Fernando A. M. Marinho

The Filter Paper Method Revisited

The calibration curve for any device or method to infer suction is the most important characteristic to evaluate for reliable measurement to be obtained. Some published literature (e.g. Houston et al. 1994; Bulut et al. 2001; Leong et al. 2002) have presented results in which the calibration for the filter paper method is not unique in relation to the type of suction (i.e., total or matric). A review of the data from the literature is presented, together with data showing the reason that justifies a unique calibration curve for the filter paper. It is also shown that it is necessary to check the calibration curve by performing a quick calibration check. A procedure for calibrating the filter paper for this check is given. Fundamental matters necessary for understanding the concept on which the filter paper method is based are also presented.

Suction Equilibration Time for a High Capacity Tensiometer

It is well established, the importance of the measurement of soil suction for the assessment of mechanical and hydraulic behavior of unsaturated soils. Among the methods to obtain the soil suction, the tensiometer is one of the most convenient and reliable. However conventional tensiometer has a limitation related to the maximum suction it is capable of measure. This limitation was overcome by Ridley and Burland (1993), with the development of a high capacity tensiometer, which is capable of measure suction well above 100 kPa. The equipment has a quick response time, allowing the determination of suction in minutes. This paper presents a study about the factors that affect the equilibrium time for high capacity tensiometers in the laboratory. Soil specimens were prepared at three different conditions, creating different soil structures. In addition to that an investigation about the characteristic of the interface that is required between the soil sample and the porous ceramic of the tensiometer was carried out; showing the role of the paste on the technique. The results also suggested that it is possible to infer the hydraulic conductivity function using the equilibrium curve obtained during the measurement of the soil suction using the high capacity tensiometer.

Unsaturated Infinite Slope Stability Considering Surface Flux Conditions

A slope stability model is derived for an infinite slope subjected to unsaturated infiltration flow above a phreatic surface. Closed form steady state solutions are derived for the matric suction and degree of saturation profiles. Soil unit weight, consistent with the degree of saturation profile, is also directly calculated and introduced into the analyzes, resulting in closed-form solutions for typical soil parameters and an infinite series solution for arbitrary soil parameters. The solutions are coupled with the infinite slope stability equations to establish a fully realized safety factor function. In general, consideration of soil suction results in higher factor of safety. The increase in shear strength due to the inclusion of soil suction is analogous to making an addition to the cohesion, which, of course, increases the factor of safety against sliding. However, for cohesive soils, the results show lower safety factors for slip surfaces approaching the phreatic surface compared to those produced by common safety factor calculations. The lower factor of safety is due to the increased soil unit weight considered in the matric suction model but not usually accounted for in practice wherein the soil is treated as dry above the phreatic surface. The developed model is verified with a published case study, correctly predicting stability under dry conditions and correctly predicting failure for a particular storm.

DETERMINATION OF THE SOIL WATER CHARACTERISTIC CURVE OF HIGHLY COMPRESSIBLE MATERIALS: CASE STUDY OF PULP AND PAPER BY-PRODUCT

A technique was developed for the determination of various points of the soil-water characteristic curve (SWCC)-including the air entry value (AEV)-of compressible materials from one single test. The testing setup, which employs the axis-translation technique, is presented and the testing methodology, explained. With the proposed methodology, it is possible to determine the volume of the sample at various stages ef the desaturation process, thus making it possible to determine the degree of saturation and volumetric water content for each level of suction applied. The results of some tests performed on deinking residues (DR), a fibrous and highly compressible industrial by-product used in geoenvironmental works, are presented and discussed. It is shown that if volume changes that samples undergo during desaturation are not considered, the volumetric water content and degree of saturation of the sample is underestimated at all suction values. One important consequence is that lower hydraulic conductivities are obtained from mathematical models based on the SWCC.

Shear strength behavior of compacted unsaturated residual soil

Abstract The focus of this paper is to study the shear strength behavior of a compacted residual soil from a site of the University of São Paulo in São Paulo, Brazil by conducting several series of experiments both under saturated and unsaturated conditions. The shear strength is a key property required in the design of foundations, construction of several geotechnical structures such as pavements, backfill material and embankments. A total of 57 unconfined compression tests and 57 constant water (CW) content triaxial tests were conducted on specimens compacted at three different initial water content conditions to study the shear strength behavior. One of the key objectives of the present study is to examine the influence of soil structure associated with different water contents and also the initial stress state on the shear strength. The matric suction in the soil specimens was precisely measured during the tests by using high capacity tensiometer (HCT). The experimental results suggest that the shear strength behavior is significantly influenced by the different initial compaction water contents. An empirical model is proposed using the results of the present study to estimate the three-dimensional (3D) failure envelope of the tested compacted residual soil for any condition of the initial stress state. Such a model is useful for the practicing engineers to estimate the shear strength behavior of the São Paulo residual soil for different loading and initial compaction water content conditions.

Temperature effects on suction measurement using the filter paper technique

This paper presents the results of an experimental study of thermal effects on filter paper calibration curves used to obtain the soil suction. When the temperature is significantly different from ambient values, it is essential to consider the influence of temperature on the filter paper calibration curves to obtain a reliable soil suction measurement. The calibration curve of Whatman No. 42 filter paper was determined at 10°C, 25°C, and 50°C using the vapor equilibrium technique with sodium chloride solutions at different concentrations and the axis translation technique. The experimental results showed a major influence of temperature on the filter paper calibration curves. Using the obtained experimental data a calibration equation was proposed, taking into account the effect of temperature. The obtained calibration curves were then used to determine the soil water retention curve of kaolin clay, which showed lower retention capacity at higher temperatures.

The Effect of Contact on the Filter Paper Method for Measuring Soil Suction

The filter paper method is one of the most commonly used and critiqued techniques for measuring soil suction. However, many aspects related to its use still require some clarification. The results of a comprehensive study on the effect of the contact between the soil grains and soil water and the filter paper are presented herein. We investigated the influence of the equilibration time, the texture of the porous material and the degree of contact, or lack thereof, between the soil grains and the filter paper using Whatman #42 and three different types of porous material. To enhance the difference between the total suction and the matrix suction, osmotic suction was induced by saturating the specimens with a sodium chloride solution.

Undrained Shear of Plastic Soils under Suction

From the very beginning of the soil mechanics the interest for understanding the role of the pore water pressure is evident. In fact, the role of pore water pressure has been investigated even earlier by the agronomic sciences, which prepared a strong base for us. It has been a challenger to bring to practice the entire framework that has been developed for unsaturated soils. The primary purpose of this paper is to discuss undrained shear the behaviour of plastic soils, when the pore water pressure is negative and even under tension, in order to establish a relation between some fundamental characteristics of the soils with the undrained shear strength for plastic soils, and in particular for a residual compacted soil. Data from different type of soils are presented and specifically from a residual soil. Results from triaxial tests demonstrate that suction developed by unloading and by drying the specimen may produce different effects on the undrained shear strength. The results also indicate that the undrained shear test in unsaturated soils can be a valuable tool to bring theory into practice.

Constant Water Content Compression Tests on Unsaturated Compacted Soil with Suction Measurement Using a HCT

Abstract Soil compaction is a process used in different engineering works; from the most complex to the simplest, the quality of soil compaction indiscriminately affects the final condition of the compacted soil in terms of engineering projects. Moreover, aspects such as time of construction and cost are extremely important in works that require compacted soils, particularly because of the climate in tropical regions. In these regions, the abundance of residual soils makes this material a preferential candidate for use in engineering projects. Considering that most compacted soils will remain in an unsaturated condition, it is extremely important to study and to characterize these materials, not only in the saturated condition, but also in an unsaturated state. Within this context, this study analyzes the shear strength behavior of a compacted residual soil of gneiss, collected in two sampling campaigns from an area located at the experimental site of the University of Sao Paulo in Sao Paulo, Brazil. The ...

Discussion of "measurements of suction versus water content for bentonite-sand mixtures"

The authors have presented a very interesting research paper on the comparison of various techniques for suction measurements of a compacted bentonite–sand mixture. One of the important techniques discussed in the paper is the filter paper method, and the authors used the filter paper calibration equations presented by Agus (2005). Different aspects of using the calibration equations proposed by the authors are examined and discussed here in light of previously published data and our laboratory test results. According to Agus (2005), the calibration curves for the Whatman No. 42 filter paper were obtained using the pressure plate technique for suctions less than 1500 kPa and using a desiccator (vapour equilibrium technique) for suctions higher than 2000 kPa. To determine what the authors called the ‘‘matric suction calibration curve’’ (eq. [2]), a filter paper was been placed directly over a ceramic disc in a pressure plate during matric suction equalization, and a dead weight was placed on top of the filter paper to maintain good contact between the filter paper and the ceramic disc. An equilibration period of 3 weeks was adopted for this calibration test. The noncontact filter paper calibration data (eq. [1]) was obtained using different concentrations of NaCl solutions, and a 5 week equilibration time was adopted for this technique. The authors performed the calibration test for two different sizes of filter paper: a standard-sized (55 mm diameter) and a small-sized (25 mm diameter) filter paper. Figure D1 presents the calibration curves for the Whatman No. 42 filter paper obtained by Fawcett and CollisGeorge (1967) at 22 8C, Chandler and Gutierrez (1986) at 21 8C, Chandler et al. (1992) at 21 8C, Leong et al. (2002) at 25 8C, the proposed calibration curves by Agus (2005) at 22 8C, and also the writers’ experimental data at 25 8C. The difference in the filter paper calibration curves may be due to factors such as equilibration time, temperature, and the use of different batches of filter paper as indicated by Marinho and Oliveira (2005). Using eq. 1 proposed by Agus (2005) for suctions less than 2000 kPa can cause significantly misleading results as the equation is based on experimental data by Agus (2005) for suctions greater than 4000 kPa. The proposed calibration curve for suctions less than 2000 kPa strongly disagrees with the calibration curves found in literature and also shows a very small sensitivity of suction in relation to filter paper water content (see Fig. D1). Although the authors reported the use of the proposed calibration curve only for suctions greater than 2000 kPa, it seems the proposed calibration curve has been used for total suction measurements lower than 2000 kPa, as data presented on Figs. 4a, 4b, and 5a (in the paper under discussion), which is not applicable. The writers obtained the filter paper (Whatman 42) calibration curve using two procedures. One was via vapour equilibrium using sodium chloride solutions at different concentrations, for suctions higher than 300 kPa.3 The calibration was performed in a temperature-controlled chamber, which maintained the desired temperature to ±0.1 8C. The other procedure used the pressure plate with the filter paper to be calibrated placed between two soil samples. The filter paper was protected by two larger-size filter papers. The arrangement of the soil samples and the filter papers was subsequently placed on a fully saturated high air-entry value ceramic disc in the pressure plate. An equilibration period of 2 weeks was adopted for both calibration procedures. The results obtained from the two procedures are shown in Fig. D1. From the results it can be observed that the data from the vapour equilibrium method for inducing suction agree with the calibration equation proposed by the authors only for suctions higher than approximately 5000 kPa. The data presented by the writers agree well with the data presented by the authors, when compared with the calibration curve represented by eq. [2], regardless of the method used for generating the suction (i.e., vapour flow or capillary flow). Therefore it can be concluded that a 2 week equilibration time is sufficient for a capillary flow of any level and for a vapour flow higher than 500 kPa. From Fig. D1 it can be observed that the last two points obtained using the vapour flow method (suction lower than 500 kPa) require a longer time to reach equilibrium. This is in agreement with Received 24 September 2010. Accepted 26 November 2010. Published on the NRC Research Press Web site at cgj.nrc.ca on 1 February 2011.