Reliability-Based Design Applied to Retaining Walls

Critical Factors for Retaining Walls

• Reliability-based design for any project involving retaining walls rests on three critical factors concerning the area to be retained: the type of soil composition involved, the cubic footage of the area, and the characteristics of the groundwater surrounding the area. These critical factors are best left to soil engineers for study and recommendation when considering retaining wall projects. You may or may not feel it necessary to hire this kind of expert help, or you may not have the resources to do so. If either of these scenarios applies to your situation, it is still wise to do some research concerning reliability-based design for retaining walls. This research can be done in the library or online, as well as through contact with your local building code department.

Soil Pressure

• Soil composition and movement of the soil are perhaps the most important factors to consider. In fact, the force of the pressure created by soil movement against retaining walls is the No. 1 cause of failure. This force, known as lateral earth pressure, is driven by the natural movement of soil that is affected by gravity, groundwater factors and the type and extent of plant growth in the area, as well as the overall size of the area to be retained. It is generally true for building codes within the United States that all retaining walls exceeding 4 feet in height have to be designed and approved by licensed soil engineers. However, reliability-based design for retaining walls of any size and type depends wholly on your understanding of the soil pressure characteristics involved, including smaller projects of 1 or 2 feet in height.

Groundwater Factors

• The proper drainage of the groundwater present in the area of the project is also crucial. This groundwater consists not only of the natural water table found below the soil in your area but all water added to this supply by both rainfall and any sprinkler or irrigation systems you use. This is why it is important to include sufficient drainage for retaining walls for a reliability-based design. This is accomplished by creating holes — known as weepholes — across the bottom layer of the retaining walls to collect and remove water buildup behind the retaining walls. Weepholes, supported by drain tiles or perforated drain lines built into and on top of the backfill, ensure that the hydrostatic pressure of the groundwater will not cause failure in these projects.

Building Material Factors

• The type of building materials you choose is yet another important factor in a reliability-based design for retaining walls. The rule of thumb for retaining walls is that the amount of soil behind the wall determines the height of the wall. Smaller retaining walls under 3 feet in height are well served by using wood or masonry as building materials. However, larger soil expanses require more stable and complex building materials, including preformed or poured concrete bolstered by reinforcing steel. Examples of these types of retaining walls include cantilevered, buttressed and counterfort templates to ensure reliability-based design.