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.
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.
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.