The impact of changes in the water table and soil moisture on structural stability of buildings and foundation systems : systematic review CEE10-005 (SR90).

This Systematic Review aims to consider the impact of changes in the ground water table and soil moisture regime on structural stability of buildings and foundation systems. The possible changes in the water table levels and soil moisture conditions are expected as a result of environmental change. Building and infrastructure damage occurs where differential movements exceed the thresholds that the buildings or infrastructure can sustain. At locations where uniform vertical settlement dominates, buildings often move vertically with the subsiding ground surface and little damage occurs. It is when excessive differential deformation occurs that buildings and infrastructure are more prone to damage. A number of criteria for damage risk assessment are described. The expectation is that that settlement (downward movement of the ground surface) will occur during groundwater lowering and heave (upward movement of the ground surface) during groundwater rise. However, no cases of damage due to heave resulting from groundwater level rise per se were found in the literature reviewed. However, there were a significant number of cases of damage due to collapse settlements due to inundation during groundwater level rise. Collapse settlements in fill materials due to rising ground water levels are of major concern in the UK. Capillary rise may occur in soil above the water table. Capillary rise can cause deterioration to structures formed from monumental sandstone through dissolution of cementing minerals reducing the strength of stone and recrystallisation of dissolved salts leading to expansion of the stone. Flooding, where surface water exists above the ground surface, can be one circumstance that can lead to wetting and ground water table rise within the soil. In the first stage of flooding, the building structure is subject to the destructive impacts of water streams. In the aftermath of flooding, when water levels subside, the subsoil remains saturated with water. A further effect of flooding is that of soil erosion and scour which can do significant damage to foundations. Rises in groundwater level, can cause reductions in strength of the soil that can lead to failures of slopes. In regions of significant slope instability, significant damage to buildings can occur as a result of landslides. Lowering of the groundwater table can cause the soil to consolidate, which induces settlement. With softer, more compressible soils, settlements can become large. Many of the cases of damage reported are due to large scale land-surface subsidence induced by ground water abstraction. In some of these examples, the ground surface has fallen by as much as 8m. Other cases deal with more localised ground water control measures, usually associated with dewatering during construction of a tunnel or deep excavation such as an underground car park or metro station. The evidence suggests that significant consolidation settlements can be induced by groundwater lowering. In soft compressible soils, very large settlements can be induced. Settlements of the order of metres can be induced by large drops in groundwater level (30+ metres). Even land subsidence of less than 1 metre can induce significant damage to buildings. Much of the damage reported that is associated with groundwater lowering occurs in buildings on shallow foundations. However, deep foundations on piles can also be affected. If soil settles relative to the pile, this can result in downdrag on the pile (known as “negative skin friction”). This additional load could potentially overstress the pile and lead to failure. A further particular problem occurs with wooden piles when the groundwater level is lowered. If the water table is lowered, this exposes the upper part of the pile to aerobic conditions and rotting and decay can start to take place. There are examples of building damage due to rotting of wooden piles Karstic conditions exist in soluble rocks such as limestone and dolomite, where ground water flow causes dissolution of the rock leading to the formation of caverns. Sinkhole formation and ground surface subsidence due to dissolution of soluble rocks is a major cause of damage to buildings in karstic areas. This is often associated with groundwater lowering changing the dynamics of the hydrogeology in calcareous rocks. However, there are examples where damage has resulted from additional flow under high water table conditions, as the greater flow causes more dissolution of the soluble rocks and erosion or removal of clay filling from the fissures. Shrinkage and swelling of clay soils is the single most common cause of foundation-related damage to low-rise buildings in the UK. Vegetation, particularly larger trees, has a significant effect of removing water from the soil and inducing shrinkage. Seasonal shrinkage and swelling will be a major factor of concern if climate change produces drier summers and wetter winters, as predicted for the UK, since greater extremes of wetting and drying will induce greater cycles of swelling and shrinkage. The evidence suggests that shrinkage during periods of drought causes the greatest degree of damage. There is evidence from France that soil conditions are becoming progressively drier and this is consistent with a long-term drying trend predicted for the UK. Peat poses particular geotechnical problems due to its high compressibility. It is made from decaying vegetation and can be very fibrous with a very open, compressible structure. Due it its high compressibility, any changes in stress resulting from groundwater level changes are likely to result in large surface settlement or heave. To be able to assess the future implications of damage to structures due to environmental change it is important to understand the economic cost of damage to buildings due the mechanisms of groundwater level change, shrink/swell etc. The costs of damage due to shrink/swell movements on clay soils have resulted in economic losses of over £1.6 billion in the UK during drought years in the 1990s. Similar figures are evident from France where losses have been as high as 3.3 billion Є (£2.7 billion) in a single year. In China, losses due to land subsidence in Shanghai are estimated to be about £10 billion over a decade with £0.3 billion a year in losses in three other cities. Consideration has been given by researchers and strategists to the impacts of climate change on the UK built environment and what might be needed for adaption. A consensus is that potential problems to foundations could be addressed through higher specification of foundations, including greater depths for foundations, as well as by new construction methods. It is also possible that higher [increasing] minimum temperatures and fewer cold days could reduce problems associated with frost heave. It may be that an increase in the number of properties suffering damage could result in changes in the perception of the severity of damage and householders may become willing to accept minor levels of damage. Discussions about building performance in New Zealand also lead to the suggestion that risks of future climate change to buildings should be managed and this means that building codes and practices around the world will need to change to suit new climate conditions. However, changing codes and practices requires a good foundation of evidence and research. This is difficult to establish given the uncertainty of current climate change scenarios and their long timescale. There is also an awareness that buildings built now will still be in use in 50-100 years time. This produces a need for early action in the construction sector.