When should you use geogrid for a retaining wall?
A retaining wall is a structure designed to hold back soil or other materials and prevent them from collapsing. Retaining walls are near-vertical structures designed to hold back soil on one side and create level areas on both sides of the slope. These structures must remain stable, endure changing climatic conditions, and withstand installation with regular wear and tear. These structures are erected to maintain soil stability on one side, thereby facilitating construction on the level areas. Under defined loading conditions, retaining walls alone are not enough to ensure long-term stability or to prevent slope failures. Slope stabilization requires the integration of geogrids, which stabilize and reinforce layers of soil and similar materials. These grids are made from polymer plastics such as polypropylene, polyethylene, or polyester.
What are retaining walls?
The primary role of a retaining wall is to create level ground near sloping terrain, prevent soil erosion, and provide structural support for buildings, roads, or other structures built on or near slopes. By restraining soil laterally at different elevations, retaining walls enable construction on challenging topography. Retaining walls create stable foundations for buildings on slopes, shaping the land for agriculture and transportation infrastructure. These walls are used to construct bridges, roads, basements, and other structures, where retaining embankments or soil in a nearly vertical position is critical. The wall’s load distribution must be managed carefully, considering the active and passive pressures being applied. Active pressure determines the wall’s minimum lateral resistance, while passive pressure determines its maximum lateral resistance. This is where geogrids become a solution, redistributing loads through their tensile strength.
When does a retaining wall need reinforcement?
While some retaining walls (like gravity walls) rely solely on their mass to hold back soil, others require reinforcement to withstand lateral earth pressures. Here’s a breakdown of factors that should influence your decision on reinforcing a retaining wall:
1. Wall height:
An increase in wall height results in higher pressure exerted by the soil on the wall. Walls that are not reinforced and rely on their own mass for stability have a limit beyond which the wall’s mass becomes inadequate to resist overturning forces. This limit depends on soil properties but typically ranges between 1 m (3.3 ft) and 4 m (13 ft). While permissible limits vary by local building codes, these are general guidelines.
2. Soil type and stability:
The restrained soil directly impacts the retaining wall’s strength, as each soil type has a unique angle of repose and friction. Since soil varies in its particle size and other factors, the need for reinforcement also varies accordingly.
Cohesive soils (Clay):
Cohesive soils, such as clay, stick together due to fine particles and moisture, which increases their internal friction and raises soil stability. They resist deformation, which raises the soil’s internal stability. Reinforcement is recommended for taller walls (> 3m) or when dealing with loose cohesive soils with friction angles below 20 degrees.
Cohesionless soils (Sand):
Cohesionless soils, such as sand and gravel, consist of loose, granular particles that do not stick together and rely on external forces for stability. Retaining walls holding back such soils almost always require reinforcement, particularly for taller walls.
The geocells manufactured by Strata are strategically perforated to confine the in-fill material while allowing liquids to pass through. Thus, material migration is minimized while reducing hydrostatic pressure. It’s worth mentioning that the design of Strata’s geocells takes into account the optimal cavity distance to ensure the structure meets core confinement requirements without deformation. This fundamental aspect of geocell design makes StrataWeb a preferred product for a nuanced understanding of geocell perforations.
3. Surcharge loads:
Additional weight acting on the top of the wall or behind the backfill (e.g., traffic loads, stockpiles) significantly increases the design pressure. Surcharge loads, like traffic which have a pressure of 200-800 psf* or stockpiles (300-1000 psf), can dramatically increase the pressure on retaining walls by up to 50%. Geogrid reinforcement becomes a requisite in such cases to ensure the wall can handle these added loads.
4. Drainage considerations:
Excessive water buildup behind the wall raises hydrostatic pressure. Again, this is a function of the soil type, which must also be considered while building a retaining wall. Proper drainage systems become key in such cases to minimize the hydrostatic pressure to considered level In some instances, geogrids also reduce the amount of backfill needed through improved distribution of loads, thus reducing costs. However, in the event of high moisture content, geotextiles are recommended in cases of high moisture content, as they provide drainage, reinforcement, and separation depending on their application.
*psf= pounds per square foot
Types of retaining walls
Gravity walls:
Gravity walls depend on their own mass to resist earth pressure. They are made of materials such as concrete or stone and are appropriate for lower walls (under 4 meters) because of their reliance on weight. Gravity wall design needs to consider the risk of overturning, sliding, and bearing capacity to ensure stability. Furthermore, poor soil conditions, high surcharge, or high loads can lead to a situation where a geogrid is needed for reinforcement and load distribution.
Cantilever walls:
Cantilever walls use a thin, reinforced concrete slab that cantilevers out from a base slab to restrain soil. Highly efficient in terms of material usage when compared to gravity walls (typically used for 8–10 meters), they also require proper foundation design to overcome overturning moments exceeding 150% of the stabilizing moment (weight of the wall multiplied by the distance to its center of gravity). While the structure itself is stable, many civil engineers consider the use of a geogrid to stabilize the basal structure in embankments over soft soils. This is done to ensure robust bearing capacity, prevent failure of the base, mitigate settlement issues, and achieve overall stability of the cantilever base.
Counterfort walls and buttress walls:
Counterfort walls and buttress walls are similar, as they incorporate vertical elements (counterforts or buttresses) at regular intervals behind the wall to bolster against overturning. The difference lies in the location of the vertical elements. Counterforts are constructed within the backfill, while buttresses are found on the front face of the wall. These walls are appropriate for taller retaining walls above 8 meters in height.
Gabion walls:
Gabion walls consist of rectangular wire baskets filled with rocks or stones. These structures are flexible and can accommodate some settlement. They are well-suited for applications where aesthetics or drainage take precedence. Gabion walls are usually constructed using galvanized steel or stainless steel coated wires for durability.
Crib retaining walls:
These walls utilize interlocking precast concrete, forming a cellular structure that is filled with granular material. Crib retaining walls are helpful in draining water and are used for lower to mid-height retaining walls (typically under 6 meters).
Selecting geogrids based on wall-height:
One key component in choosing geogrids for wall retention is the height of the wall to be reinforced. Heights pose a challenge owing to the slope height and the amount of pressure they are likely to experience.
Low walls (under 3 meters):
Relatively shorter walls may suffice with geogrids of moderate tensile strength. When combined with well-compacted, high-quality soil and a stable wall design, these geogrids effectively minimize lateral earth pressure.
Medium walls (3-6 meters):
An increase in wall height leads to a corresponding increase in lateral earth pressure. A geogrid with a higher tensile strength may be used to evenly distribute and absorb these forces. Additionally, engineers may choose a multi-layered geogrid system, placing layers strategically within the backfill for taller sections of the wall.
High walls (over 6 meters):
Such imposing structures require a durable geogrid reinforcement system for stability and safety. These high-tensile layers are positioned strategically within the backfill to create a composite mass that can handle the large amount of lateral earth pressure. Collaboration with geotechnical engineers is advisable, as their expertise is essential in selecting the optimal type, strength, and configuration for such demanding situations.
A safer future with geogrids
Reinforcing retaining walls is crucial to ensuring the integrity of these structures. By carefully considering the factors requiring reinforcement, engineers can determine the best course of action regarding material and design. Geogrids create a composite mass with the surrounding soil, making them a popular choice to bolster retaining walls. Constructing such structures should be done with meticulous planning and collaboration with experts owing to the high stakes involved. Work with India’s leading geosynthetics brand known for its manufacturing capabilities, alongside zero failure RS Walls. Contact us today to see how we can help you find the ideal solution for your site.