A494 Llanycil, Retaining Wall Construction

When earlier remedition of an embankment on the shore of Bala Lake in Wales failed to address the underlying ground movement, Griffiths delivered a solution that features the world’s first installation of a Smart Anchor Monitoring System developed by DYWIDAG Ltd. The system allows remote surveillance of the asset’s performance, supported by early warning alarm notifications.

Erosion and cracking within the footpath at Llanycil. Erosion and cracking within the footpath prior to remedial works and Smart Anchor installation.
Smart Anchor in the retaining wall at Llanycil. Smart Anchor head arrangement and readout unit within the anchor pocket of the retaining wall at Llanycil.
Griffiths' work at Llanycil Smart anchor installation at Llanycil.

The northern section of the A494 is one of the busiest trunk roads in Wales and is crucial to tourism and the local economy of Gwynedd.

Significant erosion had weakened an embankment that that carries a section of the road alongside the northern shore of Bala Lake. A scheme to rectify and protect the embankment from further damage was completed in 2008, however, in late 2009 settlement occurred and surveys were commissioned by the North and Mid Wales Trunk Road Agent (NMWTRA) to monitor and record the settlement.

The data recorded showed that movement had continued and was affecting the pathway. This resulted in the reconstruction of the vehicle restraint barrier running along the section. Engineers were engaged to investigate the cause of the movement and to provide a detailed design to stabilise the embankment over the entire 180m length.

In 2018, a soil nail trial was carried out using self-drilling soil nails that proved to be partially successful in slowing the ground movement considerably, but not entirely. But in early 2020, the condition of the safety barrier foundation rendered the soil nail solution ineffective. As NMWTRA’s primary concern was the integrity of the vehicle restraint system, temporary traffic lights were installed to close the southbound lane. Due to the importance of the A494, NMWTRA was keen to identify a permanent solution that would allow the re-opening of the road as early as possible.

Alun Griffiths (Construction) Ltd was engaged by NMWTRA at the ECI stage to develop a solution. Working with Ymgynghoriaeth Gwynedd Consultancy (Gwynedd County Council’s design office) and DYWIDAG, several options were considered for the remediation of the embankment, with the preferred solution comprising a restrained/anchored reinforced concrete pile cap/retaining wall over a length of approximately 75m, with anchor piles spaced at 2m centres and embedded into the bedrock. The vehicle restraint system and pedestrian barrier were to be supported by the structure with both the highway and footpath regraded and resurfaced.

The proposed solution would improve the factor of safety of the embankment, reducing any ground movement and resultant cracking in the highway and footpath, and delivering a 120 year design life.

In total, 36 permanent Double Corrosion Protected (DCP) ground anchors were installed at an angle of 45o, with an average length of 18m, including approximate 15m free length (including 1.7m for the structure) and 3m bonded length.

Of the 36 anchors installed at Llanycil, 12 are Smart Anchors that feature an integrated monitoring device that acquires, processes and alerts on real-time sensor data to provide an integrated view of the foundation condition of the structure.

The Smart Anchor’s load is frequently monitored by a unique force measurement sensor, with the field data being transmitted to a cloud-based platform. This data is then distributed via alerts at predetermined intervals and can be accessed on the platform 24/7. The transmitter and battery are located externally and are fixed within the anchor pocket of the retaining wall. The transmitter relays each anchor’s real-time sensor data to a nearby readout unit to be sent through a central gateway via the cloud-based platform called ‘Infrastructure Intelligence’.

A screenshot of the Llanycil Infrastructure Intelligence platform interface.

The frequency or point-in-time measurements are taken at regular intervals, for example every 15 minutes, when the Smart Anchor will take a load reading. The frequency can be reduced to extend the transmitter’s battery life. Alert trigger levels are predetermined by the client and their engineering team, so notifications are sent once these levels are breached.

Infrastructure Intelligence presents real-time field data on all devices from desktop, tablet and smartphone, using the device’s native browser. The 3D ‘digital twin’ enables the user to navigate the virtual site remotely as if they were at the physical site. This includes navigating over and under structures and looking at every element from a 360o perspective. It provides an up-to-date digital representation of the site’s environment using live telemetry updates.

Llanycil has advanced ground anchor technology to the next level. The client can now identify minor issues before they become major problems.

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