Methods of test for soils, Part 19: Determination of centrifuge moisture equivalent
1992 Edition

The centrifuge moisture equivalent (CME) is the percentage of water retained by a saturated soil sample after being subjected to centrifugation at 1000 times the gravitational force for one hour. It represents the moisture held against this centrifugal force, reflecting the soil’s capillary water retention capacity. CME is crucial for evaluating soil permeability and relative permeability, which are important factors in foundation engineering, drainage design, and assessing soil stability. The calculation uses the formula:

[ \text{CME} = \frac{(M_2 - M_1 - 0.02) - (M_3 - M_1)}{M_3 - M_1} \times 100 ]

where:

• (M_1) is the mass of crucible plus dry filter paper (g),
• (M_2) is the mass after centrifuging (g),
• (M_3) is the mass after oven drying (g),
• and 0.02 g accounts for moisture in the filter paper.

Test setup involves saturating the soil in a Gooch crucible, centrifuging at 1000g for one hour, weighing post-centrifugation, oven drying at 105–110°C, and final weighing.

The test requires a 5-gram soil sample consisting of particles passing through a 425-micron IS sieve, representing the fine soil fraction. The sample should be well mixed and fully saturated with water prior to testing. The soil is placed in a Gooch crucible, a porcelain vessel approximately 40 mm in height, with a top diameter of 25 mm and bottom diameter of 20 mm, lined with filter paper (e.g., Whatman No. 42). The centrifugation is conducted at 1000g for a duration of one hour at a controlled temperature of 27 ± 2°C to ensure consistency and reproducibility.

The apparatus specified for the CME test include:

• A Gooch crucible made of porcelain with a perforated base, approximately 40 mm tall, 25 mm top diameter, and 20 mm bottom diameter.
• Circular filter paper (e.g., Whatman No. 42) placed over the crucible base.
• A Babcock trunnion cup equipped with a brass cap and a support device to hold the Gooch crucible elevated about 12 mm above the cup bottom, allowing free air circulation and preventing contact with expelled water.
• A centrifuge capable of generating a centrifugal force of 1000 times gravity at the soil sample’s center of gravity.
• A precision balance conforming to IS 1433:1965 with accuracy up to 0.001 g.
• A thermostatically controlled oven with non-corrosive interiors maintaining temperature between 105°C and 110°C for drying samples to constant weight.

This setup ensures precise measurement of moisture content retained after centrifugation.

The standard requires applying a centrifugal force equivalent to 1000 times the acceleration due to gravity (1000g) at the soil sample’s center of gravity. This force must be applied continuously for one hour. The soil sample, after saturation, is placed in a Gooch crucible within a Babcock trunnion cup apparatus. The test temperature should be maintained at 27 ± 2°C. After centrifugation, the crucible and contents are weighed prior to oven drying at 105–110°C to determine dry mass. The CME is then calculated using the prescribed formula considering mass measurements before and after centrifugation and drying.

Test results should be reported as the average of two independent determinations to ensure accuracy and reliability. Each determination must include the mass of the crucible plus dry filter paper (M1), mass after centrifugation (M2), and mass after oven drying (M3). The centrifuge moisture equivalent (CME) is computed for each test using the formula:

[ \text{CME} = \frac{(M_2 - M_1 - 0.02) - (M_3 - M_1)}{M_3 - M_1} \times 100 ]

Final values must be rounded following IS 2:1960 guidelines, maintaining the specified number of significant figures. Regarding variability, for CME values up to 15%, the allowable difference between duplicates is 1%; for CME values exceeding 15%, the maximum permissible difference is 2%. Should the variation exceed these limits, the test must be repeated to ensure data reliability.