Total Suction Research Articles

Total suction measurement of unsaturated soils with a device using the chilled-mirror dew-point technique

The total suction of a soil consists of two components: matric suction and osmotic suction. There are different techniques for measuring the total, matric and osmotic suction. The total suction of the soil is related to the water vapour pressure in the air space of the soil, and can be measured by devices capable of measuring relative humidity. A range of devices is available to measure relative humidity. In this paper, a device using the chilled-mirror dew-point technique with a selectable temperature environment was used to measure the relative humidity of a soil sample. Two residual soils of Singapore were used in the evaluation of the device for total suction measurement. The residual soils were compacted at various water contents and with three compaction efforts to provide soil samples of various suction values. The total suction of the compacted soil was measured and compared with its matric and osmotic suctions as independently measured using the null-type axis-translation apparatus and the pore f...

Suction effects on rockfill compressibility

A conceptual mechanism of rockfill deformation was postulated by Oldecop & Alonso (2001) on the basis of the observed rockfill behaviour in suction-controlled oedometer tests. Rockfill mechanical behaviour was linked to water action by means of some crack propagation phenomena, usually known as subcritical crack growth (Atkinson, 1984). For most rocks, the propagation velocity of cracks depends on the applied loads and the chemical action of water contained within the rock particles. Water action is conveniently measured by the relative humidity or by the total suction. It is believed that such phenomena are involved in rockfill particle breakage, an experimental fact that is well recognised as being part of rockfill volumetric deformation under a wide range of stress states (Kjaernsli & Sande, 1963; Sowers et al., 1965; Fumagalli, 1969; Marsal, 1973). A phenomenological constitutive model was proposed (Oldecop & Alonso, 2001) for one-dimensional compression, in which total suction and total stress are the relevant variables. This initial approach was based on a reduced number of laboratory tests, in which the vertical stress was limited to a maximum of 1 MPa. A new experimental programme was performed on the same material, using a newly developed testing device. A large-diameter (300 mm) oedometer, specially designed for the control of relative humidity by means of an air-flow circulation through the specimen, was built. The maximum vertical load was extended to 2·8 MPa in order to investigate the material behaviour in a load range common to rockfill dam structures. New features of rockfill behaviour were observed during this experimental programme. These are described in the paper. Moreover, the previously proposed constitutive model has been reformulated in order to extend its capabilities to cover these new features.

Suction effects on rockfill compressibility

A conceptual mechanism of rockfill deformation was postulated by Oldecop & Alonso (2001) on the basis of the observed rockfill behaviour in suction-controlled oedometer tests. Rockfill mechanical behaviour was linked to water action by means of some crack propagation phenomena, usually known as subcritical crack growth (Atkinson, 1984). For most rocks, the propagation velocity of cracks depends on the applied loads and the chemical action of water contained within the rock particles. Water action is conveniently measured by the relative humidity or by the total suction. It is believed that such phenomena are involved in rockfill particle breakage, an experimental fact that is well recognised as being part of rockfill volumetric deformation under a wide range of stress states (Kjaernsli & Sande, 1963; Sowers et al., 1965; Fumagalli, 1969; Marsal, 1973). A phenomenological constitutive model was proposed (Oldecop & Alonso, 2001) for one-dimensional compression, in which total suction and total stress are the relevant variables. This initial approach was based on a reduced number of laboratory tests, in which the vertical stress was limited to a maximum of 1 MPa. A new experimental programme was performed on the same material, using a newly developed testing device. A large-diameter (300 mm) oedometer, specially designed for the control of relative humidity by means of an air-flow circulation through the specimen, was built. The maximum vertical load was extended to 2·8 MPa in order to investigate the material behaviour in a load range common to rockfill dam structures. New features of rockfill behaviour were observed during this experimental programme. These are described in the paper. Moreover, the previously proposed constitutive model has been reformulated in order to extend its capabilities to cover these new features.

Polymer Capacitance Sensors for Measuring Soil Gas Humidity in Drier Soils

Abstract Polymer capacitance sensors were assessed in this study for measuring soil gas relative humidity and total suction in soils that are too dry for thermocouple psychrometers. Capacitance of the sensors varies as the relative humidity of the gas surrounding the sensor is varied. Soil gas relative humidity is inferred through a calibration curve relating soil gas relative humidity and capacitance. Tests conducted with six finer textured soils and three sands showed that the capacitance sensors can be used to obtain unbiased estimates of relative humidity for a broad variety of soils with a precision of approximately ±3%. Tests also showed that the sensors are non-hysteretic and relatively insensitive to temperature, except when the temperature approaches the freezing point of water. An example is provided illustrating how capacitance sensors can be used to monitor soil gas relative humidities in alternative earthen final covers that employ passive flow of atmospheric air to remove water.

Filter Paper Column for Measuring Transient Suction Profiles in Expansive Clay

An inexpensive experimental system for measuring suction profiles during transient moisture movement processes in unsaturated expansive clays was developed. The system relies on measurements of total suction using the noncontact filter paper technique at several stations along a onedimensional soil column. Performance of the system was demonstrated for a column of compacted Ca2+-smectite undergoing evaporation at the top boundary under ambient laboratory conditions for 91 days. The system was most applicable during latter stages of drying when the rate of moisture movement was slow enough to be adequately captured by the required 7-day measurement interval and the suction values are large enough to offset scatter. Final water content and suction profiles were determined gravimetrically and compared with values obtained from the filter paper measurements. Desiccation cracking did not significantly affect the accuracy of the filter paper system. The research forms an experimental basis to develop or verify theoretical models aimed at predicting moisture flux for in situ expansive soil subject to infiltration or evaporation processes.

Factors Affecting the Filter Paper Method for Total and Matric Suction Measurements

Abstract The filter paper method is used as an indirect means of measuring soil suctions. The advantages of the method include its simplicity, its low cost, and its ability to measure a wide range of suctions. The filter paper method has also been used in the field to measure soil suctions. However, the simplicity of the filter paper method has led to an inadequate understanding of the method and, therefore, improper usage. Recent findings have highlighted the need for more precautions in the use of the filter paper method for suction measurements. This paper presents a literature review of the factors affecting the accuracy of the filter paper method and identifies gaps in the current knowledge of the method. An experimental study was also conducted to clarify some of the factors affecting the response of the two most commonly used filter papers for suction measurements. Whatman No. 42 and Schleicher and Schuell No. 589. Calibration curves of the filter papers show that the water content-suction relationships are different for total and matric suctions. The total suction calibration curve is not very sensitive to applied suctions less than 1000 kPa. The calibration curves obtained for total suction are different from those recommended in ASTM D 5298-94, suggesting that the ASTM D 5298-94 equations should not be used for total suction. Instead of existing bilinear equations, new equations are proposed to describe the total and matric suction calibration curves. All the equations were evaluated using the filter paper method to measure the suction of soil specimens of known matric suctions.

The Influence of Hydraulic Nonequilibrium on Pressure Plate Data

Pressure plates are used routinely to measure water‐retention characteristics of soils. Plates of varying porosity are used, depending on the pressure range of interest. For applied pressures up to 1.5 MPa, 15‐bar porous ceramic plates with fine porosity are used because of their high bubbling pressure (>1.5 MPa), which limits airflow through the plate. The typical saturated hydraulic conductivity of the 15‐bar plate is <3 × 10−11 m s−1 Low plate conductance coupled with decreasing soil hydraulic conductivities at high pressures strongly influence equilibrium times, which theoretically may extend to months or years. We measured the soil water pressures (suctions) for three soils, a sand, a silt loam, and a clay, placed on 15‐bar pressure plates for 10 d or longer, with and without static loads and with and without using a kaolinite slurry to improve plate contact. Total matric suctions, inferred from peltier psychrometry data, were always <1.0 MPa. When sample height was increased from 1.5 to 3 cm, the water contents increased and total suctions decreased to 0.15 MPa for sand, 0.3 MPa for silt loam, and 0.55 MPa for clay. These data suggest that alternative methods other than pressure plates may be required to measure equilibrium water suctions of soils in reasonable times in the 1.5‐MPa (15‐bar) pressure range and that loading of the samples and use of kaolinite slurry appear to be ineffective in speeding equilibrium.

Filter Paper Technique for Measuring Total Soil Suction

An experimental program was conducted to investigate the noncontact filter paper technique for total suction testing. Calibration curves for seven different batches of Whatman #42 filter paper were obtained to evaluate the variation in the calibration characteristics from one batch of paper to another. Additional tests using salt solutions for controlling suction were conducted to evaluate the measurement uncertainty as a function of suction magnitude. Calibration curves for the various papers varied significantly from batch to batch. This variation can be as much as 11% for filter paper water content, which results in a potential measurement error as high as 92% for suction. Scatter in the measurements appeared to increase with decreasing total suction. At relatively high values of suction (32 000 kPa), the standard deviation for five independent measurements was as much as 11.6% of the mean. At relatively low values (270 kPa), the deviation appeared to increase to 23.2% of the mean. One hundred measurements over a wide range of values for an expansive clayey soil confirmed the observation that the scatter in the measurements increases as total suction decreases.

The Influence of Hydraulic Nonequilibrium on Pressure Plate Data

Pressure plates are used routinely to measure water retention characteristics of soils. Plates of varying porosity are used depending on the pressure range of interest. For applied pressures up to 1.5 MPa, 15-bar porous ceramic plates with fine porosity are used because of their high bubbling pressure (>15 bar), which limits airflow through the plate. The typical saturated hydraulic conductivity of the 15-bar plate is less than 1x10-13 m s-1. Low plate conductance coupled with decreasing soil hydraulic conductivities (e.g., <1x10-13 m s-1) at high pressures strongly influence equilibrium times, which theoretically may extend to months or years. We measured the soil water pressures (suctions) for three soils, a sand, a silt loam, and a clay placed on 15-bar pressure plates for 10 days or longer, with and without static loads and with and without use of a kaolinite slurry for improved plate contact. Total matric suctions, inferred from peltier psychrometry data, were always less than 1.5 MPa. When sample height was increased from 1.5 cm to 3 cm, the water contents increased and total suctions decreased to 0.15 MPa for sand, 0.3 MPa for silt loam, and 0.55 MPa for clay. These data suggest that alternative methods to pressuremore » plates may be require to measure equilibrium water suctions of soils in reasonable times in the 15-bar pressure range and that loading of the samples and use of kaolinite appear to be ineffective in speeding equilibrium.« less

Three-Dimensional Simulations of Airfoil Clocking in a 1-1/2 Stage Turbine

Axial turbines have inherently unsteady flow fields because of the relative motion between rotor and stator airfoils. This relative motion leads to viscous and inviscid interactions between blade rows. As the number of stages increases in a turbomachine, the buildup of convected wakes can lead to progressively more complex wake/wake and wake/airfoil interactions. Variations in the relative circumferential positions of airfoils in adjacent rotating or non-rotating blade rows can change these interactions, leading to different unsteady forcing functions on airfoils and different turbine efficiencies. In addition, as the Mach number increases the interaction between blade rows can be intensified due to potential effects. It has been shown, both experimentally and computationally, that airfoil clocking can be used to improve the efficiency and reduce the unsteadiness in multiple-stage axial turbomachines with equal blade counts. While previous investigations have provided an improved understanding of the physics associated with airfoil clocking, most of the work focused on two-dimensional effects. More research is needed to determine how airfoil clocking can be incorporated in the design of three-dimensional airfoils. In the current study three-dimensional simulations of airfoil clocking have been performed for a 1-1/2 stage high-pressure turbine operating at subsonic flow conditions. Comparisons have been made with the available time-averaged experimental data at a single clocking position. The predicted results indicate approximately a 1% efficiency variation as the secondstage stator airfoils are clocked with respect to the first-stage stator airfoils. / cv Cp P P ps Pt f S ss t U a /? 7 TJ in min max ms out st tt oo 1/2 1 2 3 4 NOMENCLATURE Frequency Specific heat at constant volume (P-Pt)/(l/2pUms) Static pressure Pressure surface Total pressure Entropy Suction surface Time Rotor velocity Absolute frame flow angle (from axial) Relative frame flow angle Ratio of specific heats Efficiency Flow coefficient Density

Automated Measurement of Total Suction Characteristics in High-Suction Range: Application to Assessment of Swelling Potential

An automated experimental system to characterize moisture-content-totalsuction relationships for unsaturated clayey soils is described. Unlike traditional techniques requiring independent measurement of total suction and moisture content (e.g., thermocouple psychrometry and filter paper), suction is directly controlled and corresponding moisture contents are continuously monitored. The system is applicable for total-suction values ranging between approximately 620 000 and 14 000 kPa. Total suction is controlled during testing by computer-automated rationing of “dry” and “wet” nitrogen gas in a closed environmental chamber. To develop characteristic curves, the equilibrium moisture content of soils placed in the chamber is measured with an integrated electronic balance as water vapor is absorbed or desorbed. The system requires no user intervention and is capable of generating characteristic curves, including wetting and drying loops, in a time span on the order of one to two weeks. Concurrent measurements using the noncontact filter paper method (ASTM D5298) are shown to agree very closely with those obtained from the automated system. Total-suction characteristic curves are compared for a highly expansive smectite clay and nonexpansive kaolinite clay. Hysteresis between the wetting and drying cycles and stepwise dehydration phenomena, which have been noted by previous researchers, are reproduced in the results. Comparison of the smectite and kaolinite characteristic curves demonstrates the system’s potential to become a simple, non-labor-intensive, quick, and reliable platform to qualitatively assess the relative swelling potential of clayey soils.

Optimum Suction Distribution for Transition Control

The optimum suction distribution which gives the longest laminar region for a given total suction is computed. The goal here is to provide the designer with a method of finding the best suction distribution subject to some overall constraints applied to the suction. We formulate the problem using the Lagrangian multiplier method with constraints. The resulting nonlinear system of equations is solved using the Newton–Raphson technique. The computations are performed for a Blasius boundary layer on flat-plate and crossflow cases. For the Blasius boundary layer, the optimum suction distribution peaks upstream of the maximum growth rate region and remains flat in the middle before it decreases to zero at the end of the transition point. For the stationary and travelling crossflow instability, the optimum suction peaks upstream of the maximum growth rate region and decreases gradually to zero.

Side Shear Induced in Drilled Shaft by Suction Change

This paper describes an experimental study in which a 305 mm diameter instrumented drilled shaft was installed in a moderately expansive clay soil during the dry season and monitored over a period of about 15 months. The purpose of the study was to investigate the effects of seasonal moisture changes in the soil on the unit shear stresses imposed on the sides of the drilled shaft. The soil in the vicinity of the test shaft was instrumented to measure suction and ground surface movement. The relation between total suction, total stress, and shear strength of the soil at the test site was determined through laboratory triaxial compression testing. Daily rainfall and temperatures were also monitored. Over the course of the study, induced side shear stresses of up to 54 kPa were measured in the test shaft. A simple computational model was developed to relate observed suction changes to unit side shear stresses induced by the expansion of the soil through the use of the laboratory total suction-total stress-shear strength relation.

The effect of soil suction on evaporative fluxes from soil surfaces: Discussion

The authors have employed an elegant approach to the prediction of evaporation rates from freshwater soils by using a modified form of the Dalton mass transfer equation. This has suggested that a unique value of total suction exists at the soil‐air interface during the transition from constant rate evaporation to falling rate evaporation (AE/PE < 1). The discussers have been conducting investigations of a similar, but slightly more complex problem of evaporation from highly saline soil surfaces. Saline soils tend to occur in arid regions of the world, e.g., coastal sabkha (Arabian Gulf, Mexico, and Australia) and inland playas or salt lakes (U.S.A., Australia, Egypt, and Mexico). Artificial saline soils (tailings) are also created in various mining industries as a waste product. The source of this salinity is either the ore processing (e.g., bauxite) or scarcity of freshwater (e.g., Chile, Australia). The most characteristic feature of these soils is the development of surficial efflorescent salt crusts during periods of drying. It has been found that these crusts interfere with the evaporation process from saline soils to such an extent that in some cases the evaporation rate can be reduced to less than 10% of potential

Influence of pressure, saturation, and temperature on the behaviour of unsaturated sand-bentonite

Triaxial compression tests were performed on unsaturated sand-bentonite buffer at elevated temperatures and pressures. Air and water drainage lines from the specimen were generally kept closed through (i) heating, (ii) pressuring, and (iii) shearing. The tests can therefore be characterized as undrained-undrained-undrained tests. Effects of open drainage were also explored. Confining pressures ranged from 0.2 to 3.0 MPa at temperatures of 26, 65, and 100°C. Specimens were statically compacted to a constant dry density of 1.67 Mg/m3, and to degrees of saturation between 35 and 98%. Suction - water content relationships were established to determine the initial total suction before testing. Results indicate that undrained strengths increase with decreasing degree of saturation (50 &lt;= Sr &lt;= 100%), with increasing confining pressure, and with decreasing temperature. The normalized compression modulus E50/su does not respond monotonically to changes in saturation, pressure, and temperature. Changes in strength and stiffness have been described in relation to net mean stress and suction in shear strength - net mean stress - suction space.Key words: sand-bentonite, unsaturated, suctions, temperature, strength, stiffness, structure.

Variation of Soil Suction with Depth in Dallas and Fort Worth, Texas

The variation of soil suction and the estimate of constant soil suction with depth is used to design many slab-on-grade foundations and pavement moisture barriers. The Post-Tensioning Institute’s design procedures for slab-on-grade foundations and design of vertical pavement moisture barriers use the constant suction at depth to predict differential soil movements that influence shear, deflection, and moment magnitudes and the effective barrier depth. Constant soil suction estimates can be correlated to the climate or long-term weather conditions at any given site by using the Thornthwaite moisture index (TMI), which estimates the amount of net moisture surplus or deficit from precipitation and evapotranspiration of moisture from the ground surface. On the basis of the empirical curves, the constant value of total soil suction for the Dallas–Fort Worth, Texas (DFW) area is about 246 kPa based on an average TMI of 0. Analysis of more than 1,200 total soil suction laboratory tests performed on developed and undeveloped lots indicates that the measured average total soil suction value in the upper 6 m is closer to 979 kPa for the DFW area ranging between 55 kPa and 11,246 kPa during the 1995–1997 period. Some hypothesized reasons for the difference between the empirical and measured equilibrium (constant) soil suction values are amounts of clay, clay origin, variable plasticity indexes, soluble salt content, and equilibration curve differences.

The effect of soil suction on evaporative fluxes from soil surfaces

This paper presents a theoretical approach in which a Dalton-type mass transfer equation is used to predict the evaporative fluxes from nonvegetated soil surfaces. Soil evaporation tests were conducted in the laboratory on three different soil samples of Beaver Creek sand, Custom silt, and Regina clay. The soil surfaces were saturated and allowed to evaporate to a completely air-dried state. The actual evaporation rate for each soil surface was measured along with the potential evaporation rate for an adjacent water surface. The ratio of actual evaporation to potential evaporation or normalized soil evaporation was then evaluated with respect to drying time, soil-water content, and soil suction. The value of the normalized soil evaporation was found to be approximately equal to unity for all soils until the total suction in the soil surfaces reached approximately 3000 kPa. The rate of actual soil evaporation was observed to decline when the total suction exceeded 3000 kPa. A relationship between the actual evaporation rate and total suction was found to exist for all three soil types which appears to be unique and independent of soil texture, drying time, and water content.

Soil Suction Dispersion and Statistical Analysis with Depth at the Medford Airport

The use of soil suction measurements has steadily increased in geotechnical practice. A current standard test procedure that describes the use of laboratory filter papers as passive sensors to measure the soil matric and total suction is ASTM D5298-94. The principal objective of the research presented here was to evaluate dispersion of suction measurements with depth using the filter paper method described in the ASTM standard. In a single test boring to a depth of 5.33 m, 35 samples were taken at intervals of 152.4 mm. Each sample was separated into four replicate samples. The matric and total suction of each of the 35 sample intervals were then measured, resulting in a total of 278 suction measurements. To manage this volume of tests, the sample size was split approximately in half, and each portion was separately tested. This approach led to severall insights into the test procedure, such as the effect of delay in transferring the filter paper from the specimen container into the metal container. Sampling and testing are described. The dispersion of both the total and the matric suction measurements was characterized in three ways: the range of measurements in each sample interval, the standard deviation of measurements in each sample interval, and an error function defined as the difference between each measurement and the mean for the sample interval. Each of these statistics was plotted as a histogram. A chi-squared test was applied to measure the goodness-of-fit of a t-distribution fitted to the error as defined by the difference between each measurement and the mean for that sample interval. Results indicate precision for total and matric suction measurements of 0.0540 log kPa within one standard deviation. Analysis does not indicate a substantial difference between the error associated with total suction measurements and that associated with matric suction measurements. A brief description of the conventional geotechnical site characterization is presented for the Medford Airport.

Determination of aircraft stall margins during takeoff

Numerical Results In numerical examples a is assumed to be 1 rad. Table 1 shows a comparison study of the total induced drag between the present method and two kinds of lifting-line theories. Two lifting-line theories are unable to predict the near-field induced drag. This result confirms the validity of the present analysis because the agreement of two kinds of induced drags is good. Figures 2a and 2b show the spanwise distributions of the sectional induced drags by the present analysis for the cases of Table 1. The sectional distributions of two methods are different near the wingtip of even the large aspect ratio elliptic wing. Figure 3 shows a comparison result of the total suction force for various values of k. Even the extended lifting-line theory will not estimate induced drag sufficiently at the values of k above about 0.25, that is, of an aspect ratio below about 6.4.

Laboratory Filter Paper Suction Measurements

Abstract Results of filter paper measurements for both matric and total suction values are reported for a sand, silt, and clay. Calibration curves for the matric and total suction are given for the filter paper used in this study, and comparisons with the results of several other researchers are made. For low suction values, the matric suction is more easily measured than the total suction using the filter paper method. This difference is due primarily to the insensitivity of the water content of the noncontact filter paper to changes in suction when the suction is low. A pressure membrane, saturated salt solutions, and tensiometers were used in developing the filter paper calibration curves. Potential pitfalls in making filter paper measurements and limitations of the method are discussed. A procedure for obtaining filter paper suction measurements is given.