Retaining walls are structures designed to bound soils between two different elevations. A retaining wall is then mainly exposed to lateral pressures from the retained soil plus any other surcharge. Many retaining walls are cantilever-type, but it’s also common to find in practice walls that are laterally restrained at the top, such as in the case of a basement retaining wall supported laterally by an elevated floor slab. This article provides engineering background for the design of either concrete or masonry top restrained retaining walls. Our software ASDIP RETAIN will be used to support our discussion.
- Click here to download ASDIP RETAIN free 15-day trial.
What are the components of a basement retaining wall?
A typical basement retaining wall is composed of four main components: the Stem which is the vertical portion, the Toe is the portion of the footing at the front of the wall, the Heel at the backfill side, and the Shear Key below the footing. Frequently the basement walls are also laterally restrained at the base by the slab on grade, so the shear key is seldom required. The images below show the geometry of a typical top restrained retaining wall.
What lateral earth pressure theory to use?
There are two well-known classical earth pressure theories: Rankine and Coulomb. This subject has been discussed in our previous post Cantilever Retaining Walls: Overview of the Design Process. Since a restrained wall cannot tilt away from the soil to mobilize its shear strength, the active pressure cannot be developed. As a result, a basement retaining wall should be designed using the At-rest Ko pressure coefficient, instead of the Active Ka commonly used in cantilever walls.
ASDIP RETAIN uses any of the theories described above. In addition, the Equivalent Fluid method may be selected, which assumes the backfill as a fluid of a given density.
What are the typical loads on a basement retaining wall?
Being the purpose of a retaining wall to maximize the land usage, normally there are surcharge loads on top of the retained mass. These loads may be dead or live, uniformly distributed or concentrated. Driveways, parking lots, equipment, etc, are examples of surcharges in a basement wall. A uniform surcharge will produce a uniform lateral pressure on the wall. A roadway running parallel to the wall may be modeled with a Strip load, and it may be calculated using the Boussinesq approach.
Sometimes the stem extends above the backfill level and this portion of the wall could be exposed to a wind pressure. If the wall is located in a seismic zone, then the seismic effects need to be considered. In this case the Mononobe-Okabe approach is usually followed, which is based on the Coulomb theory previously discussed. The picture below shows schematically the external loads in a typical restrained retaining wall.
How do you check the basement wall stability?
In the post referenced above we discussed the four basic instability modes in a typical retaining wall:
- Sliding – The backfill and the other applied loads exert a lateral pressure against the wall, so it will tend to slide. If the wall is not laterally restrained at the base, then the sliding mode should be checked for a minimum safety factor of 1.50.
- Overturning – Since a restrained retaining wall is laterally supported at the top, the overturning mode of failure is therefore prevented.
- Soil bearing – Usually the bearing pressure under the footing is rectangular or trapezoidal, with the maximum bearing pressure at the end of the toe. The allowable soil bearing pressure should be provided by the soils report, which already includes a safety factor of about 3.0.
- Global instability – It assumes that a failure surface develops under the wall, causing a massive disturbance and movement of the soil along this surface. This check is a complex analysis that falls in the field of the geotechnical engineering.
The image below shows a restrained retaining wall designed by ASDIP RETAIN with the magnitude and location of the loads that affect the stability analysis, sorted by load combination. The calculated safety factors are also shown for immediate check.
How do you design the stem?
Since the stem is laterally supported at the top, it can be modeled either as pinned or fixed at base, so it may act as either a pin-pin or a fix-pin beam, and the resulting shears and moments will be completely different. In some cases it may be convenient to fix the base, so that the stem will be lighter even if the footing gets penalized. The stem must be designed for the maximum positive and negative moments, and the shear must be checked at the critical section located a distance d above the top of the footing.
Sometimes a basement wall extends two levels down, which means that the retaining wall needs to be designed considering lateral supports at the base and at the top, plus an additional intermediate support. ASDIP RETAIN allows to model this kind of walls as well.
The image below shows the different pressures on the stem of a typical two-story basement retaining wall, sorted by load combination. Note the shear and moment diagrams generated by ASDIP RETAIN for a fix-pin stem, where the shaded area represents the structural capacity of the stem. If the stem is overloaded at any point, the problem can be immediately identified.
How do you design the footing?
Most basement walls are supported on ground, but they could also be supported on piles. ASDIP RETAIN allows to model both scenarios. The discussion below assumes that the wall is supported directly on ground.
The heel is subjected to the vertical loads acting on the back side of the wall, including the backfill weight and any surcharge. All these loads tend to push the heel down, which acts as a cantilever beam where both the maximum moment and the critical section for shear occur at the face of the stem. The reinforcing steel at the heel should be placed at the top side of the footing.
The toe is another cantilever beam subjected to an upward pressure from the soil reaction. The weight of the fill material on top of the toe should also be considered. In this case, the maximum moment occurs at the front face of the stem, but the shear critical section occurs at a distance d from the stem face. The reinforcing steel at the toe should be placed at the bottom side of the footing.
The picture below shows the construction information of a typical restrained retaining wall. In some cases may be convenient to cut off the alternate backfill vertical rebars at a certain height in order to optimize the design.
The design of restrained retaining walls may be cumbersome and time-consuming, particularly for two-story basement walls. In such cases the calculation of shears and moments as a result of the backfill pressures may become complex. ASDIP RETAIN includes the design of basement retaining walls, with multiple options to optimize the design in less time.
For software usage, please read the blog post How to Design Basement Retaining Wall Using ASDIP RETAIN. For a footing design example, please see the blog post Basement Wall Design Example Using ASDIP RETAIN. For our collection of blog posts about retaining walls please visit Structural Retaining Wall Design.
Javier Encinas, PE
ASDIP Structural Software