The stability of the work is checked as a whole considering it as a rigid body. The work is secure when when is verified the safety to:
•Overturning
•Sliding
•Limit load
•Global stability
Overturning verification
The overturning is represented by the possible rotation of the work with respect to the downstream point.
The action that determines the overturning is given by the horizontal component of the earth's thrust plus eventual external actions.
The stabilizing action is given by the vertical component of earth's thrust, by work's own weight.
The stabilizing action represented by the action of the earth's passive thrust is not taken into consideration.
In analytical terms the overturning verification can be expressed with the condition that the stabilizing moment Ms, with respect to the rotation center, is non inferior to the moment induced by the overturning forces Mr, with respect to the rotation center.
The security of this equilibrium must be ensured with a due safety coefficient.
Sliding verification
The sliding depends on the possibility that the parallel forces to the contact plan between foundation and soil are superior to the soil-foundation friction forces.
The force that determines the sliding T is the horizontal component of the thrust plus eventual surcharges, while the force that opposes the sliding is given by the resultant of the normal forces N to the contact plan multiplied by the friction coefficient.
The friction coefficient f is the tangent of the foundation-soil friction angle.
To reduce sliding danger the foundation footing can be inclined.
In analytical terms it can be expressed as:
FS, safety factor, varies with the standard.
Limit load verification
It is carried out comparing the maximum normal stress on the foundation's footing with the ultimate shear stress of the soil.
This condition is considered verified if the ratio between the limit stress and the maximum stress is superior to a fixed safety factor.
Global stability verification
Consists in checking the rotation of a soil cylinder containing both the work and the thrust wedge. The verification is performed by software Slope made by GeoStru.
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