Geosynthetics, unknown and unexercised only recently, are not strange anymore, being used more and more often for subgrade and road pavement construction. Yet, in spite of the successful experience of their usage and a further range expansion, many experts of the road industry are either insufficiently knowledgeable about, or totally unaware of the geosynthetics benefits. This is why the experts are losing sound opportunities to solve simply and efficiently many of the road construction problems, such as a swamp and weak soil, surface deformation, rutting, frost heaving, reflective surface cracks and so on. Geosynthetics is a certainly promising trend of road construction, and their functions, application terms and calculation methods should be scrutinized in order to utilize the potential of these game-changing materials to the highest possible extent.
Geosynthetic material selection: conditions, purpose and goals
The effectiveness of using various geosynthetic layers in road pavement structures is defined by their ability to perform reinforcement and separation of the layer materials so as to cut the volume of conventional aggregate used and increase the road structure service life. Depending on the geomaterial purpose, the benefit derived can lie in the initial (construction) cost reduction and/or the repair cost reduction over the service life.
Woven and nonwoven geotextile fabrics, flat biaxial geogrids, geomats, volumetric geogrids (geocells) and composite geomaterials are used as geosynthetic interlayers in road pavements. The selection of geosynthetic material is done by comparison of technical and economic features of a road pavement versions with and without geosynthetic materials. The technical effect as compared to conventional solutions should be paid due attention. The use of geosynthetics boosts the reliability of road structures as well as construction quality, which sometimes cannot be numerically evaluated.
The benefit of geosynthetics application is especially tangible in construction, renovation and repair of motorways of a high standard, under extreme Northern weather conditions (handling earth at temperatures below zero) and unfavourable soil and hydrology conditions (weak foundations, swamp soil, special soils). For technical feasibility purposes, this benefit, on account of a road structure fitness to work and operational properties, can be evaluated higher than the initial expenditure saving of the alternative variants under comparison. As generally known, geosynthetics are used in road pavements as:
- a reinforcement and separation layer for reinforcement of aggregate of loose, granular materials (macadam, gravel, macadam-gravel-sand mixtures, slag, etc.) or temporary pavement;
- a separation layer at the border of large-grained materials and soil;
- protection-drainage layers where the draining sand layer contacts the embankment soil;
- protection layers under precast concrete slabs;
- a reinforcement (crack preventing) layer for reinforcement of asphalt pavements.
Geosynthetics effect on a road pavement structure as per Road Industry Norms (RIN) 218.046-2001 is measured by three methods (CREDO RADON 3.4 program):
Road Industry Method (RIM) 218.5.002-2008 ‘Recommendations for usage of polymer geomats (geogrids) in reinforcement of road pavement layers of granular materials’;
RIM 218.5.003-2010 ‘Recommendations for geosynthetic material usage in motorway construction and repair’;
RIM 218.5.001-2009 ‘Recommendations for usage of geomats and flat geogrids in reinforcement of asphalt and concrete layers of advanced pavements during motorway overhaul and repair’.
Calculations per RIM 218.5.002-2008 method
Reinforcement of road pavements and layer separation of road pavement aggregates of untreated granular materials and underlying soil layers is carried out through geosynthetic materials, i.e. flat geogrids and geomats, woven geosynthetic fabrics, geocomposites. As a result, the road pavement reliability and durability grow, while the thickness of road pavement layers of conventional materials, particularly granular-material aggregates, is reduced.
The most rational way of using reinforcing geosynthetics is achieved under the following conditions:
- when an aggregate of large-grained material is placed right on the subgrade (no sub-base layer), or a sub-base layer of uniform sand is placed instead of a protective aggregate layer of 10-to-20-cm thickness required in this case,
- for heavy-duty roads;
- for construction under unfavourable conditions (swamp subgrade, aggregate use for construction vehicles traffic and a long period of time between the aggregate placement and its covering by overlying road pavement layers, phased construction and renovation, a heavy-traffic road repair);
- for multilane highways with most of the truck traffic along the outer lanes equipped with underneath interlayers;
- in cases when the safety factor values per RIN 218.046-01 in terms of sub-base sand layer shear are the lowest for flexible pavements as compared to the other criteria.
Reinforced road pavements are calculatedd per RIN 218.046-01, with amplifier ratio depending on geomaterial deformation properties, layer thicknesses, road pavement and subgrade mechanical properties. The reinforcing geomaterial has the highest influence on the value of active shear stresses on the subgrade layer located right under the reinforced layer.
