Moisture Content Test

A moisture content test on soil is a test that determines the amount of water present in a sample of soil. Moisture content is typically measured as a percentage of the dry weight of the soil sample.

There are different ways to perform a moisture content test on soil. The most common method is called the oven-dry method, which involves taking a sample of the soil, drying it in an oven at a specific temperature and weight it before and after the drying process. Moisture content is then calculated as the percentage of the weight of the sample that is lost due to evaporation during the drying process. The method is widely used because is simple, fast and very accurate in most cases.

Other methods include using a microwave, an infrared moisture meter, or a field test such as the hand method, this method is simple, affordable, but less accurate in some cases.

The moisture content of the soil is important to determine because it can affect the strength, compressibility, and stability of the soil, and also affect the behaviour of foundations and slopes. Moisture content also affects the suitability of soil for agriculture, engineering or construction purposes.

Soil Moisture Content Test Method

SCOPE

This document outlines the laboratory procedure for determining the moisture content of soil, expressed as a percentage of its oven-dried weight. The test is suitable for soils with particle sizes ranging from 2 mm to over 10 mm, including fine, medium, and coarse-grained soils.

PRINCIPLE

The soil moisture content is determined by oven-drying a soil sample until its weight stabilizes. The percentage of moisture content is then calculated based on the weight difference before and after the drying process.

SPECIAL APPARATUS

Depending on the soil granularity, different equipment is required:

For Fine-Grained Soils (maximum particle size 2 mm):

Thermostatically controlled oven, preferably forced-draught, maintaining 105-110 °C.
Balance, readable and accurate to 0.01 g.
Numbered aluminium weighing tins with lids.
Desiccator with anhydrous self-indicating silica gel.

For Medium-Grained Soils (maximum particle size 10 mm):

The same type of oven as above.
Balance, readable and accurate to 0.2 g.
Airtight, corrosion-resistant container (about 400 g capacity).
Scoop for handling the soil.

For Coarse-Grained Soils (maximum particle size >10 mm):

The same type of oven as for fine-grained soils.
Balance, readable and accurate to 1 g.
Corrosion-resistant container (about 3.5 kg capacity).
Scoop for handling the soil.

PROCEDURE

Each type of soil requires a different procedure due to its distinct characteristics.

Fine-Grained Soils:

Clean and dry the weighing tin and lid, then weigh (W1). Place at least 30 g of moist soil in the tin, replace the lid, and weigh again (W2).
Oven-dry the tin and soil at 105-110 °C until weight remains constant.
Cool in a desiccator and weigh the dried soil and tin (W3).

Medium-Grained Soils:

Clean and dry the container, then weigh (W1). Place about 300 g of soil in the container, replace the lid, and weigh again (W2).
Oven-dry the container and soil at 105-110 °C until the weight remains constant.
Allow it to cool and weigh the dried soil and container (W3).

Coarse-Grained Soils:

Clean and dry the container, weigh (W1). Place about 3 kg of soil in the container and weigh again (W2).
Oven-dry at 105-110 °C until weight remains constant.
Allow it to cool and weigh the dried soil and container (W3).

CALCULATIONS

REPORTING THE RESULTS

Report the moisture content to three significant figures and reference the standards employed during the testing, e.g., AS 1289 B1.1-1977 for the Oven Drying Method.

NOTES

Balance precision depends on the sample size and should be within 0.03% of the sample weight.
Maintain a record of weighing tin weights. Clean and re-weigh tins every six months or as needed.
Crumble samples for easier drying, taking care to avoid soil loss.
For soils containing gypsum, adjust the drying temperature and report the method used.
Overnight drying is usually sufficient; extend as needed to reach constant weight.
Use a desiccator for cooling if necessary.
Discard moisture content samples post-test; they are unsuitable for further testing.

REFERENCE

AS 1289 B1.1-1977, Determination of the Moisture Content of a Soil: Oven Drying Method (standard method), Standards Association of Australia.