When should you use geogrid for a retaining wall?

A retaining wall is defined as a structure that holds back the soil or other materials from collapsing. Since they handle lateral pressure, these structures need to be stable, must handle pressure despite changing climatic conditions and survive installation and regular wear and tear. These structures are ere cted to maintain soil stability on one side, and in turn helping with construction on the level areas. Under specific loading conditions, retaining walls alone are not enough to ensure long-term stability, avoid slope failures. This requires the integration of geogrids to stabilize and reinforce layers of soils and similar materials. They are made from polymer plastics like polypropylene, polyethylene, or polyester.

What are retaining walls?

Retaining walls are near-vertical structures designed to hold back soil on one side to create level areas on both sides of slopes. Principally, the role of a retaining wall is to create level areas near sloping terrain, prevent soil erosion, and provide structural support to buildings, roads, or other structures built on or near sloping ground. By laterally restraining soil at different elevations, retaining walls allow construction over challenging topographical terrains. Retaining walls create stable foundations for buildings on slopes to shape land for agriculture and transportation infrastructure. Such walls are used in the constructions of structures such as bridges, roads, basements etc., where it is critical to retain embankments or soil in an approximately vertical position. Depending on the active and passive pressure being applied, the load distribution must be managed. Active pressure is used to determine the wall’s minimum required lateral resistance, while passive pressure is used to determine the wall’s maximum required lateral resistance. This is where geogrids come in as a useful solution to redistribute loads, with responsive tensile strength.

When does a retaining wall need reinforcement?

The integrity of retaining walls is vital for the safety and longevity of construction projects. While some retaining walls (like gravity walls) can hold back soil solely on their own mass, others often need reinforcement to withstand the lateral earth pressures exerted by the retained soil.

Here’s a breakdown of factors that should influence your decision on reinforcing a retaining wall:

1. Wall height:

An increase in the height of the wall is consequently marked with 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 is dependent on soil properties but generally falls between 1 m (3.3 ft) and 4 m (13 ft). While the exact permissible limits will vary based on area and local building codes, these are general guidelines to be followed.

2. Soil type and stability:

The soil being restrained itself has an impact on the retaining wall’s strength since each soil type has a different angle of repose and angle of friction. Since soil varies in their particle size, and other factors, correspondingly, the need for reinforcement also varies.

Cohesive soils (Clay):

Cohesive soils, such as clay, are known for their ability to stick together due to the presence of fine particles and moisture. This internal friction and cohesion make them quite different from non-cohesive soils like sand. They can resist deformation which raises the internal stability of the soil. Reinforcement is recommended for taller walls (> 3m) or loose cohesive soils with low friction angles (less than 20 degrees).

Cohesionless soils (Sand):

Cohesionless soil is a type of soil that does not stick together and is composed of loose, granular particles such as sand and gravel. These soils rely on external forces to maintain stability. Retaining walls that are set to hold back cohesionless soils almost always need reinforcement, more so for taller walls.

Since the geocells manufactured by Strata are perforated, strategically, they confine the in-fill material, while also letting liquids pass through. Thus, the material migration is minimised, relatively, while reducing hydrostatic pressure. It’s worth mentioning that the design for Strata’s geocells take into account the optimal distance of the cavities to ensure the structure meets the core confinement without deformation requirement. This is one of the most fundamental aspects of geocell design that makes our StrataWeb a chosen product –for a nuanced understanding of the implications 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 buildup of water behind the retaining wall can also raise the pressure by adding hydrostatic pressure. Again, this is a function of the soil type, too which must 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, we recommend geotextiles since they serve multiple purposes of drainage, reinforcement, and separation, depending on the layer where they are applied.

*psf= pounds per square foot

Types of retaining walls

Gravity walls:

Gravity Walls depend on their own mass to resist earth pressure. They are constructed from 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. Further 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 need 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 that bearing capacity is robust, prevent failure of the base, settlement issues, and 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 constitute 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 seek precedence. Gabion walls are usually constructed using galvanized steel or stainless steel coated wires for durability.

Crib retaining walls:

These walls make use of interlocking precast concrete that form 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:

Apart from the soil conditions one key component in choosing geogrids for wall retention is the height of the wall that will be reinforced. Heights pose a challenge due to the slope height, and the amount of pressure it’s likely to experience.

Low walls (under 3 meters):

Walls that are relatively shorter, may suffice with geogrids with moderate tensile strength. When combined with well-compacted, high-quality soil and a stable wall design, these geogrids will work just fine to minimize lateral earth pressure.

Medium walls (3-6 meters):

An increase in wall height means a subsequent increase in the lateral earth pressure exerted. A geogrid with a higher tensile strength may be used to evenly distribute and absorb these forces. To add to it, engineers might choose a multi-layered geogrid system with layers placed strategically within the backfill for taller sections of the wall.

High walls (over 6 meters):

More imposing structures like such require a highly robust geogrid reinforcement system to ensure 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 should also be considered as their specific expertise is helpful in selecting the optimal type, strength and configuration for such demanding situations.

A safer future with geogrids

The reinforcement of retaining walls is principal in ensuring the integrity of these structures remains intact. Through careful consideration of the factors that contribute to the need for reinforcement, engineers can choose the best possible course of action in terms of material and design. Geogrids have the ability to create a composite mass with the surrounding soil which makes 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.

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When should you use geogrid for a retaining wall?

What are retaining walls?