Land subsidence on Magnesian Limestone terrain in County Durham, England

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1 Land Subsidence (Proceedings of the Fifth International Symposium on Land Subsidence, The Hague, October 1995). IAHS Publ. no. 234, Land subsidence on Magnesian Limestone terrain in County Durham, England M. R. GREEN Soil Mechanics Ltd, Glossop House, Hogwood Lane, Finchampstead, Berkshire, UK R. A. FORTH Department of Civil Engineering, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK D. BEAUMONT Durham County Council, UK Abstract The east of County Durham, England, is underlain by a complex group of Permian age Magnesian Limestones. Under certain circumstances these limestones may be prone to dissolution by a range of processes; the result often being subsidence and collapse of roadways and other engineered structures. It has become important at the site investigation stage to reveal the presence of solution voids and conduits so as to avoid the possibility of subsidence or collapse occurring during construction or the economic life of the structure. A review of the controls on the processes of solution has been carried out and the findings are summarized. A study of investigation and exploration techniques is presented, along with a review of current construction practice and methods applicable to solution feature remediation. Associated case histories from County Durham are given. A weighted factor hazard map, showing the risk of encountering solution features, has been produced for the benefit of engineers and planners dealing with projects lying in limestone areas, and its main features are summarized. INTRODUCTION County Durham is situated in the northeast of England and much of the east of the county is underlain by a complex group of Permian age Magnesian Limestones. These limestones can be prone to dissolution by a range of processes. The result of which can often be the subsidence and, in extreme cases, the collapse of roadways and other engineered structures. Remedial measures to rectify the effects of subsidence or collapse can be extremely costly and it is, therefore, desirable at the site investigation stage of a project to quantify the ground conditions on a site with as high a degree of confidence as possible, so that any likely problems can be taken into account and if necessary acted upon to minimize the possibility of subsidence or collapse occurring during the construction or economic life of the structure.

2 424 M. R. Green et al. CONTROLS ON THE SOLUTION OF LIMESTONE There are many different factors that affect the solution of limestone. The particular physical and chemical characteristics of a rock will affect the degree of weathering and erosional processes. These processes are many and varied but the most important factor that controls their effectiveness is the amount of surface area that exists in the rock which is penetrable by water. Thus the frequency and degree of jointing and bedding within a rock are very important in determining the amount of water that may enter the system facilitating dissolution and hydration. Discontinuities provide the major pathways for solutional and erosive waters, but deposition of metals and minerals often occurs in joints and fissures and this can effectively seal them, thereby reducing the overall permeability of the rock. The weatherability of a rock is increased with larger pore spaces within the fabric of the rock and thus the size of grains, their degree of interlocking and cementation between grains will all influence the weatherability. The rate of dissolution depends on the total amount of calcium carbonate held in solution by water passing through the limestone. This in tarn is dependent on the temperature and partial pressure of C0 2 with a low temperature and high pressure favourable for increased activity (Dickson, 1993). In northern England limestone is dissolved chiefly by the direct act of acid rainwater and snow and by the action of C0 2 and humic acids in association with a cover of soil and vegetation (Sweeting, 1966). The Magnesian Limestones of County Durham are on the whole a dolomite rich limestone. Dolomite rocks tend to be more porous than calcitic rocks as a 12% reduction in molecular volume is effected when calcite is replaced by dolomite. However the solubility of a rock may not be a major factor in its erodibility as its petrological properties may be more important. Other factors also affect the weatherability of the rocks. The age of a rock plays an important role as young unlithified rocks will erode differently due to shelly constituents with differing solubilities being present. Environmental controls such as the availability of C0 2 and contact time between solvent and solid are also very important. These factors are controlled by the type and depth of soil cover, slope angles, temperature and rainfall regimes. Figure 1 shows the key variables in the control of limestone solution processes (Trudgill, 1985). A factor that may be particularly important in areas prone to snowfall, such as Durham, is the C0 2 content of snow which is very high and therefore may increase the solution potential of limestone. Dissolution rates can be controlled by either the rates of transport of ions or by the rate of the surface reactions. The dissolution rate is accelerated by the flushing of the solution products as a drop in the concentration of dissolved solute is achieved. Other factors affect the solution processes to a lesser degree. Dissolution potential is increased when water leaches out acids from organic material such as leaf litter in woods. Solution erosion processes are more likely to operate at the top of slopes than at the foot of them due to mechanical slope processes producing alkaline lower slopes and acidic upper slopes. Altitude and aspect can affect biological processes. For example, soil C0 2 productivity is lower on north facing slopes and at higher altitudes. It can be seen that the prediction of degrees of limestone solution in an area is difficult as the situation is multivariate. Dissolution potential in combination with water flow rate and volume provide the chief control on the solution of limestone. High

3 Land subsidence on Magnesian Limestone terrain in County Durham, England 425 PRECIPITATION-EVAPORATION i EFFECTIVE RUNOFF PERMEABILITY OF/ SOIL AND ROCK \ j SOIL MOISTURE \k SOIL CARBON DIOXIDE f NL- WATER FLOW RATES A, SOIL i ACIDITY I.CHEMICAL TEMPERATURE \ BIOLOGICAL ACTIVITY ORGANIC ACIDS REMOVAL OF PRODUCTS IN SOLUTION i DISSOLUTION RATE AND SOLUTE LOAD REACTION \ MINERALOGY AND SURFACE AREA LITHOLOGY Fig. 1 Key variables in the control of limestone solution processes. volumes of runoff increase limestone dissolution rates as does high soil acidity and high C0 2 productivity in permeable soils. The balance between dissolution potential and waterflow controls the overall rate of erosion of the limestone. INVESTIGATION/EXPLORATION OF LIMESTONE SITES The most important step before undertaking investigations is to define the scope and nature of the proposed development. Limestone terrain is prone to solution features such as sinkholes and dolines and as such the investigation should aim to locate these features and define their profile and stability. The first stage in any investigation should be a review of all geological information, aerial photographs and historical information. At this stage a reconnaissance walk over survey of the area should also be carried out (Fischer et al., 1987). At Stage 2 of an investigation consideration of the planning alternatives that are available based on the information gained from Stage 1 should be addressed. Possible or probable development problems can be identified and the next stage of the investigation can be tailored to prove or disprove these. Stage 3 should employ both direct and indirect methods of exploration. Stage 4 is further detailed to increase the confidence of any previous findings and also to obtain any planning or design parameters. At all times during the investigation it is critical that the programme is flexible to take account of unforeseen ground conditions. The following parameters may need to be determined in an investigation: (a) depth, thickness and engineering properties of the soils and rock stratum; (b) groundwater levels, any fluctuation and direction of movement;

4 426 M. R. Green et al. (c) rock joint distribution; (d) nature and extent of defects. The initial site investigation should aim to delineate groundwater regimes and forecast any changes in these regimes that are likely to occur due to any construction works. All site investigation work should be followed up with laboratory testing to give an indication of the susceptibility of a formation to dissolution. Investigation techniques can be divided into direct (boring/penetration tests, etc.), indirect (remote sensing/geophysics) and statistical methods. In terms of cost per km 2 coverage, indirect and statistical methods are more effective than direct methods. Table 1 shows some of the many methods available for cavity detection. Table 1 Methods available for cavity detection. Direct methods Percussion drilling Diamond drilling Pumping tests Trial pits Percussion probe CPT Indirect methods Aerial photographs Thermal imagery Satellite imagery Multispectral scanning Microgravity Radar Electrical resistivity Electromagnetic Seismic reflection Spontaneous potential CONSTRUCTION AND REMEDIATION PRACTICE When building on ground that is prone to solutional features, several different approaches can be taken to minimize the effect of these features: (a) optimization of site location, (b) correction or mitigation of any defects that are present, (c) use of modified shallow foundations allowing for defects, (d) use of deep foundations to overcome defects, (e) minimization of future activation of defects. Often a degree of uncertainty exists as to the exact location of solution features so it is not uncommon to find more than one of the above design approaches being adopted to take account of any uncertainty that does exist. The approach or approaches,that are taken should correct or mitigate the existing solution features and minimize the activation of old features whilst halting the development of new features. There are numerous methods that are applied to either correct or treat ground that contains solution features, including correcting the hazards by filling or collapsing them, bridging over small hazards, reinforcing the rock, bypassing shallow hazards with deeper foundations and minimizing activation of the processes that form the hazards (Savers, 1984). Any found improvement measures that are adopted need to take into account three major factors:

Land subsidence on Magnesian Limestone terrain in County Durham, England 427 the aerial extent of the surface depression, depth of bedrock, the size of the cavity in the bedrock (Parate, 1984).

5 Land subsidence on Magnesian Limestone terrain in County Durham, England 427 the aerial extent of the surface depression, depth of bedrock, the size of the cavity in the bedrock (Parate, 1984). There are several methods that can be used to remediate solution features, including grouting, dental filling, high volume grouting, compaction grouting, dynamic compaction, vibrocompaction and vibroreplacement. Structures can be strengthened so they do not deflect if nearby cavities fail and existing cavities may be cleaned out and a series of engineered filters and fills used to stabilize them. Foundations may be engineered to bridge any cavities with beams, geogrids, reinforced earth or rock. When relocation of a construction is not feasible and shallow foundations are not suitable, deep piled foundations are often used to overcome the real surface defects in the underlying limestone. Future defect development can be minimized by preventing access of water by using cutoffs or diverting flows. Water can also be chemically altered to stop it being corrosive, although this is usually prohibitively expensive. County Durham is currently undergoing a large programme of redevelopment and this involves the construction of several new link roads to existing main routes. The nature of the Magnesian Limestone in certain areas may lead to instability of batter slopes and a loss of integrity in the carriageway (Buist & Ineson, 1992). In County Durham the Magnesian Limestone possesses a well developed system of vertical or near vertical joints with a regional trend. Where the limestone is underlain by coal workings the joints may develop into open fissures as a result of subsidence and chemical weathering (Shadbolt & Mabe, in Buist & Ineson, 1992). Reactivation of Pleistocene fractures can be caused by the mining subsidence and this is likely to have occurred in the East Durham area. Where fracturing occurs in the limestones the following recommendations for cuttings have been made by Buist & Ineson (1992): for areas of little fracturing a near vertical slope may be cut into the limestone; for moderately fractured rock a 60 slope is considered to be safe; for highly fractured rock a 45 slope is considered safe; where gulls or fissures are present all slopes should be cut to less than 45. Embankment stability can be affected by flooding within sinkholes and solution features which saturate the embankment toe. Collapse features often occur along the edge of highways if unpaired ditches are employed as the method of drainage and they approach the soil rock interface. A contributory factor to all the above is that highway development often causes an increase in the drainage volumes (Moore, 1984). Where void development is a possibility, tensile reinforcement is often used to support earth structures to resist complete collapse into the void and secondly to limit deformations so that serviceability of structure is maintained. CASE HISTORIES FROM COUNTY DURHAM Fox Cover, Dawdon Redevelopment The Fox Cover site in the east of the county has been chosen for the development of an industrial estate. A link road is planned to connect the development to existing major

6 428 M. R. Green et al. routes. The planned route of the roads has been found to lie very close to solution features mapped during a reconnaissance survey by Dickson (1993) and a further investigation by Green (1994) (Fig. 2). It is noted that in the south west of the area a fault is believed to exist and trends NNW-SSE, and the depressed ground areas appear to run on the inferred position of the fault. From preliminary borings limestone appears to be close to the surface in this area and solutional features and ground movements correlate closely with areas where limestone is close to the surface. A sewerage interceptor tunnel is being bored in the immediate vicinity and the local water company has been pumping considerable volumes of water to assist in this boring. It is believed that the pumping of water from within the limestone is leading to solution as limes are washed out. This may be a contributory factor of the solution features, particularly where faults/joints and fissures extend to the surface. Previous experience in the East Durham area has shown that problems with dissolution of the limestone occur when construction has led to exposure of the Magnesian Limestone resulting in rainfall causing rapid erosion of the weaker material along joints and fissures. For this reason it is important that the limestone is not exposed for long periods during construction work. The probable cessation of pumping in the area following the closure of the last coal pits will lead to groundwater rebound which in turn may increase the rate of dissolution of the limestone, especially along joints and faults. Current dissolution rates are in the order of 0.05 mm year" 1. The tunnelling contractor for the water company described the limestone they were encountering as soft yellow dolomite that was able to be mined out with picks. Hard bands were also encountered but soft rock was dominant. A report by local consulting engineers notes that rainfall is low in the area and significant solution of the limestone is taking place at the present time and most of the solution features observed developed during periods of heavier rainfall in the Quaternary. Small areas of collapse are unlikely to be caused by mining subsidence as the limestone covering the coal deposits is greater than 100 m thick. As a precaution, the following recommendations have been made for the Dawdon redevelopment scheme. (a) All buildings should be built on reinforced raft foundations. (b) All new drainage should be a closed system with flexible jointing or contain impervious linings to prevent infiltration of water into the underlying limestone. (c) No surface water drainage should be taken into soakaways. (d) During construction all foundations should be kept dry and protected from inflows of surface or groundwater at all times. This can be achieved by the use of a concrete binding layer. (e) Development should avoid linear problem areas. (f) Reducing ground levels where Magnesian Limestone is at or near surface should be avoided, if possible. (g) Any sand layers with flowing water encountered at foundation depth should be replaced with concrete. (h) Any loose completely weathered powdery limestone encountered should be removed and replaced with suitably compacted granular fill material. (i) Slopes with completely weathered limestone should be protected by a clay capping.

7 Land subsidence on Magnesian Limestone terrain in County Durham, England 429 Fig. 2 Detailed plan of features observed at Fox Cover.

8 430 M. R. Green et al. Wheatley Hill Bypass Scheme (A181) This road scheme passes through several cuttings and in these Type 1 subbase (local Dolomite Limestone) has been laid directly upon the dolomitic country rock. This country rock appears to be well fissured and jointed and there is also some evidence of karstic activity. The site investigation for the road revealed that the limestone is bedded and close to the surface (0.4 to 4.0 m below ground). Water seepage was detected in silt layers in the area. During construction of the carriageway for the bypass the subbase material settled within fissures in the Magnesian Limestone and was subsequently washed out leaving voids which could promote subsidence of the flexible pavement material above. Voids formed, the largest being some 2 m $. As a remedial measure, the fissures were grouted with concrete or filled with granular material. It was deemed necessary for a survey to be carried out along the road to identify any further open fissures or voids that existed beneath the pavement which might lead to failure in the flexible pavement. The method of investigation chosen was non-destructive indirect impulse radio echo sounding. The transducers used in the survey were selected for their penetration range and resolution which would allow the detection of the construction materials, any voiding of the subbase/rockhead boundary and any deeper features within the bedrock. The results of the survey indicate that there are areas of homogeneous rock but the majority of the limestone appears to contain discontinuities, with the high amplitude responses obtained being consistent with a well jointed fractured rock into which the overburden has penetrated. These areas highlighted by the survey showed larger features which appear to plot transversely to the pavement and are possibly a result of karstic solution. There is evidence of vertical formations and also orientations in the bedding planes indicating that regional folding may also be important. All the possible solution features were located where the road has been laid directly on the bedrock. Large features that are associated with high amplitude signals that are consistent with the presence of voids or fissures were found and these features may warrant further investigation and probable remedial action. Remedial measures have been adopted as a result of this survey and take the form of: (a) grouting the fissures by drilling through the road surface; (b) excavating to formation level and then grouting; (c) excavating to roadbase level and laying a continuously reinforced concrete slab overlain by a macadam basecourse. This option is very expensive but any voids would be spanned, maintaining serviceability of the road. No further action was taken as deflectograph testing showed the road to be well supported. HAZARD MAP Whatever the methodology used for the estimation of hazard, a useful hazard map should be: (a) Applicable dynamics of the sinkhole must be clear so that adequate assumptions

Land subsidence on Magnesian Limestone terrain in County Durham, England 431 about the type and magnitude of the sinkholes can be used to predict distribution of events.

9 Land subsidence on Magnesian Limestone terrain in County Durham, England 431 about the type and magnitude of the sinkholes can be used to predict distribution of events. (b) Reliable - database should accurately reflect the past distribution of sinkholes. (c) Flexible parameters should be easily changed when new data becomes available to provide new information. (d) Rapid - hazard maps should be produced in a short enough time to be effective and usable. The area of County Durham included in the study was some 600 km 2. Due to the large area and time constraints, the method of weighted factors was chosen. In this method a whole series of factors that may be contributing to sinkhole/solution feature development are assessed and mapped onto individual maps. A database was setup on a grid square basis, every square being 1 km 2 in this study. Each factor that affects, or is believed to affect, sinkhole development is scored or weighted according to its degree of importance. The degree of importance is decided upon by a combination of visual examination, professional experience and literature studies. If a factor occurs in a square a value is attributed to that square. This is done for all factors on all squares. A totals of risk values is obtained and a contour plot can then be produced. A plan using risk, based on a 1 km 2 area, was produced for the benefit of engineers and planners. With this method, choice of weighting factors was subjective and the method would have benefited from a probability analysis of the factors causing dissolution. The method would have been better suited to a site specific investigation where all factors could be subjected to a rigorous statistical analysis. Time dependency is not important in this application as it is the location of sinkholes that is important for planners. The hazard maps produced give the user an indication of the likelihood of encountering a solution feature that already exists. It does not indicate when new sinkholes will develop or the likely severity of the problem if a sinkhole is encountered. REFERENCES Buist, D. S. & Ineson, P. R. (1992) Problems in highway construction in Magnesian Limestone - a case history from Pleasley, Derbyshire. Highways and Transportation, April 1992, Dickson, R. (1993) East Durham Regeneration Scheme site investigation. MSc Dissertation, University of Newcastle upon Tyne (unpublished). Fischer, J. A., Greene, R. W., Ottoson.R. S. & Graham, T. C. (1987) Planning and design considerations in karst terrain. Proc. of the Second Multidisciplinary Conf. on Sinkholes and the Environmental Impacts of Karst (Orlando, Florida, 9-11 February). A. A. Balkema, Rotterdam. Moore, H. L. (1984) Geotechnical considerations in the location, design and construction of highways in karst terrain - the Pellissippi Parkway extension, Knox Blount Counties, Tennessee. Proc. of the First Multidisciplinary Conf. on Sinkholes (Orlando, Florida, October). A. A. Balkema, Rotterdam. Parate, N. S. (1984) Sinkhole subsidence damage and protective measures. Proc. of the First Multidisciplinary Conf. on Sinkholes (Orlando, Florida, October). A. A. Balkema, Rotterdam. Sweeting, M. M. (1966) The weathering of limestones. In: Essays in Geomorphology (ed. by G. H. Dury). Heinemann, London. Trudgill, S. T. (1985) Limestone Geomorphology. Longman, London.

Land Subsidence. Land subsidence is defined as the lowering of the land surface.

Land Subsidence. Land subsidence is defined as the lowering of the land surface. Land Subsidence Land subsidence is defined as the lowering of the land surface. Many different factors can cause the land surface to subside. Subsidence can occur rapidly due to: a sinkhole or under ground