The Publication of UK codes and design advice, such as BS 8006 and HA 68/94 has helped increase the acceptance of reinforced soil techniques within civil and geotechnical disciplines.
These codes, however, give very little guidance on the factors influencing the establishment of vegetation on steepened slopes up to 70degrees. Yet the visual appearance and success of vegetation is becoming of increasing importance to clients, such as the Highways Agency.
varying degrees of success in greening-up reinforced soil slopes is apparent on the motorway widening schemes carried in the mid-1990s. Even on the same motorway, some sections have very good vegetative cover, with little or no visible evidence of the front face constraint (ie geogrid or metal mesh) while other sections have not.
From a geotechnical viewpoint both sections may have equally adequate factors of safety. Increasingly, the 'clever stuff ' is no longer ensuring sufficient reinforcement to prevent instability, but getting a front face detail to ensure a green, well-vegetated, slope.
Help is on the way, however.
The Highways Agency has commissioned TRL to produce a revision to HA68/94 which will include a section specifically discussing the factors influencing the establishment of vegetation and current best practice.
Innovations are also coming through from practitioners. For example, a pre-seeded hessian mat has been used on a slope of a new section of the M60 near Manchester.
The concept of spraying site-specific seed mix onto hessian is not a new one. Many previous reinforced soil schemes have used hydroseeding, but with spraying on to the front of the hessian after construction of the wall.
On the M60 project, contractor Balfour Beatty in conjunction with Cambridge Direct Tree Services, developed a pre-seeded hessian with the site-specific mix attached to the rear of the hessian, incorporated during construction of the Fortrac geogrid reinforced slopes.This has the added benefit of the seed being in direct contact with the topsoil and resulted in quicker germination and hence quicker establishment of lush green vegetation.