Design and construction of a permanent shotcrete lining—The A3 Hindhead Project, UK

摘要

This paper discusses the design and construction of a permanent shotcrete lining for the 1.8 km long A3 Hindhead twin bore highway tunnel that commenced construction in January 2008 and for which excavation and support was completed in March 2009. The tunnel is excavated within the Hythe Beds—a 90 m thick sequence within the Lower Greensand Series formation consisting of ‘weak, to very weak sandstone with occasional thin beds of fine sand’. The proposed construction methodology was progressive mechanical excavation of a horseshoe shaped tunnel with shotcrete installed in each excavation cycle using robotic spraying equipment. This paper describes the development of the innovative ‘permanent’ primary lining design including the modelling of the lining, specification of a durable shotcrete mix including early strength requirements, as well as details of the construction. This construction technique has proved to be very successful in ‘soft’ rock, and has local applicability to Auckland’s bedrock material, East Coast Bays Formation.