Using Geo-grid in Pavement

Engineers are continually faced with maintaining and developing pavement infrastructure with limited financial resources. Traditional pavement design and construction practices require high-quality materials for fulfilment of construction standards. In many areas of the world, quality materials are unavailable or in short supply. Due to these constraints, engineers are often forced to seek alternative designs using substandard materials, commercial construction aids, and innovative design practices. One category of commercial construction aids is geo-synthetics. Geo-synthetics include a large variety of products composed of polymers and are designed to enhance geotechnical and transportation projects.

A geo-grid is defined as a geo-synthetic material consisting of connected parallel sets of tensile ribs with apertures of sufficient size to allow strike-through of surrounding soil, stone, or other geotechnical material. Existing commercial geo-grid products include extruded geo-grids, woven geo-grids, welded geo-grids, and geo-grid composites. Geo-girds can be manufactured from steel, glass fibre, polyester or polypropylene, as well as natural organic products such as bamboos.

Geo-grids in Pavement

Geo-grids used within a pavement system perform two of the primary functions of geo-synthetics: separation and reinforcement. The primary function of geo-grids used in pavements is reinforcement, in which the geo-grid mechanically improves the engineering properties of the pavement system.

Research has shown that the required base course thickness for a given design may be reduced when a geo-grid is included in the design. Relative agreement exists that substantial benefits can be achieved from the inclusion of geo-grids within pavement systems.

Geo-grids have traditionally been used in three different pavement applications: (a) mechanical sub-grade stabilization, (b) aggregate base reinforcement, and (c) asphalt concrete (AC) overlay reinforcement.

For mechanical sub-grade stabilization and base reinforcement applications the geo-grid should be placed at the bottom of the base for aggregate layers less than 14 in. If a geo-textile is to be used for separation of the sub-grade and base materials, the geo-textile should be placed directly on top of the sub-grade. The reinforcement geo-grid is then placed directly on top of the separation geo-textile for aggregate layers less than 14 in. For pavements with a design base thickness greater than or equal to 14 in., the geo-grid should be placed in the middle of the base course.

Regardless of the placement location of the geo-grid, the separation geo-textile is always placed at the sub-grade base interface.

Reinforcement Mechanisms

Three fundamental reinforcement mechanisms have been identified involving the use of geo-grids to reinforce pavement materials: (a) lateral restraint, (b) improved bearing capacity, and (c) tensioned membrane effect. Lateral restraint refers to the confinement of the aggregate material during loading, which restricts lateral flow of the material from beneath the load. Since most aggregates used in pavement systems are stress-dependent materials, improved lateral confinement results in an increase in the modulus of the base course material. The effect of increasing the modulus of the base course is an improved vertical stress distribution applied to the sub-grade and a corresponding reduction in the vertical strain on the top of the sub-grade. The second mechanism, improved bearing capacity, is achieved by shifting the failure envelope of the pavement system from the relatively weak sub-grade to the relatively strong base course material. The third fundamental reinforcement mechanism has been termed the “tensioned membrane effect.” The tensioned membrane effect is based upon the concept of an improved vertical stress distribution resulting from tensile stress in a deformed membrane. In the early stages of research regarding geo-grid reinforcement of pavement systems, the tensioned membrane effect was thought to be the primary reinforcement mechanism. However subsequent investigations have shown that reinforcement benefits are obtained without significant deformation of the pavement section. Thus, lateral restraint has been identified as the primary reinforcement mechanism, followed by the improved bearing capacity concept and the tensioned membrane effect. The actual contribution of each of these mechanisms to the overall reinforcement provided to the pavement system has yet to be quantified.

Aggregate-Surfaced Reinforced Pavement

Geo-grids in aggregate-surfaced roads can be used to support two pavement applications: mechanical sub-grade stabilization and aggregate base reinforcement. The application is determined by the sub-grade soil strength, and the type of geo-synthetic recommended for used in aggregate-surfaced roads is based upon the sub-grade soil conditions. Geosynthetic used to construct pavement over very soft sub-grade conditions typically serve to mechanically stabilize the sub-grade. As the design sub-grade strength increases, the primary application of the geo-synthetic transitions from mechanical sub-grade stabilization to base reinforcement.

Reinforced Flexible Pavement

Geo-grids can be used to accomplish both mechanical sub-grade stabilization and aggregate base reinforcement in flexible pavements. Like the aggregate-surfaced pavement design, the application is typically predetermined by the sub-grade soil strength. Different combinations of geo-synthetics are recommended for use in flexible pavements based upon the sub-grade soil conditions. Geo-synthetics used to construct roads and airfields over very soft sub-grade conditions typically serve to mechanically stabilize the sub-grade. As the design sub-grade strength increases, the primary application of the geo-synthetics transitions from mechanical sub-grade stabilization to base reinforcement.

Asphalt Concrete (AC) Overlay Reinforcement

Geo-textiles, geo-synthetic composites, and fibreglass grids have demonstrated success in reinforcing AC overlays, primarily in regard to reflective crack retardation. A secondary benefit, particularly of those geo-grids which develop an efficient interlock with surrounding asphalt, is that cracks are prevented from opening even after penetrating full depth.

Life Cycle Cost Analysis

A detailed cost analysis should be performed to determine if the geo-grid-reinforced pavement design is justified. The cost of the geo-synthetic-reinforced pavement section should be compared to the cost of an unreinforced pavement section. A direct cost comparison based upon material savings alone, however, does not include the indirect benefits of using geo-grid reinforcement. These indirect benefits include increased site mobility, improved ease of construction, reduced haul costs for additional aggregate and an improved ability to meet compaction requirements over soft sub-grades. These indirect benefits may compensate for slight increase in material costs.