§ REPORT ON THE FRACTURE OF A 6" DIAMETER MEDIUM PRESSURE MAIN AT CONGRESBURY, SOMERSET, ON SUNDAY, 17TH DECEMBER, 1967.
§ The main is 6" in diameter and of spun grey iron material manufactured to B.S. 1211 of 1945. Pipes are 18' in length with flexible joints of the Staveley hook bolt type.
§ 20 years (laid between June 1947 and January 1948).
§ The main provides a bulk supply of gas from Weston-super-Mare to Congresbury, Yatton, Clevedon, Nailsea and Portishead. It is reinforced at Yatton by supplies from the supergrid main between Seabank Works and Weston-super-Mare. At the time of the incident the main was operating at a pressure of approximately 20 p.s.i.
§ The main is laid approximately 3' 0" from the footpath in the carriageway of the A370 road through Congresbury at a depth of 2' 6" to the top of the main. (The "Recommendations for Mainlaying" issued by the Institution of Gas Engineers in 1956 recommended a depth of not less than 2' 0" in carriageways, but in 1966 a revised edition of this publication recommended a depth of 2' 6". 249W The main, therefore, is laid in accordance with modern practice in respect of depth.)
§ An examination of the main, both at the point of fracture and in adjacent excavations, revealed that the pipe was in excellent condition with no evidence of corrosion.
§ Description of fracture:
§ A complete circumferential fracture had occurred near the centre of an 18' length of pipe. The crack was open approximately 3/32" at the top reducing to approximately 1/32" at the bottom.
§ Cause of fracture:
§ About 2" away from the point of fracture a large stone was located underneath the main on which the pipe was resting.
§ A 4" earthenware stormwater pipe, at a depth of 2' 4", and an old stone stormwater drain at a depth of 2' 0", each within 12" of the point of fracture, were located in the excavation. Both drains had been out of use for many years, probably since a date prior to the laying of the gas main.
§ It is considered that water seeping through one, or both of the disused drains loosened the ground beneath the main causing an erosion of the bed of the main over part of the pipe length with the result that the main dropped slightly to rest directly on the stone. It is considered that external stresses caused by this central pivot in the length of pipe resulted in the fracture.