Ground movement had been occurring on the B4570 at Treforgan, near Cardigan, over a period of years. The scale of movement eventually resulted in a road failure and route closure.
We were instructed to undertake a Preliminary Sources Study Report (PSSR), Ground Investigation Scoping Report (GISR), Ground Investigation Report (GIR) and Geotechnical Design Report (GDR) in line with CD-622 (2019) by Ceredigion County Council.
Due to the speed of the final failure and requirement for it to be remediated quickly to enable the road to be reopened, our scope of works was adapted to the client’s needs and a site investigation was carried out which enabled an Remediation Options Report.
The site investigation, which comprised, dynamic sampling with rotary coring, windowless sampling and trial pitting was focussed towards confirming the ground model and obtaining sufficient information for an Remediation Options Report to be produced. Information was also obtained to enable final design of a suitable remediation scheme.
A Conceptual Ground Model of the wider valley and landslide was generated and showed Landslide Deposits underlying the road which were underlain by a variable thickness of Glaciolacustrine Deposits (fine-grained soils laid down in a post-glacial lake) that covered the surrounding area and lower valley areas.
Following provision of our Options Report and discussions with the client, the preferred remediation design was chosen and we worked with civil engineering design partners to enable this.
We produced an Earthworks Specification in accordance with the Specification for Road Works Series 600 –
Earthworks for the remediation. In order to provide sufficient strength to the formation and to limit the potential for future road failure, two different types of geosynthetic reinforcements were selected to provide the most economic approach. The chosen design factors of safety (EC7 adequacy factors) were checked using limit equilibrium modelling.
A design for the reconstruction of the road was provided which included a drainage scheme. This enabled issue for tender by local civil engineering contractors.
Cut off drains to intercept and lower groundwater are
visible and protection of the toe of the slope is also notable where a small stream is present.
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