24.2 Sliding, Overturning, Bearing, and Global Stability

External stability asks whether the wall-soil system slides, rotates, overstresses its foundation contact, settles, or fails along a deep surface. These are related but distinct modes. For July 2026, use the April 2024 PE Civil: Structural specification, current PE Civil Reference Handbook, AASHTO LRFD 8th edition with the listed May 2018 errata, and IBC 2018. Do not import April 2027 standards.

Draw One Complete Free Body

Choose the body before summing forces. For a cantilever wall, an external-stability body commonly includes wall concrete and soil above the heel, but internal stem or heel design uses different cuts. Label wall and footing weights, retained soil, surcharge effects, lateral earth force, water pressure, uplift, vertical components, toe and heel locations, base width, and every lever arm.

Do not count the same soil both as weight on the heel and as an external vertical force across a cut. Water behind the wall creates horizontal load; water beneath the base can reduce effective vertical reaction and friction. If water exists in front, its direction may be stabilizing for one stage and absent after drawdown. Stage and drainage assumptions control.

Choose ASD or LRFD Before Calculating

In a traditional ASD service-load check, sliding factor of safety can be written

FS_slide = ΣR/ΣD

and overturning factor of safety

FS_OT = ΣM_resist/ΣM_overturn

using unfactored service actions and the geotechnical safety-factor basis specified by the problem. Allowable bearing pressure and settlement criteria then use their compatible service basis.

AASHTO LRFD instead applies load combinations, load factors, resistance factors, and limit-state rules. Do not factor earth load, divide soil resistance by an ASD factor of safety, and then apply an LRFD resistance factor. That double- or mismatched treatment has no valid calibration. Use one complete framework.

Sliding Resistance

Base friction is often modeled as R_f = μN', where N' is effective normal force after relevant uplift and μ is the permitted interface coefficient. Cohesion or adhesion needs explicit geotechnical support. A shear key changes the passive and bearing mechanism and requires structural design.

Passive soil in front of the toe is unavailable if it may be excavated for utilities, eroded, scoured, frost-disturbed, or not move enough to mobilize passive pressure. Codes or owners may limit or prohibit its use. Never add full passive resistance merely because a coefficient can be calculated.

Worked ASD External-Stability Example

A 10 ft-wide wall base is checked with service loads per foot of wall. Total effective vertical force is V = 100 kips/ft, acting 6.0 ft from the toe before lateral overturning is included. The lateral driving force is H = 20 kips/ft, acting 4.0 ft above the base. Use permitted base friction coefficient μ = 0.45, ignore passive resistance, and use a problem-given allowable bearing pressure of 12.0 ksf.

Sliding:

R_f = μV = 0.45(100) = 45 kips/ft

FS_slide = 45/20 = 2.25

Overturning about the toe:

M_resist = 100(6.0) = 600 kip-ft/ft

M_overturn = 20(4.0) = 80 kip-ft/ft

FS_OT = 600/80 = 7.50

Net resultant location from the toe:

x_R = (600 - 80)/100 = 5.20 ft

The resultant is 0.20 ft toward the heel from the 5.0 ft base center, so e = 0.20 ft. Because e < B/6 = 1.667 ft, full compression contact is predicted.

Average pressure is V/B = 100/10 = 10.0 ksf. Extreme pressures are

q_max,min = (V/B)(1 ± 6e/B)

q_max = 10[1 + 6(0.20)/10] = 11.2 ksf

q_min = 10[1 - 6(0.20)/10] = 8.8 ksf

The stated bearing check passes because 11.2 < 12.0 ksf; both pressures are compressive. These results do not establish that required project safety factors are met unless their acceptance values are given, nor do they check settlement or global stability.

When the Resultant Leaves the Kern

If |e| > B/6, the linear full-width formula predicts tension at one edge. Soil contact does not resist tension, so use the permitted partial-contact distribution and effective bearing width. Do not report negative bearing as a stabilizing tensile force. Large eccentricity can also increase settlement and rotation even when peak bearing remains below an ultimate value.

Bearing, Settlement, and Global Stability

Bearing capacity addresses a shallow shear failure beneath the base. Settlement considers compression and distortion at service load and may govern on compressible or variable soils. Check resultant direction, inclination, footing embedment, groundwater, and layered foundation soil using the geotechnical basis provided.

Global stability examines a deep slip surface passing through retained soil, foundation soil, and possibly beneath or behind the wall. A wall can have high sliding and overturning factors on its base yet move with a larger soil mass. Weak seams, sloping ground, surcharge, water, seismic stage, and adjacent excavation affect this mode. It generally requires a slope-stability analysis, not a base-friction shortcut.

Final Audit

Write ASD or LRFD at the top. Then check sliding, overturning, contact, bearing, settlement, and global stability separately. Identify every credited resistance and why it remains available for the governing stage.

A base key or anchor is not free resistance: check its structural shear and flexure, surrounding-soil failure, construction tolerance, and compatibility with the movement needed to mobilize friction or passive pressure.

External-Stability Gate Map