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 primarily reinforce weak soils, improving stability and load-bearing capacity, and can also contribute to separation functions (though geogrids are primarily for reinforcement; geotextiles are more commonly used for separation). 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 soil is made of particles like sand and gravel, which makes it a bit rough. When the interaction between the soil particles and the geogrid is enhanced, greater friction and interlock are produced. 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.
- A rougher geogrid surface means that this material has a larger area to touch the soil, which in turn enhances shear resistance through extra friction.
- 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 big, soil particles can slip right through, which decreases interlock potential. 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
Design review:
The understanding of the project design is fundamental. These involve the geogrid type, dimensions, placement locations, and overlaps.
Soil properties:
Geogrids are affected by the soil’s attributes, such as its type, strength, and moisture content. These properties are the main considerations that can affect the selection and installation of the geogrid.
Site preparation:
The base surface should be clean and even. If there is any existing vegetation or unsuitable materials, they should be removed.
Equipment and personnel:
The equipment required must be checked first, such as rollers, excavators, and spreading machines. Employees who are involved in the installation process need to be well informed and trained.
Installation process
Geogrid placement
It is important to make sure that the geogrid is installed smoothly and uniformly, without any wrinkles or creases. Geogrid sheets normally have overlaps. The overlap length should stick to the 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.
To protect the slope surface before the establishment of permanent vegetation or final facing, a temporary erosion control measure (e.g., blanket or mat) was used. If a vegetated facing was part of the design, grass seed would be incorporated with the facing elements and topsoil. If a concrete facing was applied over a reinforced soil mass, vegetation might be established on areas adjacent to or above the concrete. 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!