Retaining wall design considerations to IBC 2018 and ASCE 7-16

What a retaining wall does

A retaining wall acts like a dam for soil: it holds back earth to allow height differentials across a site. Without one, soil at a higher elevation naturally disperses over time. Retaining walls make it possible to create stepped terrain, landscaping features, patios, and other civil site features.

Of all the components in a retaining wall design, the retained soil is arguably the most important element. Its properties govern the lateral pressures that drive every subsequent design decision.

How lateral soil pressure is calculated

Three methods are available for calculating lateral soil pressure: equivalent fluid pressure, Rankine, and Coulomb. The equivalent fluid pressure method is the most prescriptive and the most straightforward to apply, and it is the approach codified in IBC 2018.

The forces acting on the wall extend beyond lateral earth pressure alone. Surcharge loads at grade add a horizontal pressure component over the retained height. Where drainage is not positively controlled, the design must also account for high water table conditions.

Common wall types and failure modes

Cantilevered retaining walls come in three common configurations: standard cantilever (with both toe and heel), cantilever without toe, and cantilever without heel. The geometry chosen affects which failure modes are most critical to the design.

Five failure modes must be checked: wall fracture, footing bending, overturning, soil bearing, and sliding. Each has a corresponding design lever. Wall fracture and footing bending are addressed by adding or resizing reinforcement, or by increasing wall thickness. Overturning and soil bearing failures are handled by increasing the footing size or adjusting the lever arm. Sliding resistance can be improved through footing geometry or soil improvement measures.

Worked example: a 4-foot retaining wall in Calcs.com

The session walked through a complete design example: a wall retaining a 4-foot height differential, designed for a factor of safety of 1.5 against both sliding and overturning. The design used the equivalent fluid pressure method with poorly graded soil.

The Calcs.com calculator returned the required reinforcement directly: #5 bars at specified spacings in the stem, heel, and toe, plus temperature reinforcement across the section. Those results were then carried through to a construction detail, showing exactly where each bar sits in the wall section.