Shear Strength: Why and When Does Soil Slide?

1. Why does soil slide?

Soils carry no tension and rarely crush under compression; failure in soil almost always happens by shear. A slope sliding, the soil under a footing pushed sideways, the wedge behind a retaining wall moving — all are the shear strength being exceeded. Shear strength is therefore the central quantity of geotechnical engineering; bearing capacity, slope stability and earth pressure all rest on it.

2. The Mohr–Coulomb criterion

The shear strength on a plane increases with the normal effective stress acting on it:

where c' is the effective cohesion and φ' the effective angle of internal friction. In sands c' is practically zero; strength comes entirely from friction. For a sand with φ' = 32° under σ' = 100 kPa, τ_f = 100 × tan 32° = 62.5 kPa. The criterion is clearest together with the Mohr circle: failure occurs when the Mohr circle touches this line. The site's Mohr circle tool shows the stress transformation.

3. Effective stress: water's share

The σ' in the criterion is effective, not total, stress: σ' = σ − u. Pore water carries no shear; the part with which the grains press on each other — the effective stress — sets the friction. The consequence is striking: at a point with total stress 150 kPa and pore pressure 50 kPa, the correct strength is 100 × tan 32° = 62.5 kPa. Computed with total stress it comes out as 93.7 kPa — 50% too high. This is also why slopes slide after rain: pore pressure rises, effective stress and with it strength fall.

4. Drained and undrained behaviour

Because clay has low permeability, water cannot escape when load is applied quickly; pore pressure changes and the soil behaves undrained. Strength is then expressed by one parameter, the undrained shear strength c_u (s_u), in a total-stress analysis (φ_u = 0). If the load is applied slowly, or enough time passes, the water equilibrates and the soil behaves drained; an effective-stress analysis with c' and φ' applies.

Which is critical depends on the type of loading:

• Fill or footing on soft clay (loading): the most critical moment is right at the end of construction. Loading raises pore pressure; as water drains, effective stress and strength increase. Undrained analysis governs.
• Excavation or cut slope (unloading): conversely, the long term is most critical. Unloading lowers pore pressure; water gradually returns and strength falls. Drained analysis governs. A cut that stands for years and then suddenly slides is therefore no surprise.

5. The triaxial test

Shear strength parameters are usually measured in the triaxial test. A cylindrical specimen is held under cell pressure σ₃ and the axial stress is increased to failure. The test type sets the drainage condition: UU (unconsolidated–undrained) gives c_u; CU (consolidated–undrained, with pore pressure measured) and CD (consolidated–drained) give c' and φ'.

The Mohr–Coulomb criterion links the principal stresses at failure:

For a sand with c' = 0 and φ' = 32°, at σ'₃ = 100 kPa failure occurs at σ'₁ = 325.5 kPa; the failure plane makes 45° + φ'/2 = 61° with the major principal plane. For a clay with c' = 10 kPa and φ' = 25° at the same cell pressure, σ'₁ = 277.8 kPa.

6. Uncertainty: how sensitive is φ'?

φ' is never known exactly; sample disturbance, test type and natural variability create an uncertainty of a few degrees. Its effect on strength:

±2° means about ±8% in strength. Because bearing capacity factors grow much faster with φ', the same uncertainty becomes a much larger difference in bearing capacity — try changing φ' in the site's bearing capacity tool. Cohesion calls for more caution: a small c' measured in the laboratory is often a by-product of curve fitting, and relying on it in long-term analysis is risky.

All values in this article are checked by tests/v583-shear-strength.test.js.