RC Wood, Bsc, MICE, CEng. This paper was first published in GE’s March 1986 edition.
In the vicinity of the Reigate Hill Interchange (Fig. 1) the M25 London Orbital Motorway runs close to the South Downs Escarpment at Colley Hill, which is an area of outstanding natural beauty. As a result of a public inquirythe proposed carriageway level of the motorway was lowered to lessen its impact on the area. There was insufficient time to purchase additional land to facilitate conventional construction and consequently the increased depth of cutting had to be supported by a retaining structure.
Construction of the walls started in October 1983 and was substantially complete by December 1984. Each of the two walls comprises a row of discrete, bored in situ, reinforced concrete (r.c.) piles which are restrained at the capping beams by single rows of permanent ground anchors. The traffic face of each row of piles was excavated to road formation level; this left the embedment length in chalk undisturbed (see Fig. 2). By incorporating a system of ground anchors, the consulting engineers W.S. Atkins & Partners (WSA&P) were able to reduce the size of the piles.
During installation of the 200 ground anchorages WSA&P collated valuable observations concerning specification and installation procedures in medium to hard chalk. The irregular depth of the overlying clay at this location made installation more difficult. Steep sided clay filled valleys, within the chalk, meant that logs from bores less than a metre apart recorded different stratification.