Project Sheet: Ysbyty Ifan

View of the patio from downslope; note over-steep slope at the edges
View of the patio from downslope; note over-steep slope at the edges

Client

  • Cartrefi Conwy

Contractor

  • C. C. Geotechnical and Earth Engineering

Works Undertaken

  • Produce a geotechnical report
  • Desk Study
  • Topographical Survey
  • Obtain and review aerial photography of the area
  • Site Investigation
  • Trial pits
  • Detailed design, drawing & specification of a retaining wall at the crest of the slope.

Description of Project

A property in the village of Ysbyty Ifan is located at the crest of a slope, and its rear patio was showing extensive signs of movement, tilting downslope towards the Afon Conwy. Cracks along the interface of the property were also visible. Even though the house has not experienced any distress so far, its future is possibly at danger so stabilisation of the slope is required.

A desk study was carried out to try to fully understand the failure mechanism at the site. The purpose of a desk study is to highlight changes in land use such as the river course or development of properties, which in this case could have potentially been the cause of the over-steep slope. Aerial pictures of the site in 1948 identified an elongated hollow within the steep bank leading from the bungalow towards a small pond/marshland at its base.

The area at the toe of the slope has also been suggested to be susceptible to flooding during flash flood events of the river Conwy. It is likely that such events have eroded the base of the slope causing further steepening.

GroundSolve Ltd initially supervised the excavation of several boreholes and four machine-dug trial pits as deep as 2m were carried out by Earth Engineering Ltd. These revealed layers of soft and firm clay overlying glacial till.

It was suggested that in preparation for the construction of the bungalow, the ground level had been altered. The hollow was backfilled with poor quality topsoil, extending beneath the patio at the rear of the property, and it is unlikely that any compaction was carried out.

Recent site investigation has observed that the material has been settling under its own weight and has begun to creep due to the over-steepness of the slope. This has caused the overlying concrete slabs to tilt unevenly and crack.

Following the investigation, GroundSolve Ltd were required to replace the material beneath the patio area and to provide lateral support to prevent the loss of ground due to hill creep processes. The most cost-effective solution was to excavate the made ground from beneath the patio area and raise the level using suitable compacted granular fill.

The slope was also stabilised by soil nailing and a reinforced concrete beam constructed at the crest of the slope.

A new concrete slab was then constructed for the patio and topsoil lain over the exposed slope adjacent to the patio to complete the works.

Trial pit revealing soft clay
Trial pit revealing soft clay
The patio at the rear of the property
The patio at the rear of the property