--- title: "When to Use Geogrid in a Retaining Wall: A Decision Guide" date: 2021-09-28 author: "Pawan Khatri" url: https://oceangeosynthetics.com/when-to-use-geogrid-in-retaining-wall/ --- September 28, 2021 | [Geogrids](https://oceangeosynthetics.com/category/geogrids/), [Insights](https://oceangeosynthetics.com/category/insights/) # When to Use Geogrid in a Retaining Wall: A Decision Guide Use geogrid in a retaining wall when the wall’s own weight can no longer hold back the soil behind it: when it gets tall, carries load on top, has a slope above it, or sits on ground that needs a wider base. Below that point a gravity wall of blocks, stone or concrete is enough. Above it, geogrid in the retaining wall turns the backfill into part of the structure and is usually the cheaper way to gain height. This guide helps you decide which side of the line your wall falls on, and when geogrid is the wrong answer even for a tall wall. ## Why a gravity wall runs out of height A gravity wall stands because its weight resists sliding and overturning. The trouble is how the two sides grow. Earth pressure on the back of a wall rises with the square of its height. The wall’s weight, for a given block depth, rises only in direct proportion to height. Double the height and the push from the soil roughly quadruples while the wall’s weight only doubles. Sooner or later the soil wins, and the only ways to keep up are a much deeper wall or reinforcement. ### An illustrative check Illustrative numbers only. Take a wall of solid, filled units 0.6 m deep (front to back) weighing about 20 kN/cu m, retaining level sandy fill of 18 kN/cu m with a friction angle of 32 degrees (Ka about 0.31), no surcharge and no batter. | Height | Soil thrust (kN/m) | Wall weight (kN/m) | Sliding safety factor (base friction 0.6) | Overturning safety factor | | --- | --- | --- | --- | --- | | 1.0 m | 2.8 | 12 | 2.6 | 3.9 | | 1.5 m | 6.3 | 18 | 1.7 | 1.7 | | 2.0 m | 11.2 | 24 | 1.3 | 1.0 | Against the safety factors commonly sought (around 1.5 for sliding and 2 for overturning), this wall is already short on overturning at 1.5 m and fails both checks by 2 m. Batter, interlocking units and heavier blocks push the limit up a little; a road or a parked truck above pulls it down. Block manufacturers publish their own unreinforced height limits for each unit, and those charts, checked by an engineer, should govern a real wall. ## Signs your wall needs geogrid Work through these. If any one applies, plan for a reinforced wall and get it designed. 1. **Height beyond the gravity limit for the unit.** Check the unit maker’s chart; as the example shows, even a deep unit can reach its limit below 2 m. 2. **Surcharge on top.** A road, parking area, building footing, crane pad or stockpile within a distance of about the wall height from the face adds a lot of lateral pressure. 3. **A slope above the wall.** Sloping backfill increases thrust sharply compared with level ground. 4. **A slope below the wall.** The toe has less support and the wall needs a wider effective base. 5. **Tiered walls.** An upper wall set back a short distance loads the lower one. Treat tiers as one system unless they are far apart. 6. **Weak or variable foundation.** A wide reinforced block spreads load and tolerates settlement better than a narrow, stiff wall. 7. **Seismic region.** Earthquake load adds to the thrust, and the seismic zone under IS 1893 should be considered in the design. Reinforced soil walls are flexible and generally tolerate shaking well when properly designed. 8. **Poor drainage or clayey backfill.** Geogrid does not cure bad drainage, but a reinforced design forces the select fill and drainage details that a casual gravity wall often skips. ## When geogrid is not the right choice A reinforced soil wall needs room behind the face. IRC:SP:102-2014 sets a minimum reinforcement length of 0.7 times the design height or 3 m, whichever is greater, so a 5 m wall needs about 3.5 m of excavation and fill behind it. Look at other systems when: - **There is no space behind the wall**, for example against a property line, an existing building or a rock cut that cannot be excavated. RCC cantilever walls, soil nailing or anchored walls fit tighter sites. - **You are retaining a natural cut, not building up fill.** A reinforced soil wall is built from the bottom up in placed fill. Cutting into a hillside to make room for it can destabilise the slope. - **The face is in flowing water.** Riverbanks and canal sides need scour protection; gabions, which tolerate settlement and drain freely, often suit better. - **Good fill is not available and cannot be bought economically.** SP:102 allows some silty or clayey gravels with extra drainage, but plastic clay is a poor reinforced fill. ## Comparing the options | Wall type | Practical height range | Space needed behind face | Tolerates settlement | Notes | | --- | --- | --- | --- | --- | | Gravity blocks or masonry | Low walls only | Small | Poorly | Simple, limited by weight | | RCC cantilever | Medium to tall | Heel width only | Poorly | Needs formwork, steel, curing | | [Gabion box](https://oceangeosynthetics.com/products/gabions/) gravity wall | Low to medium | Base width | Well | Free-draining, suits hill roads and waterways | | Geogrid reinforced soil | Tall walls, highway approaches | 0.7H or 3 m minimum | Well | Uses site or local fill, thin facing | For a cost view of the gabion route, see [are gabions cheaper than a retaining wall](https://oceangeosynthetics.com/are-gabions-cheaper-than-a-retaining-wall/). Ocean has supplied [rope gabions for retaining walls at Talcher, Odisha](https://oceangeosynthetics.com/ocean-rope-gabions-for-retaining-walls-in-talcher-odisha/) and [gabion boxes for slope stability in Srinagar](https://oceangeosynthetics.com/mountain-engineering-in-srinagar-using-gabion-boxes/). For a wider comparison of wall types, read the [retaining walls guide](https://oceangeosynthetics.com/retaining-walls-guide-how-to-choose-the-right-walls/). ## Choosing the geogrid once you have decided - **Primary reinforcement** is normally uniaxial geogrid, with its strong direction running back from the face. Strength grades step down from the bottom of the wall to the top. - **Foundation and secondary layers** often use biaxial or [polypropylene geogrid](https://oceangeosynthetics.com/products/geogrid-manufacturers/polypropylene-geogrid/). - **Filter and separation** layers are non-woven geotextile, not geogrid. The design steps, with a worked example of length and spacing, are in our article on [retaining walls with geotextile and geogrid reinforcement](https://oceangeosynthetics.com/retaining-wall-with-geotextile-and-geogrid-reinforcement/). The site sequence is in [how to install geogrid in a retaining wall](https://oceangeosynthetics.com/how-do-you-install-geogrid-on-a-retaining-wall/). ## Frequently asked questions ### At what height does a retaining wall need geogrid? There is no single number. It depends on the block depth and weight, the soil, any surcharge and any slope. A simple illustrative check shows a 0.6 m deep gravity wall running short of safety margin at about 1.5 m. Use the block maker’s charts and an engineer’s design. ### Does a short wall ever need geogrid? Yes, when it carries a driveway, road, building or slope above it, when walls are tiered, or when the foundation is weak. These loads can make a 1 m wall behave like a much taller one. ### Can geogrid be added to an existing wall that is leaning? Not by itself. Geogrid has to be built into the fill in layers as the wall rises. A leaning wall is usually taken down and rebuilt as a reinforced wall, or strengthened by other means such as soil nails. ### Is a geogrid wall suitable for a riverbank? Only with scour protection at the toe and a design for water level changes. Gabions or geobag revetments are often a better fit where the wall faces flowing water. ## Get advice on your wall Send the wall height, what sits above it and the site soil to Ocean through the [contact page](https://oceangeosynthetics.com/contact/), and we will help you work out whether a reinforced wall and which geogrid grades suit the job. 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