Abstract
An area around Horndean in SE Hampshire, England, has been studied to determine the reasons for an unusually high density of sinkholes related to dissolution of the underlying chalk. Ground investigations and excavations for a variety of engineering and construction projects in this area have provided detailed information that gives an insight into their formation and development. Contouring of the chalk surface indicates the presence of a north–south-trending palaeovalley, beneath Tertiary cover deposits, to the south of Horndean. The significance of this feature to chalk karstification and continuing sinkhole development is discussed, along with the significance of other geological factors, such as fracture zones in the chalk and the nature and distribution of cover deposits. This forms the basis of a risk assessment of ground subsidence in this area, with particular reference to collapse. The engineering significance of the sinkholes and the design of ground investigations are discussed in the light of experience gained from past projects in this area and the established risk of pollution of the chalk aquifer
The occurrence of ground subsidence related to dissolution of chalk has become increasingly significant in England over the last 20 years, with progressive urbanization and development of associated infrastructure. This is particularly the case in SE England, where there is considerable planned and current development, and where the Cretaceous Chalk Formation is more common in outcrop, or beneath a thin cover of residual soils or post-Cretaceous sediments, than in other parts of the country. Figure 1 shows the outcrop of the Chalk in southern England and the distribution of associated solution features, based upon a database developed by Edmonds (1983, 2001b), Edmonds et al. (1987) and Edmonds & Kirkwood (1990).
Density of solution features in chalkland areas of southern England.
Karst-related circular depressions in the ground surface, usually called dolines by geomorphologists, are commonplace in these areas, particularly in the vicinity of the Chalk–Palaeocene boundary. The database indicates that 97% of these features occur where there is a ‘cover deposit’ over the chalk, and that they usually occur within 200 m of a ‘cover deposit’ margin. Where dolines are known, or suspected, to have been formed by ground subsidence it is more appropriate to call them sinkholes, in accordance with international practice (Waltham et al. 2005). However, sinkholes in chalk areas that have surface water inflow are often called swallow-holes, because of their hydrogeological significance in terms of groundwater recharge and potential pollution of the chalk aquifer (Banks et al. 1995).
In England, there are a variety of man-made voids that can constitute a subsidence risk to buildings and engineering works in chalk areas (Edmonds 1988) but, because the chalk usually has a high fracture density and low strength, and is very susceptibile to weathering processes, discrete natural voids are usually much smaller than in other limestones. A number of natural cave systems have been recorded within chalk in the UK, the most extensive being at Beachy Head in East Sussex, where 354 m of accessible passages were observed, but these were generally less than 1 m in diameter (Bradshaw et al. 1991). However, dissolution of bedding plane discontinuities, tectonic joints, fault planes and fissures can result in a zone of increased void space within the chalk mass. In southern England, this mainly occurs within the upper few tens of metres of the Upper Chalk division of the Chalk Formation (Owen & Robinson 1978). Soil suffusion into this void space can lead to void formation within the overlying cover deposits and the subsequent formation and development of sinkholes by progressive settlement or collapse (McDowell 1989; McDowell & Poulsom 1996; Edmonds 2001a). Examples of ground subsidence by sudden collapse have been recorded in Dorset (Sperling et al. 1977), in West Sussex (McDowell & Poulsom 1996), and in Hertfordshire (Banks et al. 1995), generally producing steep-sided, circular, surface depressions varying in depth and diameter between 1.0 and 10 m.
In SE Hampshire there have been anecdotal accounts of small-scale ground collapse in chalk areas but very few published examples of karst-related ground subsidence. This is surprising because the density of sinkholes in this area is significantly higher than the average of 15 per 100 km2 for the South Downs chalk escarpment (Edmonds 2001b), and an anomalously high density of 55 per km2 has been recorded between Horndean and Rowlands Castle (Fig. 1). Most interest in these surface depressions has been due to their potential to act as swallow-holes, particularly since dye tracer tests carried out by Atkinson & Smith (1974) proved a direct hydrological link between some of them and the very important underlying Upper Chalk aquifer. However, in more recent years, there have been several schemes that have also required an evaluation of the ground subsidence risk associated with these features. These include the A3(M) trunk road and service roads around the Hazelton Interchange, the supermarket and car parks at Dell Piece West, the Hazelton housing estate, a large diameter storm water pipe and the proposed, but rejected, landfill site at Hazelton Farm (Fig. 2). Ground investigations for these schemes and, in some cases, excavations prior to construction, have provided detailed information that not only helps to explain the high density of sinkholes in this area but also allows further insight into their formation and development. This, in turn, assists the assessment of ground subsidence risk and the evaluation of ground investigation procedures and techniques in such areas.
Location of engineering and development schemes in the Horndean area, SE Hampshire.
Landscape and geology of SE Hampshire
The area of interest in SE Hampshire, is c. 16 km to the north of Portsmouth, in the vicinity of Horndean (Fig. 1). To the north and east of Horndean the topography is characteristic of the southern part of the dip-slope of the South Downs chalk escarpment, with a southerly slope dissected by north–south-trending dry chalk valleys (Fig. 3). Some dolines occur on these chalk slopes, or within the dry valleys, but the density is significantly greater to the south of Horndean where the ground surface is lower and gently undulating, and the chalk is covered by drift deposits and/or sediments ofthe Lambeth Formation (formerly known in this area as the Reading Beds; Table 1). Figure 4 shows a large doline at Dell Piece West (Fig. 2) prior to infilling and the construction of a supermarket car-park. Ground investigations at this site revealed an average thickness of c. 10 m of cover deposits.
Topographic map of the Horndean area.
. Description of the soils and stratigraphic units around Horndean
Sinkhole at supermarket site, Dell Piece West, Horndean.
Most of these dolines are known or suspected to be sinkholes and their distribution, based upon the Natural Cavity Archive held by Peter Brett Associates, is shown in Figure 5. This map clearly shows that a large majority of the sinkholes are associated with some form of cover deposit over the Chalk and usually occur within the outcrop of the Lambeth Formation. A linear set of sinkholes follow the Lambeth Formation–Upper Chalk boundary between Horndean and Rowlands Castle, but a large number have also been recorded further away from the feather- edge, where the cover is much thicker. The map also shows the broad V-shaped extension of the Lambeth Formation outcrop to the north, which can only be partly attributed to topography.
Geology and sinkhole distribution in the Horndean area.
The relatively high density of boreholes in this area, and detailed mapping of the Lambeth Formation–Upper Chalk boundary using trial pits and geophysical investigations, has allowed contouring of the Chalk surface and shows the thickness variation of the Lambeth Formation and other cover deposits within this area of high sinkhole density (Fig. 6). For reference, the outcrop of the Upper Chalk–Lambeth Formation interface is indicated on each contour plot. The plots indicate the following significant points.
Contour plots of the ground surface, the chalk surface and thickness variations of the Lambeth Formation in the Horndean area.
(i) The ground surface contours show a broad ridge aligned in a north–south direction, with a relief of up to 10 m across the width of the plot.
(ii) The depth to Chalk and the thickness of the Lambeth Formation correlate well, indicating that drift deposits, topsoil and made ground are only thin layers in this area.
(iii) The Chalk surface elevation contours suggest a north–south-trending buried valley, which widens to the south and has a relief of up to 8 m.
Data points clearly associated with sinkholes were removed from the data set prior to analysis, and, although there may still be some distortion of the contours because of localized chalk dissolution, the results are indicative of the existence of a buried, broad, shallow palaeovalley within the Chalk.
The ground investigations and excavations in this area also provided detailed descriptions of soils and hydrogeological information. A description of the deposits overlying the Chalk around Horndean is shown in Table 1. Generally the deposits are argillaceous, particularly within the upper part of the Lambeth Formation (Reading Formation) and the residual clay-with-flints at the chalk surface. However, sand and silt horizons have been recorded within the Reading Formation, which appear from all of the available evidence to be mainly in the form of channel deposits. Also, the sediments within the Upnor Formation, between the Reading Formation and the Chalk, are predominantly sands, clayey sands and gravels. These more arenaceous horizons improve the drainage of surface water down to the chalk, particularly in those areas where clay-with-flints is not present at the chalk surface. Water can then move readily through the weathered chalk, via open dissolution widened pipes, joints and fissures, to the water table, which is typically at a depth of between 20 and 30 m in this area.
The tectonic structures of this area can be broadly divided into three groups, with different trends. First, deep-seated east–west-trending basement faults, originating from tectonic activity between the early Oligocene and early Miocene epochs (Bevan & Hancock 1986), are expressed at the surface as east–west folds. These folds are considered to be Cretaceous and Tertiary strata draped over basement downthrows (Melville & Freshney 1982). Second, a series of NW–SE-trending fault zones stretch from southern England, across the English Channel, into northern France. These features were formed after the east–west structures (Bevan & Hancock 1986). Third, a set of predominantly NE–SW-trending fractures is indicated by a series of linear, deep, dry valleys in the Chalk of the South Downs, identified by the interpretation of Landsat imagery (Greenbaum et al. 1993). As a result of this tectonic deformation, the fracture system within the Upper Chalk in the Horndean area is likely to be heterogeneous, with pervasive joints within fault-bounded blocks (Bloomfield 1996).
Major fissures have also been observed within the Reading Formation during excavations of cuttings for the A3(M) road near the Hazelton Interchange. They were mainly slickensided and may, at least in part, be the result of differential settlement of clay associated with ground subsidence (Edmonds 1997).
Chalk dissolution
The initial karstification of the chalk in this area was probably during the late Cretaceous and early Palaeocene (i.e. from 70 to 60 Ma), when the Chalk was first exposed to subaerial erosion and weathering. A considerable thickness of chalk, possibly as much as 100 m, could have been removed by aggressive dissolution related to the tropical conditions prevailing during that period (Jones 1981; Bevan & Hancock 1986; Gibbard & Lewin 2003). High rainfall and soil organic activity would have more than compensated for the effect of reduced solubility of carbon dioxide in water at higher temperatures (Sowers 1996). The most active dissolution would have occurred at the intersection of vertical joint sets and along fracture zones, where water ingress into and through the chalk was more localized, leading to the formation of open pipes and fissures (West & Dumbleton1972). These features can be observed at Alum Bay and Whitecliffe Bay in the Isle of Wight, where the unconformity between the Upper Chalk and the overlying Lambeth Formation is exposed. At these locations the steeply dipping Chalk surface is very irregular and has solution pipes filled with grey–brown argillaceous sand and flint cobbles. The irregularity of the Chalk surface in the Horndean area, as at Hazelton Interchange (Fig. 7), is thought to be partly the result of dissolution during this period.
Section through the sinkholes at Hazelton Interchange (A3M, Junction 2).
The presence of the north–south-trending palaeovalley in the Chalk surface to the south of Horndean (Fig. 6), may have concentrated drainage and increased localized dissolution in this area, as water circulation has a significant impact on dissolution (Drew 1985; Smart & Whitaker1991). The zones of relatively high permeability recorded for some present-day dry valleys in chalk areas (Younger1989), and for the chalk in the Horndean area (Atkinson & Smith1974) appear to support this model.
The depositional environment of the Lambeth Formation is of particular interest and suggests that conditions were suitable for the focusing of drainage and dissolution, especially within the palaeovalley. Gibbard & Lewin (2003) collated the research of Hester (1965), Buurman (1980), Daley (1999) and Anderton (2000) to produce the following model of deposition within the Hampshire Basin.
(i) Sediments were deposited during a period of marine regression.
(ii) Sediments were deposited by rivers draining from the NW.
(iii) Coarse-grained sediments were deposited in meandering channels within fine-grained lower energy floodplain and channel plug deposits.
(iv) Clays were mottled as a result of subaerial weathering.
(v) Sediments were deposited under a warm, wet climate with distinct rainy seasons.
(vi) Diverse flora is indicative of a dense floodplain swamp forest.
(vii) Drainage paths extended their courses seawards to follow the regressing sea.
In summary, during the deposition of the Lambeth Formation (60–56 Ma), the environmental conditions provided large quantities of runoff, dense vegetation producing CO2-rich water, and a significant hydraulic gradient during the periods when the shoreline was regressing. In contrast, the subsequent stage of Thames Group deposition is thought to have occurred during a period of significant marine transgression, which would have reduced the hydraulic gradient, and the deposition of predominantly argillaceous sediments in this area during the London Clay stage would have restricted water ingress to the chalk.
Another possibly significant period of chalk dissolution in this area was during the Pleistocene Ice Age (i.e. within most of the last 2.5 Ma). It has been suggested by Higginbottom & Fookes (1971), and others, that solution features in the Chalk were initiated, developed and reactivated during this period as a result of the abundance of meltwater, which, because of its low temperature, had an enhanced capability of dissolving CO2. In the Horndean area, most of the Tertiary sediments had, by this time, been removed by erosion, which exposed the Chalk to the north of Horndean and left only cover deposits of the Lambeth Formation above the Chalk in the Hazelton area to the south of Horndean. Also, by this stage, the major fracture sets and linear fracture zones within the Chalk had been fully developed, which increased its permeability, except when it was affected by permafrost during the Pleistocene cold phases. During interglacial stages, large volumes of frozen surface water, and groundwater held within the Lambeth Formation, could have been released by melting and found its way down to the Chalk, primarily through the basal arenaceous layer of the Lambeth Formation, locally dipping into the palaeovalley. It is also possible, although not directly confirmed in this area, that pingos of frost-shattered chalk, or frozen water-bearing sand, may have penetrated the clay horizons and, upon melting, provided drainage routes between the ground surface and the Chalk. A further related suggestion is that ice-wedging has contributed to the widening of fissures within the weathered zone of the Chalk (Rhodes & Marychurch 1998). Where chalk was exposed, existing dissolution features were subdued and removed by a general surface lowering by solution weathering and erosion, resulting in most remaining features within the Chalk outcrop being situated close to the feather-edge, where these processes have been relatively short-lived.
Sinkhole formation and distribution
Collapse sinkholes are rare in chalk, except over old chalk mines, because the rock mass strength is generally low and unable to sustain large cave passages. The sudden collapse that created the Bime des Enfants in France, a 16 m deep and 10 m diameter sinkhole, in an area of fractured chalk and on a gently sloping hillside, is a very unusual event. Waltham et al. (2005) stated that although no cave was observed around the edge of the mound of fallen debris, the immediate area is distinguished by having some long stream caves with passages generally 2 m high and 1 m wide, which are large for chalk in Europe. As the exposed walls of the sinkhole are described as putty-chalk and rubble-chalk, and the failure occurred during a cold wet winter, it is conjectured that roof stoping above a cave passage led to the formation of the large void required for a catastrophic collapse of this size. Events of this type are even less likely to occur in the Chalk of southern England, given that the most extensive recorded natural cave system is the 354 m length of accessible passage at Beachy Head, which is generally less than 1 m in diameter (Bradshaw et al. 1991).
Detailed investigation of several sites of ground subsidence in chalk areas of southern England has clearly shown that the washing of sands down into open fractures in underlying chalk has led to void formation within the cover deposits and caused sinkhole development (McDowell & Poulsom 1996). Voids formed within the basal arenaceous layer increase in size until the roof collapses to produce either a sinkhole or an extended column of loosened cover deposits. Where the sand is overlain by cohesive soils, slabbing of clay from the void roof (Sowers 1996) can cause the void to migrate upwards until a plug-collapse occurs. These processes may explain the formation of sinkholes close to the outcrop of the Lambeth Formation–Chalk boundary in the Horndean area, where Head Gravels and sandy soils of the Lambeth Formation form a major part of the cover deposits. Detailed ground investigations carried out by the University of Portsmouth at Pyle Farm (Fig. 2), which included geomorphological mapping, electrical and electromagnetic surveying, and drilling, clearly indicated that the majority of the sinkholes are on the outcrop of the Lambeth Formation. It appears from this evidence that the position of the boundary of the Chalk outcrop, as shown on geological maps (e.g. Geological Survey Sheet 316) between Horndean and Rowlands Castle, can be in error by up to 100 m. Presumably, as the sinkholes are called swallow-holes on these maps, it was originally assumed that these features were formed by dissolution of chalk at outcrop by surface water runoff from the Lambeth Formation and not by ground subsidence. Some of the sinkholes do have streams flowing into them from the Lambeth Formation, and incorporate swallow-holes, but most are circular hollows without stream entry. The concentration of sinkholes on a linear trend between Horndean and Rowlands Castle (Fig. 5) may be due to a linear fracture zone in the Chalk at this location. Support for this suggestion is provided by the existence of NW–SE-trending fault zones in this area (Bevan & Hancock 1986) and the results of tracer tests between sinkholes at Horndean and Rowlands Castle and the Bedhampton Springs, which indicated rapid open fissure flow through the Chalk (Atkinson & Smith1974; Barton et al.1996).
Localized linear trends of sinkholes were also observed along nearby minor valleys where the thickness of cover deposits has been reduced by erosion; for example, at Dell Piece West (Fig. 2), where the sinkholes were located along the western flank of a wide, shallow valley with a NNE–SSW trend (Fig. 8). These features have now been backfilled with clay excavated from an adjacent industrial estate, but many sinkholes are still to be seen further to the south, along the same valley (Fig. 5). Sinkholes along WNW–ESE and WSW–ENE linear trends (Fig. 5) were developed within narrow, steep-sided valleys, with drainage leading into swallow-holes with direct links to the underlying chalk. In the Hazelton Farm proposed landfill area (Fig. 2), there are two small valleys with a NE–SW trend, which may partially explain the high density of recorded sinkholes in that vicinity (Fig. 5).
Sinkholes along side of valley, Dell Piece West, Horndean.
Where the cover deposits are thicker, and arenaceous sediments constitute a smaller proportion of the cover deposits, the formation of the sinkholes by the soil suffusion mechanism is more difficult to explain. At Hazelton Interchange (Junction 2 on the A3(M) trunk road), where the Lambeth Formation is c. 12 m thick, extensive drilling has allowed a detailed cross-section (Fig. 7) to be drawn through the two surface depressions, which were mistakenly interpreted as clay pits on Ordnance Survey maps. The irregular chalk surface clearly indicates that these are karst features, and the disturbed ground and small voids within the cover deposits suggest that the sinkhole formation process is still active. The relatively high permeability of the soils within the sinkhole was proved by the tracer tests carried out from this location (Atkinson & Smith 1974). During construction of the road interchange, excavation exposed a layer of water-bearing sand beneath clay and above chalk (Fig. 9). As this sand horizon was not encountered in any of the boreholes within the confines of the sinkholes, it is possible that sand was washed down into open fissures in the underlying chalk, leaving a void beneath the overlying clays, which eventually resulted in ground subsidence. In this case, groundwater probably flowed laterally to this location through the basal sand layer from its outcrop at the sides of the buried valley, rather than through conjectured vertical links with sand horizons and lenses higher up in the cover deposits. However, ground investigations at Dell Piece West, by Listers Geotechnical Consultants and McDowell (1981), indicate that water-bearing east–west-trending channels of sand within the Reading Formation, formed by tributary streams of the palaeovalley, focus the drainage and may have had a role in the formation of both this sinkhole and others in this area.
Excavated face at Hazelton Interchange, Horndean.
Further explanations can be provided for the more random distribution of sinkholes where there are thicker cover deposits. Some boreholes have found residual clay-with-flints between the basal sands and the chalk, whereas, at other locations, the basal sands are absent. Periglacial disturbance, such as ice-wedging, may have initiated some localized palaeokarst features in the chalk, such as open or sand-filled pipes and fissures, which could influence continuing sinkhole development in both linear and random patterns (Rhodes & Marychurch 1998).
Engineering significance
The engineering risk of ground subsidence associated with sinkholes, in terms of the type, scale and rate of subsidence, needs to be evaluated for most building and engineering projects so that the appropriate remedial measures or specialist engineering design can be undertaken. The types of subsidence associated with chalk dissolution terrains were considered by Matthews et al. (2000) and can be summarized as follows: (i) differential settlement as a result of variations in the thickness and density of cover deposits; (ii) large settlements as a result of loose and metastable materials overlying solution pipes; (iii) collapse associated with voids within the Chalk or cover deposits.
It is recommended that these types of subsidence be considered for all sites in chalk areas with a high density of sinkholes, even where there is no surface depression. This is illustrated by Figure 10, which is a schematic diagram of the wide variety of natural solution features that can form in chalk areas, most being incipient rather than actual sinkholes. Two examples, 6 and 9, depict large voids that potentially could lead to sudden ground collapse, and other examples (e.g. 5 and 7) show metastable ground over solution pipes, which can result in ground settlement and smaller-scale collapse.
Schematic diagram of chalkland natural solution features.
Example 9 illustrates the problem of a solution cavity within the Chalk whose roof has become destabilized causing collapse of cover deposits into the cavity, leading to the formation of a collapse sinkhole at the ground surface. A possible case in point is the sudden collapse event at Darnicle Hill in Hertfordshire (Banks et al. 1995), which produced a steep-sided sinkhole 9 m wide and 9 m deep within a 14 m thick sequence of Tertiary deposits over chalk. Although the reason for the onset of the Darnicle Hill event has not been established, it may have been formed by this process, as void space in excess of 600 m3 would be required to accommodate the full volume of collapsed sediment following bulking.
Example 6 illustrates the formation of a void within the cover deposits above a solution pipe in chalk, which is depicted as having migrated upwards until arrested by a stronger layer; hence it has not, as yet, produced a sinkhole. Such a void can be formed by the soil suffusion mechanism; that is, where sands and silts are washed down into open pipes and fissures into the chalk (McDowell & Poulsom 1996). In the Horndean area, the authors consider that a void several metres in diameter could potentially develop within the basal sand horizon beneath the clays of the Reading Formation. Whether or not catastrophic subsidence would occur through plug failure would depend upon the lithology and geotechnical properties of the Reading Formation at that site. The presence of sand and silt lenses within the Reading Formation reduces the overall shear strength of the formation, both directly and by the introduction of water, and slickensided microfissures in the clays, produced by periglacial disturbance or subsidence, also reduce the shear strength. Close to the feather-edge of the Lambeth Formation, the basal sand layer will have a thinner covering of clay and/or Head Gravels, which is very similar to the ground conditions at Fontwell in West Sussex, where a cluster of over 70 dropout sinkholes suddenly occurred following the burst of a main water pipe (McDowell & Poulsom 1996). Other factors, such as surcharge, ground vibration, groundwater lowering and localized increased water ingress associated with engineering works, are all potential triggers for this type of failure, as in the environment depicted in example 9.
Example 7 depicts a solution pipe beneath cover deposits where upward migration of metastable ground conditions has resulted in ground subsidence forming a sinkhole at the ground surface. In a closed system, where drainage to the water table in the Chalk is impeded (e.g. by clay in the solution pipe), subsidence will progress slowly by settlement and collapse of minor voids within the soil column. This process will be more rapid in an open system, where there is a direct hydraulic connection between the ground surface and the water table zone in the Chalk. In such cases, the associated deepening of a sinkhole may be offset by the infilling of the depression with in-washed soils or from slippage of the side slopes. Hazelton Interchange (Fig. 7) is a good example of an open system with this type of subsidence. Once the mechanism had been identified at this location, appropriate remedial works were designed that included removal of the loose soil, construction of a concrete capping to the pipe at the chalk surface, and backfilling of the depression with compacted clay.
Example 5 depicts a situation where there is a column of disturbed ground above a solution pipe but there is no surface depression. The soil column of disturbed ground is characterized by high porosity, high permeability, low density and low strength. In such cases, localized concentrated water ingress can result in large settlement and structural damage. This again is more likely to develop where the cover deposits are more arenaceous, such as close to the feather-edge of the Lambeth Formation and the Upper Chalk. An example of this type of subsidence is provided in a case study by Edmonds (Waltham et al. 2005), which describes and explains structural damage to a house at Chalfont St. Peter, Buckinghamshire, UK, following the emptying of a swimming pool onto the ground adjacent to the house. A ground investigation, incorporating a borehole and extensive dynamic probing, identified a metastable soil pipe, with an elliptical cross-section of 10 m × 6 m, within a 10 m thick sequence of sands and gravels over chalk. A similar situation occurred in Chichester, West Sussex, where Quaternary and Tertiary sediments overlie the Upper Chalk (McDowell & Poulsom 1996). However, in this case a wide, shallow surface depression was produced, causing damage to houses, gas and water pipes, as well as the highway.
The potential for pollution of groundwater also needs to be considered for engineering schemes and housing developments in chalk areas with sinkholes. In SE Hampshire, many of the sinkholes act as swallow-holes and provide a direct hydraulic link between the ground surface and the water table in the Upper Chalk aquifer. Tracer tests have established that there is a rapid hydraulic connection between several of the sinkholes in the Horndean and Rowlands Castle area and an important group of springs in the Bedhampton–Havant area, which are the main source of water supply for some 600 000 people in and around Portsmouth. (Atkinson & Smith 1974; Barton et al. 1996). The maximum velocity of water flow between the swallow-holes tested and the springs, assuming a direct flow path, varied between 0.6 and 12.0 km day−1, which is in accord with the values recorded in other parts of SE England (Banks et al. 1995). Special drainage measures were required to isolate the A3(M) trunk road, and developments at Dell Piece West, from the underlying Chalk aquifer.
Discussion
The unusually high density of sinkholes in SE Hampshire appears to have been caused by the combination of several geological and topographic factors. Sinkholes tend to be concentrated along the feather-edge of the Lambeth Formation and the Upper Chalk (Edmonds 2001b) because of both (i) the lithology of the Lambeth Formation in this area and its effect upon drainage and acidity of water moving down through the cover deposits to the Chalk, and (ii) ground subsidence mechanisms. As the feather-edge receded southwards because of erosion, the zone of increased sinkhole development moved accordingly. Erosion and weathering subsequently removed or degraded sinkholes in the areas of chalk outcrop to the north. The other major factors are the postulated NW–SE-trending fractures in the Upper Chalk and a north–south-trending palaeovalley in the chalk surface. Figure 11 is a simplified model to indicate where these factors combine to create a zone of increased sinkhole activity in the Hazelton area to the south of Horndean.
Geological factors producing zone of high sinkhole density.
The factors that appear to be particularly significant in terms of sinkhole formation in this area are the presence of a palaeovalley and the nature and distribution of arenaceous units of the Lambeth Formation within the valley. The basal layer of sand of the Lambeth Formation and arenaceous channel deposits within the Reading Formation focus drainage of water down to the Chalk within the palaeovalley. As arenaceous units are also an important component of the soil-suffusion subsidence mechanism, it is suggested that their presence increases the risk of sinkhole development and collapse. They are, therefore, considered to be particularly important targets for ground investigations in this area, and also in other areas where similar ground conditions are anticipated from the desk study.
Although sand layers with a large areal extent can be readily located by boreholes, a combination of geophysical surveying and boreholes has been found to be a more cost-effective approach for locating narrow, linear sand units. Electrical resistivity and electromagnetic traversing, which are both rapid, low-cost methods (McDowell et al.2002) indicated the presence of a sand channel feature within the Reading Formation at Dell Piece West, prior to construction of the A3(M) trunk road (McDowell 1981). A borehole at the site of the geophysical anomaly proved a sand horizon, with water under sub-artesian pressure, within otherwise predominantly argillaceous sediments. Electrical resistivity imaging is also appropriate for ground investigations of chalk areas with dissolution features (Rigby-Jones et al. 1997), and was used to locate a large sand channel feature at the proposed Hazelton Farm landfill site (McDowell 1996).
Surface topography and drainage involved in the formation of the sinkholes can easily be obscured by engineering works, such as the dumping of soils from the A3(M) trunk road cuttings at Hazelton Farm. This aspect needs to be carefully appraised by the use of ancient maps, aerial photographs, historical records and local knowledge. Such information, along with an evaluation of subsurface drainage potential by a desk study and site investigation, can then be used in a numerical risk assessment for solution subsidence, such as the ones developed by Edmonds (1983, 2001b) and Edmonds et al. (1987).
Conclusions
An unusually high density of sinkholes in SE Hampshire, England, has been produced by a combination of cover deposits with arenaceous units within a buried palaeovalley in the Chalk surface, proximity to the feather-edge between the cover deposits and the Chalk, and localized karstification along a fracture zone in the Chalk. The buried valley and arenaceous units within the infill cover deposits are considered to be particularly important targets for ground investigations in this area, and also in other areas where similar ground conditions are anticipated from the desk study.
Although there is little recorded evidence of ground collapse at this location, this is considered to be a high-risk area for ground subsidence because of the many recorded examples of ground subsidence from locations with similar ground conditions in other parts of SE England.
This study also highlights the significance of surface water drainage for sinkhole development and the instigation of ground subsidence associated with these features. In chalk areas identified as high risk, it is important to carefully consider the location and construction of any drainage works to minimize the risk of ground subsidence and to avoid pollution of aquifers.
Acknowledgements
The authors would like to thank Peter Brett Associates for permitting access to their Natural Cavity Archive. They are also grateful to Hampshire County Council, The Environment Agency and Listers Geotechnical Consultants for ground investigation information for the A3(M), the proposed Hazelton Farm Landfill Site, and the Dell Piece West Development Site, respectively.
- © 2008 The Geological Society of London
References
The nature, formation and engineering significance of sinkholes related to dissolution of chalk in SE Hampshire, England
