Channel Tunnel Geology

by

C. S. Harris

A summary of the geology of the Channel Tunnel, compiled for you by a professional geologist who was consultant geologist during Channel Tunnel construction.

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

This page is an attempt to provide a summary, based on existing published data, of the general geology of the Channel Tunnel. It has been published with the kind permission of Eurotunnel.

Introduction

In 1833 the French engineer Thome de Gamond began the first systematic geological and hydrographic survey of the Channel. The data obtained allowed him to propose a number of designs for a crossing. His work is the first example of an engineer recognising the importance of the regional geological framework to the design of a Channel crossing. Many more were to follow in the footsteps of de Gamond culminating in the work for the site investigation for the completed project. Much of this work concentrated quite sensibly on the continuity of the potential tunnelling horizons across the Channel based on surface mapping, shallow borehole data and geophysical information. Thus the overwhelming majority of work concentrated on rocks of Late Cretaceous age, with some work on the Gault Clay and Folkestone Beds. Apart from a study for the potential influx of hazardous gases, mainly from rocks of Carboniferous age, very little work on pre-Cretaceous data was undertaken, even in the later site investigations.

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Map showing the regional geology of SE England and NW France.

Structural data were considered to be important both for the stability of the excavations and for long term stability of the tunnels (earthquakes). The structural control exercised on the Mesozoic and Palaeogene rocks by the largely concealed Palaeozoic rocks of the London-Brabant massif was not fully documented (Shephard-Thorn et al, 1972) even by the time of the later site investigations. Results obtained by TML' s (the contractor) geotechnical department together with a number of key papers by leading academics (Mortimore and Pomerol, 1991) since construction began suggest that lateral changes in the stratigraphy and structure can be related back to observable structures in the underlying Palaeozoic rocks.

Figure showing the configuartion of the three tunnels.

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Regional Geology

The region includes southeast Kent, the Strait of Dover and adjacent parts of northwest France. It is located at the eastern end of the major Wealden-Boulonnais Anticlinorium (Anticline), which straddles the region. This major structure separates the Palaeogene basins of the southern North Sea and the eastern English Channel. The sea-bed outcrops of the Jurassic, Cretaceous and Palaeogene rocks are adequately defined by recent research involving bottom sampling, seismic profiling and some shallow boreholes, but in general the stratigraphy is less well known than for the onshore successions. In Kent and Sussex, the Weald Anticline is bounded to the north and south by the North and South Downs respectively; its continuation across the Channel is seen in the Boulonnais, where it is delimited by the horseshoe-shaped Chalk uplands of the Haut Boulonnais. East Kent forms the northeastern edge of the post-Mesozoic anticline, with the Mesozoic rocks exposed at the surface dipping away from the axis beneath the North Downs. To the north of the main Chalk outcrop, Tertiary strata are preserved in the Richborough Syncline, beyond which the Chalk again rises to the surface to form the anticlinal Isle of Thanet.

In both the Weald and the Boulonnais, the Anticline has been deeply eroded to expose the core of earlier Cretaceous and Jurassic rocks. In the Boulonnais, not only does the axial inclination of the major structure bring the Palaeozoic floor much nearer the surface than in east Kent, but normal faulting during the Tertiary along the lines of reactivated N 110 degree faults initiated at the close of the Variscan Orogeny has raised the Palaeozoic core to an even higher level in a horst-like structure (Auffret and Colbeaux, 1977), so that it is actually exposed in an inlier centred on Ferques.

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Site Investigations

The first proposals for a fixed link between the two countries were made in the early nineteenth century and included various highly imaginative combinations of bored tunnels, immersed tubes, bridges and artificial islands. Most schemes, however, were far ahead of the engineering techniques and geological knowledge of the time to be considered seriously. One scheme even proposed the boring of a short direct route between Folkestone and Cap Gris Nez in Jurassic rocks of mixed lithologies, not ideally suited for tunnelling. The more reasonable proposals did at least recognise the importance of gaining a sound knowledge of both seabed topography and geology along the route, and it was these factors that ultimately would determine the success of any scheme.

Geological section along the length of the Channel Tunnel

Despite the early history and attempts made in the late nineteenth century, the first modern investigation directly associated with the building of the Channel Tunnel was undertaken in 1958/59. This comprised both geophysical and borehole surveys with further work being carried out during the periods 1962-65, 1972-74 and more recently in 1986-88 as part of the present scheme. The initial two campaigns formed part of feasibility studies and were not intended to provide the complete information necessary for design purposes, and they therefore covered very large areas and had fairly broad specifications. They were, however, of considerable significance at the time and have been very useful in terms of their general contribution to the total database. The 1972-74 survey was the first to investigate a specific tunnel route as part of a tunnel design and was restricted in its area of coverage, as were the recent 1986-88 surveys.

In all, a total of 116 marine and 68 land boreholes have been drilled along the alignment and over 4000 line kilometres of geophysical survey completed.

Continuous seismic profiling was and still is considered to be the most acceptable means of providing geological information relatively quickly for the large areas to be covered, but it provides only indirect information about the structure and nature of geological strata. Direct information may only be obtained from boreholes, which also provide representative samples of the ground for testing and one-dimensional information to control the geophysical survey.

The marine site investigations differ from the land investigations in that:

(a) Geophysical surveys are a very cost-effective method of obtaining geological data offshore. Consequently, these methods form a major part of the marine investigations carried out for the Channel Tunnel.

(b) Information derived from land sited boreholes is much cheaper to acquire than from marine boreholes (e.g. 1987 comparative costs of £20 000 as against £0.5 million for a typical Channel Tunnel borehole). Thus, boreholes tend to dominate in this instance.

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The Geology of the Dover of Straits

The seabed of the Strait, which reaches a maximum depth of 60 m along the tunnel alignment, is essentially the result of Quaternary erosion after several periods of emergence. It intersects (approximately at a right angle) the large Weald-Boulonnais Anticline, which trends WNW-ESE (parallel to the tunnel). The core of this anticlinal structure is occupied by rocks of Jurassic age, which are themselves bounded by Cretaceous strata (chalks, clays and sandstones). The whole assemblage rests on the primary basement, which outcrops in the Marquise region but was only encountered during site investigations at a depth of 114 m below natural ground level adjacent to the Sangatte access shaft.

Figure showing the seafloor geology of the Dover Straits

The Weald-Boulonnais anticline is in fact composed of several main flexures which have resulted in numerous small secondary anticlines and synclines, in particular the Sangatte-Quenocs anticline, which directly affects the French side of the project.

The Strait may be divided into two basic structural units, with continuity between them:

(a) the UK part, comprising a relatively undisturbed structure representing the offshore extension of the Kent basin

(b) the more deformed French part featuring the Sangatte-Quenocs anticline and some relatively substantial faults

The whole of the Channel Tunnel route is within the Cretaceous layers that form the north slope of the Weald-Boulonnais Anticline. In France all three tunnels start within the flinty chalks of the Lower Senonian at the portal at Frethun. Proceeding generally down sequence the tunnels pass through Turonian chalks and into the clayey chalks of the Cenomanian, which then form the main tunnelling horizon right up to the UK portal. The UK portal at Castle Hill was constructed in a mixed face of the lowest Chalk Marl, Glauconitic Marl and topmost Gault Clay. The UK portal posed specific difficulties during construction as the tunnel passed through a major landslip at the base of Castle Hill. This was continually monitored by TML during construction while major remedial works were undertaken, including toe-weighting and drainage channels, in order to sufficiently stabilise it.

The general dip of these Cretaceous beds is very slight (1 to 4 degrees maximum eastwards) in the Sangatte area, but from the beginning of the undersea section the NNE dip (i.e. beds strike roughly parallel to the tunnels) increases rapidly over the space of 1km to reach 20 degrees in the French half. Towards the UK section it then decreases just as quickly to 2-5 degrees, dips characteristic of the greater majority of the UK section.

The sea floor of the Channel, which is generally very flat and mostly slopes gently southwestwards, is often notched by deep fossil valleys filled with sandy-gravelly alluvia or flints. These valleys are the result of glacial erosion and their position and form .are imposed by the main structural features of the geology. In this area the tunnel route carefully avoided the 80 m deep Fosse Dangeard trench, situated 500 m to the south in the middle of the Strait at the dividing line between the French and UK sectors.

Tunnelling hazards in the Straits of Dover

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Structural Geology

The fracturing of the Chalk formations through which the tunnels were driven is related both to the re-activation of faults that were present in the Jurassic and basement rocks beneath, and to the development of the Weald-Boulonnais Anticline.

Tectonic control of sedimentation also occurred as the Weald-Boulonnais Anticline began to form, during the deposition of the Cretaceous formations. The specific consequence of this was that the total thickness of the various Cretaceous layers is less on the French side, where sedimentation took place in a more marginal shallow shelf environment.

From the end of the Palaeozoic era the Channel successively underwent two extensional tectonic phases, a compressive phase and a further extensional phase:

(a) The first extensional episode gave rise, from the beginning of the Mesozoic era, to the Western Channel semi-graben.

(b) The second extensional phase lasted from the end of the Jurassic to the Middle Cretaceous (Albian) and may be related to the beginning of the opening of the Gulf of Biscay. Relatively minor tectonic events have been recorded in the Late Cretaceous. These have been proven to be controlled by major structures in the Palaeozoic basement (Mortimore & Pomerol, 1991).

(c) Just after the end of the Cretaceous (beginning of the Tertiary era) the cover was subjected to slight compression, resulting in deformation with a large radius of curvature (broad anticlines and synclines of the Weald-Boulonnais Anticlinorium), contemporaneous with the Pyrenean folding.

(d) Finally, the majority of the faults now affecting the cover and concerning the tunnel appeared at the end of the Tertiary/beginning of the Quaternary on the occasion of a third extensional phase at the time of formation of the tectonic trenches of Alsace, the Bresse and the Massif Central. This third phase reactivated many of the old faults in the basement.

The intensity of these movements, particularly the last extensional phase, is not the same everywhere, and it should be noted that the tunnel route is situated to the north and just outside of the most highly tectonised zone. This explains why the route actually intersects only a few major faults (with throws of a few metres) on the French side, while on the UK side no fault with a throw greater than 1m was recorded during tunnelling.

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Jointing

In the cliffs between Dover and Folkestone the dominant discontinuities (joints) dip steeply, between 60 and 70 degrees (to subvertical) with a WNW-ESE strike. These according to Bevan & Hancock (1986) correspond with the fracture type 'conjugate steeply inclined hybrid joint' and are attributed to gentle flexuring of the strata during geologically recent times.

A second set of WNW-ESE trending discontinuities strike the coast at intervals of 1.0 km to 1.5 km and extend inland for 10km or more. Each discontinuity or lineation comprises a group of closely spaced subvertical joints and corresponds to the fracture type 'vertical extension joints' also described by Bevan and Hancock. A third set of major joints running NE-SW, associated with the formation of the dry valleys, is not so evident in the section of cliff studied because of their proximity with the predominant orientation of the cliffs.

Further discontinuities occur subparallel to the cliffs. However, these are not related to regional structural trends but to a continuous process of stress relief taking place within metres of the cliff face. Furthermore, the initiation of discontinuities subparallel to the cliffs maybe attributed to creep movements within underlying more malleable Gault Clay, particularly where the cover to the clay is limited, i.e. towards the western end of Abbot's Cliff.

In the Lower Chalk, exposed towards the base of the cliffs, joints are locally seen to 'sole out' as they penetrate the more clayey strata.

During construction of the Channel Tunnel only two main conjugate sets were recognised, high angle to vertical WNW-ESE and NE-SW. These, together with a major subhorizontal set of joints were of major significance in the stability of the tunnels during excavation. This was particularly evident where the tunnel trend was parallel to one of the major joint sets, increasing the possibility for wedge failure in the sidewalls of the tunnel. Channel tunnel data did not allow the separation of the joint sets as proposed by Bevan & Hancock (1986) nor was there any proof within the tunnels of significant changes in joint orientation or joint abundance directly under the dry valleys.

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Stratigraphy

Folding, faulting and fracturing of the Palaeozoic rocks occurred during the formation of the Variscan Armorican massif. The Jurassic rocks and Cretaceous formations were subsequently deposited on these structurally complex basement rocks.

While the Cretaceous strata, within which the whole tunnel route is situated, includes sequences that differ greatly both in their characteristics and in their properties (overconsolidated clays, glauconitic marls, flinty chalks etc) and are therefore easy to distinguish from each other, they also include sequences whose boundaries are difficult to establish, as their changes in nature or properties are very gradual. Such is the case with the boundary between the Chalk Marl (Craie Bleue) and the overlying Grey Chalk and the boundaries between the flinty chalks of the Senonian and the flintless Turonian.

From the top downwards (i.e. from the most recent to the oldest) the layers that concern the tunnel route are described below. This is also the order in which the tunnel route encountered them from the French Portal at Frethun.

Figure showing the major geological units plotted against their calcimetry

Senonian chalks with continuous layers of flint

The flints are decimetric in size and form almost continuous layers at spacings of 0.50 to 1 m. Only the underland part of the tunnel route on the French side passed through these White Chalks (for approximately 1.5 km from the Fréthun portal).

Turonian chalks

The upper part of the Turonian comprises White Chalks 10 to 15 m thick containing layers of flint, similar to the facies in the Senonian chalks. After these came White Chalks with few flints: a 12 m thick sequence consisting of a granulose White Chalk with greenish clayey streaks. After passing through this formation, the tunnels encountered no more flint.

Finally came 24 m of marly chalks and a nodular chalk, which is 19 m thick on the French side, 15 m on the UK side, consisting of nodules of hardened yellowish chalks in a chalky matrix containing greenish marly streaks. The total thickness of the Turonian is 60-70 m.

Cenomanian chalks

The tunnel route encountered few chalks of this age before the Sangatte shaft, after which the tunnels were excavated entirely within rocks of Cenomanian age (minor exceptions being the UK service tunnel, which beneath the UK Crossover is within Albian strata; the pump stations and all three tunnels towards the UK portal which are all partly within Albian strata) all the way to the UK portal.

One characteristic of the series of Cenomanian chalks is the overall decrease in their calcium carbonate content (and corresponding increase in clay content) with increasing depth towards the Gault Clay.

This variation is not entirely progressive, as the sedimentation of the chalks is cyclic from a clayey phase at the base to a calcareous phase at the top. Each cycle is 0.2 to 2 m thick. The progressive increase in clay content takes the form of clayey intermediate beds, which increase in thickness and become increasingly clayey towards the base of the series. The Chalk Marl is specifically characterised by its extensive marly intermediate beds which have made it less sensitive to fracturing and alteration and hence less permeable than the overlying chalks.

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Upper and Middle Cenomanian Chalks

These are the equivalent of what were previously known as the 'White and Grey' Chalks. They can be divided into four units with quite distinct lithologies:

a) At the top, beneath the Turonian, occurs a unit,1 to 2 m thick, which comprises a succession of thin marly layers.

 (b) Below this a 10 to 15 m thick unit of solid greyish White Chalk with some thin many intermediate layers is present.

(c) Next, a unit 15 to 18 m thick comprising a finely rhythmic assemblage containing chalk beds, several decimetres thick, with thin bluish to greenish marly layers at the base.

(d) Finally a 6 to 8 m thick unit, which is a granular chalk with frequent small hardgrounds, together with metre thick beds of more calcareous chalk, separated by very thin marly layers. It is often in the middle of this unit that the bluish colour associated with the Chalk Marl (Craie Bleue) first appears. Frequently there is indurated chalk at the base of the unit.

The tunnel route encountered these chalks between the Sangatte shaft and the portal and at some points in the undersea section.

Lower Cenomanian Chalks

Essentially equivalent to the Chalk Marl, the Middle Cenomanian comprises three distinct units:

(a) An upper unit which is 7 to 10 m thick and comprises metre thick beds of marly chalk, often bluish in colour, separated by extensive darker blue intermediate marly layers, identifiable in borehole samples by the occurrence of two microfossil species characteristic of the top of unit (Rotalipora. reicheli and C. formosus).

 (h) A middle unit, 6 to 9 m thick, often lighter in colour, comprising solid clay beds separated by very marly intermediate layers.

 (c) A basal unit, 6 to 9 m thick, each bed with a base of marls which change progressively upwards to marly chalks. Beds of sponges are frequently present at the chalky tops of the sedimentation cycles and form small very indurated decimetre thick levels.

Variation of bulk density and calcimetry in a typical chalk marl sedimentary cycle

The thickness of the Chalk Marl is least towards the Quenocs anticline, 18-20 m at the beginning of the undersea tunnel, with a subsequent steady increase to 28-30 m on the UK side.

Basal Cenomanian Chalks

All of the previously described layers occurred in both the French and UK sectors alike, although in the UK sector only the Lower and Basal Cenomanian chalks were encountered during tunnelling. In contrast the basal units of the Cenomanian exhibit marked lateral changes.

(a) an upper unit of very clayey homogenous chalk, typically 5 to 7 m thick but up to 12m thick in the vicinity of the Shakespeare Underground Development, is always present on the UK side, but only represented on the French side by a thickness of 1-2 m between adjacent to the UK sector.

(b) the Glauconitic Marl (Tourtia), is 1 to 12 m thick. The facies of the Glauconitic Marl are highly irregular, ranging from compact and indurated sandstone to clayey-calcareous sands, and the transition to the formation above is often very gradual. In contrast, the base is consistently indurated and very dense, with phosphate nodules that reflect seismic waves. A major facies change within this unit occurred at the UK Crossover, where directly under the Crossover a hard indurated calcareous sandstone with a sharp top and base was recorded on an Early Cenomanian structural high, whilst eastward and off this high the glauconitic marl was observed to thicken rapidly to around 12m with a sharp base and very transitional top.

The total thickness of the Lower and Basal Cenomanian chalks, frequently reaches 32-35 m. Due to the optimisation of the tunnel route, these comprise over 90 % of the formations through which the undersea section of all three tunnels pass.

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Gault clay (Albian)

Zone 6a, a greyish clayey chalk with facies intermediate between the Chalk Marl and the underlying Gault Clays, is unevenly distributed on the UK side, present in traces on the French side (1-2 m), and attains maximum thickness of 6-7 m in the vicinity of the UK Crossover. Although some authors have included this intermediate stratigraphic unit in the overlying Cenomanian, a major unconformity which is associated regionally with minor folding occurs at the top of this unit. This unconformity is the most distinctive event between the base of Bed XII and the Mid-Cenomanian unconformity. As Zone 6a occurs below this event it is considered to be part of the Albian. This topic was extensively researched during the construction of the Channel Tunnel, mainly because of the need to estimate the relative height of the tunnel above the base of the Glauconitic Marl, a major seismic reflector and one which was used to geostatistically contour (isopachyte maps) the subsurface geology across the Dover Straits. It was proven that the greatest thickness of Zone 6a occurred in a NNW-SSE trending channel or 'graben like' structure 5-6 Km in width. This structural/depositional low is parallel to one of the major structural trends of the region. As, by the time of Glauconitic Marl deposition, the Crossover area became a structural/depositional high, a minor inversion is implied at the boundary. This structural event is consistent with regional information indicative of an angular unconformity at the Zone 6A/Glauconitic Marl boundary.

Below Zone 6a occurs the more typical Gault Clay. Dark grey overconsolidated clays at the base, pass into lighter grey clays towards the top (due to their higher calcium carbonate content), containing numerous phosphatic horizons which represent periods of non deposition. The minimum thickness on the French side is between 10-12 m with a steady increase to 40 m or more at some locations on the UK side. Towards the French sector the Late Albian above Bed XII reduces to several metres or less with sedimentary sequences as thin as a few centimetres separated by marked unconformities.

The base of the formation is characterised by a thin layer of cemented glauconitic sands which reflect seismic waves.

Greensands (Aptian)

Glauconitic clayey sands with crossbedded stratification (15 m thick on the French side and 25 m on the UK side). Large areas of Greensand outcrop on the sea floor a few kilometres to the southwest of the tunnel route. As the Greensands are permeable and because they were potentially in direct hydraulic contact with the seafloor with a Gault Clay cap rock overlying them, it was possible that if encountered in the tunnels they could be under significant hydrostatic pressure (equivalent to their depth below sea level). While this was not anticipated to be a problem in any of the tunnels, it did pose a potential threat during probing undertaken to verify the safety of the tunnels.

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Geomorphology

The present position of the cliffs between Folkestone and Dover was established during Late Glacial times, probably during the later stages of the Flandrian transgression when weathering processes were at their most aggressive. Since then the cliffs have been subjected to marine erosion, subaerial weathering and more recently the works of man. The attack by the sea tends to have a destabilising effect whilst the process of subaerial weathering would, given no further marine erosion, ultimately result in a stable cliff the shape of which would resemble the escarpment of the North Downs to the west of Folkestone.

The rate of erosion of the toe of the cliffs between Folkestone and Dover has been measured as up to 0.75 m per year (May, 1966). However, the rate and amount of erosion at any specific locality is dependent on the protection of the cliff foot by shingle or debris from recurrent cliff falls. Clearly the process of toe erosion is halted, albeit temporarily, by sea defences with sufficient height to provide protection from wave splash and spray or with sufficient width to keep the shoreline remote from the cliff base.

Studies of the effects of the spoil reclamation platform on beach processes and cliff erosion have established that the net west to east longshore drift will create a build up of beach deposits at the western end of the reclamation platform. In the course of time shingle will bypass the reclamation platform to replenish the foreshore to the east of the existing platform where the cliffs are undergoing severe attack at the toe.

The mean rate of cliff top retreat between Folkestone and Dover has been estimated by May (1966) to be 0.09 m/yr. More detailed estimates, made by comparing the position of the cliff top shown on the first edition of the Ordnance Survey of 1872 with that shown on the coastal mapping prepared for TML in 1986, indicate that the mean rate of cliff top retreat was greater above the protected section of coast (0.13 m/yr) than above Abbot's Cliff (0.08 m/yr) and Shakespeare Cliff (0.06 m/yr). This seemingly anomalous result may be attributed to the gradual regression of the drift deposits, which form the upper few metres of the cliff face, back to a more stable angle than that at which they were left by the slope trimming operations of the railway company in 1843.

Figure showing the geomorphology, landslips and tunnel construction at the UK terminal

Subaerial weathering in the form of water erosion, wetting and drying, wind attack and frost action causes the gradual frittering away of the cliff face, both the chalk and overlying drift deposits. Aided by the process of stress relief, the more gradual processes of material removal can be accompanied by the occasional toppling of loosened blocks, particularly where support has been lost above the more readily degraded marl bands. This process is particularly prevalent at the horizon of the Plenus Marl where the overlying and more brittle Melbourn Rock becomes undermined by erosion of the softer marl. These processes are at their most active throughout the winter months but particularly following periods of heavy rain and or ground freezing, i.e. January, February and March. The chemical processes of chalk solution by naturally slightly acidic rainfall and the growth of crystalline salt in cracks on the lower slopes may also be contributory, albeit to a minor extent, to the gradual degradation of the chalk cliffs.

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Weathering and fracturing

The original lithological and geotechnical characteristics of the chalks have been modified by weathering processes which were most intense during the interglacial times when the Strait was above sea level. Classification of weathering grades and the identification of these grades in boreholes during the various site investigation campaigns allowed a weathering profile to be defined across the Strait. As a broad generalization, it was possible to show that the sound chalk thickness from the top of the Gault is typically a linear function of the ground thickness above the Gault. It was also found to penetrate more deeply into the chalk rock mass in the vicinity of faults and in the vicinity of highly fractured zones.

The permeability of the chalk mass results from the network of fractures running through it. The chalk matrix itself is almost impermeable; laboratory measurements on samples produced values less than 10-9 for both the Grey Chalk and Chalk Marl. The permeability of the fractured rock mass, which is at least 1000 times greater, depends on two factors:

(a) the geometry of the fracture network (density, orientation and especially degree of interconnection between the various discontinuity sets)

(b) the hydraulic conductivity of each individual fracture (aperture, filling, roughness, continuity etc).

Greater depths of weathering associated with such zones are readily visible on the Sussex coast to the east of Brighton where increased fracturing and deep weathering can be seen associated with the dry valley systems where they intersect the cliffs. These are in the much purer and highly permeable white chalk where permeability of the rockmass can easily be in the order of 10-3 m/s.

The main reason for the choice of the Chalk Marl as the main tunnelling horizon was because of its relatively low permeability typically in the range 10-7 to 10-8 m/s. The lower permeability also results in a reduced susceptibility to penetration by water and therefore deep weathering. It was never anticipated that deep weathering as seen in the white chalk would occur at the Channel Tunnel.

The fracture spacing recorded in the Grade II chalk (99%+ of the UK sector) during tunnelling varied from around 1 per m to as high as 1 per 3.0 m. Typically, in the Chalk, fracture abundance greatly reduced with depth.

On the UK side every attempt was made to record weathering at tunnel horizon. No clear evidence was found, apart from where it was expected, close to the portal and the entrance to Adit A2 at Shakespeare cliff.

Prof. R Mortimore undertook SEM analyses on behalf of TML on a limited number of fracture surface samples recovered from the Marine Service Tunnel in the area of the worst tunnelling conditions encountered. The results proved the presence of limited mineralisation of the fracture surfaces which might be attributable to early stages of weathering. Even with this slight mineralisation, the fracture spacing clearly indicated Grade II chalk.

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Hydrogeology

Despite the relatively high porosity of the Lower, Middle and Upper Chalk the primary permeability remains very low due to the absence of continuity between pore spaces. Reynolds (1947) recognised the impermeable nature of the Lower Chalk in the Folkestone-Dover chalk block stating that the movement of water is prevented in any direction other than along fissures. This secondary permeability is governed by lithology, structure and topography.

Lithology

Zones of high secondary permeability are associated with the more brittle and consequently more jointed strata above the Grey Chalk, notably in the Melbourn Rock. Water movement is related to the density and aperture of the fissures. Both decrease with increasing clay content towards the base of the Lower Chalk and there is no measurable activity within the lowest 30 m of the Chalk, i.e. within the Chalk Marl.

The top of the Chalk Marl corresponds to a spring line which is observed widely in the Chalk of both SE England and NW France. In the cliffs between Folkestone and Dover the spring line falls gradually eastwards emerging at about +20 m OD at the western end of Abbot's Cliff and falling to about -45 m OD at the eastern end of Shakespeare Cliff.

More recent research has indicated that water movement in Chalk is very strongly influenced by even relatively minor aquicludes and major subhorizontal joints. These typically lead to water migration in a dip direction. Where the structure of the chalk comprises a series of domes, the intervening structural low may become a conduit for the runoff from the dome structures themselves facilitating the formation of valley systems. This theory does nor require the valley systems to be the result of preferential weathering of more faulted and jointed and faulted.

Structure

The direction of water movement is preferentially along those fissures which originated as extension joints in response to gentle flexuring of the strata. There is some correlation between the points at which water issues from the cliff and the emergence of the WNW-ESE fissure systems, notably Steady Hole spring - in the Folkestone Warren, Lydden Spout Spring - Abbot's Cliff, and a former submarine spring now buried beneath the reclamation. As previously described, this latter spring is understood to have issued at a higher elevation behind the railway following periods of heavy rainfall at a point which corresponds to the spring noted by Reynolds (1947, 1970,1972).

Topography

The topographic control on ground water levels behind the cliffs is significant in that the Aycliff dry valley, which approaches the back of the cliff from the northeast, has the effect of lowering the potential for high ground water or perched conditions in the cliffs behind Round Down and Shakespeare Cliff. Furthermore, the steep landward-facing slopes behind these sections of cliff have the effect of directing run-off inland and away from the cliff.

The levels of the main groundwater table in the Folkestone-Dover chalk block have been determined by observations over many years of the standing water levels in wells and boreholes. Minimum ground-water contours indicate levels behind the cliffs to be +15 m OD at Abbot's Cliff falling to 0 m OD at Shakespeare Cliff. The nearest point for which long term observation are available is in a well at Church Hougham, 1.1 km inland from Abbot's Cliff, where a fluctuation of 15 m has been recorded over a period of 13 years. Records from Dover Castle, 3.5 km east of the site, show a variation of only 2 m between maximum and minimum levels over a period of 8 years. Extrapolating to the area of interest one can infer fluctuation in ground water of between 5 m and 10 m behind Abbot's Cliff and between 0 m and 5 m at Shakespeare Cliff. These general water levels are consistent with a standing water level of 25 m OD encountered in a borehole sunk by British Rail in the cliff behind Lydden Spout.

Of significance to cliff fall processes is the occasional development of high transient water pressures, associated with infiltration, perched above marl bands (aquicludes) within the Middle Chalk and upper part of the Lower Chalk following periods of intensive and or prolonged rainfall. It is considered that high transient pressures above the Chalk Marl and the Plenus Marl following heavy rainfall was the final trigger for the substantial fall at the western end of Abbot's Cliff in January 1988. Forewarning of collapse in this part of Abbot's Cliff was provided by signs of distress at the cliff top and by the shearing of ventilation shafts detected as early as the 1950s.

During Construction

The long-term influence of the Channel Tunnel service and running tunnels on regional ground water levels has been demonstrated by long-term borehole monitoring to be insignificant as the tunnels are located within the relatively impermeable Chalk Marl horizon and the tunnels linings were in any event back-grouted during construction rendering the tunnel effectively watertight.

One surprise encountered during construction was a minor acquiclude which occurred above the crown of the UK Crossover. This marl seam was a mere several cms in thickness yet it effectively prevented downward migration of groundwater into the cavern. The relatively high water pressure above this marl seam triggered a minor slab failure in the crown during construction. Once recognised the problem was simply dealt with by installing drainage.

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Geological predictions compared with the encountered geology

The original and very practical approach adopted for optimising the route of the 150 km of tunnels that now link France and the UK some 900,000 years after the Strait came into existence was to ascertain at every point within a 1 km corridor the accuracy of prediction of each of the main parameters of the project (top of Gault, top of Craie Bleue, permeability of Craie Bleue, etc.) with a view to placing the tunnel route in the zone of least risk, If this was not possible, then either the risk would be met by adapting the tunnel location or the work procedures to the degree of confidence; or the accuracy would be improved by conducting further exploration, an approach that was adopted for the location of the two crossovers.

The accuracy depended on that of the actual data, their distribution, the complexity of the parameters to be represented and, above all, the inevitable interpolation between data.

Geostatistical methods were adopted for contouring the main stratigraphic boundaries and made it possible to optimise the results from the exploratory work. This resulted in, for example, the position of the top of the Gault being defined with a standard deviation of only ±2 to 3m in the tunnels and ±1 to 1.5 m at the crossovers.

On the French side this parameter was checked during the tunnelling approximately every 250 m and in 44% of cases the deviation from the prediction was within ±1 m, and in 82% of cases within ±2 m, i.e. substantially better results than the deviations predicted statistically that were used as a basis for design of the project. Similar results were obtained for the UK side apart from a section past the UK Crossover where the error was up to 6m. This was one of the few geological surprises encountered and it may be no coincidence that it occurred in an area of much expanded Glauconitic Marl, which thickened rapidly away from the UK Crossover 'high'.

The successful completion of the tunnel while meeting few geological surprises validates not only the whole of the chain of exploratory operations, each of the phases of which had been rigorously optimised and pushed to its technological limits, but also the geostatistical methods that resulted in tunnelling in the best possible geological conditions.

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