Swinden Quarry: A case study of detecting, surveying, and managing caves within quarries SWINDEN QUARRY: A CASE STUDY OF DETECTING, SURVEYING AND MANAGING CAVES WITHIN QUARRIES C.S. ARDITTO1 AND M. HUDSON2 1 Lafarge Tarmac, Fell Bank, Birtley, Chester le Street, County Durham, DH3 2ST. 2 Subsurface Laser Scanning Ltd, Whitehall Business Centre, 75 School Lane, Hartford, Northwich, Cheshire CW8 1PF. ABSTRACT Caves within quarries are difficult to detect and their presence can cause serious problems for quarry operators. When caves have historically been found, it has been difficult to fully determine their extent and nature in order to devise a practical and safe solution for managing them. Using a recent case study; Swinden Quarry in North Yorkshire, this paper describes the techniques employed to manage caves at the site, and how new subsurface 3D surveying technology can be used to accurately map their extent and enabling a targeted solution to be designed, to make them safe. Arditto, C. S. and Hudson, M., 2014. Swinden Quarry: A case study of detecting, surveying, and managing caves within quarries. Pp. 156-167 in Hunger, E., Brown, T. J. and Lucas, G. (Eds.) Proceedings of the 17th Extractive Industry Geology Conference, EIG Conferences Ltd. 202pp. e-mail: [email protected] INTRODUCTION Swinden Quarry is located immediately north of the Quarry, part of the Craven Reef Belt sits within Cracoe village of Cracoe, c.3.5km south west of Grassington and Hill which gives its name to the cluster of Cracoean Reefs c.10km north of Skipton, within the Yorkshire Dales formed in the area between Grassington and Cracoe, National Park in North Yorkshire, as shown on Figure 1. which includes Skelterton, Stebden and Elbolton. These The deposit at Swinden Quarry is a Carboniferous reef reefs are shown on Figure 2 and on Figure 1 as distinct limestone complex with unpredictable karst topography. hill features. These hills appear to be associated with To date, most cavities encountered have been either underlying anticlinal structures and their current surface small or naturally filled with debris and so have not expression broadly represents the shape of the original presented a safety issue. However, in 2011 a seemingly reef geometry. innocuous cavity was encountered that, when probed Locally, the stratigraphy of Swinden Quarry can be appeared to be in the region of 28m deep. With summarised as Carboniferous aged rocks starting with a machinery in excess of 200 tonnes traversing the quarry foundation of pre-reef bedded limestone (Skelterton floor, gaining an accurate understanding of the cave’s and/or Threapland limestone) upon which a reef like geometry was essential in order to mitigate the risk it complex has evolved, comprising both massive and posed. This paper is a case study of how caves are bedded limestone (Mundy, 2000). Flanking this reef managed at Swinden quarry, highlighting associated complex are a series of limestone debris beds or breccias problems and how, in one particular instance, a large comprising reworked limestone and black shale. The cave was dealt with from detection, through investigation Bowland Shale overlies the limestone to the north, west to its ultimate remediation to make the cave safe. and east. Recent superficial deposits, including boulder clay and alluvium, infill valley features incised in to the Bowland Shale along the north west and south eastern GEOLOGICAL SETTING flanks of Cracoe Hill. As can be seen on the schematic Swinden Quarry extracts Lower Carboniferous cross sections on Figure 3, it can be considered (Mississippian) aged limestones that were deposited somewhat isolated from other limestone outcrops in the between the shallow ocean shelf environments of the area. Askrigg Block to the north and the deeper marine The caves within the limestone are dissolution environments of the Craven Basin to the South, as shown features, cross cutting all strata thus far encountered in Figure 2. The margin between the two was actively within the quarry. Typically they are infilled with clay being deformed both during and after the deposition of and the age of their formation is unknown. The main the limestone, resulting in the bedrock being folded and problem posed by the presence of the caves relates to the faulted. Along this margin, a series of reefs, known as the production process. The soft infill within the caves means Craven Reef Belt (Mundy, 2000) were formed. Swinden 156 C.S. Arditto and M. Hudson Figure 1. Location of Swinden Quarry. The top image shows its regional position and the bottom image the local situation with the quarry, the village of Cracoe and the reef knolls of Elbolton, Stebden and Skelton visible (as on Figure 2). 157 Swinden Quarry: A case study of detecting, surveying, and managing caves within quarries Figure 2. Regional structural geology (modified from Arthurton et al., 1988). The settlements of Settle and Cracoe are shown. Solid black areas denote reef structures within the Craven Reef Belt. that drill holes fail to remain open and thus prevents characterise the nature of the problem in the first charging with explosives. Once blasted, the presence of instance. Karstic caves are unpredictable and notoriously the clay in the blast pile does not pose quality issues as difficult to detect through proactive investigation alone; the clayey material is removed via a scalping (screening) therefore any approach needs to be a mix of proactive process within the fixed processing plant. Prior to investigation and ongoing observation. Once a potential encountering the large open cave in 2011, no large open cave is detected, further reactive investigation will be cave had been encountered within the quarry. required to determine the risk it represents and to inform how the cave will be managed and remediated. MANAGING CAVES AT SWINDEN QUARRY Characterisation In quarrying, unlike in civil engineering where static To understand the characteristics of voids that may be features are built, quarries are dynamic working present at a particular site, several points need to be environments where, using mobile equipment the considered. Appraising the regional and local geology thickness of the supporting roof beam is progressively will identify strata prone to natural dissolution, whilst removed from above the cave. also highlighting the potential for problematical man- The finding of caves at Swinden is relatively common made voids primarily resulting from mining activities. and the process by which these caves are managed is In the case of Swinden, man-made voids have been shown in Figure 4. ranked unlikely. Cracoe Hill, whilst on the edge of one Any system for detecting caves has to be proportionate of the most historic productive lead mining regions in the to the risk and be practicable, therefore it is important to UK, seemingly was not itself part of the ore field, with 158 C.S. Arditto and M. Hudson Figure 3. Geological cross sections, stratigraphy and solid geology map of the quarry. 159 Swinden Quarry: A case study of detecting, surveying, and managing caves within quarries intact piece of rock. In the case of Swinden the rock has been assessed using Laubschers Mining Rock Mass Rating (MRMR(2)) (1990). Mohr (2008) states that the limestone rock mass contains widely to very widely, very irregular shaped, rough stepped jointing. Often the joints are open and vuggy due to karstic weathering. The limestone is very competent and extremely strong, showing an average intact rock strength of between 80 and 130MPa. The Laubscher MRMR rock mass classification indicates the limestone to be a good to very good quality rock mass. Waltham and Fookes (2005) refer to an informal guideline known as the 70% rule which states that stability can be expected if the thickness of the roof beam above the cave is at least the equivalent of 70% of the width of the cave. This makes the assumption as outlined Figure 4. Flow diagram for cave management at Swinden Quarry. in Table 1, that the rock is strong (with an unconfined compressive strength of at least 80MPa) with good rock only minor mineralisation being noted within the quarry mass characteristics. This is considered to be a and mining records showing no significant operations. conservative approach based upon destructive testing Field mapping only identified what appear to be small carried out upon model caves of 4m width using occurrences of mining spoil from what may have been limestone with a UCS of 80MPa. These tests assumed a explorative excavations across Cracoe hill. The structural Safe Bearing Pressure (SBP) of 2MPa with the 70% being setting of the Cracoe reef, south of the North Craven determined as the point where the relationship between Fault, appears to have kept the deposit isolated from the roof thickness for a jointed limestone equalled a failure processes that created the nearby ore field. load 3 times that of the SBP i.e. 6MPa, thereby building in a Factor of Safety of 3. Swinden sits within a karstic landscape. Waltham and Fookes (2005) believe the likely presence and nature of At Swinden Quarry the greatest risk from cave collapse karstic features on a given site can be judged by is within the dynamic part of the quarry; the area of appraising the regional setting using a classification extraction where two large machines are employed, a system summarised in their 2005 paper. Being on the CAT 5130 excavator and a Nordberg LT160 mobile edge of the Yorkshire Dales, home of some of the UK’s crusher, both of which routinely traverse the quarry floor largest cave systems and well known karstic landscapes, and each represent a load of 0.2MPa, considerably less Swinden has to be regarded as being potentially prone to than the SBP expected of a sound limestone as stipulated these features on a macro scale.