Retaining Wall Sliding Factor of Safety
Retaining Wall Sliding Factor of Safety
Besides tipping over, a retaining wall can also simply slide horizontally along the base of its footing if the lateral earth pressure pushing it exceeds the friction (and any passive resistance) developed at the footing-soil interface. Sliding stability is checked alongside overturning as one of the two classic rigid-body stability checks for any gravity or cantilever retaining wall.Sliding failure tends to be gradual at first — small incremental movements — but can accelerate once the footing loses full contact or the soil beneath is disturbed, and a wall that has slid even a small amount can develop cracks and drainage problems that accelerate further deterioration; this is why sliding, like overturning, is checked with a healthy safety margin rather than a bare 1.0.
The sliding factor of safety is SF = mu_s·N_v/H_d, the frictional resistance (friction coefficient times the vertical resisting force) divided by the driving horizontal force. where mu_s is the coefficient of friction between the footing base and the soil, N_v is the total vertical (resisting) force on the footing, H_d is the driving horizontal force from lateral earth pressure, and SF is the resulting factor of safety.
Inputs
mu_s := 0.5=0.5
N_v := 300 kN=300000 N
H_d := 120 kN=120000 N
Multiplying the vertical load by the friction coefficient gives the maximum frictional resistance available at the base, which is then compared with the driving horizontal force to get the factor of safety against sliding.
SF := mu_s·N_v/H_d=1.25
Results
An SF of 1.25 here is on the low side relative to the 1.5 minimum commonly required for sliding, so this wall would typically need a design adjustment — options include adding a shear key beneath the footing to mobilize passive soil resistance, increasing the footing width to raise N_v, or reducing driving pressure with a backfill drainage improvement. Because friction coefficient mu_s depends heavily on the footing base material and soil type, a conservative or field-verified value should be used rather than an optimistic assumption, since sliding resistance is very sensitive to this single parameter. As with overturning, sliding is checked independently from bearing pressure — passing one does not imply the others pass.
Assumptions & limitations
- Friction-only resistance is credited here; no passive earth pressure or shear key contribution is included.
- Coefficient of friction is a representative value; actual value depends on footing material and soil conditions.
- Vertical resisting force N_v already accounts for wall, footing, and soil self-weight.
- A minimum SF of about 1.5 is a common (not universal) code target — check the applicable local requirement.