Geomembranes for Earth Reinforcement
Reinforced soil is a composite building material made up of earth and reinforcement. This material has strong compressive and tensile strength, similar to reinforced cement concrete in theory. It can be achieved by either inserting continuous reinforcement inclusions (such as strip, bar, sheet, mat, or net) in a specific pattern inside a soil mass or by randomly mixing discrete fibres with a soil fill before installation. The former is referred to as’reinforced soil,’ but it is more correctly referred to as’systematically reinforced soil,’ whilst the latter is referred to as ‘randomly distributed/oriented fibre-reinforced soil,’ or simply ‘fibre-reinforced soil’ (Shukla et al., 2009). Although reinforced soil has been utilised in some form since ancient times, it has been increasingly popular in civil engineering applications since Henry Vidal, a French architect and engineer, developed the contemporary type of soil reinforcement in 1966.
The reinforcement in most modern civil engineering applications is often geosynthetic sheets or strips of galvanised steel, positioned horizontally or in the directions where the soil is subject to unwanted tensile stresses. Metal strips are thought to be somewhat inextensible in comparison to geosynthetic sheets at the stress levels encountered in civil engineering applications. Metal strips were employed as reinforcement in the early days, and the resulting composite material was dubbed “Reinforced Earth” by Henry Vidal (1966, 1969), who was the first to propose the idea of strengthening the strength of a soil mass by including reinforcements inside it. To avoid slippage between the soil and the reinforcement, the soil should ideally be cohesionless and have strong frictional qualities. For similar reasons, the surface roughness of the reinforcement should be as rough as feasible.
The ostensibly simple reinforced soil mechanism, as well as the cost and time savings, has made it an instant success in geotechnical and transportation engineering applications for both temporary and permanent constructions. Continuous inclusions may be used to reinforce soil-like materials like coal ashes and other waste materials, which is a cost-effective way to improve their mechanical qualities. A reinforced soil retaining wall (Fig. 1) is a frequent use of soil reinforcement that offers an alternative to a traditional heavy concrete/brick masonry/stone masonry retaining wall (Fig. 2). As a result of its incorporation, reinforcement increases the mechanical characteristics of a soil mass. In reality, in geotechnical constructions such as retaining walls, soil slopes, bridge abutments, foundation rafts, and so on, any reinforcement, whether inextensible or extensible, has the primary goal of resisting applied tensile loads or avoiding inadmissible deformations. The reinforcement functions as a tensile component in this process.
The idea of strengthening soil using fibres, particularly natural ones, dates back to antiquity. Reinforced soils made of clayey soils and natural fibres are still used to make containers, ovens, toys, and other items in several nations, including India. However, in geotechnical engineering, randomly dispersed fibre-reinforced soils have recently gotten a lot of attention. Randomly dispersed fibre-reinforced soils have a few benefits over systematically reinforced soils. Soil stability by admixtures is simulated by preparing randomly dispersed fibre-reinforced soils. Discrete fibres, like cement, lime, or other additions, are simply put to the soil and mixed in. Randomly dispersed fibres provide strength isotropy and help to prevent possible planes of weakness from developing parallel to the directed reinforcement found in systematically reinforced soil.
SOIL THAT HAS BEEN SYSTEMATICALLY REINFORCED
Systematically reinforced soil is a type of soil that has been reinforced in desirable directions by geosynthetic (woven geotextile/ geogrid/ geocomposite) sheets or strips of galvanised steel. It is now commonly utilised in civil engineering practise. It’s largely because such a reinforced soil has a number of unique properties that make it ideal for the building of geotechnical constructions. Handling, storing, and installing the reinforcement is simple. The dirt that makes up the majority of its mass may be locally accessible and easily deposited in place using contemporary hauling and compaction equipment in a short amount of time. The flexible nature of reinforced soil mass allows it to tolerate earthquake-induced vibrations as well as huge differential settlements without suffering major damage. Thus, geotechnical constructions may be built over poor and challenging subsoil conditions using systematically reinforced soil.
The behaviour of soil supplemented using extensible reinforcements, such as geosynthetics, does not totally conform to the above-mentioned ideas. In terms of the load-settlement behaviour of the reinforced soil system, the difference between the affects of inextensible and extensible reinforcements is considerable. When compared to soil alone or soil reinforced with inextensible reinforcement, dubbed “reinforced earth” by Vidal, the soil reinforced with extensible reinforcement, dubbed “ply-soil” by McGown and Andrawes (1977), has better extensibility and less post-peak strength losses (1966, 1969). Despite certain variances in their behaviour, both ply soil and reinforced earth have one thing in common: they both use tensile stresses in the reinforcement to prevent internal and border deformations of the soil mass. In other words, tensile strain inclusion occurs in both the normal soil and the reinforced earth.
Because the geosynthetic may operate as a tensile member due to two separate processes: shear and anchoring, Jewell (1996) and Koerner (2005) discuss not two but three methods for soil reinforcement. As a result, the three reinforcing mechanisms, which are only concerned with the sorts of loads sustained by the geosynthetic, are as follows:
- Shear, commonly known as sliding: The geosynthetic bears a planar load as the earth slides over it.
- Anchorage, also known as pullout: Due to its withdrawal from the earth, the geosynthetic can support a planar load.
Shukla (2002, 2004, 2012) and Shukla and Yin (2006) discuss reinforcing processes that consider the geosynthetic’s reinforcement action, or how the geosynthetic reinforcement absorbs stresses from the soil and what kind of stresses it takes. The following responsibilities of geosynthetics can be seen as examples of this concept:
- The outward horizontal loads (shear stresses) transmitted from the overlaying soil/fill to the top of the underlying foundation soil are reduced by a geosynthetic layer. Shear stress reduction is the name given to this effect of geosynthetics. This action causes a general-shear failure rather than a local-shear failure (Fig. 7(a)), resulting in an increase in the foundation’s load-bearing capability. As a result of the shear contact mechanism, the geosynthetic can increase the system’s performance with little or no rutting.
- A geosynthetic layer redistributes the surface load through restraint of the granular fill if incorporated in it, or restraint of the granular fill and the soft foundation soil if put at their interface, reducing applied normal stress on the underlying foundation soil (Fig. 7(b)). This is known as the slab effect or geosynthetic confinement effect. The friction that is created between the soil and the geosynthetic layer is crucial in restricting the soil.
- The deformed geosynthetic has a membrane force with a vertical component that resists applied loads, i.e. the deformed geosynthetic offers vertical support to the underlying soil mass subject to loading while sustaining normal and shear stresses. The membrane effect (Fig. 7(c)) is a popular term for how geosynthetics work. The membrane support can be characterised as ‘normal stress membrane support’ or ‘interfacial shear stress membrane support’ depending on the kind of stresses – normal stress and shear stress – sustained by the geosynthetic during their operation (Espinoza and Bray, 1995). To generate the membrane support contribution coming from normal stresses, the geosynthetic layer’s edges must be secured, but the membrane support contribution resulting from mobilising interfacial membrane shear stresses does not require any anchoring.
- The interlocking of the soil through the apertures (openings between the longitudinal and transverse ribs, often greater than 6.35 mm of the grid known as interlocking effect (Fig. 7(d)) of geogrids has additional benefit. Stress is transferred from the soil to the geogrid reinforcement through bearing (passive resistance) at the soil-grid crossbar contact. It’s worth noting that, because to the limited surface area and huge apertures of geogrids, contact is mostly due to interlocking rather than friction. When the soil particles are tiny, however, there is an exception. Because no passive strength is created against the geogrid in this circumstance, the interlocking effect is insignificant.