Methods of Test for Soils, Part XV: Determination of Consolidation Properties
1965 Edition

The consolidation test requires a consolidation cell (consolidometer) to hold the soil sample under vertical load, a loading device such as a frame or lever to apply incremental pressure, and a dial gauge for precise measurement of soil settlement. Consolidation rings secure the specimen, while porous stones positioned above and below allow water flow during consolidation. Filter papers are used to prevent soil particles from clogging the porous stones. Additionally, saturation apparatus, water reservoir, and tubing are necessary for specimen saturation and drainage control. A seating pressure of 0.05 kgf/cm² should be applied before starting load increments to ensure proper setup.

Soil samples should be prepared by extruding or cutting a representative soil disc with faces parallel to each other. For tube samples, the disc thickness must exceed the consolidation ring height. For block samples, the disc diameter should be at least 10 mm larger than the ring's inside diameter. The sample must be oriented to simulate the natural load direction. Trimming involves using the weighed consolidation ring as a guide to shape the specimen edges so the ring fits without voids. Inclusions are to be removed and any resultant gaps filled with soil parings, or the sample discarded if excessive. Trimming tools vary by soil type: soft to medium soils require a wire saw with a straight edge, whereas stiff soils use only a straight edge, avoiding excessive remoulding or specimen displacement to maintain sample integrity.

Load increments should approximately double each time, commonly following a sequence such as 0.1, 0.2, 0.4, 0.8, 1.6, 3.2 kgf/cm², etc. Each load must be applied smoothly within two seconds to avoid impact. The load is maintained until primary consolidation is complete or the secondary compression trend is evident, typically around 24 hours per increment. Dial gauge readings are taken at designated intervals to facilitate plotting against the square root or logarithmic time scales. Upon completion of the highest load, unloading is conducted in steps down to one-quarter of the last load, with readings recorded accordingly.

To calculate the coefficient of consolidation (cv), settlement data from dial gauge readings are plotted against time or logarithmic time. The time corresponding to 90% consolidation (t90) is identified from this plot. The formula used is cv = (Hay² × Tv) / t90, where Hay represents the average drainage path length (typically half the specimen thickness for double drainage), and Tv is the theoretical time factor for 90% consolidation, approximately 0.848. Consistent units must be used throughout to ensure correct dimensional results.

Accuracy is ensured by employing dial gauges with at least 0.01% precision relative to specimen height and sufficient travel of at least 50% of the specimen height, complying with IS 2092-1983 standards. Initial specimen height measurements should be made with instruments accurate to ±0.1 mm, such as vernier calipers or micrometer screw gauges. Loading devices must apply vertical load increments with stability within ±1%. Readings should be recorded at specified time intervals using standard data sheets. Calibration of the dial gauge before testing, ensuring perpendicular contact with the specimen surface, minimizing environmental vibrations, and strict adherence to time schedules for reading acquisition are crucial for data integrity.