In rock engineering, geogrids used for soil reinforcement are gaining increasing attention in the civil engineering community and are being used more and more widely.
The main purpose of geogrids is to improve the engineering properties of soil and to reinforce and stabilize it.
Choosing the right geogrid for your project is essential to ensure long-term durability and cost-effectiveness. When selecting the right geogrid for your specific application, there are several key factors to consider:
- Engineering design requirements
- Selection of specifications
- Selection of materials
- Influence of production process
- Structural form
- Product strength test method
Geogrid Reinforced Soil Engineering Design Requirements
The first step in selecting the right geogrid is to define the project type and its intended application clearly.
Geogrids are used in a variety of projects, including road construction, embankment reinforcement, slope stabilization, and retaining wall construction.
Different applications may require geogrids with specific characteristics and properties to meet project requirements.
Geogrids are generally divided into biaxial geogrids and uniaxial geogrids, depending on the grid structure.
Uniaxial Geogrids
Uniaxial geogrids are the most commonly used type of retaining wall and are designed to withstand high tensile stresses in one direction, with less strength in the transverse direction.
Due to their strength and durability in stabilizing soil, they are ideal for reinforced soil retaining walls. They are often used where the wall height is significant and the forces applied are primarily vertical.
Biaxial Geogrids
Depending on the design of the wall and the characteristics of the soil, biaxial geogrids that provide strength in two directions may also be used. Biaxial geogrids have equal strength in the longitudinal (machine) and transverse (cross-machine) directions.
They provide effective reinforcement in two perpendicular directions, distributing loads more evenly. Biaxial geogrids are often used in applications where soil conditions are difficult to predict and stronger reinforcement is required.
Selection of Geogrid Specifications in Geogrid Reinforced Earth Wall Projects
Tensile Strength and Modulus
Geogrids should have sufficient tensile strength to resist applied forces and provide effective reinforcement. As a key parameter of geogrids, it indicates how well the geogrid can withstand tensile forces.
Tensile strength is often specified as Ultimate Tensile Strength (UTS) and is measured in units of force per unit width (e.g., kN/m or lbs/ft).
Tensile strength is often classified as low, medium, or high strength. UTS requirements depend on expected loads, stresses, wall height, and soil conditions.
Also, consider the modulus of the geogrid, which indicates its stiffness and ability to effectively distribute loads.
Pore Size and Shape
Pore size refers to the size of the openings between geogrid tendons or strands. It is usually specified as the maximum opening size or nominal pore size.
The pore size and shape of a geogrid are key factors affecting soil interaction, compaction, and aggregate interlocking.
Geogrids with larger pores are typically used for coarse-grained soils, while those with smaller pores are better suited for fine-grained soils.
The shape of the pores, whether square, rectangular, or triangular, also affects the performance of the geogrid and its interaction with soil particles.
Select geogrids with the appropriate pore size and shape to achieve effective soil interlocking and prevent soil intrusion into the geogrid structure.
Joint Strength/Efficiency
Joint efficiency is a measure of the ability of a geogrid to transfer tensile forces from one rib or strand to another. It also often refers to the strength of the joint within a geogrid structure.
It is expressed as a percentage and represents the strength of the joint between intersecting strands.
Higher joint strength indicates better load transfer ability to distribute loads.
When selecting a geogrid, consider the design and manufacturing quality to ensure that the joint strength is sufficient to meet your project requirements.
Typically, minimum joint efficiency requirements are specified in the project specifications.
Long-term Durability
The durability of a geogrid is critical to the longevity of a project.
Consider factors such as resistance to UV radiation, chemical exposure, biodegradation, and environmental conditions prevalent in the project area.
Geogrids with enhanced durability and resistance to degradation are critical, especially for long-term applications.
Specifications often include requirements for long-term durability and resistance to installation damage.
Strain Compatibility
Geogrids should exhibit strain compatibility with the surrounding soil to prevent excessive deformation and maintain stability.
Strain compatibility refers to the ability of the geogrid to deform and elongate with the soil without experiencing significant strain differences.
This property ensures that the geogrid and soil work together as a composite system.
Appropriate roll size for installation
Geogrids are typically supplied in rolls, with the size specified by the width and length of the roll.
Roll size should be appropriate for specific project requirements, taking into account factors such as wall height, reinforcement arrangement, and ease of installation.
Different geogrids may have specific installation requirements, including overlap distance, anchor trench depth, and connection method.
Some geogrids may require specialized equipment or techniques for proper installation.
Cost Considerations
Cost is always an important factor in any construction project.
While it is critical to select a geogrid that meets your project requirements, consider the overall cost-effectiveness, including initial purchase cost, installation cost, and long-term maintenance expenses.
Higher-quality geogrids can provide better long-term value by reducing maintenance and repair costs.
Comply with Standards and Regulations
Finally, make sure the geogrid you choose complies with relevant industry standards and regulations.
Different regions may have specific requirements for geosynthetics used in construction.
Compliance ensures your project meets safety and quality standards.
Selection of Geogrid Materials in Reinforced Soil Retaining Walls
The raw materials for producing geogrids are mainly high-density polyethylene (HDPE), polypropylene (PP), glass fiber, polyester, etc. Generally speaking, unidirectional geogrids are produced with polyethylene and bidirectional geogrids are produced with polypropylene. This is mainly because:
1. The molecular structure of high-density polyethylene is linear, with few branches and a crystallinity of up to 75%~90%. It has excellent mechanical strength, high rigidity and toughness, high surface strength and use temperature (80℃), good solvent resistance, acid, alkali and steam resistance, as well as good dimensional stability and environmental stress cracking resistance. It is very suitable for the production of unidirectional grids. Its disadvantage is that the elongation when reaching tensile strength is large, generally around 12%.
2. Polypropylene copolymer is a linear structure with side methyl groups. It has better mechanical strength and impact strength than polyethylene, higher rigidity and bending resistance, higher high-temperature resistance and chemical corrosion resistance. The disadvantage is that it is brittle at low temperatures and easy to age. It needs to be modified by stretching or other methods, so it is particularly suitable for bidirectional geogrids produced by bidirectional stretching.
3. Fiberglass geogrids are made from woven or stitched glass fibers coated with a polymer. They have excellent tensile strength, high modulus, and resistance to creep and temperature changes. Fiberglass geogrids are chemically inert and highly durable, making them suitable for applications where long-term performance is critical. However, they can be more expensive than plastic geogrids.
4. Polyester geogrids are made from high-strength polyester yarns coated with a protective polymer. They have high tensile strength, low elongation, and good resistance to environmental factors such as UV radiation and chemical exposure. Polyester geogrids are often used in mechanically stabilized earth (MSE) walls where soil reinforcement is required.
The choice of geogrid material for a specific geogrid-reinforced earth retaining wall project depends on factors such as project requirements, expected loads, soil conditions, and budget.
The Influence of Production Technology on the Selection of Geogrids
Geogrids can be classified according to their processing technology into integral stretched polymer geogrids, high-strength fiber woven geogrids and composite welded geogrids, the latter two of which are woven geogrids.
Geogrid Produced by Integral Stretching
It is generally formed by heating extruded sheet – precision punching – longitudinal stretching and then transverse stretching.
This stretching effect is very important. It re-orients the polymer molecules and greatly improves the performance of the grid:
1. The directional arrangement of molecules improves the strength of the material; at the same time, the directional molecules make the nodes better in integrity.
This is different from pseudo grids (such as woven and composite welded). The nodes of pseudo grids are woven or composite welded, with poor integrity and poor longitudinal and transverse force transmission performance.
2. The tensile modulus is improved, so that the grid can exert high tensile strength performance at low strain.
3. Long-term creep tests have proved that the tendency of polymer grids treated with stretching is greatly reduced under long-term continuous load, so that the reliability of reinforcement is guaranteed.
4. Stretching is a modification treatment for polypropylene grids, which improves many properties, such as greatly reducing the disadvantages of low-temperature brittleness and easy aging, and improving the durability of use.
Geogrid Produced by Weaving
It is woven with high-strength synthetic material ribbons, also known as high-strength fiber geogrid.
It is made of polyester fiber or glass fiber with high strength and low elongation. After being woven into a grid shape on a warp knitting machine, it is impregnated with polyvinyl chloride (PVC) according to process requirements.
The ribs of this grid have high tensile strength, its pseudo-nodes have poor integrity, the node strength is very low, the longitudinal and lateral force transmission performance is very poor, and the pull-out resistance in the soil is originally lower than its strength. As a reinforcement material, its strength is not fully utilized.
Composite Welded Geogrid
It is a pseudo-grid, which is made by weaving multiple polypropylene strips or steel-plastic composite strips and welding the nodes.
Its single reinforcement strength is relatively high. Since the nodes are formed by overlapping in the warp and weft directions, the overall strength depends on the welding strength of the nodes.
The shear strength, tear strength and bursting strength of this node are relatively low. The integrity is poor, the strength is low, the performance of the node in transmitting longitudinal and transverse forces is not good, and the dimensional stability and overall performance are relatively poor.
Summary of the Geogrid Production Process
The node strength test method proposed by Drexel University in the United States, and the results of the overall tensile geogrid and pseudo geogrid tested by it (node strength is expressed as a percentage of the strength of a single rib), are shown in the following table:
| Type | Node strength/single rib strength |
| Integrally stretched bidirectional geogrid | 90% – 100% |
| Integrally stretched bidirectional geogrid | <10% |
| The nodes are pseudo-bidirectional grids compiled | 3% – 13% |
Geogrids with good production technology have a uniform appearance, smooth surfaces, and an obvious luster of carbon black.
Geogrids with poor production technology have rough surfaces.
Although surface roughness and other patterns can increase friction a little, they cannot improve the overall reinforcement performance of the grid, because friction only accounts for a small part of its reinforcement performance, and the main reinforcement performance is the interlocking force and embedding effect between the grid mesh and the filler.
In addition to indicating that the processing technology of the rough surface grid is relatively low, the surface pattern marks and notches concentrate stress when subjected to external forces, which weakens its tensile performance and affects durability.
In addition, it is not economical to determine the anchor length based on friction.
Structural Selection of Geogrids in Reinforced Soil Engineering
The influence of structural form on the performance of geogrid is mainly manifested in two aspects: node form and substrate composition.
The overall stretched polymer geogrid has a single material, no load is required, the node is integral, the strength of a single rib and the node strength match well, and the overall strength is high.
The nodes of woven geogrids are pseudo-nodes, and the interwoven warp and weft ribbons at the nodes are only bonded by impregnated polyvinyl chloride (PVC), with low strength and poor force transmission performance.
The mesh size of welded composite geogrids is about 100mm. The mesh size of this grid is too large, which reduces the bending stiffness of the ribs, makes them easy to bend and deform, and reduces the bite force. This grid is made of two materials through compounding, the nodes are also pseudo-nodes, the node strength is low, and the force transmission performance is poor.
The composite plastic-steel strips may be damaged in the process of transportation, construction and use, cracks and ruptures, and moisture and moisture around it will cause corrosive corrosion to the rigid reinforcement, making the effective section smaller, and reducing resistance and service life.
Impact of Product Strength Testing Methods
The strength indicated by the geogrid above is measured by tensile test. Therefore, the tensile test method of the material and the significance of the test data should be one of the factors that should be paid special attention to during design.
The reinforcement of the geogrid with integral tensile strength is achieved by mechanical interlocking and interlocking with the soil particles.
Based on this, the standard GRI-GGI of the Geosynthetic Research Institute of the United States stipulates:
The measurement of tensile test data must comply with the transmission relationship of the force exerted on the grid in the soil.
It is also pointed out that the acquisition of longitudinal tensile force is inseparable from the right-angle transmission of transverse reinforcement, that is, the longitudinal tensile force of the geogrid in the soil is transmitted to the longitudinal reinforcement through mechanical interlocking with the transverse reinforcement.
Therefore, in the tensile test, the longitudinal tensile force is measured by clamping the transverse reinforcement and stretching it in the longitudinal direction.
Since the longitudinal and transverse ribs of the integral geogrid are whole, of course, the tensile strength can also be measured by directly clamping the longitudinal reinforcement and stretching it. The results obtained by the two are consistent without distortion.
The other two types of pseudo-grids are damaged due to low node strength. The longitudinal and transverse components that make up the grid are not a whole. When the bite force of the transverse ribs is transmitted to the node, it is damaged due to low node strength, causing the longitudinal reinforcement to slip and the reinforcement to fail.
Because of this situation, when measuring its tensile strength, the clamp can only be clamped on the longitudinal reinforcement. What is measured is the strength of the longitudinal reinforcement, not the overall strength of the grid. It is distorted to use the strength of the longitudinal reinforcement to represent the overall strength.
This is also the important reason why the pseudo-grid has a high marked strength but low actual strength, and it is also a fatal weakness as a reinforcement material. When selecting this type of geogrid for the construction of reinforced soil retaining walls, special attention should be paid!
Eindelijk
In conclusion, choosing the right geogrid for reinforced soil projects is a critical decision that is essential to ensure the stability, long-term performance and cost-effectiveness of retaining walls.
It is essential to consult a geotechnical engineer, review the project specifications and consider soil conditions, expected loads, grid specifications, materials, production processes, test methods, structure, etc. to make an informed decision on the type and material of geogrid to be used.
QIVOC is an experienced geogrid manufacturer and supplier. Our years of accumulated product and project experience may be of great help to you in choosing the right geogrid. If you have any needs, please feel free to contact us.
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