Retaining Wall Design

Retaining structures hold back soil or other loose material where an abrupt change in ground elevation occurs. The retained material or backfill exerts a push on the structure and thus tends to overturn or slide it, or both. The cantilever is the most common type of retaining structure and is used for walls in the range of 10 to 25 ft in height. The stem, heel and toe of such a wall acts as a cantilever beam.

The program computes the soil bearing pressures under the base of a concrete cantilever retaining wall supporting any kind of backfill material with additional surcharge and concentrated external loads acting on the wall.

In addition, the program analyzes the stability of the whole structure and performs the concrete design based either on the Working Stress Design Method or on the Ultimate Strength Design Method of the ACI. The lateral pressures are calculated by either the Rankine or Coulomb theory. Seismic design as per the Mononobe-Okabe approach.

The program also provides the option to design a base shear key if the sliding of the whole structure is a possible cause of failure.

Input Data

The required input data includes all wall dimensions, backfill slope, concrete and steel strength, allowable soil bearing pressure, and backfill properties. The figure to the right shows schematically the input data required by this program.

The program also offers the option of considering other external loads applied to the wall, in addition to the backfill pressure. A surcharge can be defined as an equivalent height of backfill. Concentrated loads may also be applied to the wall. This is particularly useful in the design of bridge abutments where beams or girders reactions must be taken by supports.

Example

Consider the retaining wall shown at left supporting a backfill material with an estimated active pressure coefficient of 0.45 and a surcharge of 100 psf.

Find out if the section is adequate and design the reinforcing steel. Use an allowable soil pressure of 3.0 ksf.

Output

When the data is entered, the program automatically performs the stability check for the structure and designs the reinforcing steel. In this manner, the designer may change any dimension or property to obtain an adequate design.

The program computes the maximum soil pressures present under the toe and heel, and compares them with the allowable soil pressure provided as data.

The minimum safety factors allowed by the program are 2.0 for overturning and 1.5 for sliding. In the example, a shear key of 1'-0" depth was designed in order to increase the safety factor for sliding.

Procedure

For the reinforcement design, the program computes the required spacing for the specified bar sizes in the stem, heel and toe.

A detailed tabulation of the stem design is also provided in the output, showing for each tenth of the stem height, the steel area required by strength and the minimum allowed by the ACI Code. This allows the user to optimize the stem width.