Retaining Wall Design

Location

When choosing a location for your retaining wall, make sure you have a detailed understanding of property lines and both above ground and underground utilities including storm water management systems and irrigation. Additional factors to consider about the location might include:

Ground Slopes

If your wall is on a slope, where will you store additional infill that must be brought to the site?

Excess soil

If you are cutting into a hillside, where will the excess soil be stored?

Natural drainage patterns

Depending on the wall size, a retaining wall can impede on natural drainage patterns and have environmental consequences downstream.

If your wall is along the property boundary, will the reinforcement system encroach the property line?

Surcharge Loads. Will there be additional weight or vertical forces above the wall such as fencing, guardrails, driveways, parking lots, or swimming pools? Don’t forget about temporary construction equipment.

Soil Conditions

The soil that creates the foundation, or base, needs to be examined to ensure it meets the strength required to support your wall. You should determine the type, bearing capacity (the capacity of soil to support a load), stress parameters, and friction angle (resistance to movement) 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 it should not be moist. Wet soils such as clay soil are also not recommended for infilling. They are already saturated with water, so additional moisture cannot make its way through to the drainage channels. Also, in areas where there is freezing, wet soil can expand and contract which will damage the wall. On the other hand, sandy soils allow for good drainage. Consult the geotechnical report which describes the on-site soils, expansive soils, poor chemical properties, groundwater conditions, and more.

 Design: To begin the design, you must calculate the corresponding wall heights, footprint sizes, slopes, and the setback angle which are dependent on the site elevation and grade. You must also consider that gravity will cause the retained material to naturally move downslope. This should be counteracted within the design to minimize the amount of lateral earth pressure behind the wall, which, at maximum value, can ultimately overturn the wall. Your wall height is dependent on soil and slope, setback, and size of the block.

Wall Reinforcement: If gravity alone won’t support your wall, there are reinforcement methods available that depend on the wall type, height, design, friction, angle, soil material, and more. Mechanically stabilized earth (MSE) is soil with some means of artificial reinforcement such as steel or geosynthetics (such as geogrids). Geogrid is often made of a high-tensile fabric woven in to a grid pattern. It is placed in between the layers of retaining wall blocks and rolled back into the earth. Other types of reinforcement include soil nailing, earth anchors, rock bolts, among others

 Drainage: Considering that water is the most common reason that retaining walls fail, it’s essential to make sure your wall has good drainage and that there will be no buildup of water behind the wall. Identify potential surface water sources and make sure to drainage adjacent to the wall site has been accounted for. It is important to grade the site for drainage patterns and construct a drainage system behind the wall to minimize the amount of hydrostatic pressure the groundwater could create. An example drainage system could include backfilling with gravel, using drain pipes, and using “weep” holes to allow water to pass through the wall. Larger wall projects, such as those for transportation, should have a hydrology analysis done.

Choosing the right wall type

After evaluating the location, soil, design, and drainage, you can begin thinking about the type of retaining wall your project will need. There are a multitude of options. If the wall comes as a system, designers should work with the product manufacturer for absorption requirements, strength, height deviations, and more. Also, if needed, the appropriate reinforcement method should be used.