Site investigation · Laboratory: strength & consolidation

Oedometer consolidation test calculator

Lab sheet: e–log σ' curve, Cc, Cr, Casagrande preconsolidation pressure, mv per stage and cv by Taylor's root-time method.

Standard: AS 1289.6.6.1 · Casagrande σ'p · Taylor √t

Inputs in the worked example

The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.

Input	Value	Unit
Initial height H0	20	mm
Initial void ratio e0	0.95
Stages: σ' (kPa), cumulative ΔH (mm)	12.5, 0.10 25, 0.20 50, 0.35 100, 0.60 200, 1.10 400, 1.90 800, 2.90 1600, 3.95 400, 3.80 100, 3.55 25, 3.30
Time readings for one stage: t (min), settlement in stage (mm)	0.1, 0.040 0.25, 0.062 0.5, 0.088 1, 0.124 2, 0.172 4, 0.236 8, 0.315 15, 0.390 30, 0.452 60, 0.481 120, 0.494 240, 0.500
Drainage	Double (top & bottom)

Result	Value	Unit
Compression index Cc	0.340	(800→1600 kPa)
Recompression / swelling Cr	0.035
Preconsolidation σ'p (Casagrande)	266	kPa
mv, 100→200 kPa	0.258	MPa⁻¹
t90 (Taylor)	10.0	min
Drainage path Hdr	9.45	mm
cv	3.97	m²/yr
Cr / Cc	0.10

Diagram for the worked example

e = e0 − ΔH/H0·(1 + e0) for each stage
Cc = steepest Δe / Δlog σ' on the loading branch; Cr from the unloading branch
σ'p: at the point of maximum curvature draw horizontal + tangent, bisect, intersect with the back-extended virgin line (bisector taken in e–log σ' units, so it depends on plot scaling, as on paper)
mv = Δe / (1 + e1) / Δσ'
Taylor: straight line through early √t points, second line with abscissae × 1.15; intersection with the curve gives t90
cv = 0.848·Hdr² / t90

Triaxial test (UU / CU / CD) — Mohr circles and failure envelope from triaxial specimens: su for UU, c′ and φ′ for CU (with pore pressure)…
Unconfined compression test — Stress–strain with area correction, peak qu, su = qu/2 and E50 secant modulus.
Direct shear test — Peak (and residual) failure envelopes from shear box results.
Permeability (constant / falling head) — Hydraulic conductivity from constant- or falling-head tests, corrected to 20 °C.
Stress paths (MIT p′–q) and strength fit — Total and effective stress paths from triaxial data, with the Kf line fitted to give c′ and φ′.
Coefficient of consolidation (root-time & log-time) — cv from Taylor's t90 and Casagrande's t50, with permeability from k = cv·mv·γw.
Collapse potential (oedometer) — Collapse potential from the strain on wetting at constant stress, with the usual severity classes.
Free swell & swell pressure (oedometer) — Free swell strain under seating load and swell pressure from the constant-volume or load-back method.
Residual strength: ring shear — Residual friction angle from a regression through the origin and a free-intercept fit.
Vane shear (lab / field) — Undrained strength from peak and remoulded torque, sensitivity and Bjerrum's plasticity correction.

For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.