Module 9 · Lesson 9.3
Why the path matters
The same soil, two loadings, two routes to failure.
Why this matters
The stress path is not book-keeping for its own sake — it decides whether a loading is safe. Because the effective path depends on drainage, the same soil under the same final total stress can be safe one way and failed the other. This lesson makes that concrete.
Picture a normally consolidated clay under a foundation, loaded quickly (undrained). Shearing makes it want to contract, so positive excess pore pressure builds up; the effective stress path bends left, towards the failure line. Immediately after construction — maximum excess pore pressure, effective path furthest left — the clay is at its most critical. As the excess pressure then dissipates (consolidation, Module 10), the effective path moves right, away from failure: the clay gets safer with time. This is why the short-term (undrained) case governs a loading on clay.
For an excavation the story reverses: unloading generates negative excess pore pressure, the effective path sits to the right initially and drifts left towards failure as pressures equilibrate — so the long-term (drained) case governs.
Check yourself
An embankment is built rapidly on soft, normally consolidated clay. When is its stability most critical?
Practice
A clay is loaded undrained. Its total stress path reaches s = 200 kPa, t = 70 kPa. If the effective stress path shows s′ = 140 kPa at the same instant, what excess pore pressure has developed, in kPa?
Summary
- Drainage sets the effective stress path, and the effective path decides failure.
- Loading on clay (undrained): positive excess u, effective path bends towards failure — short-term critical, safer with time.
- Unloading (excavation): negative excess u, effective path drifts towards failure with time — long-term critical.
- Design checks both the undrained route and the drained destination, using whichever governs.
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint