Contents1.Abstract12.Introduction23.Regional setting22.1.Geology22.2.Structure Tectonic regime32.3.Hydrogeology34.Local setting45.Investigations46.Discussion65.1.Soil and Rock properties65.2.Geological Hazards75.3.Uncertainty and variability87.Conclusions & Recommendations98.References109.Appendices1110.Figures12

AbstractFolkestone Junction Shaft and Stade Outfall Tunnel are engineering works part of the Dover and Folkestone Wastewater Treatment Scheme. At the area of the proposed engineering works the near surface substratum is formed by the Mesozoic system of strata, including the Gault clay and the sands and sandstones of the Folkestone beds. Along the Tunnel line four boreholes were drilled: DF6, DF10, DF15 and DF26. The geological materials were divided into units of similar geological conditions and of similar geotechnical properties. The Gault clay formation is divided into two sub units according to their degree of weathering and the Folkestone beds are divided into three subunits according to the percentage of sandstone present in each subunit. It can be derived that the excavation of tunnel is expected to be, in its larger portion, through the sub-unit F-III (percentage of sandstone between 25%-50%) of the sandy Folkestone beds, below the ground water table, with the exception at the area of borehole DF6 where the lower sandy Sandgate beds formation, unit S, was encountered, also below the ground water table. A further investigation program can clarify uncertainties in the distribution of the sandstone inside the Folkestone beds, as well as, the extent of the appearance of unit S at the tunnel level. Geological hazards that need to be taken into account during the design of the tunnel are: ground water conditions, ground conditions at excavation level, tunneling induced surface subsidence, slope stability issues, abrasive geological materials and highly plastic geological materials.

IntroductionFolkestone is a town located on the coasts of SE England and the Folkestone Junction Shaft and Stade Outfall Tunnel are engineering works part of the Dover and Folkestone Wastewater Treatment Scheme that will enable the treatment of waste waters that are presently discharged at sea by the existing system of sewers and drains. Folkestone Junction Shaft (aka Shaft 1) is one of the main pump stations with a circular cross section of 22m in external diameter and extends from ground level at 36m OD to -5m OD. The Stade Outfall Tunnel is a tunnel which links Shaft 1 to the sea. It has a length of 573.8m, an internal diameter of 3m and joins Shaft 1 to the Stade pumping station at East Cliff in a straight line. The tunnel invert at Shaft 1 is at 0.4m OD and rises steadily to 2m OD at Stade Screen House. In the following Figure 1a the location of Folkestone is shown as well as the location of the proposed engineering works in Figure 1b, from data freely available by the Ordnance Survey. Both the Shaft and the Tunnel are engineering ground works that require detailed understanding of the geological regime, far field and near field, for their design and construction.

Figure 1 Location of Folkestone in SE England (Figure 1a, left) and location of the proposed engineering works (Figure 1b, right). Data source provided from the Ordnance Survey in TIFF and DXF formats available by EDINA (http://edina.ac.uk/).

Regional setting1. 2. 2.1. GeologyFolkestone it is a part of the Wealden District, which extends from Dover and Folkestone to Farnham in East-West direction and from Sevenoaks to Brighton in North-South direction. The most notable major characteristic in the topography of the Weald proper is the hill ridges of the North Downs, which extends from Farnham to Folkestone and the hill ridges South Downs, which in turn extend from Petersfield to Eastbourne and forms the Walden uplift. Folkestone is located at the eastern end of the North Downs. The topography in the area of interest is characterized by the steep chalk cliffs north of Folkestone that reach an altitude of 165m OD in an axis W-E until the eastern coastline and consequently turn to NNE along the coastline. Steep coastline cliffs also develop close to the city of Folkestone with altitudes of 35m OD approximately along the southeastern and eastern coasts. The area of the proposed engineering works is located at the eastern part of the city where a relative gentle topography at altitudes of 35m OD approximately is encountered with only a small stream crossing the area in a direction towards the SSW (Figure 1b). Based on the published geological map (Institute of Geological Sciences, 1974) of the Wealden area the following bedrock and superficial-drift geological formations can be encountered in the Wealden district: (a) Paleozoic rocks that have been proved in boreholes only, (b) Concealed strata of the Mesozoic system including some Jurassic rocks exposed at the surface (c) Mesozoic system of strata exposed to surface (d) Tertiary system strata and (e) Pleistocene and Recent deposits. It should be noted that between the close of Paleozoic and the earlier marine Cretaceous strata the area of Wealden was a fresh to brackish water lake and later until late Cretaceous, sea transgressions change the area into a shallow marine environment (Gallois, 1965, p.21). The Mesozoic system of strata that includes: Wealden Series, with Ashdown beds [h1a-b], Wadhurst Clay [h1c], Tunbridge Well Sands [h1d] and Weald Clay [h1e], Lower Greensand with, Atherfield Clay [h2a], Hythe Beds [h2b], Sandgate Beds [h2c] and Folkestone Beds [h2d], (3) Gault formation [h3-4] and the Chalk formations [Lower: h5a, Middle: h5b and Upper: h5c]. Finally, it is worthy to mention that under the Periglacial conditions, during Pleistocene, the main Pleistocene and recent deposits were either erosional or depositional formations and the limit between Pleistocene and recent deposits is the end of the last European glaciation and it is not easily distinguishable in the area (Gallois, 1965, p.59). 2.2. Structure Tectonic regime The Wealden District experienced various structural events under the influence of the changes in the tectonic regime of the area (Gallois, 1965, p.51). Briefly, as described by Gallois (1965) the closure of the Paleozoic era has undergone two orogenitic phases, the Caledonian and the Hercynian, both characterized by intense folding events. By the end of the Paleozoic the folded platform of Wealden subsided into a shallow depression that later, until the Middle Cretaceous, become differential movement, as this subsidence in the Wealden was accompanied by the uplift of its margins. The initial phases of the Alpine orogeny during Jurassic and Cretaceous can be observed by the transgressions and the regressions of the sea at the shorelines. The first main phase of this orogeny took place during Danian - Montian times and raised above sea level much of the area. Later at Oligocene and Miocene the main phases raised and folded the area and were followed by epeirogenic movements which continue until today and raised the area to its present state. The Wealden uplift is a large dome structure that extends to northern France separated by the English Channel. Its regional dip away from its axis is very small, one or two degrees, although smaller scale folds have steeper limbs. Notably, secondary and smaller scale faults and folds are relatively scarce at the line formed from Maidstone to Folkestone because in this area the Paleozoic rocks form a platform near the surface that inhibits the formation of folds. The present day tectonic regime can be seen in Figure 2 in the Figures section of this report. The area of interest is not close to tectonic plate boundaries and currently there is no active tectonic process. 1. 2. 2.1. 2.2. 2.3. HydrogeologyBased on data published on the hydrogeological map of the area (Institute of Geological Science, 1970), the Gault clay formation is acting like an aquiclude between the ground waters in the upper Chack formation and the aquifers in the lower Greensand formation, which Gault gradually overstep. Furthermore, Folkestone beds form a porous not fissured aquifer and Sandgate beds have varying thickness also act like an aquiclude and separate groundwater in Folkestone beds above and Hythe beds below. Local settingAt the area of the proposed engineering works, at Folkestone, the near surface geological substratum is formed by the Mesozoic system of strata exposed to surface including: (1) Wealden Series, with Ashdown beds [h1a-b], Wadhurst Clay [h1c], Tunbridge Well Sands [h1d] and Weald Clay [h1e], (2) Lower Greensand with, Atherfield Clay [h2a], Hythe Beds [h2b], Sandgate Beds [h2c] and Folkestone Beds [h2d], (3) the Gault formation [h3-4] and (4) Chalk formations [Lower: h5a, Middle: h5b and Upper: h5c]. Details on the above geological materials, their structure and depositional environment as well as, the major geological events that are associated with them, can be found on published data by Gallois (1965). From that data it was derived that, the Wealden series formations are characterized by the rhythmic depositions (cyclothems) of sandstones, siltstones, shales, limestones and mudstones, also, by the massive, cross-bedded, sandstones in Lacustrine and deltaic environments with distinct horizons of transgressions and regressions. Wealden series are deposited on the deepen platform of folded Paleozoic strata. Later Lower Greensand, Gault and Chalk formations are deposited in shallow water, near-shore and marine environments. Lower Greensand and Gault are characterized as lateral lithological variations of arenaceous and argillaceous deposits, respectively also with distinct sea transgression that overstepped locally earlier formations. As far as Pleistocene and recent deposits, at the Folkestone area the following are found: (1) the Coombe deposits, (2) the Head deposits, (3) River gravels, (4) storm beach gravels and (5) marine beach deposits. Details on the above geological materials, also found on published data by Gallois (1965). From that data it can be seen that these deposits are indicative of the terrigenous environment that prevails until today. Furthermore, modern process that still affect the area of interest include mass wasting (landslides, rock falls, slope-washes), stream drainage and sea wave erosion. On Figure 3, found in the figures section of this report, the geological map of the area in interest based on the geological map of B.S.G. (Institute of Geological Sciences, 1974). Similarly, on Figure 4, found in the figures section of this report, a representative vertical crosses section of the subsurface geology is presented showing the relationships between the strata and the major depositional characteristics associated with them.

InvestigationsAlong the Stade Outfall Tunnel line four boreholes were drilled: DF6, DF10, DF15 and DF26. Their exact locations and depths, as well as, their field description and field and lab testing on their samples, is presented on the material given for this report found on the Appendices of this report. Briefly the major stratigraphic units encountered at the location of each borehole, with the respective results of field and lab testing on their samples, are presented as follows: Borehole DF6: depth 40.0m, location: 623492.60E 136426.00N 33.50m0.0-1.70mMADE GROUND1. 70-9.90mGAULT: grey, brown grey, CLAY, stiff to very stiff, with extremely closely spaced sub-horizontal fissures. Zones of weathering are identified as follows: 1.70-4.60m Zone IV, 4.60-9.20m Zone II and 9.20-9.90m Basal Gault, mottled yellow grey, sandy CLAY to clayey SAND, very stiff. 9.90-28.05mFOLKESTONE beds: grey to green grey, SAND, fine to medium coarse, dense, glauconitic. Green grey, fine to medium, calcareous SANDSTONE bands, moderate weak to moderate strong, in varying amounts inside the sand formation. Noted that in this report, we will group the Sandstone bands according to their amount is the soil mass as follows: 9.90-25.30m: less than 10% sandstone, 25.30-28.05m: 25% to 50% sandstone28.05-40.0mSANDGATE beds: grey, fine SAND to silty SAND, dense. Groundwater measurements on piezometer installed show WT depth varying between 24.80m to 27.78m. Borehole DF10: depth 40.2m, location: 623556.32E 136299.87N 35.70m0.0-1.20mINSPECTION PIT1.20-16.40mGAULT: grey, brown, CLAY, firm to stiff, with extremely closely spaced randomly oriented (from 1.20 to 9.80), sub-vertical to sub-horizontal (from 9.80 to 10.90) and sub-horizontal (from 10.90 to 15.30) fissures. Zones of weathering are identified as follows: 1.20-5.20m Zone IV, 5.20-9.80m Zone II, 9.80-10.90m Zone III, 10.90-15.30m Zone II and 15.30-16.40m Basal Gault, black, yellow green, orange clayey SAND fine to medium, dense.16.40-34.95mFOLKESTONE beds: green, yellow green grey, SAND, fine to medium coarse, dense, glauconitic. Green grey, fine to medium, calcareous SANDSTONE bands, moderate weak to moderate strong, in varying amounts inside the sand formation. Noted that in this report, we will group the Sandstone bands according to their amount is the soil mass as follows: 16.40-30.40m: 10% to 25% sandstone, with very closely space sub-horizontal discontinuities, planar, rough, 30.40-34.95m: 25% to 50% sandstone, with very closely space sub-horizontal discontinuities, planar, rough.In one point load test on sandstone sample the value of Is(50)-axial was found to 5.68MPa. Noted that in this report this sample can be characterized as of very high strength based on the classification of point load strength by Franklin and Broch (Bell, 2007, p.255).34.95-40.2mSANDGATE beds: grey, fine SAND to silty SAND, dense. Groundwater measurements on piezometer installed show WT depth varying between 32.30m to 32.80m. Borehole DF15: depth 15.0m, location: 623592.40E 136232.80N 3.90m0.0-6.00mBeach deposits6.00-15.00mSANDGATE beds: grey, silty to clayey fine SANDS, dense. SPT values range from N=61 to N=111. Borehole DF26: depth 65m, location: 63363.01E 136761.34N 36.33m0.0-5.10mmade ground, brown, grey sandy CLAY. SPT values range from N=5 to N=6. 5. 10-20.40mGAULT: grey, brown, CLAY, firm to stiff, with extremely closely spaced sub-horizontal planar, clean, smooth, polished fissures. SPT value found N=19. Zones of weathering are identified as follows: 5.10-7.00m Zone II/I, 7.00-20.00m Zone I and 20.00-20.40m Basal Gault, grey, SANDSTONE fine to medium, with disseminated pyrite and shell debris 20.40-38.20mFOLKESTONE beds: green grey, grey, SAND, fine to medium coarse, dense, glauconitic. Green grey, fine to medium, calcareous SANDSTONE bands, weak to moderate strong, in varying amounts inside the sand formation. Noted that in this report, we will group the Sandstone bands according to their amount is the soil mass as follows: 20.40-30.40m: less than 10% sandstone, 30.40-38.20m: 25% to 50% sandstone, with very closely space sub-horizontal discontinuities, planar, rough.In five point load test on sandstone samples the value of Is(50)-axial was found to vary between 0.11MPa to 3.68MPa. Noted that in this report these samples can be characterized as low to very high strength based on the classification of point load strength by Franklin and Broch (Bell, 2007, p.255).38.20-65.0mSANDGATE beds: grey, fine SAND to silty SAND, dense.Groundwater measurements on piezometer installed show WT depth varying between 25.64m to 28.50m.

Discussion 3. 4. 5. 5.1. Soil and Rock propertiesBased on the local geological setting, borehole field logs, field tests and lab testing the geological materials in the subsurface can be divided into units of similar geological conditions and of similar geotechnical properties. Each unit is expected to behave in a uniform way during the construction of the engineering works. In some cases the units coincide with the major geological strata found in the boreholes. In other case a formation is divided in sub-units with similar geological and geotechnical characteristics. All these units are presented in a model cross-section along the tunnel and along the vertical section of the shaft in Figures 5 and 6, found in the figures section of this report. Specifically, the Gault clay formation is divided into two sub units according to their degree of weathering and the Folkestone beds are divided into three subunits according to the percentage of sandstone present in each subunit. Thus the following units are created and presented along with their geological and engineering properties:

Unit MMADE GROUND, range from sandy GRAVEL to sandy CLAY, moisture content varies from 5% to 46% with an average of 26%. Cohesive materials have intermediate to high plasticity. Bulk density ranges from 1.69 Mg/m3 to 2.16 Mg/m3 with an average of 1.91 Mg/m3. Undrained shear strength measured from triaxial compression range from 22 kN/m2 to 92 kN/m2 and increases with depth. Unit GGAULT, CLAY, high to very high plasticity, average bulk density of 1.95Mg/m3, undrained shear strength measured by triaxial compression which increase from 80 kN/m2 near ground level to 250 kN/m2 near its base. Measurements of range from 6deg to 12deg assuming zero cohesion. Filter paper suction tests gave average suction values of 98 kN/m2 for moisture contents of 23%, 370 kN/m2 for 25% and 441 kN/m2 for 25%. For Gault clay Ko is 0.5 to 1.0, Youngs Modulus range from 165 to 270 MN/m2 and in situ permeability k from 10-6 to 10-8 m/s.Sub-unit G-I: weathering zones I and II, moisture content from 19% to 29%.Sub-unit G-II: weathering zones III and IV, moisture content from 16% to 33%.Unit F: FOLKESTONE beds, green grey, grey, SAND, fine to medium coarse, dense, glauconitic. In the formation natural cemented layers (doggers) are found. Moisture content from 19% to 27%, bulk density for doggers is 2.37 Mg/m3. For Folkestone beds Ko is 0.5, Youngs Modulus 700 MN/m2 and in situ permeability k from 10-6 to 10-8 m/s.Sub-unit F-I: less than 10%, green grey, fine to medium, calcareous SANDSTONE in bands, weak to moderate strong. Found partially under the water table. Sub-unit F-II: 10% to 25%, green grey, fine to medium, calcareous SANDSTONE in bands, weak to moderate strong. Found partially under the water table.Sub-unit F-III: 25% to 50%, green grey, fine to medium, calcareous SANDSTONE in bands, weak to moderate strong. Found either partially or fully under the water table. Unit SSANDGATE beds, grey, fine SAND to silty SAND, dense. Moisture content found to have an average of 23%. Found below the water table.

5.2. Geological HazardsGood knowledge of the geological conditions is essential as the type of ground encountered along the tunnel line affect both the selection of the tunnel type as well as its method of construction. Based on the above data, geological hazards that need to be taken into account during the design of the tunnel and the shaft are: (a) Ground water conditions. Ground water effects are of the upmost importance as it can flood a tunnel, break in an exposed face. Also, ground pore water pressure adds load to the tunnel lining and also modifies the strength of soils or may reduce the strength of a rock by lubricating joints or other discontinuities and furthermore water may act on minerals, like anhydrite, and cause swelling or disruption (Megaw and Bartlett, 1981). In particular, the tunnel is expected to be excavated below the water table at its greater length, with the exception possibly of its final part. Measures should be taken to control ground water inflow and also measures should be taken to prevent surface water to enter from the shaft into the tunnel.(b) Ground conditions at excavation level. When the ground material is below the water table, it lacks sufficient cohesion or cementation and the behavior is more subjective and can easily run or flow into the excavation (Hung, Monsees, Munfah and Wisniewski, 2009). The tunnel will be excavated inside the sandy formation of Folkestone beds, below the water table. According to the tunnel behavior for sands and gravels by Terzaghi (Bickel, Kussel and King, 1996, p.98) sands with binders under the water table can be firm or exhibit slow raveling. Also, the approximate ground behavior trends of various soiles bu Deere et al. suggest either firm or slowly raveling conditions (Bickel, Kussel and King, 1996, p.98). Slow raveling according to the Tunnelmans Ground Classification for soils, by Terzaghi, is when chunks or flakes of material begin to drop out of the arch or the walls sometime after the ground has been exposed (Hung, Monsees, Munfah and Wisniewski, 2009, p.7-2).(c) Tunneling induced surface subsidence. Subsidence is always greater for soft ground tunnels. Sources of subsidence are ground water depression and those caused by lost ground. Groundwater depression may be caused by intentional lowering of the water during construction or by the tunnel itself (or other construction) acting as a drain. Lost ground refers to the act of taking (or losing) more ground into the tunneling operation than is represented by the volume of the tunnel conditions (Bickel, Kussel and King, 1996). The presence of the tunnel below the ground water table and the possibility of raveling in the sandy material of the excavations can lead to subsidence problems. Also it should be noted that above the tunnel and near the surface the material to be encountered is the high plasticity Gault clay and at the ground surface buildings and facilities of the Folkestone town is found. Thus measures should be taken in order to address these issues. (d) Stability issues at surface slopes. The presence of a small overburden at the tunnels end in relation with the surface slope gradient rise. Thus, an open excavation may be problematic in terms of slope stability and measures for its stability should be taken. (e) Abrasive geological materials. Based on published geological data, Folkestone beds contain veins of hard ferruginous sandstone, also known as Carstone (Gallois, 1965). Furthermore the presence of cobbles and doggers, which are associated also with large diameter nodules, has been verified by the ground investigation results. Thus, it is expected to excavate into hard and abrasive materials especially in F-II and F-III subunits where the concentration of the sandstone is greater than 10% and reaches up to 50% of the total mass of the formation. (f) Highly plastic geological materials. The excavation of the shaft through the highly plastic Gault Clay can cause problems to nearby facilities and buildings because the high plasticity ground tends to plastic yielding which can occur below stress associated with shear failure (Bickel, Kussel and King, 1996). Measures to support the excavation and prevent plastic yielding should be taken.

5.3. Uncertainty and variability Based on the local geological conditions the findings of the subsurface investigation and the division of the geological formations in units of similar geotechnical characteristics, as mentioned above, a profile along the tunnel line is presented on Figure 5 and a vertical section at the Folkestone junction shaft (shaft 1) is presented on figure 6. From these figures it can be derived that the excavation of tunnel is expected to be, in its larger portion, through the sub-unit F-III of the sandy Folkestone beds, below the ground water table, with the exception at the area of borehole DF6 where the lower Sandgate beds formation, unit S, was encountered also below the ground water table. The shaft is expected to be excavated through made ground, the Gault clay units G-I and G-II, the Folkestone bed units F-I and F-III. All the above are based on the findings of investigation and the local geological setting. Folkestone beds and the Gault clay as formation exhibit little variability in the scale of the engineering works yet, inside each formation there is variability. In particular, Folkestone beds vary in the amount of sandstone bands inside the formation, even though the sandstone percentage increases with the depth. Gault clay was found to vary in the degree of weathering. Weathering patterns can be very irregular and control by various factors, like the surface drainage conditions and the density of the fissures inside the clay formations. Another important issue is the appearance of unit S in the tunnel level at DF6.

Conclusions & Recommendations a) A further investigation program can clarify uncertainties in the distribution of the sandstone inside the Folkestone beds, as well as, the extent of the appearance of unit S at the tunnel level. Thus, two boreholes along the tunnel and placed evenly between DF26 and DF6 are proposed. Their depth should reach below the tunnel invert. b) During the tunnel excavation, based on the presence of the ground water level above the tunnels level, measures should be taken to control ground water. Available methods for controlling ground water are dewatering, compressed air, grouting and freezing (Bickel, Kussel and King, 1996). c) Ground tunneling machine selection depends on ground conditions. According to Bickel, Kussel and King (1996) slurry face machine and earth pressure balance (EPB) machines are suitable for the sandy soils that are likely to be encountered during the excavation. Another option is the use of soft ground TBM machines that have the function to excavate the ground, remove the excavated material and to support the excavated tunnel temporarily until permanent support can be installed. Also they can handle adverse ground conditions. d) The shaft can be excavated using various methods of support like steel sheet piling, liner plates or slurry walls in order to prevent the occurrence of plastic yielding in the clay formations (Bickel, Kussel and King, 1996). e) The shaft excavation below the ground water table should be accompanied by measures to lower the ground water table or other measures like freezing the soil, using slurry and grouting(Bickel, Kussel and King, 1996).

References

Bell, F. G., 2007. Engineering Geology, 2nd ed., London: Butterworth-Heinemann

Bickel, J.O., Kussel, T.R. and King, E.H., 1996. Tunnel Engineering Handbook, 2nd ed.US: Chapman & Hall

Gallois, R.W., 1965. The Wealden district, 4th ed. 1978 reprint, London: Institute of Geological Sciences (Geological Survey and Museum)

Hung, C. J., Monsees, J., Munfah, N. and Wisniewski, J., December 2009, Technical Manual for Design and Construction of Road Tunnels Civil Elements, Publication No FHWA-NHI-10-034 U.S. Department of Transportation Federal Highway Administration

Institute of Geological Science, 1970, Hydrogeological map of the Chalk and Lower Greensand of Kent, 1:50000, London BGS

Institute of Geological Sciences, 1974, Folkestone & Dover solid and drift, Sheets 305 and 306, 1:50000, London BGS

Megaw, T.M. and Bartlett, J.V., 1981. Tunnels, planning, design, construction, volume 1, UK: Ellis Horwood Series in Engineering

Appendices

Figures

Figure 2, Stress map of the overall region. No active tectonic process can be found in the area of interest. Data available on the Internet from the World Stress Map project, Helmholtz Centre Potsdam, GFZ German Research Centre for Geoscience (http://dc-app3-14.gfz-potsdam.de/index.html).

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