Due to the groundwater conditions, construction of the Sizewell B station could not begin until the UK's deepest ever diaphragm wall was installed to isolate the site. The foundations would need excavations nearly 18m below the water table, and dewatering consultants were appointed in 1984. Conventional dewatering techniques were rejected for several reasons. The next idea was to construct a diaphragm wall, extending into the London Clay, and linking with a cofferdam to form a 1260m-long all-encompasing cut-off wall around the whole site. The established techniques for diaphragm walling had only been used down to 30m at that time, but trenching with new reverse circulation rigs could go down to more than 100m very accurately. This solution had several advantages, and had been selected by July 1985. The wall had to have a controlled maximum permeability and virtually leak proof construction joints. Performance was monitored via a network of observation wells and piezometers. After more than 4000000m³ of water had been pumped away, the excavation was dry until the pumps were switched off in the spring of 1992. The water table had been kept at least 2m below the deepest excavation.
50m of dense sands and silts known as the Norwich Crag deposits overlay London Clay and form a natural aquifer. The site is also uncomfortably close to the North sea.
Preliminary calculations showed that even with 52 wells (rather than 6 used for the A station), it would still only be possible to lower the water by 16m. The excessive draw down below adjacent bird reserves, settlement beneath Sizewell A, heavy encrustation on the pipework due to high iron content in the groundwater, and a cost of at least £16M, all made this option unacceptable.
Only nine dewatering wells would be needed, and the construction period would be halved to six months with a saving of £2M.
A record 15GJ/h refrigeration plant formed a tube of ice-stabilised strata reaching 528m down to a dry zechstein formation overlying the Ruhr coalfield: the world's largest ground freeze. The artificially stabilised section of ground, with its steel and concrete lining structure, is the start of a 7.5m diameter shaft continuing down to 1.3km. Its prime purpose is ventilation of the nearby mines. Proximity of the Rhine demanded that a 4m platform should be built to lift the working area above any flood level. But the rise and fall of groundwater levels in the 14m of terrace sands and gravels is insignificant compared with conditions deeper down. Tertiary deposits of sands, silts and clays go down to 100m where there is about 30m chalk. Then there is Bunter sandstone, which is very soft and charged with water right down to 511m where the dry and stable zechstein occurs.
The 44 freeze pipe holes, with 127mm diameter casing, were drilled on a 22m diameter circle, and the freeze was computer-monitored. A central drill hole formed a pressure relief drain for groundwater expelled within the constricting ice front.
When the freeze is switched off, the 30000t of steel and concrete composite tube lining the shaft will retain its structural integrity independent of long term subsidence of the the surrounding aquifer.
Refrigeration comes via nearly 400m³ of calcium chloride brine being delivered at -33ºC and returned to the plant at -28ºC.
Three additional casings were set 120º apart and at 2m, 4m and 6m outside the freeze circle for the computer-monitored thermometer installation assessing the progress of the freeze.
Installation of deep dewatering wells by hole punching is proving a cost-effective and efficient way of dewatering a borrow pit on the northern contract of the Blackwater Valley Route project. A scheme was proposed to extract sand horizons from a pit in the underlying Bracklesham Beds, creating a home for surplus material.
The groundwater lies only a metre below ground level, and the Beds are hydrostatically pressurised, so excavation without reducing the pressure would lead to rupturing. Following a series of drawdown and falling head tests, a two stage scheme was proposed. The conventional first stage produced a drawdown sufficient for excavation of the terrace gravels, reducing ground level by 3m. Working from the reduced level at the top of the Bracklesham Beds, the deep wells were installed. Pumping of these wells produced a drawdown sufficient for the pit to be stepped down a further 5m. This was followed by a second series of deep wells, finally allowing excavation to the required 15m.
Blackwater Valley Route
Surrey and Hampshire county councils' joint proposed dual carriageway bypass of Farnborough and Aldershot.
The borrow pit is needed because the 5km section requires some 600000m³ of class one fill for construction of the embankments. Importing this fill and carting away 150000m³ of unsuitable surplus material would be less efficient.
The deep wells were installed with a hole puncher. High pressure water and air are injected through a central jetting pipe. This erodes the ground at the probe's base and flushes debris up the annulus between the jetting pipe and the outer casing. At the same time, a top-acting 3t drop weight drives the outer casing into the ground.
The pit needed to be 15m deep. Trials resulted in the sand boiling when the excavation extended below 6m. The situation was complicated by the permeability in the overlying gravel being ~0.001m/s, approximately 100 times greater than that in the Bracklesham Beds.
Well points were installed for pumping at the base of the gravels, around the perimeter of the proposed pit. A garland drain was constructed around the base of the excavation to stop overbleed from the gravels recharging the top of the Brackleshams.
The 300mm diameter deep wells were installed to a depth of 24m at 30m centres.
Elaborate groundwater control measures are a key feature of Munich's new airport - one of the largest greenfield construction sites in Europe. Groundwater within 1m of the surface meant a risk of regular flooding and vulnerability to frost damage of the runways and taxiways. A system of ditches and drains has dropped the level, but environmental constraints dictated the water table should be restored outside the airport boundary. Restoration of diverted streams, and a multiple level system of dirty, contaminated and clean run off drainage is required. The solution inside the airport was to lower the groundwater table by around 2m. Two longitudinal ditches between the runways are drained by an underground pipe link to the main boundary interceptor. On the northern downstream boundary, the water is put back into the ground by a series of well point injectors. Control centre computers monitor the flow distribution and any contamination with spilt aircraft fuel etc. Glycol(used as a de-icer on the taxiways will be intercepted by 20m wide ridged aprons of impermeable geotextile buried under sand and gravel filters.
The subsoil is a 10m depth of Quaternary gravel overlying an impervious layer capping a 30m thick stratum of Tertiary gravels. The gravels each have separate groundwater movements of about 1m to 2m a day from south to north. They are valuable aquifers, and all flows need to be maintained.
The injectors are 10m deep steel diffuser
pipes 200mm in diameter
Biological action should break down the
glycol before it reaches the edge of the membrane and trickles down.
The Italian road authority ANAS' bypass includes twin bore 3.2km long tunnels passing underneath Monte Barro and approach viaducts of 200m and 950m at either end. At the longer of the two, the underlying limestone bedrock dips in a 100m deep glacial valley, now infilled with soft and loose lake deposits. A brave design concept was proposed, in which hydrostatic upthrust supports the viaduct's load.
While floating roads built on lightweight polystyrene blocks have been constructed over marshland, at Lecco the viaduct is to be built on 20 huge, jet grouted cylindrical cells, bedded 25m below the ground surface. Soil within each cell is excavated from the surface using a backhoe. The cells are lined with concrete and PVC, and a 1m thick cap completes the hollow space.
The foundation design is dependent almost entirely upon buoyancy, provided by the cells being water tight and remaining air filled. Yet water has leaked into some of the completed cells and permanent pumping may be necessary to keep some of the cells dry.
jet grouted cylindrical cells
The cells are formed by vertically drilling a 10m diameter ring of 31 overlapping 1.5m diameter jet grouted columns. Each column is reinforced by a steel pipe inserted into the soil-grout mix before it sets. A second outer ring of jet grouting is then constructed over the bottom 13m of the cell while a third phase of jet grouting forms a consolidated 5m deep plug between 20m and 25m depth.
A 300mm thick reinforced concrete wall is
cast in place after every 2m of excavation until the top of the plug section
has been reached. The base is then strengthened with a 2.5m thick reinforced
concrete slab and a PVC geomembrane is installed to waterproof the sides
of the cell. Then a second internal concrete wall is cast.
In preparation for the construction of the piling mattress for a new superstore, subcontractors commenced dewatering to lower the water table within the fine silty sands of the Lower Greensand. The Gault clay dips down towards the northern end of the site, making dewatering unnecessary in this area, so the wellpoints were installed along the bank of the stream at the east boundary, and around the southern boundary. Groundwater in that area is almost at surface level.
120 wellpoints were installed, the aim being to dewater the Greensand down to 4m, allowing excavation for the construction of the pile mat. The wellpoints were connected to a pair of six-inch pumps which discharge directly into the stream. Initially, the pumps were discharging at a rate of 40 litre/sec, but after four days this had dropped to 10-15litre/sec. The Gault clay contained a number of perched water tables, and these were sump pumped during the first week of dewatering in order to drain them.
The geology consists of a layer of made ground overlying alluvium with some peat deriving from the time when the area was marshland. This overlies Gault clay and an extremely fine sand which represents the top of the Lower Greensand.
Site investigation consisted of eight boreholes and 16 trial pits, two of which were prevented from going to full depth because of the high groundwater inflows.
Record breaking diaphragm walls has been trenched 135m under Tokyo bay at the artificial island of Kawasaki - a key feature of the £5000M Trans Tokyo Bay Highway.(Note 1) The island will serve a dual role as the main pit for tunnel driving during construction, and later as a ventilator opening into the highway tunnel.
Loose marine sands have been stabilised by compaction from a barge before construction of the working platforms. Note 2 The support framework functions as part of a retaining wall when fill is placed to form a mounded island to sea level. The wall is supported and protected from erosion by rockfill armour on the outside.
A rig worked from the platform to trench out the 98m diameter, 135m deep ring of diaphragm wall panels, slicing down through the placed fill, then marine material, to form a cut off in hard rock. Once the wall was watertight, the interior was excavated to form a pitNote 3 reaching 70m below sea level. Tunnel drives then started from the dry chamber.
The Yokohama Bay bridge carries the new road 860m across the water from Kisarazu. The rest of the distance to a second artificial island is spanned by a 4.4km low level viaduct. From there the road plunges beneath the sea carried through twin 9.1km tunnels to the mainland.
A 200m diameter ring of steel trestle working platforms, founded on tubular steel piles driven into the treated material.
Over 0.5M.m³ of fill and soft sediment will have to be dug out while in situ reinforced concrete supports are erected inside.
More than twice the depth of London's deepest tube station, the tunnel was being driven for the Streatham to Brixton section of Thames Water's ring main. Groundwater pressures approaching 400kN/m² meant that compressed air fed into the workings to stem the flow broke through the legal limit of 345kN/m² for man working. The 2.5m diameter drive was started in dry London Clay before breaking into the underlying Woolwich & Reading beds. Saturated sand beds up to 0.5m thick needed more than the 100kN/m² air pressures predicted to keep the tunnel dry. Even raising the air to over 200kN/m² did not prevent water ingress.
The miners were probing to see how much Woolwich & Reading bed material protected them from the tricky Thanet Sands below, when the tunnel blew, letting 400kN/m² pressurised groundwater blast in, bringing with it more than 10m³ of sand and exposing the Chalk below. The volumes and rate of tunnel inflow, coupled with the fact that there is no sign of depressurising the Thanet Sands, indicates that the water is coming from the Chalk.
Fortunately, the tunneller was stranded below open ground, so a 50m vertical shaft can be sunk for machine recovery. The base of the 7m diameter shaft will be frozen to allow a short horizontal drive to the machine.
Many major cities obtain water by pumping from the ground. Changed industrial practices and water-supply systems over the years have led to much less water being taken from these sources. As a result, water levels which had been drawn down are now rising.
During the past two centuries, the pumping from the deep aquifer lowered the groundwater level by as much as 70m. The level is now rising, in many areas by about 1m/yr. If the rise continues for 20 to 30 years, the water pressures in the sands and clays above the Chalk will increase, causing ground movements in the clays. This could damage some large buildings and tunnels and increase leakage into them. These problems can be prevented by additional pumping, for a capital expenditure which is small compared with the potential cost of damage.
The water-bearing Basal Sands and Chalk below London are referred to as the deep aquifer. The aquifer is confined over much of the Basin by the overlying thick, relatively impermeable layers of clay, which separate the deep aquifer from the perched groundwater in the overlying gravels.
Future water levels
A regional groundwater computer model has been used to estimate the rates of rise likely to occur over the next 90 years, together with the likely maximum future groundwater levels. It shows that water levels could return almost to their original values within the next 30 to
40 years, unless action is taken.
Additional pumping of water from the aquifer in Central London not exceeding 30 mega-litres per day would be sufficient. This is about 5% of the normal daily consumption of water in Central London.
The pumping would require about 30 wells at a capital cost of £10M to £30M.
The cumulative cost of remedial works for tunnels could exceed £10M. Repairs to large buildings could cost tens of millions of pounds. Costs of new construction would also increase.
Although older buildings are unlikely to be damaged, modern structural forms, such as those in the diagram, would be vulnerable. If water levels rise causing the clay to swell, the two parts of the building with differing foundation levels will heave by amounts which could differ by up to 100mm.
There are some 130km of tunnels under London which are located near
the base of the London Clay or in the sandy deposits. Lengths totalling
about 40km have been identified which could suffer increased seepage, loading
and chemical attack.
Concern over rising groundwater at Canary Warf has led to the testing of a variable water pressure trial pipe: The piezometric head is being artificially increased in the Thanet sands pile founding strata by injecting water into five 150mm diameter wells. Bourdon gauges connected to three piezometers monitor changes in head during testing. The experiment models the change in pile base effective stress which is predicted if London ground water levels continue to rise, plus the effect of unloading under the 9m deep excavation for Founders Court car park. Trial excavation has been made redundant.
The total change in vertical effective stress at the pile base is equal to the sum of changes in total vertical stress and water pressure. The reduction in vertical effective stress is calculated to be about 180kPa, roughly equivalent to the 17m increase in water head induced by injection into Thanet Sands. This compares with a current vertical effective stress of 300kPa. The potential effect could be very serious if the factor of safety and design were inadequate.
The 2.5km dual carriageway road has been designed to relieve traffic congestion in eastern Hull and provide a direct link to the docks. Its planned route beneath the River Hull lies through an overall 361m long retained cutting, including a central 81m twin-celled concrete box section constructed in cut-and-cover through the river bed.
Original site investigation suggested a 4.3m deep layer of alluvium was underlain by a minimum 4m thick bank of impermeable boulder clay. This protected and confined a large aquifer in the chalk beneath it. The aim was to found the tunnel box in the clay zone without affecting the chalk. To construct the tunnel in two halves across the river, the contractor drove the first of two sheetpiled cofferdams on the eastern bank. With the cofferdam dewatered and being bottomed up, a 2m diameter sinkhole suddenly appeared in the river bed close to the cofferdam's north west corner, and a second hole formed nearby inside the structure. The excavation totally flooded overnight.
The cofferdam was quickly plugged with tremmied concrete and following a detailed three month second site investigation, urgent work started on jet grouting down the three exposed faces of the cofferdam, while the possible solutions were debated.
The second investigation
The second investigation revealed the boulder clay to be thinner than expected. It may contain perched water lenses and a prime concern is its integrity to protect the aquifer beneath. Water in the cofferdam has come from the river, not the aquifer, and it is possible that it has leaked through the sheetpiling rather than migrated beneath it.
Protecting the 21m wide tunnel from uplift hydrostatic pressure from the aquifer would need extensive mass concrete loading to the 1m thick tunnel floor. Ground anchors to hold the tunnel down are an option, but could puncture the aquifer.
The tunnel was eventually abandoned in favour of a lift bridge. In excess of £10m has been spent on the aborted work prior to starting on the bridge.