Oedometer

An oedometer is a laboratory device used to study the one-dimensional consolidation behavior of soils. A cylindrical soil specimen is placed in a rigid ring between porous discs, with drainage provided through the base and sometimes the top. Vertical load is applied with a loading frame, and the specimen’s height (or thickness) is measured as it consolidates under the applied stress.

During a test, a sequence of vertical pressures is applied, with each load maintained until primary consolidation is largely complete. Settlements and time data are recorded, producing settlement-time and pressure-settlement curves. From these data, engineers derive key soil properties, including the coefficient of consolidation (cv), the coefficient of volume compressibility (mv), and the compression index (Cc) and recompression index (Cr) from e-log p' relationships. The preconsolidation pressure (σ'p) is often determined via a Casagrande-type plot, and the overconsolidation ratio (OCR) can be estimated as σ'p/σ'0. The test can use single drainage (one end exposed to drainage) or double drainage (both ends), affecting consolidation rates.

Typical specimens are cylindrical, about 38–50 mm in diameter and 20–40 mm high. The oedometer is widely used in geotechnical practice to predict settlements of foundations, embankments, and other earth structures, and to characterize soil compressibility and drainage behavior. Standards such as ASTM D2435 and related methods govern procedure and reporting. Limitations include potential sample disturbance, simplified one-dimensional drainage assumptions, and sensitivity to boundary conditions not present in actual field conditions.