🪨 Consolidation Settlement

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 2 independent reference calculations · how PhDino checks its numbers

How much a clay layer compresses over time as a new load squeezes pore water out of it, and roughly how long that takes.

Fine-grained soils (clays especially) compress slowly under a new load because the load is initially carried by pore water pressure, which only dissipates as water drains out of the soil skeleton over time — this is Terzaghi's one-dimensional consolidation theory. The total (primary) settlement depends on how much the effective stress increases relative to what the soil has already experienced; the rate at which that settlement actually happens depends separately on the soil's permeability and the drainage path length.

This is a fundamentally different, much slower process than the immediate (elastic) settlement of granular soils — a structure on clay can keep settling for months or years after construction.

Key formula

Sc = [Cc·H / (1+e₀)] × log₁₀[(σ′₀+Δσ)/σ′₀]
t₉₀ = 0.848 × Hdr² / Cv

Variables

Cc

compression index (from a lab consolidation/oedometer test)

e₀

initial void ratio

σ′₀

initial effective overburden stress at the middle of the clay layer

Δσ

stress increase from the new load

Cv

coefficient of consolidation (governs drainage rate)

Hdr

drainage path length (half the layer thickness if drained on both faces)

Notes & limitations

• Skipping σ′₀ is a common mistake — settlement depends on the ratio of final to initial effective stress, not on Δσ alone, and a clay layer already under high overburden stress settles much less for the same Δσ than a shallow, lightly loaded one.
• t₉₀ (time to 90% of primary settlement) can be months for a thin, well-drained layer or decades for a thick clay layer with only single-face drainage — this is often the practical schedule-driving number in a geotechnical report, not just the final settlement magnitude.