The advantages of the structure being made offsite in factory conditions includes; walls can be thinner because placement of reinforcement and cover can be controlled, self compacting concrete is used producing a quality finish throughout, and any accessories required can be assembled offsite to make sure they are going to fit when they get to site. If changes need to be made they can be done without affecting the works programme. Depending on the size of the required structure it will normally be supplied in modules for assembling on site. The size of the modules being governed by weight and the capacity of the delivery vehicle. The drawings on the right show a Headwall in excess of 3m wide and 8m long designed in place of an insitu structure specified. The modules are cast with lifting anchors enabling them to be lifted off the delivery vehicle and put in place. Fixings for a 1500mm Square Composite Flap Valve and a Stainless Steel Grating and Access Ladder were cast in, and dissipation blocks were cast on the apron.