1、Standard Method of Test for Soil Suction AASHTO Designation: T 273-86 (2013) American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-1a T 273-1 AASHTO Standard Method of Test for Soil Suction AASHTO Designation: T 273-86 (
2、2013) 1. SCOPE 1.1. This method covers the procedure for determining total soil suction force using thermocouple psychrometers. The test results of this method are to be used in conjunction with T 258. 1.2. The thermocouple psychrometer measures the relative humidity in the soil by a technique calle
3、d Peltier cooling. By causing a small direct current of approximately four to eight milliamperes to flow through the thermocouple junction for approximately 15 s in the correct direction, this junction will cool and water will condense on it when the dewpoint temperature is reached. Condensation of
4、this water inhibits further cooling of the junction and the voltage difference developed between the thermocouple and the reference junctions can be measured using a microvoltmeter. With proper calibration, the thermocouple psychrometer output in microvolts can be converted directly to soil suction
5、in convenient units of pressure. Typical thermocouple psychrometer output voltages vary from less than one microvolt for relative humidities close to 100 percent or total soil suction less than 95.76 kPa (1 tsf) to about 25 microvolts for relative humidities of about 95 percent or total soil suction
6、s of about 5746 kPa (60 tsf). 1.3. The following applies to all specified limits in this standard: For the purposes of determining conformance with these specifications, an observed value or a calculated value shall be rounded off “to the nearest unit” in the last right-hand place of figures used in
7、 expressing the limiting value, in accordance with ASTM E29. 1.4. The values stated in SI units are to be regarded as the standard. 2. REFERENCED DOCUMENTS 2.1. AASHTO Standards: T 207, Thin-Walled Tube Sampling of Soils T 233, Density of Soil In-Place by Block, Chunk, or Core Sampling T 258, Determ
8、ining Expansive Soils 2.2. ASTM Standard: E29, Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications 3. TERMINOLOGY 3.1. Definitions: 3.1.1. matrix suctionthe matrix suction is the result of clay mineral surface attractive forces for water and catio
9、ns (i.e., ion hydration) and the surface tension effects of water in soil. The matrix suction is both water content and surcharge pressure dependent. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a
10、 T 273-2 AASHTO 3.1.2. osmotic suctionthe osmotic suction arises from the presence of soluble salts in the pore water of the soil and is the result of the difference in the ion concentration between the pore water and the clay particles double-layer water. Osmotic suction is independent of water con
11、tent and surcharge pressure. 3.1.3. soil suctionsoil suction is a quantity that can be used to characterize the effect of moisture on the volume and strength properties of soils; that is, soil suction quantitatively describes the interaction between soil particles and water, which determines the phy
12、sical behavior of the soil mass. Soil suction, expressed in units of pressure, is a measure of the pulling force exerted on water by the soil mass. 3.1.4. total soil suctiontotal soil suction, as determined by this method of test, is the force responsible for soil water retention and is the sum of t
13、he matrix and osmotic components. 4. APPARATUS 4.1. MicrovoltmeterMinimum range of 30 V and readable to the nearest 0.01 V. 4.2. MilliammeterMinimum range of 0 to 25mA. 4.3. Thermocouple PsychrometersWESCOR Model PT 51-10, or equal. 4.4. Polystyrene Thermal ContainersChest or box insulated with at l
14、east 38.1 mm (1.5 in.) of foamed polystyrene. One container approximately 304.8 mm by 304.8 mm by 381 mm (12 in. by 12 in. by 15 in.) will accommodate six samples. 4.5. Sample Containers0.5-L (1-pt) metal sample containers with interior coated with melted wax to prevent corrosion. One-half-liter pai
15、nt cans are satisfactory. 4.6. Rubber StoppersSize 131/2rubber stoppers with an approximately 6-mm (0.25-in.) diameter hole through the center. 4.7. Electrical SuppliesSwitch box, selector switches, electrical connectors, two 1.5-V dry cell batteries, and a 1 K ohm variable potentiometer. 4.8. Calib
16、ration StandardsMinimum of three WESCOR Osmolality Standards (290, 1000, and 1800 mOs/kg) (Note 1). Standard sodium chloride solutions of various molalities may be prepared in the laboratory. 4.9. Stopwatch. 4.10. Tare Containers. 4.11. Specimen Cutting EquipmentWire saw, knife, bandsaw, etc. 4.12.
17、BalanceThe balance shall have sufficient capacity, be readable to 0.1 percent of the sample mass, or better, and conform to the requirements of M 231. Note 1mOs/kg is the abbreviation for milliosmos per kilogram. 5. ASSEMBLING EQUIPMENT 5.1. The assembled apparatus is as shown in Figure 1. The appar
18、atus should be assembled to accommodate multiple samples to be tested simultaneously. The wiring diagram shown in 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 273-3 AASHTO Figure 2 permits the
19、 testing of twelve samples simultaneously by utilizing a 12-position selector switch. Figure 1Soil Suction Apparatus Figure 2Wiring Diagram for Soil Suction Apparatus 5.2. The thermocouple psychrometer wires are fed through an approximately 12-mm (0.5-in.) diameter hole in the center of the thermal
20、container cover. The thermocouple psychrometer wires are then fed through the hole in the rubber stoppers so the psychrometer tips extend approximately 25 mm (1 in.) from the bottom (small diameter end) of the stopper. The protective sheathing around the psychrometer tip must form an airtight seal i
21、n the hole in the rubber stopper. 5.3. The milliammeter, dry-cell batteries, and the variable potentiometer form the cooling circuit. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 273-4 AASHTO
22、5.4. The electrical connectors are affixed to the psychrometer wires for easy connection to the switch box. 5.5. The switches are wired so the output voltages (temperature and soil suction) can be monitored on each of the psychrometers in turn. 5.6. The rubber stoppers are placed in the metal sample
23、 containers, which are placed in the thermal containers to minimize temperature variations. The temperature inside the thermal container is monitored by a thermometer placed through the container cover. 5.7. The equipment should be kept in a room where ambient temperature variations are minimal. 6.
24、CALIBRATION OF EQUIPMENT 6.1. Place a small piece of filter paper (type and grade variable) in the bottom of each sample container along with 3 mL of the calibration standard. A minimum of three calibration standard concentrations should be used to adequately define the calibration line (i.e., 290,
25、1000, and 1800 mOs/kg). 6.2. Seal the sample containers, containing the standards, with the rubber stopper psychrometers installed and place them in the thermal containers. Allow the temperature to come to equilibrium, which requires approximately 24 h. 6.3. After the temperature has come to equilib
26、rium, begin taking temperature (millivolt) and soil suction (microvolt) output readings at least three times per day until the output readings stabilize. A cooling current of 8 mA is applied to the psychrometer for 15 s prior to reading the soil suction output reading. Time to stabilization varies w
27、ith concentrations of the calibration standard but will generally be in the range of 7 to 10 days. 6.4. Obtain at least three stable output readings on each standard. Perform the following calculations on the average of the three readings. 6.4.1. Convert the thermocouple voltage output (millivolts)
28、to temperature (C), using the equation in Section 9.1. 6.4.2. Convert the psychrometer (soil suction) voltage output, ET(microvolts) to the equivalent output at the calibration temperature of 25C, E25, using the equation in Section 9.2. 6.4.3. Calculate the equivalent moisture retention force or suc
29、tion according to one of the following: SI Units: Multiply each concentration by 2.509 (i.e., 1800 mOs/kg 2.509 = 4516 kPa). U.S. Customary Units: Multiply each concentration by 2.62 102(i.e., 1800 mOs/kg 0.0262 = 47.2 tsf). 6.5. Plot the calibration curve for each psychrometer using E25(microvolts)
30、 as the abscissa and the soil suction, , kPa (tsf), for each calibration standard as the ordinate. Typical thermocouple psychrometer calibration curves (Figure 3) are linear and can be expressed using the following equation: ( )25soil suction, , kPa or tsf ,mE n= (1) where: m = slope of the calibrat
31、ion curve; and n = y-intercept of the calibration curve. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 273-5 AASHTO The slope of the calibration curve will always be positive and the y-intercep
32、t should be equal to or less than zero. The calibration curve is good for the useful life of the thermocouple psychrometer; however, under normal use, an annual check of the calibration by at least one point will assure that the equipment is operating properly. Figure 3Typical Thermocouple Psychrome
33、ter Calibration Line 7. SAMPLING 7.1. Undisturbed samples shall be obtained, sealed, and protected in accordance with T 207. 8. TEST PROCEDURE 8.1. Set up apparatus according to Section 5. 8.2. Cut the sample selected for testing into nine (9) cubes approximately 38.1 mm by 38.1 mm (1.5 in. by 1.5 i
34、n.). 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 273-6 AASHTO 8.3. Place two of the specimens directly into the metal sample containers, seal with the rubber stoppers containing the thermocou
35、ple psychrometers, and place in the thermal containers. These two specimens represent the natural conditions of the soil. 8.4. The remaining seven specimens, depending on their natural water contents, are either wetted with varying amounts of distilled water or dried at room temperature for varying
36、lengths of time to establish a range of water content conditions. For example, consider a soil that appears relatively dry (i.e., well below the plastic limit): To establish the necessary water content range, three of the specimens should be dried at room temperature (i.e., 1, 2, and 4 h, respective
37、ly, would be reasonable times) and the remaining four specimens should be wetted with distilled water (i.e., 0.5, 1, 2, and 4 mL, respectively, would be reasonable amounts). 8.4.1. Place the specimen to be wetted into the metal sample containers; add varying amounts of water to the specimens as desc
38、ribed above. Immediately seal the wetted specimens with the rubber stoppers containing the thermocouple psychrometers and place them in the thermal containers. 8.4.2. Allow the remaining specimens to dry at room temperature for varying lengths of time as described above. Place each dried specimen in
39、to the metal sample container and seal with a rubber stopper containing a thermocouple psychrometer and place in the thermal container. 8.5. Allow the specimens to come to equilibrium in the sealed containers. Temperature equilibrium is attained within a few hours after placing the cover of the ther
40、mal container. Equilibrium of the relative humidity of the air measured by the psychrometer and the relative humidity in the soil specimen is usually obtained within 48 to 72 h. 8.6. Using the appropriate switch, read and record the temperature output of the thermocouple psychrometer in millivolts.
41、8.7. Change the switch from thermocouple to psychrometer, set the meter to zero, apply a cooling current of approximately 8 mA for 15 s, then read and record the psychrometer output in microvolts. The cooling currents and times should be identical to those used to determine the calibration curves. 8
42、.8. Repeat Sections 8.6 and 8.7 for each of the thermocouple psychrometers in the equipment setup. 8.9. After the readings are completed, remove the specimens from the containers. Determine the dry density (volume displacement method) and the water content of each specimen in accordance with T 233.
43、(A suggested data sheet that assures correct collection of the required data is shown in Figure 4.) 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.Soil, Suction, Water Content, and Specific Volume Proje
44、ct _ Boring/Sample/Depth _ Date _ SoilSuctionPsychrometer No. Sample Container No. Water Content Increment (0, +, ) Thermocouple Output Psychrometer Output , Millivolts T, C E, Microvolts E25, Microvolts Soil Suction, kPa WaterContentTare No. Mass in Grams Tare Plus Wet Soil Tare Plus Dry Soil Water
45、 MwTare Dry Soil MsWater Content, Percent w Weight-VolumeRelationsTest Temperature of Water, C Mass in Grams Wet Soil and Wax in Air Wet Soil M Wax Wet Soil and Wax in Water Dry Soil MsSpecific Gravity of Soil GsVolume in cm3Wet Soil and Wax Wax Wet Soil V Dry Soil = Ms/GsVsDensity g/cm3Wet Density
46、= (M/V) wDry Density = (Ms/V) dSpecific Volume = 1/dVT Figure 4Suggested Data Sheet for Collection of Soil Suction Data TS-1aT273-7AASHTO 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 273-8 AAS
47、HTO 9. DATA REDUCTION AND INTERPRETATION 9.1. Convert the thermocouple voltage output (millivolts) to temperature (C) using the following equation: output in millivolts, C 0.0395 millivolts CT =(2) 9.2. Convert the psychrometer voltage output, E(microvolts) to the equivalent output at the calibratio
48、n temperature of 25C, E25, using the following equation: 250.325 0.027EET=+(3) 9.3. Determine the Soil Suction, , of each individual specimen by entering the respective psychrometer calibration curve with E25and reading the corresponding soil suction. The soil suction may also be calculated using th
49、e respective psychrometer calibration equation (Section 6). 9.4. Plot the soil suction (ordinate, log scale) versus the water content (abscissa) on a semilog plot to establish the log soil suction versus water content relationship, Figure 5, which is linear and can be expressed using the following equation: log A Bw= (4) where:
