Radial collector wells in alluvium project. Final Report (no 3) on trenchless moling trials at Carmer Wood, Laughton, Lincolnshire

This report documents the results of the field trials carried out by BGS on behalr or OOA~to deve16p-an° rntetmed'iate~ technolog-y mettrodof constructing collector wells in unconsolidated sandy alluvium. A system has been developed which uses a thrust-boring mole of the type now in common use in the construction industry to install sub-surface service mains without excavation. The system has been developed with the aim of ut il is i ng the groundwater resources which are present in thin shallow uncemented sandy aquifers but whose exploitation by borehole or shaft-only dugwell would be marginal due to small available drawdowns and moderate to low permeabilities. Mesh-wrapped plastic screen is emplaced inside jacked-out temporary steel casing which is then retracted under a small positive hydraulic head in order to avoid sandlocking and formation ingress into the main shaft. 38 mm 10 pipe has been used, and rapid autodevelopment by the radials has provided coll~ctors with yields of 1.5 lis/radial from fine running sands. Arrays have been emplaced to over 20 m length with a 98 mm ~ head at jacking forces of less than 7 tonnes f. The equipment required for the construction of the collectors is light to transport and install and significantly less expensive than a downhole rotary drill ing rig to buy and operate. The method is however restricted _ to uoconsolidated_ fine-grained alluvium, as the method is essentially one of formation displacement, not removal.