Project Outline:

During Storm Dennis in February 2020, a failure occurred on the Tylorstown upper colliery tip. A large rotational slip occurred resulting in a land slide of 60,000 tonnes of material slipping down the mountain and blocking the river and footpaths below. RCTCBC appointed Walters to complete emergency works aimed at preventing further collapse of the coal tip by making improvements to the drainage network on the hillside.

In collaboration with the client, RCTCBC, and their designer, we developed a drainage solution that we could install rapidly in challenging terrain.

Upon completion of the emergency works, Walters assisted RCTCBC and their designers throughout the design phase to collaboratively work up cost-effective remediation and reinstatement solutions, considering stakeholders and regulatory requirements. We drew up water management protocols and procedures which we submitted to NRW for consideration and acceptance, to ensure the removal of the slipped material did not constitute further environmental damage.

The final design saw the land slip material being removed from the river to receptor sites adjacent to the footpath and within the RCTCBC land boundaries. The new landforms required drainage ditches to manage and control the flow of surface run off without degrading or scouring the material. A series of lined ditches were required out falling to a rock filled pond slowing the flow and filtering before the runoff reached the river.  

The riverbanks were reinstated with 200m of new block stone. Following the rebuilding of the river banks the footpaths could be reinstated to both banks and new drainage installations across the mountain side and receptor sites.

Project Delivery:

We worked closely with NRW throughout the delivery phase, which included an extensive water management strategy including several innovative solutions. NRW regularly attended the site to review control measures. We liaised with NRW to establish a safe and collaborative method of working that enabled us to successfully deliver the project and protect the environment. The site team completed daily visual checks up and downstream from the works and installed other measures which maintained water quality. We implemented a range of silt mitigation measures including silt fencing, straw bales, baffles and silt matting. We used intermittent working to allow the sediment to settle before works recommenced.

The design included the installation of Concrete Canvas. Walters worked with Redstart and directly with Concrete Canvas to produce a buildable lining system. The installation of the canvas was carried out in stages to manage the channel’s mountain location, with steep batters either side.

The land slip material had also covered over a strategic Welsh Water asset feeding thousands of properties from the Maerdy reservoir. Walters engaged with Welsh water and their designers to agree methodology and safe systems of work to enable the material to be removed from the slip area without risk to the vital infrastructure.

Added Value and Client Partnerships:

We designed a bespoke manageable drainage solution the day following the slip and were able to mobilise to be on site within a week of the landslide, installing the pipe work across the mountain in challenging weather conditions.

Our team, working with the client and their designers provided expertise and buildability advice ensuring a suitable design that could be constructed and offered support and solutions that prevented the need to export any material from the site reducing the need for road lorries travelling through the adjacent villages, reducing the carbon footprint for the scheme. We also collaboratively, agreed a bespoke Option C target cost contract with the Employer that afforded all parties reassurance when tackling what was largely a resolution to an emergency scenario, carrying a steep risk profile.

We utilised our experience in earthworks to remove the material in the most efficient manor minimising trucks and optimising plant movements, utilising our extensive local plant fleet. The volumetrics were managed by the onsite team and continually surveyed and adjusted the three-dimensional models to suit the changing requirements. Walters utilised GPS machine control to ensure the new landforms could be constructed quickly and precisely.

We worked closely with Welsh Water tracking their assets along the new design alignment to ensure wherever possible the route was away from the main ensuring the asset was protected. Walters installed a protection mat utilising the tree trunks removed as part of the works producing a cost effective and sustainable solution enabling the works to continue.