Road and Bridge Construction across Gypsum Karst in England

Gypsum karst problems in the Permian and Triassic sequences of England have caused difficult conditions for bridge and road construction. In Northern England, the Ripon Bypass crosses Permian strata affected by active gypsum karst and severe subsidence problems. Here, the initial borehole site investigation for the road was supplemented by resistivity tomography studies. The roadway was reinforced with two layers of tensile membrane material within the earth embankment. This will prevent dangerous catastrophic collapse, but will allow sagging to show where problems exist. The River Ure Bridge was constructed across an area of subsidence pipes filled with alluvial deposits. It was built with extra strength, larger than normal foundations. If one pier fails the bridge is designed for adjacent arches to span the gap without collapse. The bridge piers are also fitted with electronic load monitoring to warn of failure. In the Midlands area of England, road construction over Triassic gypsum has required a phase of ground improvement on the Derby Southern Bypass. Here, the gypsum caps a hill where it was formerly mined; it dips through a karstic dissolution zone into an area of complete dissolution and collapse. The road and an associated flyover were built across these ground conditions. A major grouting program before the earthworks began treated the cavities in the mine workings and the cavernous margin of the gypsum mass. Within the karstic dissolution zone gypsum blocks and cavities along the route were identified by conductivity and resistivity geophysical surveys, excavated and backfilled. In the areas of complete dissolution and collapse the road foundation was strengthened with vibrated stone columns and a reinforced concrete road deck was used. INTRODUCTION Gypsum (CaSO4 .2H2O) is readily soluble in water and develops karst features much more quickly than does limestone. Road construction over gypsiferous terranes has to deal with karst problems including progressive dissolution, sinkhole formation and poor ground conditions caused by collapsed strata. Furthermore, the complete dissolution of gypsiferous beds can leave a residue of weak and brecciated strata which also produces difficult conditions for construction. Where gypsum is present in the bedrock, either as massive beds or as veins, it can be associated with sulphate-rich groundwater that can be harmful to concrete, and precautions to prevent damage should be considered (Forster et al., 1995). The Permian and Triassic strata in England both contain thick gypsum sequences, but individually their associated rocks and respective engineering problems are different. The most problematical gypsum karst conditions are found in the Permian rocks, notably in the Ripon area of North Yorkshire. Gypsum karst and difficult ground conditions have been encountered in the Triassic rocks of the English Midlands in the area to the south-west of Derby. These are described below with details of the difficulties encountered during road construction and their engineering solutions. RIPON BYPASS Geology and gypsum karst problems The Ripon Bypass was constructed to the east of Ripon. It crosses the Permian sequence which includes approximately 35m of gypsum in the Edlington Formation (formerly the Middle Marl) and 10m of gypsum in the higher Roxby Formation (formerly Figure 1. The distribution of the Permian and Triassic gypsiferous sequences in the United Kingdom and locations of the Ripon Bypass and the Derby Southern Bypass. Nottingham Gypsiferous strata Hartlepool Carlisle