered to militate against the risk of subsidence. Raft or piled foundations may be suitable options, or jet grouting depending on circumstances and economics.
Should subsidence be encountered, the remedies all fall into the category of underpinning, although this can take many different forms:
- Continuous mass concrete underpinning, generally installed in a hit-and-miss sequence for large areas
- Beam and base underpinning
- Mini-piled underpinning, including pile and beam, cantilever pile caps and piled rafts
- Displacement piles
- Expanding resin injection
- Groundbearing raft
- Structural strengthening – tie rods, resin bonding, masonry stitching, corseting with reinforced concrete or post-tensioned ground beams
- Jet grouting
- Geopolymer injection or fluid cement.
Once again, specialist advice should be sought from a consulting engineer in order to determine the most suitable strategy.
There are a number of standards and reports which relate to subsidence and underpinning. The Building Research Establishment (BRE) publishes a number of pieces of guidance:
- BRE Report 184 Foundation movement and remedial underpinning in low-rise buildings1
- BRE Digest 240 Low-rise buildings on shrinkable clay soils part 12
- BRE Digest 241 Low-rise buildings on shrinkable clay soils part 23
- BRE Digest 251 Assessment of damage in low-rise buildings4
- BRE Digest 298 Low-rise building foundations: the influence of trees in clay soils5
- BRE Digest 318 Site investigation for low-rise building: desk studies6
- BRE Digest 343 Simple measuring and monitoring of movement in low-rise buildings. Part 1: cracks7
- BRE Digest 344 Simple measuring and monitoring: Part 2 settlement, heave and out-of-plumb8
- BRE Digest 348 Site investigation for low-rise building: the walk-over survey9
- BRE Digest 352 Underpinning10
- BRE Digest 386 Monitoring building and ground movement by precise levelling11
In addition, the Construction Industry Research and Information Association (CIRIA) has published four reports:
- R111 Structural renovation of traditional buildings12
- PG1 Review of bearing pile types. 2nd edition13
- C573 Guide to ground treatment14
- C514 Grouting for ground engineering15
There are also a number of British Standards applicable to foundation work and ground stabilisation:
- BS EN 1536 Execution of special geotechnical works. Bored piles16
- BS EN 12699 Execution of special geotechnical work. Displacement piles17
- BS EN 14199 Execution of special geotechnical works. Micropiles18
- BS EN 12715 Execution of special geotechnical work. Grouting19
- BS EN 12716 Execution of special geotechnical works. Jet grouting20
Regardless of personal opinion on the validity or otherwise of the theory of climate change, the fact remains that during extreme weather events buildings have a tendency to fail. This can be due to a number of factors, and under certain circumstances can be due to conflicting requirements on the building envelope. However, as has been identified, there are numerous systems and products available with which to combat weather and climate extremes. In order to assist in producing buildings that are better able to withstand the vagaries of the weather over their design lives, the NBS offers a number of subscription products from which systems and products may be specified to mitigate the effects of climate change.