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How geological variations govern local expertise this month focuses on South east England

REGIONAL FOCUS

The strata of greatest engineering significance in south east England are the extensive deposits of over-consolidated clays - principally the Wadhurst, Weald, Gault and London Clays - and chalk. The interspersed weak rocks and unlithified granular soils also contribute to the geomorphological character of the area.

Much of the coastline of south east England is in retreat. The best known examples of landslides caused by marine erosion include Ventnor and Blackgang on the Isle of Wight (Upper Greensand, Gault and Lower Greensand), Folkestone Warren (Chalk over Gault) and Warden Point on the Isle of Sheppey (London Clay).

The substantial chalk fall from Beachy Head on 11 January made the national headlines, yet it merely represented the continuing process of cliff retreat which has taken place since the English Channel was formed about 10,000 years ago.

Sea defence works range from simple shingle walls protecting low-lying areas to the Thames Barrier. Works to stabilise cliffs, or reduce their rate of retreat, have traditionally comprised sea defence structures at the foot of the cliffs together with reprofiling and/or installation of land drainage. More recently wave energy attenuating structures have been used.

Ancient landslides are present on many inland slopes which contain overconsolidated clays, and especially on the scarp faces of the chalk and Greensand escarpments. Multiple failure surfaces are often present, so appropriate investigations are essential to ensure that works affecting such landslides are not detrimental to the stability of any part of the slip mass.

For example, the portal to the Channel Tunnel was built within the Castle Hill landslide and all the rail lines from the Channel Tunnel cross the Cherry Garden Hill landslide. Both were investigated in great detail to ensure future stability.

Solifluction slip surfaces are present in most clay slopes in the South East and have been responsible for many slope failures. Their existence was first identified by Alan Weeks and Professor Skempton in 1968 on the Sevenoaks bypass.

Soliflucted slopes are generally stable in the summer months but become marginally unstable in wet winters, when excavations as shallow as 1m, such as those required for house foundations, can be sufficient to trigger slope failures. However, most of these soliflucted slopes can be developed safely and economically if appropriate investigation and preventative stabilisation works are undertaken.

It is now widely known that the chalk has suffered extensive solution, although recent experience indicates that the density of features is much greater than previously believed. These solution features may be only partially infilled with overlying materials so they are a major potential hazard to development projects. Microgravity surveys, backed up by intrusive investigations, have been proven to be a cost-effective means of checking for solution features.

'Gulls' are joints in competent strata which have been opened, and infilled, by the process of cambering. The Hythe Beds between Sevenoaks and Charing contain extensive gulls wherever movement has been possible on the underlying Atherfield Clay. The infill to these gulls is vulnerable to erosion by water from soakaways and defective services, which leads to holes appearing at surface. This is an ongoing problem on many small development sites as well as for major roads.

The overconsolidated clays in south east England are more susceptible to moisture content changes than the geologically older clays and mudstones found elsewhere in the UK. The South East therefore has the greatest incidence of subsidence/heave damage in the UK. Numerically, the London Clay is responsible for the largest number of subsidence/heave claims because of the density of development on this formation. The Wadhurst Clay has also given rise to a disproportionate number of insurance claims.

Historically there has been minimal development on the Gault Clay because of its propensity for shrinkage/ swelling and landslip movements. The outcrop of the Gault at the foot of the North Downs escarpment has therefore provided a corridor for construction of extensive sections of the M25, M26 and M20 motorways, and now the Channel Tunnel Rail Link.

Soft alluvium including peat layers is present in all major river valleys and estuaries. In this respect south east England is similar to many other parts of the country. Peats are present within the River Terrace deposits of the Thames Valley, so boreholes drilled to investigate foundation conditions in these deposits are usually continued to the underlying strata. Blanket bog peat also affected construction of the M20, where its route crossed part of the Hothfield bog near Ashford.

South east England has the usual range of contamination problems, dependent primarily on past and present usage of the site. The characteristic feature of contamination problems in south east England is the importance of avoiding pollution of groundwater, because most of the region's public water supplies are from groundwater sources.

Prevention of contamination of the chalk aquifer is a major issue for the Channel Tunnel Rail Link because its route crosses several groundwater protection zones.

Mining in the area has been more widespread than many people appreciate, with records dating back to Roman times.

Deneholes were dug from Roman to medieval times through the Thanet Beds to extract chalk for 'marling' the fields. On completion the shafts were typically blocked with a bush, so many deneholes have remained open, especially in wooded areas. Cruder versions, known as chalkwells, appeared in the 17th century. These chalkwells were larger and were often interlinked, giving a greater risk of collapse.

Ragstone was extracted from the Hythe Beds by mining as well as from quarries. The mines appear to have been worked as a series of passages, locally opening into irregular pillar and stall workings. Roof collapses in the Ragstone mines appear to have been quite common and large voids have opened in gulls intersected by mine passages.

Similar, but more regular, mines were dug in the Upper Greensand Formation in order to extract Reigate Stone. Two lithologies were worked, firestones and hearthstones. Collapses in the Reigate Stone mines appear to have been less common than in the ragstone mines.

Iron was mined in the Weald from the 1490s to the 18th century using 'mine' pits.

The shafts were typically 1.8m to 2.4m in diameter and up to 12m deep; they were usually widened out at their base as far as possible and then back-filled with waste from the next pit. The resultant hazard is the presence of soft backfill and loosened ground around the belled-out base.

A deep coal mining industry operated in East Kent for much of this century. The presence of coal was first proven in 1896, after Parliament had banned Beaumont's Channel Tunnel workings. Beaumont's shaft at Lower Shakespeare Cliff was extended and the Coal Measures strata were eventually encountered at 1158 feet below ground level.

The only remaining active mines are near Robertsbridge in East Sussex.

These mines work evaporite seams in the basal Purbeck Beds to produce sulphate for use in the manufacture of plasterboard. Quarrying in the South East has exploited aggregates, brickearths, fuller's earth, building stones and chalk for the manufacture of lime and cement. From an engineering point of view the most interesting are the sand quarries in the Folkestone Beds.

These are a 'locked' sand so steep face angles can generally be maintained, but major failures have occurred as a result of minor faults which have destroyed the locking on the fault plane. The South East therefore presents engineering geologists and geotechnical engineers with many challenges, several of which are unique to the area. Appropriate desk studies and ground investigations, combined with innovative geotechnical design, enable cost-effective solutions to be found to these challenges.

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