BAM Ritchies have supported Network Rail in securing better, more reliable journeys on the key route between London and Hastings for at least the next 60 years

Our teams have designed and delivered new strength and resilience to this 170-year old cutting, using a combination of active and passive rock bolting, netting and soil nails in an area with protected species and logistical challenges with no safety incidents.

Wadhurst Cutting is located on the Tonbridge to Hastings rail line, running approximately 300 metres from the north and south tunnel portals. The cutting height ranges between eight to 18 metres and the geology consists of weathered Wadhurst Clay, underlain by the Wadhurst Clay Formation and Ashdown Formation. Following failures of the over steepened cutting in 2014, 2016, and 2019, BAM was engaged to install a Network Rail approved design to remediate the cutting.

The project was in two phases, the Country end (Initial Emergency Works) and London end. It involved a wide range of geotechnical stabilisation installations which you would expect to see on failing earthwork infrastructure. There were a number of independent systems working alongside each other. There was an active soil nail system on the upper slopes of the cutting consisting of nails that vary in length from five metres to 14 metres. This was meshed using Greenaxe integrated mesh and erosion matting on the active soil nails. There was also a passive rock bolt drape net system with rock bolts top and bottom, with the top row following the undulation of the rock crop from the tunnel portal until it drops away to ground level. The third system is an active rock bolt system for the first 20 yards either side of the portal as it opens up. The soil nails head up the bulk of the installation with approximately 13,000 l/m installed and in excess of 1,000 l/m of rock bolts. The meshed area in total is approximately 15,000m2 when all systems are combined. Subsequent to vegetation removal an additional LiDAR survey was completed. We used the results to further refine the design, resulting in the soil nails of two sections being shortened.

During analysis of the emergency work Network Rail design, it became clear that there was an opportunity to challenge the design and significant value engineering was possible for the majority of the stabilisation project. BAM Ritchies then approached specialist geotechnical design consultant Byrne Looby and requested they work with us on a value engineering feasibility study to confirm if, and to what degree, value engineering was possible.

Through the team’s analysis we confirmed savings were possible by increasing the reinforcement mesh strength which allowed for the soil nails spacing to be increased from a one metre by 1.5 metre grid to a 1.5 metre by two metre grid. The horizontal spacing of the rock bolts over the lower portion of the cutting was also increased from 2.2 metres to three metres. These proposed alterations to the design offered a 46 per cent reduction in the liner meters of soil nails and rock bolts installed and reduced the programme by 20 weeks, reducing overall cost by around £1.5 million. Network Rail were impressed with the level of savings offered whilst maintaining compliance with specifications, standard details and Eurocodes.

We used a blockade at the country end in October 2021 to deliver the emergency works that enabled the installation of lower rock bolts and soil nails, which were not possible during earlier ALO working. It was a very busy half-term on the blockade. An incident-free 7 days of digitally rehearsed, predictable, sustainable delivery providing resilience to the earthwork’s infrastructure of the Kent and Sussex Railway. We installed over 450 soil nails and rock bolts, with a total meterage in excess of 2,200 metres. Three drillings rigs, working over 2,700 hours, were fuelled by HVO instead of diesel, reducing our carbon footprint by 90 per cent.

The split in soil nails and rock bolts is Weighing in at Wadhurst BAM Ritchies have supported Network Rail in securing better, more reliable journeys on the key route between London and Hastings for at least the next 60 years due to the challenging strata which has Wadhurst clay sited over the Ashdown rock formation. After some test drilling with the self-drilling nails it became apparent our preferred system for production drilling and installation was not going to be as straight forward with the Wadhurst clay becoming very, very hard Wadhurst clay. This problem
was overcome by the application of hydraulic drill heads fitted to the traditional air masts and a hybrid system was installed that still used air flush to open hole but with the speed associated with a hydraulic terrapin face rig, production as a result exceeded expectation. This was an example of daily innovation delivering improving outcomes for Network Rail.

Sustainability was a key driver for this project and this can be seen in the efforts made to ensure any overengineering of the design was eliminated. Efforts were also made in the type of erosion protection used on the slope. The original scope indicated a plastic based erosion mat should be incorporated into the reinforcement mesh. Following conversations with the erosion mat supplier it became known that their products begin to degrade after 2 years; resulting in microplastics being discharged into the environment. Once this potential pollutant was known BAM Ritchies engineers investigated alternative solutions and with the assistance of Bryne Looby we convinced Network Rail to approve the switch to woven coir matting which did not present an environmental risk. Ecological challenges consisted of managing a resident badger sett which needing a licence and permission to close the set, in advance of our works. Also on site were Dormice,
which required the creation of a dead hedge mouse highway to avoid the works and also a licence to plant Dormice friendly plants on completion of the works.

While at Wadhurst we also worked in partnership with EAVE as they looked to improve their technology with their GEN3 ear defenders. The trials were carried out over 2 weeks, with Eave leaving site with some excellent data regarding noise exposure and where it is most prevalent upon our site operations. This will support them in developing and delivering the protection to meet future demands of the geotechnical sector and the ground engineering industry.

Benefits of work
Reduced risk of landslips on the line – in the past decade alone, engineers have visited Wadhurst tunnel on numerous occasions to secure the railway and install temporary fixes to weather-related movements. This seven-day emergency blockade during October half-term meant engineers could strengthen the cutting, making it as secure as possible as quickly as possible.

Less overall disruption to passengers and the benefits delivered sooner – the blockade condensed essential work into the shortest period so the overall disruption to passengers was significantly reduced.

Better, more reliable journeys – this essential work will help reduce the number of delays in the long term and therefore improve passenger journeys.

The Wadhurst Cutting project is an excellent example of a specialist geotechnical contractor working in close collaboration with the designer to produce outstanding results for the client. The value engineering work carried out significantly reduced both the cost and programme of the project and the close relationship between BAM Nuttall, BAM Ritchies and Byne Looby allowed for any construction issues to be mitigated quickly and correctly. A fantastic effort by all parties involved included our key supply chain partners, true collaboration across all parties creating one team focused on the project’s success.

Tel: 07740 771075