By: Javier Encinas, PE | February 21, 2016

ASDIP RETAIN is structural engineering software for design of retaining walls. Retaining walls are structures designed to bound soils between two different elevations. A retaining wall is mainly exposed to lateral pressures from the retained soil plus any other surcharge. This article discusses the design of either concrete or masonry cantilever retaining walls.

A typical retaining wall is composed of four main components: the Stem, the Toe at the front of the wall, the Heel at the backfill side, and the Shear Key. The images below show the geometry of a typical cantilever retaining wall.

cantilever_retaining_wall_geometry

What lateral earth pressure theory to use?

There are two well-known classical earth pressure theories: Rankine and Coulomb. Both theories provide expressions for the pressure states described below. Rankine is a simplified version of Coulomb. Rankine ignores the friction between the wall and the soil, and it assumes a vertical wall face. Coulomb takes all these factors into consideration.

Depending on the relative movement of the wall and the backfill, three different states of lateral earth pressure may develop:

  • At-rest Ko – It occurs when the wall experiences no lateral movement.
  • Active Ka – It occurs when the wall tilts away from the soil sufficiently to mobilize its shear strength.
  • Passive Kp – It occurs when the wall is pushed into the soil, compressing the mass and mobilizing its shear strength.

In order to develop the full active or passive pressures, the wall has to move, as shown below. For granular soils, the full active pressure is developed when the wall top moves between 0.001H-0.004H, being H the wall height. Most cantilever walls will move this amount as the sum of the stem deflection plus the base rotation, therefore it’s common practice to use the active pressure in the design of cantilever retaining walls, otherwise the at-rest pressure should be used.

cantilever_retaining_wall_lateral_pressures
cantilever_retaining_wall_lateral_deflections

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.

How do you check the overall stability?

There are four basic instability modes to be checked: Sliding, Overturning, Soil bearing, and Global instability, as shown schematically in the picture below.

cantilever_retaining_wall_stability_failure_modes

Sliding – The backfill exerts a lateral pressure against the wall. This sliding force is resisted by the friction between the soil and the footing. and by the passive pressure at the front of the wall. When more sliding resistance is required, a shear key may be provided. The factor of safety against sliding equals the resisting force divided by the driving force, and the minimum value should be 1.50.

Overturning – The overturning moment from the applied forces must be resisted by an opposite moment produced by the vertical forces, including the wall selfweight and the weight of the backfill over the heel. The factor of safety against overturning is defined as the resisting moment divided by the overturning moment, and the minimum value should be 1.50.

As an example, the picture below shows the ASDIP RETAIN overturning calculations.

cantilever_retaining_wall_overturning_calculations

Soil bearing – The allowable soil bearing pressure should be provided by the soils report, which already includes a safety factor of about 3.0. The resultant of the bearing pressure should fall within the kern to avoid negative soil pressures at the heel.

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.

ASDIP RETAIN generates the image shown below, with the magnitude and location of the loads that affect the stability analysis, sorted by load combination. The calculated safety factors are also provided by the software for immediate check.

cantilever_retaining_wall_stability_analysis

The second part of this article deals with the typical loads in a retaining wall, as well as the structural design of the different components.

Detailed information is available about this structural engineering software by visiting ASDIP RETAIN. You are invited to download a Free 15-Day Software Trial or go ahead and Place Your Order.

Best regards,

Javier Encinas, PE
ASDIP Structural Software

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