Twenty-five years after the introduction of geogrids in the UK, they are finding increasing acceptance for road pavement structures.
Use of geogrids in UK road projects has grown rapidly in recent years, alongside the greater use of other geosynthetics in civil engineering. The products have a wide range of applications, from steep soil slopes, retaining walls and bridge abutments to load transfer platforms for embankments.
Stiff geogrids are often used to reduce the thickness of the unbound pavement layers over soft subgrade.
However, take-up of their use as part of structural pavement layers and to stabilise the capping and subbase layers of pavements has been slower in the UK than elsewhere, according to geogrid manufacturer Tensar International.
The firm manufactured the first polymer geogrids 25 years ago and since the early 1980s has been supplying soil reinforcement and mechanical ground stabilisation solutions around the world (see below).
Tensar thinks the use of geogrids in pavements will grow, with greater awareness of the performance of stiff polymer grids and growing willingness by the Highways Agency to accept innovative techniques with proven structural, cost saving and environmental benefits.
It cites a recent project on the A19 Thirsk bypass in North Yorkshire as an example. Tensar area manager Colin Thompson says: 'Four kilometres of dual carriageway, running north-south, was surfaced with 250mm thick concrete slabs over a clay composite formation.
'The surface was noisy and had degraded significantly over the 30 years since it had been completed, especially around the joints, where water had infiltrated.' In the maintenance contract that ended in May, the concrete was broken out, removed and crushed off-site to Department of Transport Type 1 grading and reused as subbase with the geogrid.
Thompson says: 'The water infiltration had caused numerous soft spots in the clay but, as the SS30-G geogrid/geotextile composite was laid over the whole roadway, no further remedial action was required.' Tensar claims its geogrids have also proven their long term value in reducing damage and maintenance requirements on vulnerable sections of road. In 1987 the surface of the L23, a rural road in Luneburg south west of Hamburg in Germany, had become badly damaged by frost heave.
The road was underlain by 1m to 2m of weathered marl. Damage had resulted in wide longitudinal cracks in the surface along the wheel tracks and in the centre of the road. Additionally, 'crocodile' cracking clearly showed asphalt fatigue, caused by a too thin layer of sub-base which was unable to offer the necessary frost resistance and bearing capacity.
During the repair works on a 250m long section, Tensar asphalt reinforcing grid was installed over a thin asphaltic regulating layer, and overlaid with a 75mm binder and a 30mm wearing course. The rest of the pavement, which had suffered less damage, was repaired with the same thickness of asphalt as per normal practice at the time.
In 2003, 16 years after repair work, a falling weight deflectometer (FWD) survey showed the unreinforced sections of pavement had less than the required minimum bearing capacity, and needed immediate repair.
However, the geogrid reinforced section was above the minimum value, and only required minor remedial action to the wearing course with a chip seal in a few spots, despite the FWD showing the inherent subgrade as weaker than in the unreinforced 'control' sections.
Tensar predicts use of geogrids will become more extensive in the roads market as the time and materials saving bene' s become more widely appreciated.
Celebrating quarter century
In the 25 years since Tensar International began manufacturing geogrids their use has grown rapidly.
The materials provide alternative solutions to heavy, rigid concrete or masonry structures when dealing with difficult ground conditions, particularly sites underlain by alluvial and estuarine deposits and very weak soft soils like peat and soft clays.
Geogrids' environmental benefits include allowing the use of marginal soils and locally available poor quality materials, and minimise the need for intrusive materials such as concrete or steel or imported aggregates. Waste materials including pulverised fuel ash can be used in flll.
'In 1980 Tensar introduced the first uniaxial geogrid for one directional loading, ' says the firm's special projects engineer John Dixon. 'In 1981 the biaxial geogrid was introduced for multi-directional shear applications associated with pavement engineering. The next major development was in the early 1990s with improved production leading to better uniaxial grid strength for weight ratio and better value per kN.' The biaxial geogrid range was also developed to provide an equally balanced stiffness in both directions, Dixon explains.
'The mid-1990s saw the development of systems, like the Tensar Wall System, as an integrated package, comprising grids, fixings and modular block masonry that had rapid acceptance. Another development was composites, bonding geogrid with geotextile to combine the best features of each, ' he adds.
Tensar chief engineer Chris Jenner says: 'Tensar geogrids opened up another dimension to reinforcement and stabilisation solutions with flexible reinforcing structures for soil reinforcement and ground stabilisation.'