landslips

The photographs included in the 2 pages above (links above) were taken in the spring of 2001, after one of the wettest winters in my memory. They show a number of recent movements of the landslips in the Folkestone area (inclusive one taken the day after the slip had occurred), plus mudslides and rockfalls. The movements, due to the heavy rainfall over the winter months were probably the greatest that I have personally witnessed in 30 years of visiting Folkestone seafront and the Warren as a geologist.

This article also includes some details of the landslips associated with the Chalk escarpment to the west of Folkestone along the Etchinghill escarpment, together with the landslips of the relict cliffline between Hythe, Lympne and Aldington.

The figure below shows the elements of a typical landslide. In this instance the basal plane of the landslide is located at Bed XII, a thin glauconitic rich bed within the upper part of the Gault Clay.

Chalk

Chalk

Upper Cretaceous

Chalk Marl

Chalk, ca 40-60% carbonate

Glauconitic Marl

Sandy clay. Some slips at base

Major unconformity

Angular in places

Lower Cretaceous

Gault Clay

Some slips on Bed XII, but mostly at the base

Folkestone Beds

Sandstones, rare clays

Sandgate Beds

Sandy clays

Hythe Beds

Limestones/sandstones. Slips at the base

Atherfield Clay

Marine, clay

Weald Clay

Freshwater clay, with sands and rare limestones

Erosion of the cliffs around Copt Point is continuous, and the top of the cliffs behind the East Cliff Hall and the pitch and putt course have probably receded 5m or so in my lifetime. Indeed, the car park on the seaward side of the pitch & putt course where I once had picnics with my parents in the 1960's & 1970's has possibly receded the quickest, with evidence for a landslip (a backscar) now developing in a large section of the car park in front of the pitch and putt kiosk itself.

The erosion of the cliffs is mainly the result of wave action on the soft erodible Gault Clay, but also of erosion of the Gault Clay by rainfall and runoff. The only beneficial result of this erosion is the uncovering of large numbers of fossils which make this one of the most popular venues for fossil collecting in the country.

In the Folkestone area there are a number of stratigraphic horizons which act as basal planes for the landslips, the main and basal of which is the top of the Folkestone Beds, Bed XII within the upper Gault Clay, and the topmost slip horizon at the top of the Gault Clay. Movement on the landslips is typically rotational and is normally triggered by excessive rainfall reducing the amount of friction along these planes, resulting in movement.

The Folkestone to Dover railway is one of the most expensive sections of railway in the country to maintain as it runs through the the Folkestone Warren, an extensive area of landslips (2 miles by up to 400 yards) at the foot of the White Cliffs between Dover and Folkestone. The railway not only suffers from the landslips, but also rockfalls from the cliffs above. Major slips occurred in 1877, 1915, 1936,37 and 40. On each occasion ridges were formed in the foreshore which were quickly eroded by the sea. The Warren foreshore itself consists of upturned blocks of Chalk Marl, Glauconitic Marl and Gault Clay. The slips were accompanied by cracks at the top of the cliffs which later collapsed as rockfalls. Slips may also have occurred preferentially at low tide due perhaps to some of the 'natural' toe-weighting being removed.

In 1915 almost the entire area of the Warren moved towards the sea, the railway line being displaced a maximum of 55 yards near Warren Halt. It was also buried by major associated rockfalls and was not reopened until 1919. The foreshore saw a chain of islands created which reached up to 30 ft above the water at low tide. Boreholes proved that the basal slip plane in this area was the base of the Gault Clay. To complicate the matter the water in the underlying Folkestone beds has a hydrostatic head, placing uplift pressure on the base of the Gault Clay as well as lubricating the contact between the Folkestone Beds and the Gault Clay. Loading of the backslips is at a maximum during heavy rainfall. Collapses from the cliffs locally increase the driving force at the back of the slips.

Attempts have been made to stabilise the landslips, by regrading parts of the undercliff, and by building a seawall, large toe-weights, groynes and drainage galleries (1877-1955). These are standard construction techniques which attempt to stop the movement of the landslips by reducing marine erosion, placing a retaining structure at the toe of the slip and at the same time reducing the flow of water/build-up of water pressure on the slip plane by drainage.

At the end of the Devensian glaciation, climatic amelioration brought about a change from arctic to temperate conditions in the British Isles and this led to a gradual melting of the deep permafrost which had characterised the periglacial conditions affecting the Etchinghill Escarpment to the west of Folkestone at the back of the Channel Tunnel terminal site. This is likely to have led to increased porewater pressures in the Chalk and Gault Clay which would have affected the stability of many of the escarpment slopes. These conditions favour landsliding and it is likely that it was during this period that the initial landslips developed which are present in the Channel Tunnel portal and terminal areas.

It is probable, however, that more than one period of landsliding has occurred along the Lower Chalk escarpment. A cyclic pattern of landsliding within the post-glacial periods of the Allerod and Flandrian has been associated with phases of large-scale slope instability.

The landslips within the portal and terminal areas, therefore, are long-established relict features and were generally believed to be stable under contemporary conditions. The changes to the landscape associated with the Channel Tunnel construction, however, were such that reactivation of the landslips was a possibility and, therefore, they needed detailed investigation.

Five areas of landsliding were mapped (Fig. 1 below) in the escarpment above the terminal area. The main role of the mapping was to refine the boundaries of the areas of relict instability and to develop an understanding of the type of movement that had occurred.

All the major landslips had their backscars in the Lower Chalk scarp slopes, and the landslide movements had involved significant dislocation of the underlying Gault Clay. The interrelationship between the landslide toes and the 'coornbe rock' deposits was not clearly defined with indications of both burial of landslide debris by solifluction deposits plus, in some areas, evidence that the landslide debris had been thrust over the same deposits. This was regarded as further evidence that all the landslips had been subject to phases of reactivation since they first developed, probably in the late-glacial or early post-glacial period.

Category 1: Castle Hill and East Cheriton - these had a multiple rotational form with a series of clearly defined benches.Category 2: Danton Pinch, West Cheriton, Cherry Carden and Sugarloaf Hill - these all had a more subdivided form and appeared to be a type of foundered strata very similar in morphology to that described by Skempton and Weeks (I 9'16) in the area of the Lower Greensand escarpment around Sevenoaks.

The most significant landslips with respect to the engineering were: Castle Hill, as the tunnel portal went through the centre of it; and Cherry Garden, as all the terminal tracks converged onto just two lines where they crossed over this landslip. These two landslips are discussed in more detail below as representatives of the two categories, and their spatial location is shown on.

(b) benches or platforms with minor front scarps which represented displaced blocks from the main movements

(d) the landslide accumulation zone with two clearly identified benches, each with a front scarp.

Borehole data available prior to the construction showed evidence of a whole series of shear surfaces, shear zones and disturbed strata both in the Chalk Marl and the Gault Clay. These were a reflection of the multiple landslide units and the fact that there had been more than one factor in the development of the shear surfaces. The overall evolution of the Castle Hill landslip, based on its morphological form and the subsurface data was concluded to be as follows:

(a)Lateral expansion and extension of the Gault Clay as a result of denudational unloading leading to the formation of multiple 'tectonic' shear surfaces

(b)Valley incision in the adjacent Cherry Garden Coombe causing oversteepening in the valley side slope of Castle Hill

(c)Oversteepening of the slope in conjunction with high porewater pressures during the late-glacial to early post-glacial period of climatic amelioration which led to the initial landslide movements. It is postulated that the initial failure 'adopted' some of the tectonic shear surfaces in the Gault Clay which would already have been at, or close to, residual strength

(e) General shallower degradational movements leading to the break up of the main dislocated blocks and the creation of a more subdued morphological form.

The invasive nature of the construction works meant that the overall form of the landslide, and the multiple levels and orientation of the shear surfaces, had to be allowed for in the temporary and permanent works design.

This represented a very different form of landslip and four separate landslides, each with its own individual direction of movement could be identified.

The main evidence for movement in one element of the landslide was the dislocation of the Glauconitic Marl. This dislocation created a bench feature which appeared to be a single relatively intact foundered block of Chalk Marl and Lower Chalk. This requires that a considerable amount of the underlying Gault Clay has been displaced either downslope and laterally or vertically. Under the most comprehensive classification of subaerial slope movements developed by Hutchinson (1988) this type of failure could be regarded as a form of cambering.

The general cause of the landsliding at Cherry Garden Hill is again likely to be basal erosion of slopes, probably by streams that would have flowed more vigorously both out of Cherry Garden Coombe and in front of the scarp slope during late glacial times. It is also likely that the wetter conditions that existed during the late glacial would have increased pore pressures leading generally to less stable slope conditions.

The presence of the landslips was one of the factors controlling the design of the terminal and in particular the track layout. Toe-weighting was added to each of the four major slips with the exception of the Cherry Garden Hill Slip where there is a small zone of cutting and at Castle Hill Slip within which the tunnel portal was constructed by 'top-down' methods. Thus, for the Stage 1 construction works, the factors of safety of the landslip were maintained or improved.

These maps were then used to plan the layout of the permanent facilities which include; the cooling plant, supplementary ventilation, drainage discharge and fire pumping house.

On the 30th January 2001 a large piece of chalk estimated at 100,000 tonnes off England's famed white cliffs of Dover crashed into the sea . The landslide happened along a quarter-mile stretch of coast between Dover and St. Margaret's Bay in southeast England, adjacent to the South Foreland and was one of the largest in recent times. The cliffs here recede at about half an inch every year because of erosion but large landslips and rockfalls also occur. At this locality Coast guard officials recorded that just before the fall cracks in the chalk had grown from 5cms to as much as 20cms wide.

Such landslides are often related to periods of heavy rain followed by cold temperatures. The rain is absorbed into the cliff-face, freezes, then expands when temperatures rise again, weakening the chalk.

The landslips along this relict cliffline are in a different geological setting to those of the Warren and the cliffs of Folkestone as they are formed at the junction between the Hythe Beds, the Atherfield Clay and the Weald Clay. Typically the backscar of the landslides is within the Hythe Beds and the main shear plane within the Atherfield Clay. Bromhead (1998) included some photos and figures on this topic on the www in 1998.

From GeologyShop

(Links to: Channel tunnel facts and brief history; Channel Tunnel Geology; Chalk, the basic facts; Chalk, the White Cliffs; Landslips of East Kent; Channel tunnel, a detailed sequence stratigraphy)