Measurement Of Matric Suction Research Articles

Calibration of tensiometer for direct measurement of matric suction

Calibration of tensiometer for direct measurement of matric suction

Calibration of tensiometer for direct measurement of matric suction

Matric suction is a primary variable in the analysis of the hydromechanical behaviour of unsaturated soils.

Mini suction probe for matric suction measurements

A modified triaxial apparatus with mini suction probes was fabricated to study the matric suction along the specimen height during unsaturated triaxial testing. Three mini suction probes were placed at 3/4, 1/2, and 1/4 height of the specimen, each at 120° apart in the lateral direction. This paper presents the development of the mini probe for matric suction measurements. Evaluation of the performance shows that the fabricated mini probe provides a rapid response and accurate reading under negative and positive pore-water pressure changes. Matric suctions as high as 400 kPa were successfully measured on soil specimens over a time span of 15 h. On the other hand, the mini suction probes were also found to be able to measure a matric suction of 200 kPa for a longer period of 155 h.Key words: matric suction, mini suction probe, triaxial, unsaturated soils, mid-height pore-water pressure measurement.

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.

Experimental procedures and cavitation mechanisms in tensiometer measurements

In recent years tensiometers for direct measurement of matric suction have been developed at Imperial College and later on at the University of Saskatchewan. The major drawback of these instruments is water cavitation which may occur before pressure equalisation. A better understanding of the mechanisms that control cavitation inside the tensiometer may therefore help optimise their design and define adequate experimental procedures. This paper presents some of the experiences gained over the past four years using three tensiometers manufactured by Imperial College. Some of the anomalous responses recorded by tensiometers are first discussed. These were probably due to inadequate saturation of the porous ceramic rather than malfunctioning of the instruments. The results from a series of tests conducted with two or three tensiometers positioned on the same sample are then presented. These were helpful in defining suitable experimental procedures. Finally, the history of tension breakdown of each tensiometer is examined to identify some of the factors that affect the maximum sustainable tension as well as measurement duration. On the basis of data recorded before and after cavitation, a possible mechanism of cavitation in a tensiometer is described.

Wetting-Induced Settlement of Compacted-Fill Embankments

Many highway embankments experience problematic settlements. Compression of soil under the self-weight of the embankment generally occurs during construction, but postconstruction, wetting-induced collapse can result in more long-term settlement problems, depending on climate and movement of the wetting front. A study undertaken to examine settlement associated with unsaturated soil embankments included centrifuge modeling of compacted silt embankments, laboratory testing of the embankment soil, and data interpretation with the focus on settlement prediction. A silty soil was selected to facilitate the measurement of matric suction using tensiometers. Three model embankments were constructed and tested in the Army Corps of Engineers Centrifuge Research Center in Vicksburg, Mississippi. Embankments 20 m high were simulated using centrifugal acceleration of 165 g. Embankments were constructed to achieve a relative compaction of 90 or 95 percent based on standard effort and moisture content between 2 and 5 percent dry of the optimum moisture content. Instrumentation used during self-weight compression and wetting included linear variable differential transformers and pore-pressure transducers equipped with high-air-entry porous stones. Results demonstrate the importance of the as-compacted water content and dry unit weight on the potential for wetting-induced collapse settlement. Settlement caused by self-weight compression and that caused by wetting-induced collapse are clearly discernable in results of centrifuge tests, allowing for comparison to settlement predictions. Results are discussed in light of typical compaction specifications, oedometer-based predictions, and implications for the design of compacted embankments.

A Dynamic Method for Determining the Soil Water Characteristic Curve for Coarse-Grained Soils

Abstract The air-entry suction is an extremely important parameter in the water characteristic curve, but is difficult to identify when using conventional static methods such as the pressure plate apparatus. In the dynamic method described herein, instantaneous measurement of matric suction and water content are acquired from a soil specimen undergoing a slow drying cycle with the aid of a tensiometer and a soil moisture probe, respectively. The method offers a quick and reliable way of quantifying the drying soil-water characteristic curve (SWCC) for matric suctions of up to 80 kPa. The large number of data points acquired make it easy to obtain an accurate estimate of the air-entry suction. It is particularly effective for soils ranging from medium sands to sandy silts where a nearly continuous SWCC is provided.

Discussion on “The Use of Miniature Pore Pressure Transducers in Measuring Matric Suction in Unsaturated Soils” by K. K. Muralleetharan and K. K. Granger

A discussion of an article with the aforementioned title, by K.K. Muralleetharan and K.K. Granger, published in this journal (Vol. 22, No. 3, September 1999) is presented. The discusser points out additional issues related to the measurement of matric suction in soils such as the cavitation nuclei. The discusser also presents a valuable conditioning technique to measure higher suction values and supporting experimental data. Discussion is followed by closure from one of the authors.

Water balance of a tropical woodland ecosystem, Northern Australia: A combination of micro-meteorological, soil physical and groundwater chemical approaches

A combination of micro-meteorological, soil physical and groundwater chemical methods enabled the water balance of a tropical eucalypt savanna ecosystem in Northern Australia to be estimated. Heat pulse and eddy correlation were used to determine overstory and total evapotranspiration, respectively. Measurements of soil water content, matric suction and water table variations were used to determine changes in soil moisture storage throughout the year. Groundwater dating with chlorofluorocarbons was used to estimate net groundwater recharge rates, and stream gauging was used to determine surface runoff. The wet season rainfall of 1585 mm is distributed as: evapotranspiration 810 mm, surface runoff (and shallow subsurface flow) into the river 410 mm, groundwater recharge 200 mm and increase in soil store 165 mm. Of the groundwater recharge, 160 mm enters the stream as baseflow in the wet season, 20 mm enters as baseflow in the dry season, and the balance (20 mm) is distributed to and used by minor vegetation types within the catchment or discharges to the sea. In the dry season, an evapotranspiration of 300 mm comprises 135 mm rainfall and 165 mm from the soil store. Because of the inherent errors of the different techniques, the water balance surplus (estimated at 20 mm) cannot be clearly distinguished from zero. It may also be as much as 140 mm. To our knowledge, this is the first time that such diverse methods have been combined to estimate all components of a catchment's water balance.

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.

Suction Measurements on a Saskatchewan Soil Using a Direct-Measurement, High-Range Suction Sensor

A tensiometric-type suction sensor capable of direct measurement of matric suctions greater than 100 kPa has recently been developed. The sensor has been used to measure matric suctions up to 1250 kPa. The sensor makes use of the tensile strength of water for the measurement of matric suction. The cavitation of water in conventional tensiometers occurs at a negative pressure approaching 1 atmosphere because of the presence of cavitation nuclei. In the absence of cavitation nuclei, the tensile strength of water can be on the order of several atmospheres. The direct-measurement, high-range suction sensor has been used for the measurement of matric suction on borehole samples obtained during a soils investigation at a bridge site near the town of Outlook, Saskatchewan, Canada. The method has proven to be fast and simple to use. Matric suction measurements of the samples from the Outlook bridge site are presented, along with an interpretation of the laboratory data.

Measurements of matric suction and volume changes during inundation of collapsible soil : Tadepalli, R; Rahardjo, H; Fredlund, D G Geotech Test JV15, N2, June 1992, P115–122

Measurements of matric suction and volume changes during inundation of collapsible soil : Tadepalli, R; Rahardjo, H; Fredlund, D G Geotech Test JV15, N2, June 1992, P115–122

Comparison of diffuse discharge from shallow water tables in soils and salt flats

Diffuse (evaporative) discharge of ground water is of interest in the management of local or regional ground waters, and soil salinity. However, past studies show that discharge may vary greatly between soils in agricultural areas and salt flats for similar water table depths. Low discharge from salt flats has been previously attributed to the effect of salt crusts, yet possible soil hydrological reasons for those differences have not been examined. Steady-state hydraulic theory describing the relationship between discharge and water table depth is reviewed. The minimum water table depths required for the theory to be applied are defined in terms of soil parameters. Relationships between discharge and water table depth are then used to analyse the results of previous diffuse discharge studies. It is shown that discharge from both bare agricultural soils and salt flats is consistent with this theory. Unsaturated hydraulic conductivity of three salt flat soils, determined from measurements of discharge and soil matric suction, showed that low discharge flexus recorded from the sites were due to low soil permeability. The relationship between discharge flux and water table depth calculated for these sites also described discharge from other salt flats, implying that low hydraulic conductivity caused low discharge from these areas as well. The reasons for the low hydraulic conductivity of salt flat soils are not clear, and need to be investigated to determine if it is a general property of soils in these areas, or results from the high salinity levels.

Measurements of Matric Suction and Volume Changes During Inundation of Collapsible Soil

Abstract Reduction in soil volume due to inundation under a constant total stress is a phenomenon referred to as collapse. Collapse is exhibited by soils during a change of state from an unsaturated to a saturated condition. Several researchers have postulated various theories to explain collapse behavior. Recent published research has attempted to explain the collapse phenomenon using theories of unsaturated soil mechanics. However, the theoretical explanations require further verification by experimental data. A simple experimental procedure is suggested in this paper to measure changes in matric suction and soil volume during inundation. The suggested measurements provide experimental data that can be used to verify the applicability of the unsaturated soil mechanics theories to collapsible soil behavior. Typical test results from this experimental program indicate that collapsible soil behavior can be explained using unsaturated soil mechanics theories.

Typical matric suction measurements in the laboratory and in the field using thermal conductivity sensors : Rahardjo, H; Loi, J; Fredlund, D G Proc Indian Geotechnical Conference, 1989, Geotechniques of Problematic Soils and Rocks, Visakhapatnam, 14–16 December 1989V1, P127–131. Publ New Delhi: Sarita Prakashan, 1989

Typical matric suction measurements in the laboratory and in the field using thermal conductivity sensors : Rahardjo, H; Loi, J; Fredlund, D G Proc Indian Geotechnical Conference, 1989, Geotechniques of Problematic Soils and Rocks, Visakhapatnam, 14–16 December 1989V1, P127–131. Publ New Delhi: Sarita Prakashan, 1989

Hydrologic conditions leading to debris-flow initiation

Mitigation of the hazards posed by debris flows requires an understanding of the mechanisms leading to their initiation. The objectives of this study were to evaluate and document the hydrologic response of a potential debris-flow source area to major rainstorms and to evaluate whether traditional models of hillslope hydrology can account for the observed response. A field site in an area of previous debris-flow activity was instrumented and monitored for two winter seasons. Hydrologic responses for a wide variety of antecedent conditions were recorded, including two storm events that produced well-defined positive pore-pressure pulses at the site and initiated numerous debris flows in the immediate vicinity of the site. The observed hydrologic response was highly dependent on antecedent moisture conditions which can be characterized by soil matric suction measurements. The pressure-head pulses observed had a magnitude of approximately 50 cm of water, were transient, traveled downslope, and exhibited some spatial variability. Traditional models of hillslope hydrology do not fully account for the positive pore-pressure pulses observed high on the hillslope. Key words: debris flow, hillslope hydrology, pore pressure, antecedent moisture, tensiometer, piezometer, field investigation.

Use of thermal conductivity sensors to measure matric suction in the laboratory

The measurement of soil suction is pivotal to the application of soil mechanics principles in geotechnical engineering practice related to unsaturated soils. Volume change, shear strength, and seepage analyses all require an understanding of the matric suction in the soil. This note summarizes the use of thermal conductivity sensors to measure matric suction in the laboratory. The thermal conductivity sensor is described along with its mode of operation. A brief description is given of the procedure for calibrating thermal conductivity sensors using a pressure plate apparatus. The measurement of matric suction can be performed in the laboratory on Shelby tube samples. The laboratory measurements of matric suction can be adjusted for the effect of overburden pressure in the field. The required equilibration time for suction measurements is discussed along with details of the test procedure. The applications of the measured suction values to design are briefly discussed.Key words: matric suction, negative pore-water pressure, thermal conductivity sensor, laboratory, undisturbed samples.

Hydraulic characteristics of conventionally and zero-tilled field plots

A silage corn crop was grown on field plots entering their 2nd-year cycle of zero tillage, and on control plots representing traditional tillage practices. Concurrent measurements of soil matric suction and water content were made under transient conditions of a draining profile without evaporation. The results indicated some similarity in the soil moisture characteristics of the untilled soils which produced high crop yields. The time rate of change of soil water content, the root extraction rate and the unsaturated hydraulic conductivity were always smaller in the zero-tilled plots than in the conventionally tilled plots.