On page 26 of this issue, Riley Stendel from Wolf Paving describes how they used a product called ‘Glasgrid’ on the Coffee Road project in New Berlin. The subject of base grids was mentioned again during a seminar at the National Pavement Expo (NPE) in January, and so we thought we’d dig into these products a little further.
Geosynthetics are separated into two main categories: geotextiles and geogrids. Both products can be used to separate materials in the base, allow drainage vertically and add additional strength. When used efficiently, James Curtis, president of CHEC Management Systems and NPE speaker, says these items can be more cost effective than adding additional base or asphalt, plus they preserve any elevation constraints the job may have.
Geotextiles: Geotextiles are permeable fabrics that are typically made from polypropylene or polyester. When used with soil, these ‘filter fabrics’ can help separate, drain, reinforce and protect between two layers. Geotextiles offer a cost-effective engineered alternative for scarce and difficult to locate natural construction materials. These materials are often used to improve the performance of civil engineering and construction projects including roads, railways and landfills.
Geogrids: Building a reliable foundation over poor soil can be expensive, time-consuming, labor-intensive, difficult to stage and environmentally challenging. Geogrids create stiffened platforms that distribute loads more efficiently over underlying soils, increasing their effective bearing capacity and decreasing differential settlement. The result: foundations where structural integrity is enhanced, construction schedules are reduced and project costs are lowered.
Geogrids are primarily designed for structural reinforcement. They provide a high level of stabilization which reduces the need for conventional designs, such as gradual slopes or cantilever walls. As fill is placed on the geogrids, the mesh design locks soil in place, providing differing levels of stability, depending on the type of geogrid used.
The material used in the Wolf Paving project was a glasgrid for pavement reinforcement. Since the soil was so bad, the municipality sought an economical solution to fixing the base without having to complete a total excavation. A glasgrid is composed of fiberglass strands coated with an elastomeric polymer and formed into a grid structure. Each strand has a remarkably high tensile strength and high modulus of elasticity; this is particularly important as asphalt concrete typically cracks at low strains.
When the glasgrid system is sandwiched between the leveling course and the surface course in an asphalt overlay, it becomes the hidden strength in the road, turning vertical crack stresses horizontally to effectively dissipate them.
These system can be easily installed without specialized equipment or labor. With the pressure-activated adhesive coating on the underside, it’s also one of the fastest interlayer systems installed and up to 25,000 square yards of grid can be placed in one day with a standard laydown unit. Installation readily adapts to local weather conditions or unique construction requirements.
Chec mentioned other modifications that can be made to a soil or aggregate base material that can improve structural properties for construction:
Foam Asphalt: Aggregates and hot liquid asphalt or bitumen are mixed together in this base stabilizing method. A hot bitumen truck starts the train, followed by a grinder. A small, precise amount of water is injected into the heated bitumen so millions of bubbles are formed, which enhances the coating properties when applied to the pulverized surface. The water evaporates and the machine returns the material to the roadway to be compacted, first by a pneumatic roller, then a smooth drum vibratory roller.
“The decrease of movement in the pavement is phenomenal,” says Chec.
Lime Treated Base: Soil stabilization occurs when lime is added to a reactive soil to generate long-term strength gain through a pozzolanic reaction. You must have a reactive soil, a good mix design protocol and reliable construction practices for this stabilization method to work. “Lime can substantially increase the stability, impermeability and load-bearing capacity of the subgrade,” says Chec.
Enzymes: “Enzymes are biological catalysts that speed up the natural rate of reaction between substances,” says Chec. This can be beneficial when mixed with a catalyst to speed up a reaction in soil. By mixing an enzyme with soil and water, then compacting, a reaction occurs that strengthens the material significantly.
“Enzymed soil is similar to concrete,” adds Chec. It’s environmentally friendly and up to 80% less expensive to use than traditional methods for stabilizing soil.