How is soil reinforcement done using geogrids?
The emergence of geosynthetic materials has brought about a noticeable change in civil engineering as they address the industry’s technical and economic challenges. These materials are primarily used to reinforce weak soils, improving their stability and load-bearing capacity. While geogrids have an open structure designed for interlock, other geosynthetics like geotextiles are often used for functions like separation, filtration, drainage, and reinforcement. In this blog post, we will discuss geogrid soil reinforcement, how geogrids strengthen soil, how geogrid-reinforced soils work, and the key points to consider during their design and installation.
What is geogrid soil reinforcement?
Geogrid soil reinforcement is an innovative engineering practice to improve soil strength and stability using different types of geogrids. A geogrid is a polymeric grid structure, available in uniaxial and biaxial forms, typically made from high-density polyethylene (HDPE) or polypropylene, or polyester (PET), which ensures durability and resistance to environmental factors. When installed in layers in the bulk of soil, the geogrid becomes intertwined with soil particles, creating a composite material with significantly enhanced properties. One of the main applications of geogrid-reinforced soil is in the construction of retaining walls.
Traditional retaining walls normally require deep excavation combined with concrete or masonry, which could be expensive and time-consuming. Geogrids allow the use of locally available soils for reinforcement, reducing both material costs and environmental impact. The geogrid acts as a vital reinforcement that resists tensile forces within the soil mass, preventing internal and external failure modes, thereby enabling steeper and higher walls.
What is the interaction between soil and geogrid?
Many parameters affect this complex interaction and are important for the performance of the constructed system. When a geogrid is placed in the soil, the friction between the soil particles and the geogrid’s surface, along with passive resistance from soil particles bearing against the geogrid’s transverse ribs (interlock/strike-through for some geogrid types), stops the reinforcement from being pulled out. The following factors influence this force or pullout resistance:
- Granular soils, composed of particles like sand and gravel, are ideal for geogrid interaction. Their frictional nature and particle size allow for a high degree of interlock with the geogrid apertures, generating significant pullout resistance. This is why granular soils are more effective in supporting geogrids overall.
- Clays and silts, classified as cohesive soils, are known for their binding nature. This can lead to different interaction mechanisms. While direct particle-on-polymer friction might be lower, adhesion can play a role, and the overall performance depends on the geogrid type and soil plasticity. Poorly draining cohesive soils can also lead to pore pressure issues affecting overall stability if not managed.
- The rigidity of a geogrid is the capability to withstand deformation under loads, maintaining the soil contact and enhancing the frictional resistance and interlock.
- The size of the openings, or apertures, in a geogrid is crucial for how it interacts with soil.
- If the openings are too large relative to the aggregate size, the soil particles may not effectively engage with the transverse ribs, which reduces the development of passive bearing resistance and interlock. On the other hand, if they’re too small, the soil particles won’t be able to interlock properly, which limits their interaction. Optimum aperture size allows for effective strike-through and interlock of aggregate particles, developing passive resistance against the geogrid’s transverse ribs when subjected to tensile forces. Normal stress (confining pressure from overlying soil) enhances both frictional resistance and the interlock mechanism.
- A geogrid layer with a greater embedment length provides a larger contact area for interaction with the soil mass and thus greater pullout resistance.
How does geogrid soil reinforcement work?
Geogrids use a rigid or flexible polymer grid structure to reinforce soil and improve its load-bearing capacity. To simplify, the soil provides compressive strength, and the geogrid gives the structure tensile strength to produce a composite material. First, the geogrid is embedded into the soil, allowing soil particles to interlock with the geogrid’s apertures. This interaction creates a bonded structure where the soil and geogrid work together as a single unit.
When the geogrid is under a load, it distributes stress over a larger area, helping to prevent local failure and ensuring not only local but also general stability. This magnified stability makes it a game-changer in many fields, such as road construction. A geogrid reinforcement is one of the most notable methods used in soil improvement, and one of the advantages is that it can reduce the need for high-quality imported fill materials by allowing the use of locally available soils, or it can enable steeper slopes and thinner structural sections, potentially reducing overall material quantities or excavation.
The aperture size of geogrids used in this process is a very important consideration here. Due to the design and material of the geogrids, they provide high tensile strength which is their typical behaviour. The right solution and proper installation are the two factors in each geogrid design; these are the main requirements to gain the desired reinforcement properties.
What are the guidelines for installing geogrid for soil reinforcement?
Pre-installation considerations for geogrid soil reinforcement
Successful geogrid performance depends heavily on the conditions established before installation begins. Pre-installation planning helps prevent common construction errors, ensures compatibility between the geogrid and site conditions, and allows the reinforcement system to function as designed. Do consider the following things:
Design review:
A thorough review of the project design is the first and most critical step before geogrid installation. The design specifications define the type of geogrid (uniaxial or biaxial), its strength requirements, aperture size, and orientation relative to the load direction.
Design drawings also indicate the exact placement location, layer spacing, embedment length, and overlap or connection details. Any deviation from the approved design can significantly reduce reinforcement performance, so installers must fully understand the drawings, specifications, and design intent before work begins.
Soil properties:
The performance of a geogrid is closely linked to the characteristics of the surrounding soil. Key properties such as soil type (granular or cohesive), shear strength, particle size distribution, and moisture content directly influence soil–geogrid interaction.
Poorly graded soils, excessive fines, or high moisture levels can reduce interlocking and friction, limiting the effectiveness of the reinforcement. Understanding these soil conditions helps in confirming whether the selected geogrid is suitable and whether additional measures – such as soil improvement or drainage – are required before installation.
Site preparation:
Proper site preparation ensures uniform load transfer and prevents localized stress on the geogrid. The subgrade surface should be level, smooth, and free of sharp objects that could damage the geogrid.
All vegetation, organic matter, debris, and unsuitable materials must be removed. Soft or weak areas should be identified and treated through compaction, replacement, or stabilization. A well-prepared base allows the geogrid to lie flat and perform as intended once the fill material is placed.
Equipment and personnel:
Before installation begins, all required equipment – such as excavators, rollers, compactors, and material spreaders – should be inspected to ensure proper operation. Equipment must be suitable for the site conditions and capable of placing fill material without displacing or damaging the geogrid.
Equally important is having trained and informed personnel. Installation crews should understand correct handling methods, placement orientation, overlap requirements, and safety practices. Proper training minimizes installation errors and ensures the geogrid system performs in line with the design expectations.
Installation process
Geogrid placement
It is important to make sure that the geogrid is installed smoothly and uniformly, without any wrinkles or creases. Geogrid placement must follow the design drawings precisely. For reinforcement applications like retaining walls, the geogrid is typically placed in continuous strips perpendicular to the wall face, and overlaps in this primary strength direction are generally not specified. In stabilization applications covering wide areas, overlaps between adjacent rolls may be required and must adhere to design specifications.
Soil compaction:
The fill placement and compaction should be carried out in layers, which are the design lifts specified in the project plans. Each layer of fill is required to undergo the process of being fully compacted to achieve sufficient density.
Geogrid protection:
Care must be taken during installation to prevent damage to the geogrid from construction equipment or the placement of overly sharp or large aggregate directly onto it. This usually involves ensuring a minimum lift thickness of fill material is placed and compacted over the geogrid before heavy equipment is allowed to traffic the area. In some cases, a sacrificial geotextile or a thicker initial soil layer might be used for added protection.
Draining:
Proper drainage is vital to prevent the buildup of excess pore water pressure within the reinforced soil mass, which can reduce its effective stress and stability. Geogrid performance itself is generally not decreased by water, but the soil it reinforces can be.
Quality control
Inspection of geogrid:
Perform regular checks on the geogrid to identify rips, folds, or wrong alignment.
Test for soil compaction:
Verify that the required soil density has been achieved using standard field tests such as a nuclear density gauge, sand cone method, or other approved procedures.
Documentation:
Keep accurate records of the process of installation, such as the amount of materials used, the machinery used, and the names of the personnel who were a part of it.
Case study of soil reinforcement using StrataGrid™
Client: Department of Transportation, South Carolina
Location: South Carolina, USA
Product used: StrataGridTM and StrataSlopeTM
Application: Construction of a steep soil slope
The South Carolina Department of Transportation (SCDOT) needed a new road next to a CSX railroad track. Strata’s engineering experts proposed the StrataSlope system. The StrataSlope system suggested by Strata’s engineering specialists utilizes StrataGrid 200 geogrid for geosynthetic reinforcement and a special slope covering.
The StrataSlope system incorporates a facing element to protect the slope surface from erosion. Depending on the design, this can be an erosion control blanket that aids in the establishment of vegetation, or it can serve as a foundation for a hard-armored facing like shotcrete. This allowed for options such as establishing permanent vegetation directly on the slope or applying a hard-armored concrete facing, depending on the project’s specific requirements. By doing so, we made sure the slope remained stable and resisted erosion even after construction. Strata’s solution astutely met the demand for a steep, stable slope. The design allowed for options such as concrete protection or the establishment of vegetation.
The effectiveness of these systems comes from combining the soil’s compressive strength with the geogrids’ tensile strength. The performance of geogrid-reinforced systems depends largely on proper soil-geogrid interaction, but success also relies on correct design, material choice, and installation.
Contact Strata Geosystems for reliable geogrid reinforcement solutions that improve soil stability and load-bearing capacity. Our expert team is ready to assist you with your construction projects, ensuring effective results and quality materials. Reach out today to discuss your project needs!