In general, geogrid is used to distribute concentrated loads over a wider area – sometimes under footings, roadways, or other areas that require subgrade support. Geogrid’s other most common use is to act as reinforcement in a retaining wall. In this article, I will cover both cases. If you’re a homeowner or builder, and you just want to know if geogrid is worth the investment – it is. It’s relatively inexpensive and will make your driveway, patio, subgrade much stronger.
The most common types of geogrid are uniaxial, biaxial and triaxial grids. Grid type is selected based on the type of loading the soil is expected to encounter. In a retaining wall, the primary expected loading is perpendicular to the wall face, so uniaxial grids are used in retaining walls. In applications such as roadways and parking lots, where loading can be in any direction, a biaxial or triaxial grid should be used.
How Does Geogrid Work?
We’ve had a few of clients ask us “What does geogrid do in road construction?”, or “How does geogrid help soil support more load?”. The answer is simple, but the explanation is not quite as straightforward.
In short, geogrid spreads concentrated loads from vehicles or footings over a wider area, which reduces settlement and movement. This is the answer we usually give on-site, and most of the time it satisfies whoever has asked the question. But once, one of our clients wanted a bit more information. He said:
“Well I know what geogrid is for, but how does it actually spread the load out?”
Glad you asked.
Imagine a box that you’ve loosely filled with sand. If you pushed down on the sand in the box, you’d expect your hand to sink into the sand for some distance, then stop when you met resistance. This resistance is from all the particles of sand in the box rubbing against each other and squeezing closer and closer together until finally, all the friction between all the pieces of sand equals the force you’re using to push down on the sand.
Soil giving way and moving around under roads, footings, or foundations is called settlement.
Geotechnical engineers don’t like settlement. There are a few ways we combat it, and one of them is with geogrid.
By using geogrid, the particles of sand would find something solid to push against much sooner. Something they couldn’t slide past. The force exerted onto the particles of sand would instead be transferred into the geogrid as they find themselves interlocked within the geogrid. The geogrid provides tensile resistance that is much stronger than the frictional forces that were at first providing resistance to your hand.
The geogrid is able to transfer the load experienced by the soil by allowing some of the interlocking particles within the soil to penetrate through the apertures of the grid. The particles which have penetrated through the aperture are then forced into the ribs of the geogrid by the pressures above, causing the ribs to experience tension, and spreading the loading over the surface area of the grid.
This larger surface area means less pressure on the soils below the grid, less movement for the soils above the grid, less settlement of the surface, and an all-around improved strength of the soil structure. For such basic technology, it really is quite impressive. If you’re building a driveway for your home, definitely make sure your contractor is using geogrid. It’s cheap and highly effective.
Take a look at this YouTube video to see how much of an impact geogrid can have on the strength of a subgrade.
How Does Geogrid Work in Retaining Walls?
Most of the time, uniaxial geogrids are used in retaining walls. A uniaxial geogrid doesn’t work quite the same way as a biaxial or triaxial grid that we discussed above because a uniaxial grid can only be loaded in one direction – along its strong axis. This means that it doesn’t control settlement so much as it acts as an anchor.
On a uniaxial geogrid, you can easily see which axis is the strong axis. It is thicker and stiffer than the thinner pieces of fabric/thread holding the grid together. When building a retaining wall, the thick axis should run perpendicular to the face of the retaining wall. This is critical to the performance of the grid, and by extension, the wall.
Uniaxial geogrid uses the soil’s own weight to support the wall, effectively anchoring the wall to the soil and causing the entire retained soil body to act as a singular mass with the wall, holding back the unreinforced soil behind. The grid is placed on top of a block course and then pinned by the next course of blocks, making sure that the grid is positioned such that its strong axis is perpendicular to the wall face. The approved design backfill material should then be placed at the wall and compacted towards the end of the grid, further tensioning the grid.
NOTE THAT A PROPERLY DESIGNED RETAINING WALL SHOULD HAVE A DRAINAGE SYSTEM. DRAINAGE HAS BEEN LEFT OUT IN THIS DRAWING FOR CLARITY.
In the image above, the light brown soil is being reinforced by the geogrid. This reinforced soil is compacted during construction in such a way that it actually pulls back on the retaining wall blocks.
The soil that would have been pushing the retaining wall over in an unreinforced wall, is actually helping to hold it up in a reinforced wall! This locks the retained soil and blocks together, causing the entire mass to act against the unreinforced soil.
Geogrid placement, lengths, and type are important factors and are included in all of our design packages (and are inspected during construction to make sure everything is built to specification).
Conclusion
I hope I’ve answered any questions you may have about geogrid and how it works. If you’re building a retaining wall, driveway, or anything else in Calgary and are wondering if you need geogrid – drop us a line on our contact page.
