Background

The A40 is one of the original strategic routes linking London with Wales. While many sections have since been upgraded to motorway standard, the stretch between Highnam, Gloucestershire and Ross on Wye remains single carriageway and runs along a steep hillside approaching the Wye Valley.

In March 2021, prolonged periods of heavy rainfall caused ground instability and embankment failure in the Glasshouse Lane area. National Highways monitored the site carefully to design a long term repair solution capable of withstanding future extreme rainfall events.

From October 2021 to May 2022, counterfort drains were installed to safely lower groundwater levels within the embankment. Once stabilised, full reconstruction works could begin, including soil nailing, embankment rebuild and installation of new safety barriers. Reconstruction was completed in early March 2023.

The challenge

The scheme required a robust drainage system to manage surface and groundwater within a steep embankment rising approximately 6 metres from verge to toe.

Key challenges included:

  • Managing water levels within a steep and unstable embankment
  • Intercepting road runoff and sub surface drainage to prevent future failures
  • Integrating drainage infrastructure within limited verge space
  • Designing structures robust enough for highway loading and long term durability

A bespoke solution was required to tie together counterfort drains, toe drains and runoff systems within a constrained highway corridor.

The solution

The first phase of works focused on installing the drainage system. SWH Contracting installed 100 metres of filter stone along the line of the road, incorporating four access chambers and a Marshalls bespoke precast manhole positioned at the lowest point of the system.

The drainage design included:

  • A toe drain at the base of the embankment
  • Run off drains capturing water from the carriageway
  • Counterfort drains conveying water downslope

All flows were intercepted and channelled into a bespoke verge manhole at the top of the embankment. From there, water discharged into a 1500mm Perfect Manhole located at the bottom of the slope, before flowing through a headwall into the local watercourse.

Marshalls designed and supplied a rectangular manhole as a bespoke unit to meet site specific requirements. The verge manhole specification included:

  • Reinforced concrete sides 300mm thick
  • A 500mm sump at the base
  • Two inlets (300mm and 150mm)
  • One 300mm outlet

The bespoke configuration ensured structural integrity, hydraulic performance and compatibility with the wider drainage network.

The outcome

The bespoke flat pack drainage solution played a key role in stabilising the embankment and enabling the safe reconstruction of the A40 carriageway.

  • Effective control of surface and groundwater within the embankment
  • Integration with counterfort and toe drain systems
  • Long term resilience against extreme rainfall events
  • Efficient installation within a constrained highway environment

Lee Allen, Site Manager at SWH Contracting, commented:

“The verge manhole is a bespoke unit. It basically had to be Marshalls. Marshalls came back to me within a week.”

He added:

“I use a lot of Marshalls products, so I’m confident about the quality of the products and the service.”