GEOTECHNICAL

Consolidation Test (Oedometer)

Lab test giving compressibility + rate of long-term clay settlement

Also calledconsolidation testoedometer testone dimensional consolidationsettlement test soil
Related on InfraLens
CODES
IS 2720
Definition

The consolidation (oedometer) test, per IS 2720 Part 15, loads a laterally confined saturated soil specimen in increments and measures the time-dependent compression as pore water is squeezed out. It yields the compression index (Cc), recompression index (Cr), pre-consolidation pressure (pc) and coefficient of consolidation (cv) — the parameters that predict the magnitude and the time-rate of primary consolidation settlement of foundations on clay.

These feed Terzaghi's one-dimensional consolidation theory used in IS 8009 settlement analysis. Distinguishing normally consolidated from over-consolidated clay (via pc) is critical: under-estimating long-term settlement of soft clay is a frequent cause of distress in buildings, embankments and tanks. Pre-loading/surcharge and vertical-drain designs are sized directly from cv.

Where used
  • Long-term settlement prediction on clay (IS 8009)
  • Pre-loading + vertical-drain (PVD) design
  • Embankment-on-soft-clay stability + settlement
  • Raft + pile-raft settlement estimation
  • Tank + silo foundation design on compressible soil
Acceptance / threshold
Conducted per IS 2720 Part 15. Computed settlements must satisfy IS 8009 allowable + differential-settlement limits for the structure; otherwise ground improvement is required.
Frequently asked
What does the consolidation test measure?
The compressibility (Cc, Cr), pre-consolidation pressure and coefficient of consolidation of clay — used to predict how much, and how fast, a foundation on clay will settle.
Why is pre-consolidation pressure important?
It distinguishes normally consolidated from over-consolidated clay. Loading beyond pc causes large settlement on the virgin compression line; staying below it gives only small recompression settlement.
Related terms
IS 2720 — Parts List (Methods of Test for Soils)
Complete list of all 19 parts of IS 2720 — methods of test for soils.
Proctor Compaction Test
Lab test giving the soil compaction curve (OMC + MDD)
Soil Classification
Per IS 1498 / Unified Soil Classification System
Cohesion & Friction Angle
Mohr-Coulomb shear-strength parameters. Clay: cohesive (c). Sand: frictional (φ). C-φ soil has both.
Soil Permeability Test
Measures the coefficient of permeability k governing seepage + dewatering
California Bearing Ratio (CBR)
Subgrade strength index for pavement design
Effective Stress
Stress carried by the soil skeleton — total stress minus pore water pressure
Pore Water Pressure
Water pressure in soil voids; reduces effective stress and hence strength
Atterberg Limits
Liquid limit, plastic limit + shrinkage limit defining fine-soil consistency
Field Density Test
On-site test of achieved compaction (sand-replacement / core-cutter)