Filter papersSoil sampleFilter paper in between two protective papersINTRODUCTIONDISCUSSIONREQUIRED APPARATUSFILTER PAPER CALIBRATION PROCEDURESOIL SUCTION MEASUREMENTS WITHFILTER PAPER METHODINTRODUCTIONThe filter paper method is a soil suction measurement technique. Soil suction is one of the most important parameters describing the moisture condition of unsaturated soils. The measurement of soil suction is crucial for engineering applications in unsaturated soils. The filter paper method is a laboratory test method, and it is inexpensive and relatively simple. It is also the only known method that covers the full range of suction. With the filter paper method, both total and matric suction can be measured. If the filter paper is allowed to absorb water through vapor flow (non-contact method), then only total suction is measured. However, if the filter paper is allowed to absorb water through fluid flow (contact method), then only matric suction is measured. With a reliable soil suction measurement technique, the initial and final soil suction profiles can be obtained from samples taken at convenient depth intervals. The change insuction with seasonal moisture movement is valuable information for many engineering applications.DISCUSSIONThe working principle behind the filter paper method is that the filter paper will come to equilibrium with the soil either through vapor flow or liquid flow, and at equilibrium, the suction value of the filter paper and the soil will be the same. If the filterpaper and soil are not in direct contact, then only total suction is measured. However, if the filter paper and soil are in intimate contact, then only matric suction is measured.In engineering practice, soil suction is composed of two components: matric and osmotic suction. The sum of the matric and osmotic suction is called the total suction:OtPPVRTh ln (1)where,ht= total suction (kPa)R = universal gas constant [8.31432 J/(mol K)]T = absolute temperature (in Kelvin)V = molecular volume of water (m3/kmol)P/PO= relative humidity (in percent)P = partial pressure of pore water vapor (kPa)PO= saturation pressure of water vapor over a flat surface of pure water at the same temperature (kPa).1Suction is frequently represented in cm of negative head. The conversion from kPa to cm is 1 kPa = 10,198 cm. Suction is also frequently represented on a pF – scale. The pF is log10 (cm suction). Matric suction comes from the capillarity, texture, and surface adsorption forces of the soil. Osmotic suction arises from the salts that are present in the soil pore water. In the filter paper method, the soil specimen and filter paper are brought to equilibrium either in a contact (matric suction measurement) or in a non-contact (total suction measurement) method in a constant temperature environment. After equilibrium is established between the filter paper and soil the water content of the filter paper disc is measured. Then, by using a filter paper calibration curve of water content versus suction, the corresponding suction value is found from the curve, so the filter paper method is an indirect method of measuring soil suction. Therefore, a calibration curve should be constructed or be adopted (i.e., the two curves presented for different filter papers in ASTM D 5298 – 94 Standard Test Method for Measurement of Soil Potential (Suction) Using Filter Paper) in soil suction measurements.REQUIRED APPARATUSFor Calibration Procedure:a. Filter papers; the ash-free quantitative Schleicher & Shuell No. 589 White Ribbon or Whatman No. 42 type filter papers.b. Salt solutions; sodium chloride (NaCl) solutions in a range between 0 (i.e.,distilled water) to about 2.7 molality.c. Sealed containers; 250 ml glass jars with lids which work nicely.d. Small aluminum cans; the cans with lids are used as carriers for filter papers during moisture content measurements.e. A balance; a balance with an accuracy to the nearest 0.0001 g. is used for moisture content determination.f. An oven; an oven for determining the moisture contents of the filter papersby leaving them in it for 24 hours at 105  5oC temperature in the aluminum moisture cans (as in the standard test method for water content determinations of soils).g. A temperature room; a controlled temperature room in which the temperature fluctuations are kept below 1oC is used for the equilibrium period.h. Pressure plates and tensiometers; pressure plates and tensiometers are usedfor the low suction range in the calibration process.i. An aluminum block; the block is used as a heat sink to cool the aluminum cans for about 20 seconds after removing them from the oven.In addition, latex gloves, tweezers, plastic tapes, plastic bags, ice-chests, scissors, and a knife are used to set up the test.2For Soil Suction Measurements:a. Filter papers; the ash-free quantitative Schleicher & Shuell No. 589 White Ribbon or Whatman No. 42 type filter papers.b. Sealed containers; glass jars with lids which work nicely.c. Small aluminum cans; the cans with lids are used as carriers for filter papers during moisture content measurements.d. A balance; a balance with an accuracy to the nearest 0.0001 g. is used for moisture content determination.e. An oven; an oven for determining the moisture contents of the filter papers by leaving them in it for 24 hours at 105  5oC temperature in the aluminum moisture cans (standard test method for water content determinations of soils).f. A temperature room; a controlled temperature room in which the temperature fluctuations are kept below 1oC is used for the equilibrium period.g. An aluminum block; the block is used as a heat sink to cool the aluminum cans for about 20 seconds after removing them from the oven.In addition, latex gloves, tweezers, plastic tapes, plastic bags, ice-chests, scissors, and a knife are used to set up the test.FILTER PAPER CALIBRATION PROCEDUREThe filter paper water content measurements are performed by two persons in order to decrease the time during which the filter papers are exposed to the laboratory atmosphere and, thus, the amount of moisture lost and gained during measurements is kept to a minimum. All the items related to filter paper testing are cleaned carefully. Gloves and tweezers are used to handle the materials in nearly all steps of the calibration.The filter papers and aluminum cans are never touched with bare hands. The filter paper