Slope failure on the THT line in South Wales required the installation of a landslide barrier. The task was made particularly difficult due to the difficult access, heavy vegetation and steepness of the slope. The debris fencing solution, using helical screw piles driven into the soil, has never been previously installed and secured using this system onto a soil slope.
The THT line in South Wales runs from Cardiff to Treherbert. In January 2018, a shallow landslip occurred on the embankment above the railway adjacent to the section that runs alongside the Rhonda River between Porth and Tonypandy. This occurred as the result of a localised flooding event caused by a drainage system that services residential properties at the crest of the soil slope overtopping, leading to overflows down the embankment.
The ensuing slope failure resulted in the closure of the THT for five days while the line was being cleared of debris.
The result of the failure was an 8m backscar at the natural soil slope crest. As the Network Rail boundary is located midslope the backscar was not within the Network Rail land boundary. It was considered that this feature was unstable and presented a significant safety and performance risk. The site was on Network Rail’s ‘adverse weather at risk’ list and has historically suffered from drainage and surface water inundation.
Remedial works would include the installation of an approximately 40m long landslide barrier installed across the soil slope and secured into the ground using helical piles acting as both compression and tension anchors.
Using its own direct labour, plant and transport logistics, Griffiths delivered the installation of a shallow landslide barrier secured to helical piles including all associated enabling and groundworks. The installed debris fence is a 125 kilojoule barrier that can comfortably accommodate 30 tonnes of soil and other materials sliding into it.
Due to the nature of the site on a steep slope adjacent to the operational railway and with no easy road access, plant, equipment and materials needed to be brought to site using a number of 10 hour Rules of the Route (ROTR) possessions on Saturday nights, and 7 hour possessions on Sunday nights, with a working area created on the embankment from which the works could be carried out during normal day time working.
The debris fence, which measures 3.5m in height is attached to posts which sit on pivots with wire ribs tied back to the helical piles. The whole system is designed to be flexible to allow for materials impacting and therefore is highly effective in stopping materials progressing further, for example onto tracks or roads below.
The use of this barrier configuration is unusual on a soil slope as these systems are typically installed on rock slopes. So rather than using conventional drilling techniques such as rock bolting, Griffiths opted for the helical screw piles driven into the soil. Debris fencing has never been previously installed and secured using this system onto a soil slope.
The installation of the barrier mid-slope on a soil slope was challenging and required the area to be first de-vegetated and cleared, and then a series of benches created using Griffiths plant to allow the barrier to be installed.
Excavators tracked up the slope and cleared an area from which to work and install the system. The pre-manufactured screw piles were lifted to the site using plant, and a torque attachment attached to an excavator was used to drill the piles into place. Concrete foundations were then cast around the screw piles and the barrier assembled.