Adoptable Highways and Drainage Package.

This case study reviews the detailed design of an adoptable highways, SuDS and sewer detailed design package for a residential development of 34 houses in Baschurch, Shropshire for a private developer. The main constraints for the development design involved designing around a community fire station and a number of existing properties, which would be served by the new s38 adoptable road. Also due to part of the site falling below the connection horizon to the existing gravity sewer, a s104 adoptable pumping station, rising main and connections to the sewer were designed to the Design and Construction Guidance (DCG). The site is currently in the construction phase.

How We Approached The Design

The design was carried out in two phases, and we worked closely with the client’s architecture team to develop an efficient site layout and level design. Phase 1 was designed to ensure the development and adoptable road wouldn’t increase flood risk to the existing fire station, whilst providing three possible routes to access the station by fire appliances, to ensure free access could be maintained during construction. The Phase 1 level design also focused on a smooth transition for the design of the Phase 2 part of the development. The adoptable road passed several existing properties, so a careful approach to the level design was required to ensure that the road aligned with the driveway levels, whilst complying with the road adoption standards set by the local highway authority.

The geology of the site meant infiltration SuDS were favourable. The private driveways to each plot were designed to be permeable with a stone-filled trench beneath the permeable driveway for the roof water discharge. This solution saved the circa 40% construction costs from traditional plastic crate or concrete ring systems, whilst providing a more sustainable design with a lower environment footprint. The maintenance requirements for the SuDS were carefully considered with systems designed to capture silt and debris, ensuring a long design life at minimal cost. In line with CIRIA 753 The SuDS Manual, the detail for permeable paving included an impermeable binder layer over the permeable paving which allows construction to take place on the site, while reducing risk of blocking the permeable paving during the build phase. Once construction is complete, this impermeable layer can be cleaned and cored to allow water to pass through, with the rest of the permeable construction then being built up. To meet the local highway authority requirements, a system of adoptable soakaways was designed, and these were positioned in areas of open space. To ensure the development is resilient to flood risk, all SuDS were designed to manage a 1 in 100 year plus 35% climate change event without any flooding.