AQUIFERS AND GEOLOGICAL STRUCTURE
The aquifer types described in the foregoing sections are a theoretical ideal. At many sites, more than one aquifer may be present (perhaps separated by aquicludes or aquitards), or the aquifers may be of finite extent and be influenced by their boundaries. The particular construction problems resulting from the presence of aquifers at a site will be strongly influenced by the stratification and geological structure of the soil or rock.
This section will illustrate the importance of an appreciation of geological structure to the execution of groundwater lowering works. Two case histories will be presented: the London basin shows how large-scale geological structure can allow multiple aquifer systems to exist; and a problem of base heave in a small trench excavation illustrates the importance of smaller-scale geological details.
Multiple Aquifers beneath London
The city of London is founded on river gravels and alluvial deposits associated with the River Thames, which are underlain by the very low-permeability London Clay Formation. These gravels, known as River Terrace Deposits, form a shallow (generally less than 10 m thick) water-bearing layer, termed the Upper 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 two other, deeper, aquifers to exist below the city, largely isolated from the Upper Aquifer.
Beneath the London Clay Formation lie a series of sands and clays principally comprising the Lambeth Group stratum (formerly known as the Woolwich and Reading Beds) and the Thanet Sand Formation. These are underlain by the Chalk, a fractured 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’.
It has long been known that the Chalk, Thanet Sand Formation and parts of the Lambeth Group together form an aquifer - the Lower Aquifer (Woods et al., 2004). The upper 60-100 m of the Chalk is probably the dominant part of the aquifer, where significant fracture networks in the rock readily yield water to wells. The sands above the Chalk are of moderate permeability and generally do not yield as much water as the Chalk. The overlying London Clay Formation acts as an aquiclude or confining bed of very low vertical permeability, effectively separating the Lower Aquifer from the Upper 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 (Figure 3.16). Rain falling on central London may ultimately reach the gravel of the Upper Aquifer, but the London Clay Formation prevents it from percolating down to the Chalk. The Chalk of the Lower Aquifer 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.
Figure 3.16 Chalk aquifer beneath London. (After Sumbler, M G, British Regional Geology: London and the Thames Valley, 4th edition, HMSO, London, 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 existed in many parts of the city.
Before London developed as a city, the natural rates of recharge and discharge meant that the Lower Aquifer had sufficient water pressure for it to act as a confined aquifer (see Section 3.4.2). 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 Formation 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 Lower Aquifer (where the water quality was more ‘wholesome’ than in the gravels of the Upper Aquifer). Rates of groundwater pumping increased during the eighteenth, nineteenth and early twentieth centuries. This resulted in a significant decline in the piezometric level of the Lower 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 flowing artesian conditions. In some locations, the water pressure was reduced below the base of the London Clay Formation, so the formerly confined aquifer became unconfined. The deeper water levels increased pumping costs and made well supplies less cost-effective compared with mains water. This, together with a general re-location of large water-using industries away from central London, has resulted in a significant reduction in groundwater abstraction. As a result, the piezometric level in the Lower Aquifer has recovered since then (at more than 1 m/year at some locations in the 1980s) - see Figure 25.1. By the 1990s, the piezometric level in many areas was within 55 m of ground level.
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 in the Lower Aquifer were at an all-time low. Several studies (see for example 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 ground- water specialists during the first half of the twenty-first century. The use of permanent dewatering systems as part of the strategy to deal with rising groundwater levels is discussed in Chapter 20.
The pore water pressure profile beneath London is complex because of the presence of multiple aquifers and aquitards, with the low vertical permeability of the aquitards causing quite large variations in pore water pressure over short vertical distances (Figure 3.17).
In the middle of the twentieth century, when groundwater control works were planned, the groundwater system beneath London was considered to essentially be the Upper Aquifer (the River Terrace Deposits) and the Lower Aquifer (the Chalk and associated sands), separated by the aquitard/aquiclude of the London Clay Formation. Little water was expected in the zone (many tens of metres thick) between the base of the Upper Aquifer and the top of the Lower Aquifer. But in the second half of the twentieth century and at the start of the twenty- first century, tunnelling work for projects, including the London Underground Jubilee Line Extension and the Elizabeth Line (Crossrail), encountered groundwater problems in sandy and silty beds in the Lambeth Group between the Upper and Lower Aquifers. This is now considered to be a third aquifer beneath some parts of London - the Intermediate Aquifer. The presence of these permeable zones can cause problems during shaft sinking and the
Figure 3.17 Schematic of pore water pressure distribution in multiple aquifer system below London. Historic pumping from the Lower Aquifer has lowered the piezometric level (and hence reduced pore water pressures) below original hydrostatic conditions. However, the low vertical permeability of clay beds within the Intermediate Aquifer means that pore water pressures at that level remain high relative to the Lower Aquifer.
construction of tunnel cross passages, requiring the use of wellpoint or ejector dewatering from within the tunnel (Preene and Roberts, 2002; Roberts, Smith, et al., 2015). Further details on groundwater control for cross passages can be found in Section 10.6.
In a construction context, an appreciation of the aquifer system is vital to ensure that deep structures are provided with suitable temporary works dewatering. Figure 3.18 shows a typical arrangement that might be used for a deep shaft structure in central London (in an area where the Intermediate Aquifer is absent). 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 (see Chapter 18), penetrating to the London Clay Formation, to exclude the shallow groundwater. This is possible because the London Clay Formation is at relatively shallow depth. If clay were present only at greater depth, any cut-off would need to be deeper, and it might be more economic to dewater the Upper Aquifer.
- (iii) Wells are used to pump from the Lower Aquifer to reduce 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 Formation, wells are often designed to be screened in the Chalk only and are sealed from the sands using
Figure 3.18 Groundwater control in multiple aquifers.
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 16.7). 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 Formation were dewatered without pumping directly from the sand but by pumping purely from the underlying Chalk. This might seem a rather contradictory 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 dewatered; the upper poorly draining layer will drain down into the more permeable layer (see Section 13.6.2).
Water Pressures Trapped beneath a Trench Excavation
Dr W H Ward reported some construction difficulties encountered by a contractor excavating a pipeline trench near Southampton (Ward, 1957). The trench excavation was made through an unconfined aquifer of sandy gravel overlying 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 excavation 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 7.5.1), often occurring overnight while 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 groundwater pressure in a 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 determined 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 satisfactorily by installing a system of gravel-filled relief wells (see Section 17.8) in the base of the trench to bleed off the excess groundwater pressures (Figure 3.19).
This case history illustrates the importance of identifying the small-scale geological structure around an excavation. It appears that in this case the trench was excavated through an upper aquifer (the sandy gravel), which was dealt with using a sheet-pile cutoff, and the base of the trench was dug into low-permeability clay, which is effectively an aquiclude. The problems occurred because there was a separate confined aquifer beneath the aquiclude, which contained sufficient water pressure to lift the trench formation. 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 were 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 confined aquifers that may exist beneath the proposed excavations. Guidelines on suitable depths for boreholes are given in Section 11.9.
Figure 3.19 Use of simple relief wells to maintain base stability. (AfterWard, W H, Geotechnique, 7, 134-139, 1957.)