Over the last couple of months, the Tensar representative and I have been giving presentations to a number of engineering firms. The presentation is a basic explanation of why grids, and more specifically, Triax grids perform in paved and unpaved roads. It starts out with three videos that show the three mechanisms the US Army Corps of Engineers identified and defined as to why grid performs so well. The three videos shown are as follows:
These are all fairly simplistic concepts, but they start in the middle of the story and we who promote and educate on Tensar products forget that everyone was not around in the beginning. Not everyone eats, sleeps, and breathes geogrid. So, Coleman Moore Company, and others like us, need to explain that the grid is part of a system. That system is designed to carry a dynamic load and includes soil, which is the subgrade, and the geogrid, the aggregate.
The soil, geogrid, and aggregate work in conjunction with one another to support vehicle loads and the above paved section. The paving does help support loads, but it should be apparent from driving in any city that it is in dire need of help! Let’s look at the three parts of the system.
Soil and the California Bearing Ratio
Soil (subgrade) has strength, varying from extremely solid to the consistency of pudding. Routinely if the subgrade is soft-to-unstable, it is because the soil has more than optimum moisture content for that particular soil. In general, the more moisture, the weaker the subgrade. The strength of the soil is reported by several different methods. Tensar Corp will convert those values over to CBR % (California Bearing Ratio). The lower the value, the weaker the soil. CBR values are logarithmic. If the soil with a CBR strength of 80% is compared to one with 20%, the strength difference is not that noticeable but a soil with a CBR value of 4% is compared to one with 1%, it is extreme.
The second component is geogrid. The geogrids have different-shaped apertures and varying sizes to those apertures. What geogrid to use? This then becomes tricky. Depending on soil strength, aggregate available, aggregate depth (this means budget), and traffic loads will determine the appropriate grid or grids. Meaning a lower scale grid can do the same as a higher scale grid if the more aggregate is placed on the lower scale grid. The geogrid’s raw material, orientation of ribs, aperture size and shape, and the strength of the junction of ribs all contribute what value a geogrid can contribute to the system. Comparing Tensar’s first generation of geogrids (biaxial geogrids) to the their current iteration ( triaxial geogrids), through years of large-scale independent testing, the triaxial geogrids are more efficient at carrying the loads. In essence, they had a pretty good snowshoe, but engineers figured out how to build an even better snowshoe. I use the snowshoe metaphor because both the snowshoe and geogrid make the impossible possible by spreading a load out over a larger area.
Aggregates and Geogrids
The third component of the system is aggregate. The aggregate needs to be sized appropriately for the aperture size of the geogrid. The aggregate needs to be angular in nature (a crushed aggregate). Rounded aggregate, like river stone and gravel, will perform poorly, as it will move around like marbles on a table. The system performs best when the optimum aggregate size is place with the geogrid.
How Geogrid and Aggregate Work
Geogrid and aggregate work when the aggregate can strike through the grid. The ribs of the geogrid restrict movement, aggregate is prevented from moving laterally away from the applied load. Tensar established a recommended gradation of aggregate for their grids years ago. They knew when encountering soft soil that the soil wanted to pump up into the aggregate base in conventional paving sections. So, to ensure this doesn’t happen, a well-graded aggregate is used. In effect, they designed a natural filter with the grid, thereby separating the aggregate, which is being confined by the geogrid, from a saturated subgrade. This function of geogrid and aggregate being a separator is not intuitive.
Geogrids and the Army Corp of Engineers
Typically, engineers are taught the only geosynthetic that can perform the function of separation is a geotextile. The Army Corp of Engineers have placed millions of tons of rip rap along our nation’s rivers on top of an aggregate filter base to keep the river banks from eroding under the large rip rap stone. This is the same design concept that Tensar uses to perform the function of separation in roadways. When companies promote the use of geogrids, they tend to want to talk about the geogrid itself, or in my case, the three mechanisms put forth by the Corp of Engineers or local projects that have involved Coleman Moore Company. We don’t often get a chance, due to time constraints to delve into the entire system. We need to do a better job of explaining that the geogrid is only one part of a system that has allowed Tensar and Coleman Moore Company improve millions of square yards of pavement in the State of Iowa alone.