Hollingdean Recycling Centre, Brighton

VINCI Construction were at the early stage of site strip and enabling works, when SETS were asked to look at alternatives to the construction of a mass concrete gravity type retaining wall for the support of one boundary to the Holingdean Recycling Centre in Brighton.

The site which comprised several different levels resulting from former land use, was bordered on the southern side by a 15m heavily vegetated slope. The development required part of this slope to be removed to make room for a peripheral access road and the resulting cut face was to be supported by a retaining wall.

SETS made use of the observational approach in helping to engineer an alternative solution to a concrete retaining structure. On the eastern side of the site there was a large cutting in which was situated the mainline railway. The eastern side of this cutting was noted to have cut slopes in excess of 30m height formed at an angle of some 75degrees. Apart from minor spalling of chalk from the cut face, it was evident that the cutting had been relative stable since the time it was formed in the late 19th Century.

By using this observational information, SETS investigated the proposed cutting on the site by means of a mechanical excavator. This allowed the nature of the chalk present in the slope to be analysed at several locations. Apart form the uppermost section of the slope, the majority of the chalk was found to be competent and identical to the chalk seen in the adjacent railway cutting. Discontinuities in the chalk were logged and by using the CIRIA scheme for the description of chalk the material was duly classified.

Based on the findings of the limited intrusive investigation and drawing heavily on the observational approach described above, SETS was able to recommend that the slope could be cut and left unsupported, with a finished slope angle similar to that observed in the railway cutting. This approach was accepted by the design engineer employed by Vinci Construction and the free standing cut face was formed, thus saving Vinci in excess of £200k.

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