Standing on the north-facing valley near the village of Wattstown in the Rhondda Fach Valley and at approximately 290m AOD sits Wattstown Standard colliery tip, a conspicuous conical lump consisting of an area no less than 85,000m2 with 1,000,000m3 of colliery waste generated over a 188-year period by the local Cwch (National) Colliery.
Wattstown residents had reasons for concern when a month’s worth of rain had fallen over a 48hr period in December 2020 which caused devastating landslides at several historical coal tips in the Rhondda Cynon Valley.
In recent years, increased rainfall attributed to climate change has triggered landslides at several of the 2456 coal tips in Wales with over 327 being classified as presenting a high-risk potential.
Working on the Coal Authority Framework since 2013, Griffiths were invited to scope and price the reclamation of colliery waste and remediation of the landslide using a preliminary design produced on behalf of the coal authority to prevent further failures until sufficient funds were made available to procure a permanent solution.
The preliminary design broadly consisted of recovering 500m3 of failed material from the base of the tip to a receptor site on nearby Troed-Y-Rhiw Mountain.
Following reclamation of all waste material a 70m long section of gabion baskets would be installed at the toe of the failure filled with imported material and site won material all anchored in place with 9m long soil nails at 2m centers. In addition, 140m of filter drainage required installing at the rear and in front of the gabion wall all backfilled with imported single size granular material.
To prevent future landslides the designer proposed installing 5500m2 of erosion control matting over the slip area anchored in place using 5,000No 1.5m long rope anchors supplemented with 12,000 No 600mm long wooden pegs.
Removing a significant volume of material along steep narrow tracks, some 100m in elevation using conventional earth moving equipment presented safety concerns for Griffiths. Using our extensive experience, Griffiths positively challenged the design proposal by utilising areas adjacent to the slip to form landscaped benches free of future landslides therefore eliminating risk for our workforce whilst traversing steep inclines.
In doing so, the anchored gabion wall, associated drainage and importation of granular fill along hazardous access tracks wouldn’t be required, therefore significantly reducing the overall cost for the client.
Working at Height presents safety issues in many sectors of the construction industry, but not for Griffiths who are members of the Industrial Rope Access Trade Association (IRATA). Employing some 30 rope access technicians with competencies varying from Level 3 supervisor to level 1 technicians ensures Griffiths can self deliver varying activities in any sector without an over reliance on specialist external labour suppliers.
Prior to physical works commencing, Griffiths undertook tactile inspections and topographical surveys with our own rope access trained Geotechnical Engineers to identify the absolute extremities of the failure.
Whilst undertaking these surveys a significant tension crack became apparent on the western flank of the failure, which raised concerns for the safety of our workforce during the construction phase. Working in collaboration with the client, Griffiths proposed and used a specialist spider excavator tethered to sufficient kentledge to remove the 150m3 of material to the toe of the slope to be included in the landscaped benches.
Using the survey information, our Geotechnical Engineers were able to review the ground conditions and recommend a value engineering solution by reducing the overall number of rope anchors to 3000No and timber pegs to 9000.
The benefits of using Griffiths in house Geotechnical Division were undeniably beneficial to the client and stakeholders in terms of safety, quality, programme and cost.
All works were completed without any accidents or incidents, no defects were recorded by the client during the final handover, the overall delivery programme was achieved including additional works removing the tension crack and the overall cost reduced by 30%.