Procedures for Determining a Soil s Water Content

Transcription

1 Procedures for Determining a Soil s Water Content Laboratory determination of Water Content of Soil and Rock D Abstracted, with permission, from the 1996 Annual Book of ASTM Standards, copyright American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA Scope This test method covers the laboratory determination of the water content of soil rock, and similar materials by mass. The water content of a material is defined by this standard as a ratio, expressed as a percentage, of the mass of pore or free water in a given mass of material to the mass of the solid material. This standard requires the drying of material in an oven at high temperatures. Summary of Test Method The test specimen is dried in an oven to constant mass. The loss of mass due to drying is considered to be water in the soil pores. The water content is calculated using the mass of water and the mass of the dry specimen. Significance of Use For many materials, the water content is one of the most significant index properties used in establishing a correlation between soil behavior and its properties. The water content of a material is used in expressing the phase of relationships of air, water, and solids in a given volume of material. In fine-grained (cohesive) soils, the consistency of a given soil type depends on its water content. The water content of a given soil, along with its liquid and plastic limits as determined by Test Method D 4318, is used to express its relative consistency or liquidity index. Apparatus The following items are required for this testing method: 1. A thermostatically controlled oven, preferably of the force-draft type capable of maintaining a uniform temperature of 110 ± 5 C throughout the drying chamber. 2. A balance with a readability of 0.01 g 3. Specimen containers for drying. 4. Container handling devices. 5. Miscellaneous items such as knives, spatulas, scoops, etc. Samples Samples shall be stored in jars or other containers in such a way as to prevent or minimize moisture condensation on the insides of the containers.

2 Test Specimen The minimum mass of moist martial selected to be representative of the total sample, if the total sample is not tested by this method, shall be in accordance with the following: Max Particle Size (100% passing) Standard Sieve Size Minimum Mass of Moist Test Specimen for reporting Water Content to ± 0.1% Minimum Mass of Moist Test Specimen for reporting Water Content to ± 1% 2 mm or less No g 20g 4.75 mm No g 20g 9.5 mm 3/8 in. 500g 50g 19.0 mm 3/4 in. 2.5kg 250g 37.5 mm 1 1/2 in. 10kg 1kg 75.0 mm 3 in. 50kg 5kg If the total sample is used it does not have to meet the minimum mass requirements provided in the table above and the report shall indicate that the total sample was used. If a sample smaller than the minimum is used then it must be stated in the report. Test Specimen Selection When the test specimen is a portion of a larger amount of material, the specimen must be selected to be representative of the water condition of the total amount of material. The manner in which the test sample is selected depends on the purpose and application of the test, type of material being tested, the water condition, and the type of sample (from another test, bag, block and the likes). If disturbed samples such as trimmings, bag samples and the likes are being used, obtain the test specimen by one of the following methods: 1. If the material is such that it can be manipulated and handled without significant moisture loss, the material should be mixed and then reduced to the required size by quartering or splitting. 2. If the material is such that it cannot be thoroughly mixed and/or split, from a stockpile of the material, mixing as much as possible. Take at least five portions of the material at random locations using a sampling tool and combine all portions for the test specimen. 3. If the material or conditions are such that a stockpile can not be formed, take as many portions of the material as possible at random locations and combine all the portions for the test specimen. For intact samples such as block, tube, split barrel, and the like, obtain the test specimen by the following method: Carefully trim at least 3 mm of material from the outer surface of the sample to see if the material is layered and to remove material that is drier or wetter than the main portion of the sample. Then carefully trim at least 5 mm, or a thickness equal to the maximum particle size present, from the entire exposed surface from the interval to be tested.

3 Procedure 1. Determine and record the mass of the clean and dry specimen container (and its lid if used). 2. Select a representative test specimen. 3. Place the moist test specimen in the container and, if used, set the lib securely in position. 4. Determine and record the mass of the container and the moist material. 5. Remove the lid (if used) and place the container with moist material in the drying oven. Dry the material to a constant mass. Maintaining the oven at 110 ± 5 C unless otherwise specified. The time required for obtaining a constant mass may vary depending on the type of material, size of specimen, and oven type as well as other factors. The influence of these factors generally can be established by good judgment, and experience with the material being used and the apparatus being used. 12 to 16 hours should be sufficient. 6. After the material has been dried to a constant mass remove the container from the oven (and replace the lid if used). 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 convection currents and/or its being heated. Determine and record the mass of the dried sample and the container using the same balance. Calculation Calculate the Water content of the material as follows: w = ( M cws M ) ( M M ) c M w 100 = 100 M s where: w = water content, %, M cws = mass of container and wet specimen, g, M = mass of container and oven dry specimen, g, M c = mass of container, g, M w = mass of water, g, and M s = mass of solid particles. Report The report and data sheet shall include the following: 1. Identification of the sample and material being tested, such as boring number, sample number, test number, container number etc. 2. Water content of the specimen to the nearest 1% or 0.1%, as appropriate for based on the minimum sample used. 3. Indicate if test specimen had a mass less than the minimum indicated in chart above. 4. Indicate if the test specimen contained more than one type of material (layered, etc.) 5. Indicate the method of drying if different from oven drying at 110 ± 5 C 6. Indicate if any material (size or amount) was excluded from the test specimen.

4 Statement on Precision The single operator coefficient or variation has been found to be 2.7%. Therefore, results of two properly conducted test by the same operator with the same equipment should not be considered suspect unless they differ by more than 7.8 percent of their mean.

5 Determination of Water Content of Soil By the Microwave Oven Method D Abstracted, with permission, from the 1996 Annual Book of ASTM Standards, copyright American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA Scope This test method outlines procedures for determining the water content of soils by incremental drying soil in a microwave oven. This method is not intended as a replacement of Test Method D 2216, but rather as a supplement when rapid results are required or desired to expedite other phases of testing. Test Method D 2216 is to be used as a method to compare for accuracy and correction. Summary of Test Method A moist soil specimen is place in a suitable container an its mass determined and recorded. It is then placed in a microwave oven, subjected to an interval of drying, and removed from the oven and its new mass is determined. This procedure is repeated until the mass becomes nearly constant. The difference between the mass of the moist specimen and the dried specimen is used as the mass water originally contained in the specimen. The water content is determined by dividing the mass of water by the dry mass of the soil, multiplied by 100. For a given soil and sample size, the time to achieve a constant dry mass can be noted and used as a minimum drying time for subsequent tests using the same size specimen of the same soil. Significance of Use Test Method D 2216 can be time consuming the use of this microwave method can expedite the determination of soil s water content. The principal objection to the use of the microwave oven for water content determination has been the possibility of over heating the soil, thereby yielding a water content higher than would be determined by Test Method D While not eliminating this possibility, the incremental drying procedure described in this method will minimize its effects. Apparatus The following items will be required for this testing method: 1. A microwave oven, preferably with a vented chamber, is suitable. The required size and over rating of the oven is dependent on its intended use. Ovens with variable power controls and input power ratings of about 700 W have been found to be adequate for this use. Variable power controls are important and reduce 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 dry. 4. Container handling devices such as gloves or holders. 5. Stirring tools such as putty knives, glass rods or spatulas for cutting and stirring the test specimen before and during testing.

6 Samples Keep samples that are stored prior to testing in non-corrodible airtight containers at a temperature between approximately 3 and 30 C in an area prevents direct exposure to sunlight. The water content determination should be performed as soon as practical after sampling, especially if potentially corrodible containers (such as steel thin walled tube, paint cans, and the like) or unsealed sample bags are used. Test Specimen The test specimen should be selected with the same method as described for specimen selection in Test Method D Conditioning Prepare and process the specimen as quickly as possible to minimize unrecorded moisture loss that will result in erroneous water content determinations. Cut or break up the soil into small size aggregations. If the specimens are not to be tested immediately, store them in a sealed container to prevent loss of moisture. Procedure 1. Determine the mass of a clean, dry container or dish, and record. 2. Place the soil specimen in the container, and immediately determine and record its mass. 3. Place the soil specimen in the container in a microwave oven with the heat sink and turn the oven on for 3 min. If experience with a particular soil type and specimen size indicate shorter or longer initial drying times can be used without overheating and subsequent drying time may be adjusted. 4. After the set time has elapsed, remove the container and soil from the oven, either weigh the specimen immediately, or place in dissector to cool to allow handing and to prevent damage to the balance. Determine and record the mass. 5. With a small spatula or knife or short length of glass rod carefully mix the soil, taking special precautions not to lose any soil. 6. Return the container and soil to the oven and reheat in the oven for 1 min. 7. Repeat step 4 thru 6, until the change in between two consecutive mass determinations would have 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. 8. Use the final mass determination in calculating the water content. Obtain this value immediately after the heating cycle, or, if the mass determination is to be delayed, after cooling in a dissector.

7 Calculation Calculate the Water content of the material as follows: w = ( M cws M ) ( M M ) c M w 100 = 100 M s where: w = water content, %, M cws = mass of container and wet specimen, g, M = mass of container and oven dry specimen, g, M c = mass of container, g, M w = mass of water, g, and M s = mass of solid particles. Report Report the following information: 1. Identification of the sample (material) being tested by boring number sample number, test number and the like. 2. Water content of the specimen to the nearest 1%. 3. Indicate if the test specimen has a mass less than the minimum required. 4. Indicate if the test specimen contained more than one type of soil. 5. Indication of any material (size, amount, and layer or layer sequences) excluded from the test specimen. 6. Time and setting of initial drying period and subsequent incremental drying periods. 7. Initial mass of test specimen prior to drying (wet mass of sample), and the mass after the final incremental drying periods (dry mass of soil). 8. Identification of comparison test(s) if performed. 9. Identification of microwave oven and drying settings and cycles used. Precision Studies on microwave drying have indicated single laboratory average precision, expressed as a percent moisture content of 0.96% or less, depending upon the soil type, initial moisture, and specimen size.