B4224 Hereford Stone Cottage

Following the severe weather event Storm Dennis in February 2020, there were two significant failures along the B4224 road between Mordiford and Fownhope, Herefordshire. The first was a landslip at Lechmere Ley and the second was a partial retaining wall failure at Stone Cottage. The impact of these failures meant that the B4224 had been closed to all traffic until the failures could be remediated. This had a significant effect on the local community and businesses given the diversion routes that were required to access Fownhope. Herefordshire Council contracted Griffiths via Balfour Beatty Living Places (BBLP) to develop solutions to enable the B4224 to be reopened as soon as possible.

The existing retaining wall is constructed from unreinforced stone masonry along its entire length. This is built into the sloping valley side and retains the B4224 carriageway from a number of domestic dwellings and gardens on the downslope side. Structural failure of this masonry wall meant that carriageway stability had been compromised and there was a risk of a greater global failure of the retaining wall. As such, a solution was required to improve the carriageway stability and reinstate the boundary retaining wall.

Views of the initial failure and following installation of soil nails from within the adjacent domestic property gardens.

Views of the initial failure and following installation of soil nails from within the adjacent domestic property gardens.

The solution at Stone Cottage comprised the removal of the failed section of the wall and the construction of a new reinforced earthwork using bespoke system comprising of soil nails and reinforcing steel mesh combined with geogrid materials. A new masonry wall facing could then be installed to this to reinstate the previous aesthetic and blend into the surroundings, ensuring the adjacent homeowners were not impacted by the works. The solution also included the provision of a high containment kerb to act as the edge of the carriageway.

The construction of the works required close control of buried services within the carriageway, and as such required diversion of existing BT apparatus. This was implemented in advance of the construction work, with other services able to accurately located, protected and avoided.

The designed solution was developed by WSP in collaboration with Huesker and Dywidag. The resulting design consisted of a bespoke reinforced earthwork system that utilised many of the common principles of the discipline, with specific componentry that was developed to enhance the quality of the solution in this specific application. This included specialised eye nuts and high tensile alignment bars to connect the installed soil nails to the reinforced earth elements. Griffiths worked closely with WSP, Huesker and Dywidag during construction to ensure a high-quality product was delivered in accordance with all design specifications.

In order to establish the bond stresses within the stable zone of the retained road embankment, Griffiths undertook investigation soil nail testing using the Long Nail Short Nail test method. This utilised sacrificial nails installed at different depths within the earthwork, aiming to discount any resistance provided in the unstable near-surface measures and provide confidence in the true bond stresses that the nails could provide. This gave greater overall confidence within the designed solution and allowed WSP to review their nail spacing, specification and installed depths to ensure these were set out using the most efficient arrangement.

13t excavator installing first soil nails for reinforced earthwork system.

Griffiths geotechnical specialists installing Huesker reinforcement system and backfilling this with compacted fill.

Working immediately adjacent to the failed retaining wall section gave rise to various risks during construction. Chief of this was the risk of further structural failure, resulting in masonry wall elements falling into the work areas within the domestic gardens. Furthermore, loading of the road surface by heavy plant and machinery would impart horizontal loads onto the unstable masonry wall, potentially giving rise to a larger global failure of the structure and retained backfill. Griffiths managed these risks closely in conjunction with the Permanent and Temporary Works Designers. As such, large sections of the intact masonry wall were sufficiently propped using proprietary equipment dowelled into a concrete plinth that was cast at the toe of the retaining wall. This prevented structural failure and overturning/toppling of the wall. Plant loading on the road surface was managed by phased excavation of the failed retaining wall, with soil nails installed one row at a time from the top down. The drill-mounted long reach excavator could then work ahead of the direction of travel, ensuring this would only track across the road once this had already been stabilised by the installed soil nails.

Finished project, safeguarding the road against further failure and maintaining the original masonry aesthetic of the previous wall.

Given the site locality and its close proximity to small villages and domestic properties, Griffiths had to ensure that construction proposals considered the wellbeing of site neighbours. This included various special considerations such as:

• Continuous vibration monitoring, including determining of baseline surveys and trigger levels to eliminate damage to adjacent properties, structures or utilities.
• Reduced working hours and drilling times to assist in noise management.
• Continuous air quality monitoring to ensure dust was adequately controlled.
• Protection of property within immediate vicinity of drilling works by secure site fencing, debris netting and acoustic barriers.