Determination of Moisture Content of Soils using the Oven Drying Method (BS 1377: 1990)

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General:

Water is present in most naturally occurring soils. The amount of water, expressed as a proportion by mass of the dry solid particles, known as the moisture content, has a profound effect on soil behaviour. In this context a soil is “dry” when no further water can be removed at a temperature not exceeding 110 °C. Moisture content is required as a guide to classification of natural soils and as a control criterion in recompacted soils and is measured on samples used for most field and laboratory tests.

NOTE: In some situations, for example in the field control of earthworks, a rapid method of measurement of moisture content may be required and a number of rapid methods are available, e.g. the sand bath method, the microwave oven-drying method and the calcium carbide method. It should not be assumed that these methods are appropriate for all soil types and when they are used on a particular soil some tests should also be carried out using the oven-drying method as a check.

Apparatus

1. A drying oven, capable of maintaining a temperature of 105 °C to 110 °C.

NOTE: A microwave oven should not be used for the determination of moisture content by the definitive method for soils containing clay or organic matter because of the difficulty of ensuring that the temperature of the soil does not exceed 110 °C before all the water is removed. Higher temperatures than this can alter the properties of clay material.

For fine-grained soils,

2. A glass weighing bottle, fitted with a ground glass stopper or cap, or a suitable airtight corrosion-resistant metal container.
3. A balance readable to 0.01 g.
4. A desiccator containing anhydrous silica gel.

NOTE: It is preferable to use self-indicating silica gel as the desiccant. Calcium chloride should not be used as it is known that many clays when oven dry are capable of absorbing water from it.

For medium-grained soils,

5. A corrosion-resistant container of about 500 g capacity.
6. A balance readable to 0.1 g.
7. A scoop of suitable size.

For coarse-grained soils,

8. A corrosion-resistant container of about 4 kg capacity.
9. A balance readable to 1 g.
10. A scoop of suitable size.

Procedure

For fine-grained soils

Step 1: Clean and dry the weighing bottle or metal container and weigh it to the nearest 0.01 g (m₁). Take a sample of at least 30 g of soil, crumble and place loosely in the container or weighing bottle, and replace the lid or stopper. Then weigh the container or bottle and contents to the nearest 0.01 g (m₂).

Step 2: Remove the lid or stopper, and place the container or bottle with its lid and contents in the oven and dry at 105 °C to 110 °C. The period required for drying will vary with the type of soil and the size of sample but the sample shall be deemed to be dry when the differences in successive weighings of the cooled sample at intervals of 4 h do not exceed 0.1 % of the original mass of the sample. Do not replace the lid or stopper while the sample is in the oven.

NOTE 1: Certain soils contain gypsum which on heating loses its water of crystallization. The moisture content determined by this method will be affected by approximately 0.2 % for each 1 % of gypsum. If it is suspected that gypsum is present in the soil the moisture content samples should be dried at no more than 80 °C and possibly for a longer time. The presence of gypsum can be identified by heating a small quantity of soil on a metal plate. Grains of gypsum will turn white within a few minutes, whereas most other mineral grains remain unaltered.

NOTE 2: Between 16 h and 24 h is usually a sufficient length of time for drying most soils, but certain soil types and large or very wet samples will require longer. The drying time will also depend on the amount of material in the oven and may be confirmed by trial tests.

Step 3: After drying, remove the container or bottle and contents from the oven and place the whole in the desiccator to cool.

NOTE: If the lids of the containers fit well and it is unlikely that the samples are to be left for a considerable time before weighing, the samples need not be placed in the desiccator to cool, unless glass weighing bottles with ground glass stoppers have been used.

Step 4: Replace the lid and then weigh the container or bottle and contents to the nearest 0.01 g (m₃).

For medium-grained soils

Step 5: Clean and dry the container and weigh it to the nearest 0.1 g (m₁). Take a sample of at least 300 g of soil, crumble and place loosely in the container, and replace the lid. Then weigh the container and contents to the nearest 0.1 g (m₂).

Step 6: Remove the lid and place the container and contents in the oven and dry at 105 °C to 110 °C. The period required for drying will vary with the type of soil, the size of sample and the number of samples in the oven but the sample shall be deemed to be dry when the differences in successive weighing of the cooled sample (see Step 4 and step 8) at intervals of 4 h do not exceed 0.1 % of the original mass of the soil. Do not replace the lid while the sample is in the oven.

Step 7: After drying, replace the lid and allow the whole to cool.

Step 8: Weigh the container and contents to the nearest 0.1 g (m₃).

For coarse-grained soils

Step 9: Clean and dry the container and weigh it to the nearest 1 g (m₁). Take a sample of at least 3 kg of soil, place loosely in the container and replace the lid. Weigh the container and contents to the nearest 1 g (m₂).

Step 10: Remove the lid and place the container and contents in the oven and dry at 105 °C to 110 °C. The period for drying will vary with the type of soil and the size of the sample but the sample shall be deemed to be dry when the differences in successive weighings of the cooled sample at intervals of 4 h do not exceed 0.1 % of the original mass of the soil. Do not replace the lid while the sample is in the oven.

Step 11: After drying, replace the lid and allow the whole to cool.

Step 12 Weigh the container and contents to the nearest 1 g (m₃).

Calculations and Expression of Results (see attached form)

1. Calculate the moisture content of the soil specimen, w, as a percentage of the dry soil mass to the nearest 0.1 %, from the equation:

Where,
m₁ is the mass of container (in g);
m₂ is the mass of container and wet soil (in g);
m₃ is the mass of container and dry soil (in g).

NOTE 1: If the moisture content is to be related to the Atterberg limits, e.g. for determining the liquidity index, and the soil contains material retained on a 425 μm sieve, the measured moisture content, w (in %), can be corrected to give the equivalent moisture content, w_a (in %), of the fraction passing the 425 μm sieve, using the equation:

Where,
P_a is the percentage by dry mass of the portion of the soil sample passing the 425 μm test sieve.
If the particles retained on the 425 μm sieve are porous and absorb water, the amount of absorption should be determined and the value of w_a calculated from the equation:

Where,
w_r is the moisture content of the fraction retained on the 425 μm test sieve.

NOTE 2: The presence of dissolved salts in saline water gives an incorrect value of moisture content. For these soils the “fluid content” is more appropriate, i.e. the mass of pore fluid (water plus salts) per unit dry mass of soil particles. This can be calculated from the measured moisture content, w (in %), by the following two ways.

a) If the proportion by mass of salt in the fluid is known (p parts per thousand (ppt), i.e. p in mg of salt per g of fluid), the fluid content, w_f (in %), by mass is calculated from the equation:

b) If the salt content is known in terms of volume (q in g of salt per litre of fluid), the density of the fluid, r_f (in Mg/m³), also needs to be known. The fluid content (in %) by volume can then be calculated from the equation:

In sea water having a density of 1.024 Mg/m³ and containing a salt concentration of 35 ppt by mass, the difference between the fluid content and the conventional moisture content, expressed as a percentage, is about 3 %. The difference increases with increasing moisture content. Express the moisture content of the soil for values up to 10 % to two significant figures. For moisture contents above 10 % express the result to the nearest whole number.

Test Report

The test report shall affirm that the test was carried out in accordance with BS 1377-2:1990 and shall contain the following information:

a) the method of test used;

b) the moisture content;

c) the temperature at which the soil was dried, if less than 105 °C;

d) the comparison with Atterberg limits, if required;

e) the fluid content, if appropriate;