There are a range of considerations to consider when designing and constructing retaining walls, regardless of whether it is intended for residential or commercial use. The same level of care should be extended to walls as to bridges, since they are both structures. Depending on your application and experience, you might wonder what kind of wall is best suited to the site, how deep the foundation needs to be buried, whether it needs to be strengthened, or even what material is best suited to the site.
Before you start designing your wall and choosing aesthetics, you need to consider the position and environmental factors that can make your wall fail. Building a retaining wall requires meticulous preparation and careful layout to prevent it from being unsafe or collapsing. This article discusses some of the factors that need to be addressed early in the design process along with advice on selecting the right form of retaining wall for your project.
Guidance on Design
The purpose of all retaining walls is to keep the soil behind them. The basic needs can, however, vary depending on the project. Walls can range from small landscape stone walls that surround the garden to large land-retaining projects along the highway. Others may help manage hard rain erosion or build a terraced yard to minimize maintenance. When you start preparing, there are a range of factors that will influence the material and form of wall you are constructing. Below we’re looking at four of them.
Location
When choosing a location for your wall, make sure you have a detailed understanding of the property lines and the above ground and underground infrastructure, including stormwater and irrigation systems. Additional considerations to consider on the location can include:
- If your wall is on the hill, where are you going to store the additional infill that needs to be brought to the site?
- Excess property. If you cut through a hillside, where will the excess soil be stored?
- Natural patterns of drainage. Depending on the scale of the wall, the retaining wall can interfere with natural drainage patterns and have environmental implications downstream.
- If your wall is along the property boundary, can the reinforcement device intrude on the property line?
- Surcharge of loads. Will there be additional weight or vertical forces above the wall, such as barriers, guardrails, driveways, parking lots, or swimming pools? Don’t forget about temporary building equipment.
Soil
The soil that forms the foundation, or base, must be tested to ensure that it meets the strength needed to support the wall. You can decide the form, bearing capacity (the load-bearing capacity of the soil), stress parameters, and angle of friction (moving resistance) of the soil used for the foundation and reinforced zone along with the retained soil zone.
In general, the base soil needs to be firm, solid and strong, and should not be damp. Wet soils such as clay soil are also not recommended for filling. They are already filled with water, which ensures that extra moisture cannot find its way into the drainage canals. Often, in places where there is freezing, wet soil may expand and contract to damage the wall. Sandy soils, on the other hand, allow for good drainage. Consult the geotechnical study detailing on-site soils, expansive soils, weak chemical properties, groundwater levels, and more.
Design
In order to start the design, you must measure the corresponding wall heights, footprint sizes, slopes and angles of setback that depend on the elevation and grade of the site. You must also remember that gravity would allow the retained material to travel downhill naturally. This should be counteracted within the design to minimize the amount of lateral earth pressure behind the wall, which can eventually overturn the wall at its highest value. The height of your wall depends on the soil and slope, the setback and the size of the block.
If gravity alone does not support your wall, there are available reinforcement methods that rely on the type of wall, height, design, friction, angle, soil content, and more. Mechanically stabilized soil (MSE) is a soil with some means of artificial reinforcement, such as steel or geosynthetic (such as geogrids). Geogrid is mostly made from a high-tensile fabric woven into a grid pattern. It is placed between the layers of the retaining wall blocks and rolled back into the earth. Other forms of reinforcement include soil nailing, field anchors, rock bolts, among others.
Drainage
Considering that water is the most common cause that retaining walls fails, it is important to ensure that the wall is well drained and that there is no build-up of water behind the wall. Identify possible surface water sources and ensure that drainage adjacent to the wall site is accounted for. It is necessary to identify the site for drainage patterns and to create a drainage system behind the wall to minimize the amount of hydrostatic pressure that the groundwater may cause. An example of a drainage system could involve the backfilling of gravel, the use of drain pipes and the use of “weep” holes to allow water to pass through the wall. Larger wall projects, such as transport projects, should have a hydrology analysis carried out.
Please refer to the NCMA Segmental Retaining Walls Best Practices Guide for a checklist of these considerations and more.
Choosing the right wall
After assessing the location, soil, design and drainage, you can start thinking about the type of retaining wall that your project would require. There are a multitude of choices. If the wall comes as a device, designers should collaborate with the manufacturer of the product to meet the specifications for absorption, power, height deviations and more. The required form of reinforcement should also be used, if necessary.
Gravity Retaining Walls
Applications: Straight or curved walls typically below 4-feet high, but depending on product requirements.
Gravity walls use their own weight to support the soil behind them and are usually constructed of heavy materials such as brick, large concrete blocks or cast-in concrete. They lean back into the soil with interlocking edges and use their mass to withstand pressure from behind. Gravity walls can be small (under 4 feet high) or up to 10 feet without reinforcement.
Segmental Retaining Walls
Applications: Different industrial and residential applications used for both straight and curved designs. They can be built to conform to any shape while optimizing the usable space of the site, especially on high-rise terrain. When strengthened, they normally have no height restrictions.
Segmental retaining walls (SRWs) are intended to be used as a wall of gravity with or without reinforcement and can be over 40′ in height. SRWs are modular blocks made of concrete that are mostly stacked dry without mortar. In order to prevent overturning and slipping, individual units interlock with each other. Since they are manufactured in a factory, they comply with industry standards and are standardized in terms of weight, strength and durability. They also come in a range of colors, sizes and textures to suit the esthetic requirements of your project. Some brands sell pins or clips to help create a continuous framework.
Cantilevered Retaining Walls
Applications: deep excavations up to 18 feet in height.
Cantilever retaining walls function on leveraging principles and are mostly constructed in the form of an inverted T with reinforced concrete or mortared masonry. Less building material is required for a cantilevered wall than a wall of gravity, and can be poured on site or produced in a precast concrete facility. They consist of a relatively thin stem and a base slab, divided into two parts: the heel and the toe. The heel is the part of the base under the backfill, while the toe is the other part of the base. For these walls, which are typically below 25′ high, rigid concrete bases are required.
Counterfort Retaining Walls
Applications: Tall walls of 20 to 40 feet.
Similar to the cantilever walls, the walls of the counterfort require support along the back of the wall. They use concrete webs, also known as counterforts, designed at an angle to improve the wall’s stability. These webs are located at regular intervals along the length of the wall and reduce the natural pressures on the wall from the soil while also increasing the weight of the wall. These are preferred over cantilever walls where the wall is more than 25′ wide.
Sheet or Bored Pile Retaining Walls
Applications: Temporary deep excavations in small spaces, such as around underwater sites, cofferdams, and seawalls, along with structural columns, pier shafts, and more.
Sheet pile retaining walls are constructed of precast concrete, steel, vinyl or wooden planks and are used in soft soils and in tight spaces. The boards are pushed into the ground by friction and hammering to ensure their stability and can be connected by groove and tongue. The walls of the workshop include an anchor of some kind, which is fixed to the wall and then built into the soil. They are suitable for use along the waterfront and can assist with beach erosion, shoring, excavation, or cofferdams.
Bored piles are also used when the vibrations of the battery drivers are too high to tolerate sheet piles. Anchor is often required to stabilize the walls, but not always.
Panel Retaining Walls
Applications: It can be put in small spaces with a restricted right-of-way.
You can see wall panels used in large applications, such as highway ramps, or near places where there are heavy loads. They can also be used to attach panels made of precast concrete reinforced with steel. Face panels can be custom built with different architectural finishes to blend into the surrounding environment.
Gabion Mesh Retaining Walls
Applications: In stream or water applications where there is a concern for erosion. It may also be used in military applications to protect against fire-fighting or in a modern residential or commercial environment where the wire mesh is filled with recycled or aesthetically pleasing material.
Gabion-style walls use wire mesh boxes filled with stones and rocks, and then stacked together. The boxes are attached to the wire and bent down towards the hill. Gabion retaining walls have a lifespan as long as the wire used to tie them together (the wire will eventually corrode).
Conclusion
Whichever wall system you choose is best for your project, make sure that you have thoroughly assessed the location, soil and drainage requirements of your project. Many manufacturers have their own team of engineers at their disposal, who work on wall design and can provide support.
Additionally, CMQ Consulting Engineers offers retaining wall service that can help increase your design efficiency and turn a concept into a comprehensive design.
