World in motion

Around the world, global warming and the endless quest for natural resources are causing catastrophic subsidence problems that make the UK's recent spike in claims seem almost trivial, writes Frazer Fletcher

As the UK insurance industry turns its attention to subsidence and the possibility of a surge in claim numbers over the summer months, it is worth remembering that climate change is causing some dramatic transformations in previously stable ground conditions all over the world. Regions in Europe, Australia, South Africa and the US are all experiencing drier, warmer climates, and subsidence is a global phenomenon.

However, while there is evidence that subsidence damage to domestic dwellings is on the increase globally, the problem is usually met with a more practical and pragmatic approach abroad. As subsidence insurance is generally unavailable, householders are more tolerant of minor cosmetic fractures, which are repaired as part of normal decoration.

In the UK, damage due to subsidence was unheard of prior to the 1970s, but the risk has subsequently become a 'growth industry', with insurers willing to back expensive solutions. Insurers have incurred costs in the region of £3.5bn since 1975.

National importance

The increase in subsidence during the past decade and the non-availability of insurance cover, together with the growth in other weather-related events such as major floods, has meant that insurance has become a major political issue across Europe. In France, the government has intervened and chosen to underwrite subsidence risk by creating a national 'pool' for subsidence-related claims.

The experience in Russia is even more alarming. The soils of western Siberia have been perpetually frozen for millions of years but the permafrost is now melting. The whole western Siberian sub-Arctic region has started to thaw in only the past four or five years. Western Siberia has warmed faster than almost anywhere else on the planet, with an increase in average temperatures of some 3degC in the last 40 years. Despite being a remote region, there are substantial towns in the region that have grown around mineral, coal and oil extractions. Rapid 'frost-heave' is now occurring and the ground is swelling, forming hollows and hummocks that severely damage homes and infrastructure. The ground is literally turning into a massive bog the size of France and Germany combined. The poor bearing capacity of the sub-soils means reconstruction is either impossible or prohibitively expensive. The buildings and infrastructure suffer the double peril of the initial 'heave' (expansion of the thawing soil) followed by subsidence as the ground loses its load-bearing capacity - often with catastrophic results.

Out of the earth

Climate change is also having another serious effect. There is a desperate and increasing shortage of water, beginning in developing countries and now spreading to regions where water shortages were unthinkable in recent memory. The resources that do exist are being heavily exploited and aquifers depleted.

Subsidence as a result of ground-water withdrawal is becoming common and is affecting whole regions. While there has been concern over this issue for many years, ground movements are becoming more significant given the increase in exploitation of ground-water resources, and there is consequent damage to buildings, infrastructure and the environment.

Unesco is involved in monitoring this growing problem. More than 80% of the subsidence identified worldwide is as a result of human exploitation of underground water. Ground motion endangers public and private property, and poses safety and legal issues. The overall increasing development of land and water threatens to aggravate existing ground motion problems and initiate new ones.

In Mexico City, rapid land subsidence has been caused by ground water withdrawal and associated aquifer-system compaction. It has damaged colonial-era buildings, buckled highways and disrupted water supply and waste-water drainage. Astonishingly, the maximum rates of subsidence have approached two feet per year and the total subsidence during the 20th century is as great as 30 feet. This massive movement has rendered the underground services, including the waste-water drainage system, ineffective, forcing the construction of a deeper sewer network.

In addition to water extraction, fossil fuel consumption has increased worldwide by almost 30% in the past 18 years, according to the World Bank. This increase in the extraction rate also resulted in other worldwide subsidence problems.

Watching movement

Monitoring of subsidence ground movements over large areas has proved problematic until recently. Knowledge of those areas affected enables better corridor planning for new infrastructures and development of homes in areas of stable ground conditions.

Field surveys have been the primary method of gathering data to date, but these are labour-intensive and must be repeated at regular intervals for any progression to be apparent. Monitoring of land motion can now be achieved with earth-observation satellites employing Synthetic Aperture Radar, which scans the ground in successive passes, accurately measuring the distance between the satellite and stable natural (or installed) reflectors on the planet's surface. Indeed, it is this method that has identified just how big worldwide subsidence problems have become.

These global problems put UK subsidence problems into sharp perspective - the magnitude of ground movements experienced in the UK is minor in comparison to some regions of the globe. Although the UK has suffered two successive dry winters and the aquifers and reservoirs in the South-east region were at a particularly low level in the early spring, the recent significant rainfall appears to have at least delayed any potential increase in subsidence claims. This situation, of course, could change if rainfall decreases and we have a hot summer.

- Frazer Fletcher is the director of construction and engineering at Ashworth Mairs Group.

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