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Britain's longest borehole

Almost every type of geological material in south east England is encountered on the CTRL route, explains RLE lead geologist Colin Warren.

The CTRL route passes through almost the entire geological column of south east England, from the Tertiary deposits in London, through the full depth of Chalk outcropping at surface in north Kent and finally into the sands, clays, mudstones and limestones of the Lower Cretaceous in southern Kent.

This represents a total thickness of around 500m and almost 140 million years. Superficial deposits include a significant thickness of recent alluvium and terrace gravel found along the valleys of the Thames and the Medway rivers and head deposits including claywith-flints overlying the bedrock in upland areas.

In recent times, the prevailing geological landscape has been disturbed by man's activities as shown by the numerous, deep chalk pits on the route, some disused and open, others infilled with considerable thickness of made ground.

It was realised from the start of the ground investigation that a thorough appreciation of geology along the route, including the risks associated with a particular stratum, depended on applying the most appropriate stratigraphic framework to the geological strata.

This was achieved by specialist input from experts such as Dr Richard Ellison of British Geological Survey, for the Lower London Tertiaries, and Professor Rory Mortimore from Brighton University for the Chalk.

Since the new stratigraphic framework for the Lower London Tertiaries and Chalk are based on lithostratigraphy, the new definitions are more applicable when determining engineering properties and behaviour of the soils or rocks that will be encountered during construction. This is particularly significant for the tunnels to be built under London, beneath the river Thames and through the North Downs.

In the North Downs Tunnel, for example, it is generally found that the dry density and spacing and form of jointing varies between the Lewes Chalk and underlying New Pit and Holywell Chalks - an important factor when deciding on the method of excavation and tunnel support.

The Lewes Chalk is more closely jointed and fractured and contains several prominent marl seams, along which past groundwater flow occurred.

It is also more susceptible to the effects of weathering and dissolution.

Consequently, the risk of encountering deep solution pipes is substantially increased for the length of tunnel through this strata.

Additional geological factors that required special attention included:

lThe presence of faults, solution pipes and areas of deep weathering along the route;

lExtreme lateral variation in strata thickness over short distances. This is particularly true of beds in the Lower Greensand in Kent, for example, the Folkestone Beds are reported to be around 36m thick near Ashford compared to 55m at Thurnham 20km further north;

lOccurrence and nature of fissures, joints and shear surfaces within overconsolidated clays such as the London Clay, Gault Clay and Atherfield Clay. These shear surfaces, if adversely orientated in relation to the proposed excavation, could create difficulty during construction. Failure along shear surfaces within the Gault Clay is well known and has been reported on several projects in Kent including the Channel Tunnel where it passes through Castle Hill;

lThe percentage and form of flints within the Chalk. Due to the abrasive character of flints, knowing the size and quantity is vital when designing tunnel boring machines. Quarries were inspected and measurements made of the number of flint layers likely to be met in the tunnels, the percentage of flints within each layer and an estimate of their size (maximum-mean-minimum).

Most of the geotechnical investigations for design are finished.

Monitoring of the encountered conditions on site will enable geologists and geotechnical engineers to validate the geological model, confirm their design assumptions and provide additional information to the geological fraternity.

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