M23 Concrete Replacement Scheme

The M23 Concrete Replacement Scheme involved resurfacing of the M23 London bound carriageway between Junctions 10–9. Works included the removal and replacement of 1.3-kilometre section of deep set concrete due to carriageway break up and end of life cycle.

To complete the works safely the London-bound carriageway was closed over four consecutive weekends from 10pm Friday night to 4am Monday morning allow 24 hours continuous work for the excavation of 260 defective 300mm thick concrete slabs, each weighing 2.2 tonnes which had to be removed with a specially designed excavation bucket. The surfacing was completed with 3 layers of 1.375 kilometres of asphalt re-instatement.



This section of the M23 provides a vital link to the M25 for traffic travelling to and from Brighton to London, plus essential access to Gatwick Airport; it is the second largest airport in the UK, serving over 45 million passengers annually, contributing around £2 billion to London and the surrounding economy and employing over 21,000 people.

Collaborative planning and communication was key to the successful delivery of the scheme.

Embedding safety from plan to completion.

Preventing harm near overhead cables.

During design stage we located 2 overhead cables one just before and one within the working area; one 11KV the other 132KV. Making sure our site teams could easily identify these cables we installed blue cones with yellow sleeves, warning signs and we deployed K9 lighting at night to improve visibility of the warning cones and signs. In addition, we added an adjustable boom height limiter, fixed to the excavators prior to operation. The limiter allows pre-set heights for the boom which then sounds an alarm, when the pre-set limit is reached it automatically stops the boom. 

Protecting site teams from incursions.

This is a key risk for our workforce and the travelling public. On this scheme we introduced Carnell’s SafetyCam providing a visual deterrent to potential breaches on the closed slip road. SafetyCam is an intelligent site safety innovation which protects road workers by using two complimentary vehicle detection systems both monitoring and reporting  the speeds of passing site traffic and recording beginning to end footage of unauthorised vehicle incursions into the closure.  

Enhancing capacity and capability 

The project team sourced local based sub-contractors ensuring our investment in the neighbouring community was maximised.  We obtained surfacing material through our partner Tarmac and their nearby plant. And, working closely with them we developed the specially designed excavation bucket for the concrete slabs which were heavier than anticipated.

Local based hauliers collected, crushed and recycled more than 3,900 tonnes of concrete; turning the material into a sub-base product that could be used in the construction of new roads.

Delivery on time and to budget 

With over 67 defects recorded and large pothole repairs the scheme required daytime lane closures. The urgency to get the scheme on the ground was critical to prevent further defects developing and to protect the safety of road users. Our construction and commercial team worked with Highways England project team to agree a target price in one day, allowing progression of work dates to be liaised and agreed with stakeholders, contractors allocated, and resources ordered.

Although the scheme crept over budget, the urgent timescales for designing, planning and resourcing for the lifecycle of the works was paramount, and the initial budget did not account for last minute changes of excavation depth from 200mm to 300mm. Delaying works to trial alternative solutions would have ultimately cost more in terms of safety of the stretch of carriageway, socio-economics and customer satisfaction. 

Each weekend closure was planned from 10pm Friday night to 4am Monday morning to align with the timings of staff changeovers at Gatwick and their first cascade of passengers. The support of traffic officers allowed the closure to be put out on time and for works to commence immediately and all closures were opened ahead of schedule, providing a positive message to customers and stakeholders.

Innovation and improvements 

Specialised bucket: During trials of the vacuum lifting process the concrete slabs were splitting in two. Inspired by technology used in the USA, we worked with Tarmac to design a specially adapted excavating bucket capable of lifting and scooping the concrete slabs, allowing them to be placed for easy extraction mitigating the risk of splitting. The slot in the back of the bucket allows the concrete slabs to slip into the slot and lock in, preventing any movement making the whole process a safer operation. This innovative bucket design is a first for this style of machinery adaptation the UK.

Gatwick Airport app and billboard notices: Liaising with key stakeholder Gatwick Airport the notice boards in all terminals and car parks within the airport provided clear visual notifications to passengers, staff and visitors of the upcoming weekend closures. We also provided content for Gatwick Airport’s passenger app (190,000 downloads in the last year) and their community app for staff (over 12,000 users) to provide updated information on the works, they also included an update on their webpage which included a link to the official scheme webpage.

Four hourly updates: A key contact distribution list was created to receive 4-hourly email updates during closures, allowing us to provide up to date information on the progress of works and advise of potential issues. This list included key stakeholders such as; Gatwick Airport, Regional Control Centre (RCC),  local authorities and Highways England press office and project team.

Other improvements included:

  • Free recovery on the diversion
  • Signage on diversion
  • Wide use of VMS across throughout the area
  • Ensuring no simultaneous rail closures nearby
  • Carnell’s SafetyCam
  • 10k letter-drop to residents adjacent to the works and on the diversion route (10k circa)


Early engagement began with Gatwick Airport to understand their organisational and passenger needs and to identify and agree closure dates to coincide with expected lower levels of passengers.

Up front engagement with local authorities was also crucial. The scheme had two diversion routes travelling via two different local authority networks. We had to make sure we had their full co-operation and agreement on the dates as well as their assurance that their networks were clear of any works to allow the diversion routes to run freely and safely. We also had to liaise with Connect-Plus Services (M25) to check there were no planned works within the M25/M23 junctions.

We held early planning meetings with Network Rail making sure there was no planned works on the Gatwick Express and the rail-lines from Brighton; rail works on these lines lead to an increase vehicle movement on the M23 that we didn’t want to then push onto local road diversions.

We were able to allow M23 Smart Motorways access during the closures to carry out clearance works prior to the start of the smart motorway scheme helping to minimise future impact on Gatwick Airport and the travelling public.

Every element from planning to construction of the M23 Concrete Replacement Scheme was engineered to align safety, customer and delivery imperatives, raising the bar for our industry standards.


For more information on this scheme please go to our contact page and get in touch.