Determination of Moisture Content of Soil

Introduction

     The moisture content describes the amount of water in a soil sample. It is equal to the ratio of the mass of “pore” or “free” water in a given mass of material to the mass of the solid particles in the same mass of material, expressed as a percentage. The water (or moisture) content of a soil is useful and recorded in every test in geotechnical engineering. This basic type of information provides insight on the conditions of the soil. The water content of undisturbed samples from the site is also measured and reported on boring logs and in the engineering reports. Sometimes, we need to mix a soil to a certain water content to meet specifications for construction.
     Traditionally, we used a standard oven set at a temperature of 110oC. These days we can also use a microwave oven, which gives immediate results. The two methods are only slightly different and they are both explained on this page. You will mostly use the microwave oven method, but in a few cases the standard method is more reliable. The instructions for the specific test will tell you which method to use for each laboratory experiment.

Objective(s) of the Experiment

     To determine the moisture (water) content of soil, rock, and soil-aggregate mixtures, expressed as a percentage of the mass, by means of either a conventional oven or a microwave oven.

Preparation of the Sample

Store samples prior to testing in airtight containers at a temperature between 2.8°C (37°F) and 30°C (86°F) and in an area that prevents direct contact with sunlight. Select the minimum mass of soil to use for moisture content determination based on the following table:

1. CONVENTIONAL OVEN METHOD

This part outlines the procedures for determining the moisture (water) content of soil, rock, and soil-aggregate mixtures by using the conventional oven method. Drying takes at least 12 hours in a standard oven, but the temperature is constant; avoiding problems with overheating. If you have a large sample, overheating is likely in a microwave oven, therefore the standard oven is recommended.

Equipments and Materials Needed

The equipments and materials needed for conventional oven method are:

  1. Drying oven capable of maintaining a temperature of 110oC ±5oC.
  2. A scale with readability of 0.01 g is required for specimens with mass of less than 200 g.
  3. Specimen container suitable for use in an oven. For small samples (less than 200 g) use the aluminum containers with a lid to prevent moisture loss before drying and moisture gain from the air after drying. The containers must be dry.
  4. Gloves or holders to handle the container.
  5. Tools such as knives, or spatulas.
  6. A marker, if the container does not have an identifying feature.

Procedures

  1. According to Table 1, select a representative test specimen of the soil mass
  2. Determine the mass of an empty, clean, dry container and lid, and record as MC. Remember to record the container ID or mark the container. Many similar containers are placed in the oven at the same time and may be moved. You want to make sure you will be able to find your specimen.
  3. Place the moist specimen in the container and secure the lid onto the container.
  4. Determine and record the mass of the container, lid, and wet specimen as MCWS.
  5. Remove the lid and place the container with the sample in the drying oven.
  6. Dry for a minimum of 16 hours or to a constant mass.
  7. To oven-dry large test specimens, place the material in containers having a large surface area (such as a pan) and break into smaller aggregations.
  8. After the material has dried to a constant mass, remove the container from the oven and replace the lid firmly.
  9. Allow the material and container to cool to room temperature or until the container can be handled comfortably with bare hands and the operation of the balance will not be affected by the convection currents.
  10. Determine the mass of the container, lid, and dry specimen using the same balance which had been used in the above procedures and record as MCS.

Results and Calculations

  • MCWS = mass of container and wet specimen, g
  • MCS = mass of container and oven dry specimen, g
  • MC = mass of container, g
  • MW = mass of water, g
    MW = MCWS – MCS
  • MS = mass of solid particles.
    MS = MCS – MC
  • w = water content, %
    w = (MW / MS) x 100

Report

The moisture content is presented using the type of form below:

2. MICROWAVE OVEN METHOD

This section explains the procedures for determining the moisture (water) content of soil, rock, and soil-aggregate mixtures by using the microwave oven method. This method is commonly used as a quicker alternative to the standard oven drying method; therefore it is mostly used when immediate results are needed. You cannot use the microwave oven method for soils with significant levels of organics.
The main problem with using the microwave oven for water content determination is the possibility of heating the soil to temperatures higher than 110oC. The higher temperature may actually change the chemical structure of the clay minerals and give wrong results. By drying the soil in several steps you minimize the chance of overheating.

Equipments and Materials Needed

The equipments and materials needed for microwave oven method are:

  1. A microwave oven. There should be variable power controls as this reduces the potential for overheating the test specimen.
  2. A scale having a 2000 g or greater and readability of 0.1 g is required.
  3. Specimen container suitable for use in a microwave. These containers must be clean and dry.
  4. Gloves or holders to handle hot containers.
  5. Heat sink, something that will absorb the microwave energy once all the water has been extracted from the specimen and will prevent overheating.
  6. Tools such as knives or spatulas for cutting the specimen before and during testing.

Precautions and Instructions

  • Handle hot containers with insulated container holder. Some soil types can retain considerable heat, and serious burns can result from improper handling.
  • Observe any safety precautions supplied by the manufacturer of the microwave. Pay particular attention to keeping the door sealing gasket and door interlocks clean and in good working condition.
  • Manufacturers of microwave ovens may consider using their products to dry soils to be abusive and to constitute the voiding of warranties. Drying soils containing metallic materials may cause arcing in the oven. Highly organic soils and those containing oils and coal may ignite and burn during drying. Continued operation of the oven after the soil has reached constant weight may also cause damage or premature failure of the unit.
  • Do not use metallic containers in a microwave oven. Arcing and oven damage may result.
  • Placement of the test specimens directly on the glass liner tray provided with some ovens is strongly discouraged. Concentrated heating of specimens may shatter the glass tray, possibly causing injury to the user.
  • The use of a microwave oven is acceptable in place of a 110 ±5oC (230 ±9oF) oven for drying soil specimens. Experience and good judgment should dictate sufficient drying time related to using a microwave oven.

Procedures

  1. Determine the mass of a clean, dry container or dish, and record it as MC.
  2. Place the soil specimen in the container, and immediately determine and record the total mass as MCWS.
  3. It may be necessary to split the sample into segments and dry them separately to obtain the dry mass of the total sample when very large samples are needed to represent soil containing large gravel particles.
  4. Place the soil specimen in the container in a microwave oven with the heat sink and turn the oven on for 3 min (initial setting time).
  5. After the set time has elapsed, remove the container and soil from the oven taking care not to burn yourself, and immediately record the mass.
  6. Determine and record the combined mass of the container and oven-dried specimen.
  7. Mix the soil carefully with a small spatula and make sure you do not lose any soil.
  8. Return the container and soil to the oven and reheat in the oven for one minute.
  9. Repeat drying, cooling, and weighing, until the change between two consecutive mass determinations has an insignificant effect on the calculated moisture content. A change of 0.1% or less of the initial wet mass of the soil should be acceptable for most specimens.
  10. Record the final combined mass as MCS

Results and Calculations

Use the same calculations as shown in the conventional oven method above and record in the form below:

Report

The moisture content is presented using the type of form below:

References

  1. Mamlouk, M.S. And Zaniewski J.P., Materials For Civil and Construction Engineers, 2011.
  2. Texas Department of Transportation. Test Procedure for Determining Moisture Content in Soil Materials, TEX-103-E. Effective Date: August 1999.
  3. ASTM D 4643 – Standard test method for determination of water (moisture) content of soil by the microwave oven heating.
  4. ASTM D 2216 – Standard test method for laboratory determination of water (moisture) content of soil and rock by mass.

Download “Determination of Moisture Content of Soil”

Determination-of-Moisture-Content.docx – Downloaded 0 times – 509.24 KB

Leave a Comment