Multiple aquifers beneath London

The city of London is founded on river gravels and alluvial deposits associ-ated with the River Thames, which are underlain by the very low permeabil-ity London Clay. These gravels form a shallow (generally less than 10 m thick) aquifer. Construction of utility pipelines, basements and other shallow structures often requires groundwater lowering to be employed; wellpointing and deep wells have proved to be effective expedients in these conditions.

However, the geology beneath London allows another, deeper, aquifer to exist below the city, largely isolated from the shallow aquifer.

Beneath the London Clay lie a series of sands and clays comprising the Lambeth Group stratum (formerly known as the Woolwich and Reading Beds) and the Thanet Sand stratum. These are underlain by the Chalk, a fissured white or grey limestone, which rests on the very low permeability Gault Clay. The overall geological structure is a syncline forming what is often called the ‘London Basin’. The Chalk, Thanet Sand and parts of the Lambeth Group together form an aquifer. The upper 60–100 m of the Chalk are probably the dominant part of the aquifer, where significant fissure net-works readily yield water to wells. The sands are of moderate permeability and generally do not yield as much water as the Chalk. The overlying London Clay acts as an aquiclude or confining bed, effectively separating the deep aquifer from the shallow gravel aquifer.

The Chalk has a wide exposure on the North Downs to the South of London and on the Chilterns to the North and occurs as a continuous layer beneath the Thames Valley (Fig. 3.15). Rain falling on central London may ultimately reach the gravel aquifer, but the London Clay prevents it from per-colating down to the Chalk. The Chalk obtains its recharge from rain falling on the North Downs and the Chilterns many miles from the city. Ultimately this water forms part of the reservoir of water in the chalk aquifer. If the recharge exceeds the discharge from the aquifer (either from wells, or natural discharge to springs and the River Thames) the water pressure in the aquifer will rise slowly. If discharges exceed recharge the water pressure will fall.

Before London developed as a city, the natural rates of recharge and dis-charge meant that the deep aquifer had sufficient water pressure for it to act

as a confined aquifer (see Section 3.3). In the lower lying areas of the city there was originally sufficient pressure in the aquifer to allow a well drilled through the London Clay into the Chalk to overflow naturally as a flowing artesian well. In fact, in central London there are still a few public houses called the Artesian Well, indicating that in earlier days the locals were probably supplied with water from a flowing well.

This availability of groundwater led to a large number of wells being drilled into the deep aquifer (where the water quality was more ‘whole-some’ than in the gravel aquifer). Rates of groundwater pumping increased during the eighteen, nineteenth and early part of the twentieth centuries.

This resulted in a significant decline in the piezometric level of the deep aquifer. Artesian wells ceased to flow, pumps had to be installed to allow water to continue to be obtained and over the years the pumps had to be installed lower and lower to avoid running dry. By the 1960s the water level in wells in some areas of London was 90 m below the ground surface – a huge drop relative to the original artesian conditions. In some locations the water pressure was reduced below the base of the London Clay, so the for-merly confined aquifer became unconfined. The deeper water levels increased pumping costs and made well supplies less cost-effective com-pared with mains water. This, together with a general re-location of large water-using industries away from central London, has resulted in a signifi-cant reduction in groundwater abstraction. As a result the piezometric level in the aquifer has recovered since then (at more than 1 m/year at some loca-tions in the 1980s). By the 1990s the piezometric level in many areas was within 55 m of ground level.

Figure 3.15 Chalk aquifer beneath London (after Sumbler 1996). The chalk aquifer extends beneath the London basin and receives recharge from the unconfined areas to the north and south. The London Clay deposits act as a confining layer beneath central London. Prior to the twentieth century flowing artesian conditions existing in many parts of the city.

This continuing rise of water pressures is a major concern because much of the deep infrastructure beneath London (deep basements, railway and utility tunnels) was built during the first half of the twentieth century when water pressures were at an all-time low. Several studies (see e.g. Simpson et al. 1989) have addressed the risk of flooding or overstressing of existing deep structures if water levels continue to rise. The management of water levels beneath London (and, indeed, beneath other major cities around the world) is an important challenge to be faced by groundwater specialists dur-ing the first half of the twenty-first century.

In a construction context, an appreciation of the aquifer system is vital to ensure deep structures are provided with suitable temporary works dewa-tering. Fig. 3.16 shows a typical arrangement as might be used for a deep

Figure 3.16 Groundwater control in multiple aquifers.

shaft structure in central London. Important points to note are:

i The structure penetrates two aquifers, separated by an aquiclude.

Groundwater will need to be dealt with separately in each aquifer.

ii In London it is common to deal with the upper aquifer by constructing a cut-off wall, penetrating to the London Clay, to exclude the shallow groundwater. This is possible because the London Clay is at relatively shallow depth. If clay was present only at greater depth any cut-off would need to be deeper and it may be more economic to dewater the upper aquifer.

iii Wells are used to pump from the deep aquifer to lower the piezometric level to a suitable distance below the excavation. Because the wells must be relatively deep (perhaps up to 100 m), and therefore costly, it is important to design the wells and pumps to have the maximum yield possible, so that the number of wells can be minimized.

iv Although the lower aquifer consists of both the Chalk and the various sand layers between the top of the Chalk and the base of the London Clay, wells are often designed to be screened in the Chalk only, and are sealed from the sands using casing. This is because it can be difficult to construct effective well filters in the sands (which are fine-grained and variable), yet wells screened in the Chalk are simpler to construct and can be very efficient, especially if developed by acidization (see Section 10.6).

This is the approach successfully adopted for some structures on the London Underground Jubilee Line Extension project described by Linney and Withers (1998). Excavations in the Thanet Sand were dewatered without pumping directly from the sand, but by pumping purely from the underlying Chalk. This might seem a rather contradic-tory approach, but is an example of the ‘underdrainage’ method. This is a way of using the geological structure to advantage by pumping from a more permeable layer beneath the layer that needs to be dewa-tered; the upper poorly draining layer will drain down into the more permeable layer (see Section 7.3).

3.5.2 Water pressures trapped beneath a trench excavation Dr W. H. Ward (1957) reported some construction difficulties encountered by a contractor excavating a pipeline trench near Southampton. The trench excavation was made through an unconfined aquifer of sandy gravel over-lying the clays of the Bracklesham Beds. The contractor dealt with the water in the sandy gravel by using steel sheet-piling to form a cut-off on either side of the trench and exclude the groundwater. The clay in the base of the exca-vation was not yielding water, so dewatering measures were not adopted.

The trench was approximately 6.1 m deep, to allow placement of a 760 mm diameter pipe which was laid on a 150 mm thick concrete slab in the

base of the excavation. The construction difficulties encountered consisted of uplift of the bottom of the trench (called ‘base heave’, see Section 4.3), often occurring overnight whilst the trench was open. At one location the trench formation rose by almost 150 mm before the concrete slab was cast, and a further 50 mm after casting.

When Dr Ward and his colleagues at the Building Research Station were consulted, they suggested that the problem might be due to a high ground-water pressure in a ground-water-bearing stratum below the base of the trench. This was proved to be the case when a small borehole was drilled in the base of the trench. This borehole overflowed into the trench with the flowing water bringing fine sand with it. The water pressure in the borehole was later deter-mined to be at least 1.3 m above trench formation level, but it is likely that the original piezometric level was even higher, since the flowing discharge from the borehole may have reduced pressures somewhat. Once the problem had been identified, the contractor was able to complete the works satisfac-torily by installing a system of gravel-filled relief wells (see Section 11.3) in the base of the trench to bleed off the excess groundwater pressures (Fig. 3.17).

This case history illustrates the importance of identifying the small-scale geological structure around an excavation. It appears that in this case the

Figure 3.17 Use of simple relief wells to maintain base stability (after Ward 1957).

trench was excavated through the upper aquifer (the sandy gravel), which was dealt with using a sheet-pile cut-off, and the base of the trench was dug into low permeability clay, which is effectively an aquiclude. Problems occurred because there was a separate confined aquifer beneath the aquiclude, which contained sufficient water pressure to lift the trench for-mation. Once identified and understood, the problem was solved easily using relief wells. However, because the problem was not identified in the site investigation before work started, time and money was wasted in changing the temporary works, as well as making good the damaged pipelines. A classic failing of site investigations for groundwater lowering projects is that the boreholes are not taken deep enough to identify any con-fined aquifers which may exist beneath the proposed excavations.

Guidelines on suitable depths for boreholes are given in Section 6.4.