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This case study explores the slope failure that occurred at St. Aidans Extension Opencast Coal Site in West Yorkshire, UK in 1988. It investigates the causes of the failure and its impact on coal mining operations.
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St Aidans Extension OCCS The River Aire Slope Failure, 1988. Covering material required for WJEC AS-Level Geology: GL3: Key Idea 3(c) Mining: A Case Study. Prepared by Toby White, University of Leeds, Leeds, LS2 9JT.
St Aidans Extension OCCS The River Aire Slope Failure, 1988. With thanks to Dave Hughes & Barry Clarke for permission to include information and material from their paper; “The River Aire slope failure at the St Aidans Extension Opencast Coal Site, West Yorkshire, United Kingdom.” Canadian Geotechnical Journal, 38: 239-259 (2001).
IntroductionLocation plan St Aidans Opencast Coal Site
IntroductionDescription • This part of West Yorkshire, between Leeds and Castleford has been extensively deep-mined for coal since the late 1800s and there have been several opencast coal mines since the 1940s. • The initial St Aidans site was extended in a new contract that started in 1981 and was expected to yield about 6m tonnes of coal from a number of different seams over 10 years. • River Aire • The River Aire formed the western and southern boundary of St Aidans Extension, with the canal running close-by. • Part of the river was diverted and a 4m high flood levee was constructed along most of the river next to the site. • Steel sheet piling was driven, generally down to rockhead, to further strengthen that the area between river and site and to prevent groundwater flow.
Working Method • Motor-scraper • Shovel and Truck • Rope Face Shovel • Hydraulic Backactor • Dragline
Working MethodTwo draglines working in tandem on St Aidans Shovel & Truck Draglines Highwall Overburden dump Coaling Low-wall
Slope FailureDescription • A large section between the excavation and the river (350m long, 120m wide, 50m high) moved towards the void. • Over the next 3 days, 17million m3 of water flowed into the site. • Operations were held up for 10 years and about £20m was spent before coaling could start again. • Slope failures in opencast coal sites do occur. • This was exceptional because it appeared to have been triggered by the presence of previous underground workings, which caused a loss in mass strength and an increase in mass permeability. • Maximum tensile strains were very high (calculated at 10mm/m) and so vertical cracking (especially in the river bed) and slipping along weak bedding horizons must have been major contributory factors to this slope failure.
BackgroundGeology • The geology is coal measures, with shales, some sandstone layers, coals and seatearths (including weak seatearths that are sometimes called clay bands, or intraformational shear zones!). There was also 5-8m of superficial deposits, comprising fluvioglacial gravels, sands, silts and clays. • The coal seams worked in St Aidans Extension were: • Kents Thick (not present in failure zone) • Barnsley Rider (not well developed, but with seatearth) • Barnsley Top Softs (good seam, but with extensive old workings) • Low Barnsley (good seam) • Dunsil (good seam) • There are 6 further seams below the Dunsil that had been worked previously to varying degrees.
BackgroundGeology • The whole of St Aidans lies between two major faults. The Water Haigh fault (<135m vertical displacement) lies to the north and the Methley-Saville fault (<25m vertical displacement) to the south. • Records from underground mining of the sub-Dunsil seams indicates the existence of at least 3 sets of minor faults with the area bounded by these two major faults. They also show faulting aligned sub-parallel with the course of the river beneath the failure zone. • Old plans show the presence of river meanders which have been in-filled with alluvium or made ground. This could represent a zone of weakness.
BackgroundGeology • Minor faulting recorded in coal seams below the level of the opencast mine.
BackgroundGeotechnical • Approx 1,000 boreholes were drilled down to Dunsil seam in a diamond pattern with 30-40m centres. • Stand-off distances were based to a large degree on previous experience of similar situations, rather than detailed geotechnical analysis. This was because there had been little drilling or other ground investigations carried out in the area of the river because of the presence of Swillington Ings (water meadow) covered by up to 2.5m of water. • Some boreholes were sunk in 1986 after work on the site had started. Swillington Ings
BackgroundPrevious underground (deep) mining • Many of the coal seams in this area have extensive “old workings”, with up to 75% of coal having been removed in the Barnsley Top Softs seam. • These old workings produced a zone in which the average strength was less than that of the surrounding rock.
BackgroundPrevious deep mining • The surface subsidence caused by these old workings (<3.6m) resulted in high ground strains (10mm/m or 1%). • These were sufficient to cause fault zones to open and to create new discontinuities.
BackgroundGroundwater • The opening up of joints crack and fault zones caused by the subsidence and ground strains from previous deep mining and the creation of the opencast void, will have greatly increased secondary permeability in the rock mass. • This probably meant that the groundwater system beneath the failure was constantly recharged by river water from the Ings and other surface drainage.
BackgroundOperations • In summary, shovel and truck operations were used to excavate the top few benches, with a walking dragline used to excavate the bulk of the overburden down to the Dunsil seam. • At the time of the failure, the dragline was not working due to breakdown, which meant the upper benches were more advanced than usual. This meant the zone of excavated ground exposed adjacent to the river was much longer than normal.
BackgroundEnd wall instability • In 1985 (3 years before the failure) there were a series of cracks (surface and vertical) that appeared in the western endwall near the river. • A slope failure also occurred that was associated with an anticline with a small reverse fault that dipped into the site. However, this was north of the failure zone and was not visible in the area of the failure, so it may not have had much influence other than at the northern end.
The FailureSat 19th March 1988 • 6:10am Water issuing from upper parts of excavated slopes. • 11:45am River broke through the flood levee and sheet piling. • 12 noon Breach quickly widened to 30m and then to 100m. • Surveys showed that during the failure, the block moved 0.5m down and 4m towards the void. • Water was entering the site from both directions. • Increased flow from upstream caused extensive erosion, with a threat to the canal. • Efforts were made to protect the eroding river bank by using a Chinook helicopter to dump boulders. Not effective! Trucks were used to transport material by road. • The flow of water upstream into the site shocked a number of observers who were unaware of the cause!
The FailureA deep-seated failure was observed on the river bed R.Aire flowing in from BOTH directions 350m long St Aidans Site 150m wide 50m high Slope failed – river broke through bank and poured into void
The FailureFailure mechanism • This appears to have involved sliding along a planar surface or surfaces which dipped gently towards the excavated void, these surfaces being weak bedding horizons (seatearths). • Evidence suggests the basal shear surface in the middle of the failure zone is below the dragline bench, probably at the Barnsley Top Softs seam. • At either end of the failure zone, where the endwall was buttressed by the rock, the slip plane was probably higher, possibly along the Barnsley Rider seatearth, or even on the dragline bench or at rockhead. • The driving force was the very high groundwater pressures acting in the faulted backscar zone, constantly recharged by the river. This opening of this fault zone was probably facilitated by the strains resulting from subsidence and the void.
The FailureSection through failure zone R. Aire percolates down through fractures in rock, and along old workings in BTS. Weak, impermeable, seatearths provided slip plane Opencast site – support for slope removed Old mine workings causing subsidence and opening of fractures
Types of FailureWhich most closely represents the St. Aidans failure?
Recovery of site • Approximately £20m was spent on • a new navigation (navigable river) combining the old river and canal (3.5km) • three bridges • a deep-water lock • a coal loading wharf • and a marina. • Removal of flood water took 2 years. Some of it was treated for domestic and industrial use during some water shortages! • Coaling began again in 1998 and approximately 3m tonnes of additional coal were recovered. • The restoration has included several lakes and a nature reserve.
RecoveryAerial photo after civil works, but before pumping New marina New R.Aire Navigation Old Canal Flooded void Old river New bridge New coal loading wharf
Lessons for the Future • The mining history of the area should be fully researched before direct ground investigations commence, so that the locations of possible ground strains and likely fault zones can be identified. • To establish the detailed geological structure, direct ground investigations by boreholes are absolutely necessary in all areas of the site and for an adequate distance outside the planned excavation boundaries, irrespective of how difficult surface access may be. • Adequate groundwater monitoring must be carried out prior to excavation to establish the groundwater regime and to permit changes to be recorded during the progress of the works.
SummaryMain Features • Excavation of upper benches was more advanced than normal so there was less end restraint to any potential slope failure. • Old meanders could represent zones of weakness in the superficial deposits. • Surveying showed that during the failure, the block moved up to 0.5m down and over 4m towards the void. • The lack of exploration boreholes in the failure area means that geological structure had to be extrapolated from the east. This suggests the bedrock under the river dipped gently towards the excavation void (but less than 5%). • Weak seatearths were identified under both Barnsley Top Softs and Barnsley Rider seams. • The old workings in the BTS produced a weak zone.
SummaryMain Features • Old workings in deeper seams (below the base of the opencast site) produced subsidence and ground strains. • There was evidence of cracks in the bed of the River Aire which indicates the main back scar of the failure. • There were also deep cracks running through the dragline bench at the northern end of the failure. • There was also some “lipping” (overhang) and seepage at the level of the Barnsley Top Softs and Barnsley Rider seams. • Minor faulting was likely to have undergone substantial disruption and displacement due to mining subsidence. • Faulting and subsidence gave rise to a high permeability zone, with the river providing a constant source of recharge. • The anticline and reverse fault to the north of the failure area may have provided “release” surfaces in the northern part.
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