Geology Hindhead Tunnel

geology of hindhead tunnel

the hindhead tunnel runs through sequence of fine grained strata of lower greensand group laid on wealden clay during (part of wealden group) during lower cretaceous period (70–140 million years ago) on margins of subsiding weald basin. greensand group consists of hythe beds, overlay 10 20 m (33 66 ft) layer of atherfield clay. hythe beds themselves, host tunnel, consist of upper hythe bed, has 4 substrata identified letters d , , lower hythe bed, has 2 strata identified letters , b . whole area covered burgate beds, when weald uplifted, burgate beds eroded away.

most of tunnel passes through upper hythe c , d layers , lower hythe layer described weak, locally weak moderately strong, clayey fine medium sandstone occasional thin beds of clayey/silty fine sand , typically has uniaxial compressive strength (ucs) values of 2–5 mpa. rock heavily fractured , has mean fracture centres varying 190 815 mm (7.5 32.1 inches). southern end of tunnel passes through less competent upper hythe , b layers described medium dense thinly bedded , thinly laminated, clean silty , clayey fine , medium sand subordinate weak strong sandstone, cherty sandstone , chert . of tunnel above predicted water table.

route map of hindhead tunnel scheme