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University of London Imperial college of science, technology and medicine

Coursework 1 : Engineering geology in folkstone area

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1) Introduction :The present report objectives is to provide a geological description of the area of Folkstone both on a large and on a close scale. This description aims to minimize geological hazards linked to the settlement of Folkestone new discharge system and thus in order to minimize the cost of construction.This paper will firstly be focused on the general geology of the south east of England and its geological context. Then increasingly emphasis on the geological specification of the site of Folkestone and its implication on the establishment of the discharge system.

2) South-east England geology : Far Field

Sedimentology :The south-eats of England is a region is mainly composed of sedimentary rocks from cretaceous and quaternary age (cf fig 1). Most of those rocks are sequences of bedded sands, marine clays and limestones/chalk overlying a bedrock from the Variscan orogeny. Those sedimentary rocks provides us with important information about their deposition environment. Chalk/Limestone and sands are the markers of a shallow sea, whereas marine clays are deposited in a slightly deeper open sea. Those type of sequences are typical from a succession of eustatic cycle (cycle of high and low sea water level).

Tectonic/deformationThe Alpine bedrock (Cretaceous) is slightly folded into regional anticline (Cf Fig 1). This shape is typical of compressional environment, Indeed even though the Alpes are more than 750km away, the effect of its compression are clearly visible in the south-east England. This anticline’s core, is obviously composed of oldest deposit, is highly faulted. The general motion of those fault is normal. This motion goes against the general idea of a compressive environment as it is more commonly related to an extensional environment. However, This kind of “horst” structure is really common in the core of a fold (cf Fig 2). In addition to normal faulting system visible in the core of the anticline, the area is affected by a large number of strike fault conjugated to the main stress axis σ1 (Alpes pressure toward the North). Those conjugated fault happens to an angle of approximately 45 degree depending of the coefficient of friction applicable for the material considered (cf fig 3). This statement will have an important influence later in this report.

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Figure 1 : Geology of South-east England Source : digimap

Hydrogeology In the area Chalk provides the most important aquifer of south east England and is the principal source of water in this area. However, as said, before this formation is faulted and uncovered by any impermeable layer. In this configuration groundwater flow is fast, and the aquifer subjected to pollution. In the area, Folkestone sand formation may also contain ground water. This formation is much more suitable to provide the area with a clear water because of its obvious filtration ability. The topography of the area control the water flow, which goes down the valleys toward the sea. The gault clay formation all around the anticline control the resurgence of multiple current and former spring all around the anticline.

3) Area of Folkestone : the close field

Folkestone area is a coast line village situated in the north part of the anticline (cf fig 1). Sedimentology :

The rocks that can be found in Folkestone area are, of course, of cretaceous and quaternary age. There are five formations in the area of folkstone (cf fig 5), which will be described bellow from the oldest to the youngest.- The sandgate unit which is a silty fine sand.- The Folkestone beds which are composed of a silty medium sand.- The gault clay unit which is a marine clay with some more sandy beds.- The upper chalk,

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Tectonic/deformationIn the area the strata is obviously gently dipping in the north direction (cf fig 4).The area is also affected by the system of conjugate fractures which as been mentioned before (cf fig 6). In this locality the system of faulting has a major importance, indeed, in the area, most of the cliff are N/S or NW/SE orientated. The intersection between the cliffs and the conjugate faulting system tigger major landslides along Folkestone coast.

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