“Design and Construction of the First Vegetated Steep Sloping Structures on the UK Motorway Network” by Dave Woods was presented at GeoAmericas 2016 (10 – 13 April 2016, Miami).
ABSTRACT: FIRST VEGETATED STEEP SLOPE STRUCTURES ON UK MOTORWAY
It is 20 years since the publication of HA68/94, the first UK design method and guidance for reinforced soil slopes which specifically addressed the issues that specifically affect steep slopes rather than vertical MSE walls. With steep slopes the traditional single wedge failure model is no longer sufficient whilst the increased flexibility allows for the use of more readily available marginal fill materials. At the time of the documents publication the first major reinforced soil slopes on the UK Motorway network had been approved on 3 sections of the M25 Motorway widening. Trust had to be gradually built both from a design perspective that utilised and compared all available methods at the time from Jewell’s log spiral analyses to multi part wedge analyses that mirrored the parallel development of HA68/94. Initial caution led to the construction of the first section of the works using RS slopes for the clockwise carriageway and conventional RC structures on the anticlockwise which offered an invaluable opportunity to compare and contrast the new methods against the tried and tested industry standards.
Given the notoriously slow nature of the traffic on the section of the M25 in question the slopes were built under the watching eye of both the engineering and the general community. The pace of the observer was particularly unnerving when they could practically watch as blades of grass grew or, in some cases, died.
This case study looks at the design and construction of these slopes using exceptionally poor London Clays and Gault Clays and a combination of both geogrid and non-woven geosynthetics to provide both reinforcement and drainage. It addresses the often troublesome issues with both establishing and maintaining suitable vegetative cover to the slopes and evaluates the Engineering benefits of RSS when compared with the conventional concrete structures. It will look at the lessons learned from these structures and their inclusion in current guidance documents and celebrate the fact that the RSS embankments and cuttings have stood the test of time and continue to flourish whilst the glare of the motorists has descended to the more conventional Engineering methods used in the carriageway construction which bears the test of time less well.
The full paper is available via the three-volume digital proceedings set.