The results of two studies into the effects of undermining offshore room and pillar workings by longwall faces are presented. The first study indicated no potential instability from undermining one level of room and pillar workings. On the basis of this it was decided to undermine two levels of room and pillar workings, again monitoring pillar stability. The results showed no signs of increased instability.
At Lynemouth colliery, Northumberland, England, all the coal workings are under the North Sea, and extend approximately 8 km from the coast. The mine is divided in two by an East-West fault with a throw of approximately 200 m to the South. Prior to 1977 all extraction in the Northern part of the mine had been in the Main, High Main, and Yard seams by room and pillar. In the Main seam, approximately 85 m below the sea bed, the initial extraction height was 2.5 m, in some areas this was followed by removing the supports, and blasting down the overlying 1.5 m of High Main coal. In the Yard seam, between 3 m and 15 m below the Main seam, the full height of 2.4 m was extracted. All the workings consisted of 23 m square pillars, and 5.5 m wide roadways giving an extraction of 35%. The main seam in the instrumented area was extracted between 1964 and 1966, and the overlying Yard seam was extracted between 1972 and 1974. The Brass Thill seam which underlies these workings, was 1.6 m thick at a depth of 160 m below the sea bed in areas suitable for extract•n. As this seam was more than 105 m below the sea bed, British mining regulations permitted longwall extraction providing the tensile strain on the sea bed was kept below 10 mm/m. This aquifer is exposed on the sea bed, so any fractures reaching it, could cause water inflows large enough to close the mine. Due to this danger, Newcastle University was requested to monitor pillar stability as the underlying Brass Thill was extracted. In January, 1977, extraction of the Brass Thill by longwall mining was begun. The first longwall face, KS4, was a 180 m wide retreat unit, taking the full seam height of 1.6 m. This face only undermined room and pillar workings in the Main seam and some areas of High Main extraction. The observations taken in the room and pillar workings indicated no signs of pillar instability. On completion of KS4 face, KS3 was worked by advancing longwall, also undermining Main and High Main seam workings only. This was followed by KS6, which some 300 m from the start line undermined an area in which both Main and Yard seams had been worked, with High Main coal also having been taken in some roadways. Since potential pillar and junction failures in the Yard seam could affect the overlying Main seam pillars, the University was again requested to monitor pillar stability in the Main seam.
