--- title: "Earth Reinforcement: How Reinforced Soil Works and Where Geomembranes Fit" date: 2021-11-28 author: "Pawan Khatri" url: https://oceangeosynthetics.com/geomembranes-for-earth-reinforcement/ --- November 28, 2021 | [Geomembrane](https://oceangeosynthetics.com/category/geomembrane/), [Insights](https://oceangeosynthetics.com/category/insights/) # Earth Reinforcement: How Reinforced Soil Works and Where Geomembranes Fit Earth reinforcement means placing tensile elements inside compacted fill so the soil can stand steeper, carry more load or sit on weaker ground than it could alone. Soil is good in compression and almost useless in tension. Reinforced soil fixes that by adding layers of steel strip, geogrid, woven geotextile or polymer strip that pick up the tension the soil cannot. Geomembranes are often named in this context, but a geomembrane is a barrier: it stops water and adds no useful tensile strength. This article explains how the reinforcement works, which materials do the job, and where a geomembrane does belong in a reinforced soil structure. ## How earth reinforcement works Picture a vertical cut in sandy soil. Left unsupported, a wedge of soil behind the face slides down and outwards along a failure plane. Anything that stops that wedge from stretching sideways keeps the face standing. A layer of reinforcement laid horizontally through the fill does exactly that. As the soil tries to strain outwards, it drags on the reinforcement, and the reinforcement goes into tension. The load passes from soil to reinforcement in two ways: - **Friction** along the surface of a strip or sheet. Rough surfaces and granular, high-friction fill give the most grip. - **Passive bearing** against the cross members of a geogrid. Soil particles lock into the apertures and push against the transverse ribs, much like an anchor plate in the ground. Each layer must extend far enough beyond the potential failure plane to anchor itself in stable soil. If it is too short it pulls out; if it is too weak it snaps. Design checks both. The result is a block of soil and reinforcement that behaves as one heavy, flexible mass, which then holds back the ground behind it like a gravity wall. The modern method dates from the 1960s, when the French engineer Henri Vidal developed Reinforced Earth using metal strips. Polymer reinforcement followed, and today most reinforced soil walls on Indian highway projects use either steel strips or polymeric geogrids and strips. ## Extensible and inextensible reinforcement The material changes how the composite behaves. IRC:SP:102-2014, the Indian Roads Congress guideline for reinforced soil walls, follows BS 8006 in drawing the line at strain: reinforcement that carries its design load at 1% total axial strain or less is treated as inextensible, and above that as extensible. | Reinforcement | Behaviour | Main design concern | | --- | --- | --- | | Galvanised steel strips or mesh | Inextensible, very little creep | Corrosion: SP:102 sets resistivity, chloride, sulphate and pH limits on the fill | | Polyester (PET) geogrids and strips | Extensible, low creep | Creep and hydrolysis over the design life | | HDPE or PP extruded geogrids | Extensible | Creep at site temperature, installation damage | | High-strength woven geotextile | Extensible, sheet form | Lower pullout grip than a grid; useful for wrap-around faces | | Geocells | Three-dimensional confinement | Used as foundation mattresses and stacked facings more than as tension layers | | Discrete fibres mixed into soil | Random, all directions | Mostly research and small works | Polymer reinforcement creeps, so its usable strength is far below the number printed on the roll. SP:102 builds the long-term design strength from the characteristic tensile strength (measured by the wide-width test, ISO 10319 or ASTM D6637) divided by reduction factors for manufacture, creep over the design life, installation damage and environmental damage. It asks for at least 10,000 hours of creep data on polymeric reinforcement and sets a design life of 100 years for walls on highway and bridge works. ## Where reinforced soil is used in India - **Retaining walls at flyover and ROB approaches.** The most visible use: precast concrete panel or block facings with the approach embankment built as reinforced fill behind them. For how such a wall is laid out, see our guide to a [retaining wall with geotextile and geogrid reinforcement](https://oceangeosynthetics.com/retaining-wall-with-geotextile-and-geogrid-reinforcement/). - **Steepened slopes.** SP:102 covers faces of 70 degrees or steeper. Below that, the structure is a reinforced slope, often with a vegetated or wrap-around face. - **Basal reinforcement.** A strong geogrid or geotextile at the base of an embankment on soft clay spreads the load and holds the toe from spreading. Ocean’s [basal reinforcement solutions](https://oceangeosynthetics.com/solution/basal-reinforcement-soil-stability/) cover this case. - **Pavement and formation layers.** [Biaxial geogrid](https://oceangeosynthetics.com/products/geogrid-manufacturers/biaxial-geogrid/) under granular layers restrains lateral spreading of aggregate. Ocean supplied 80,000 sq m of biaxial geogrid 30 kN for [railway work at Hotgi, Maharashtra](https://oceangeosynthetics.com/railway-with-ocean-biaxial-geogrid-30kn/), and uniaxial geogrid 40 kN for [road construction in Guwahati](https://oceangeosynthetics.com/road-reinforcement-in-guwahati-using-uniaxial-geogrids/). ## Where a geomembrane belongs in a reinforced soil structure Water is the most common reason reinforced soil misbehaves. Saturated fill is heavier, loses strength and, if it cannot drain, pushes on the facing with hydrostatic pressure that most reinforced walls are not designed for. SP:102 says as much: RS structures are normally not designed for hydrostatic pressure, so drainage has to keep it away. A geomembrane helps by keeping water out in the first place. Typical positions: 1. **A cap over the reinforced zone** where surface water would otherwise soak into the fill, for example under an unpaved verge or median at the top of a wall, with a drain to carry the water clear. 2. **The water face of a reinforced embankment** that retains a pond, canal or storage reservoir. The reinforcement holds the slope; the liner holds the water. 3. **A corrosion barrier for steel reinforcement**, where salty or aggressive water could reach the strips. 4. **Lined containment built on reinforced ground**, such as a landfill cell or effluent pond on a geogrid-reinforced base. Two cautions. First, a geomembrane is smooth, and the interface between it and soil can have a low friction angle. Placed inside or under a reinforced mass, it creates a potential slip surface, so the designer should check interface shear, usually measured with ASTM D5321 tests on the actual materials. Textured sheet helps on slopes. Second, a liner traps water as well as excluding it, so it must always have a drainage path on the side where water can collect. Ocean supplies [HDPE sheets](https://oceangeosynthetics.com/products/ocean-geomembrane/high-density-polyethylene-sheets-hdpe/) for containment and a [waterproofing sheet](https://oceangeosynthetics.com/products/ocean-geomembrane/waterproofing-sheet/) for below-ground barriers, alongside its reinforcement range. ## Choosing reinforcement: an illustrative check Illustrative only: a 6 m reinforced soil wall with granular fill of unit weight 18 kN/cu m and friction angle 32 degrees. The active earth pressure coefficient Ka = (1 – sin 32°)/(1 + sin 32°) is about 0.31, so horizontal stress at the base is about 0.31 × 18 × 6 = 33 kPa before any surcharge. With layers 0.6 m apart, the bottom layer must hold roughly 33 × 0.6 = 20 kN per metre of wall, and more once traffic surcharge and safety factors are added. The reinforcement’s long-term design strength, after all reduction factors, has to exceed that figure, and the layer must be long enough to resist pullout. SP:102 sets a minimum length of 0.7H or 3 m, whichever is more, so 4.2 m here. The same arithmetic shows why grade selection changes with depth: the top layers carry a fraction of the bottom layers’ load, so many designs step the grade down towards the crest. ## Frequently asked questions ### Is a geomembrane used for earth reinforcement? No. A geomembrane is an impermeable sheet used as a barrier. Its tensile strength is not used to reinforce soil, and its smooth surface can lower stability if placed carelessly inside fill. Reinforcement comes from geogrids, woven geotextiles, polymer strips or steel strips. ### What is the difference between reinforced soil and reinforced earth? Reinforced Earth was the name Henri Vidal gave his system using metal strips. Reinforced soil is the general term covering any reinforcement, including geogrids and geotextiles. In Indian practice the phrases are often used loosely for the same kind of wall. ### Which Indian guideline covers reinforced soil walls? IRC:SP:102-2014, Guidelines for Design and Construction of Reinforced Soil Walls, covers walls at bridge, flyover and ROB approaches and high road embankments. It refers to the MoRTH Specifications for Road and Bridge Works for materials and tolerances. ### Can fly ash be used as reinforced fill? SP:102 allows fly ash conforming to IRC:SP:58 as both reinforced and retained fill, with periodic quality checks. With steel reinforcement, the fill’s electrochemical properties must also be checked for corrosion risk. ## Talk to Ocean about your reinforced soil works Ocean supplies [geogrids](https://oceangeosynthetics.com/products/geogrid-manufacturers/), geotextiles, geocells and geomembranes for reinforced soil walls, slopes and embankments. 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