Concept
- In saturated soil, loading can create excess pore-water pressure. As drainage occurs, stress transfers to the soil skeleton and settlement develops.
- An oedometer compresses a laterally confined specimen to study deformation and consolidation rate under load increments.
Specimen and sequence
- An undisturbed sample better preserves structure and moisture; handle it carefully and record orientation and depth.
- Trim the specimen to fit the ring, mount it between porous elements, and apply the inundation condition specified.
- Apply load increments and record deformation over time until each stage meets its criterion; unloading and swelling stages may also be included.
Reading the curves
- The load-deformation relationship characterizes compressibility; the deformation-time curve describes the rate of the process.
- Derived parameters help estimate settlement and time, but depend on the model, stress history, and specimen quality.
Points to consider
- Distinguish consolidation from immediate deformation and secondary compression; do not attribute all settlement to one mechanism.
- The source also describes CRS and suction methods. Select a procedure for the soil, geotechnical question, and applicable standard.
Compressibility and settlement rate
- Compression index: Cc = (e1 − e2)/log10(σ′2/σ′1), slope of the virgin branch on an e–log σ′ plot. Recompression index Cr is obtained from the unload-reload branch.
- Primary settlement, normally consolidated soil: Sc = H0·Cc/(1+e0)·log10(σ′f/σ′0), for a stress increase within the virgin branch. For overconsolidated soil, use Cr up to σ′p and Cc beyond it.
- Coefficient of consolidation: cv = Tv·Hdr²/t, where Tv corresponds to the selected degree of consolidation, Hdr is drainage-path length, and t is time from the adopted graphical method.
- For double drainage, Hdr = H/2; for single drainage, Hdr = H. Check actual drainage thickness and keep cv units consistent.
- Project settlement sums compressible layers and requires initial/final effective stress, thickness, stress history, and drainage conditions; an oedometer result alone does not represent secondary compression.
Educational reference guide. Check the current standard, calculation assumptions, and project specification before applying a procedure or acceptance criterion.