Journal of Testing and Evaluation, July 2005, Vol. 33, No. 4 Paper ID JTE11866 Available online at: www.astm.org
Vikas K. S. Thakur1 and Devendra N. Singh2
Rapid Determination of Swelling Pressure of Clay Minerals
ABSTRACT: The present study deals with development of a rapid and easy way to apply methodology that can be employed to determine the swelling pressure of clay minerals, and hence, expansive soils. Conventional oedometer tests were performed to determine the swelling characteristics of montmorillonite and bentonite clay minerals corresponding to various molding water contents, maintaining the same dry density. With the help of a dew point potentiameter, which can be used for measuring very high suction (∼ =90 MPa), the suction of these minerals was measured. The results obtained were used to correlate suction with the swelling pressure and the correlation was validated using data reported in the literature. The study demonstrates the usefulness of the methodology for quickly determining the swelling pressure of various clay minerals and soils. KEYWORDS: montmorillonite, bentonite, swelling pressure, suction, dew point potentiometer
Nomenclature A, B, C, D Hi M P P0 Pa Ps R T
fitting parameters; initial height of the sample; molarity of the solution; vapor pressure of air; saturation vapor pressure; applied pressure; swelling pressure; universal gas constant; temperature of the sample (in K); γd dry unit weight; w water content (in %); total suction (in MPa); total suction (in pF); m matric suction; o osmotic suction; χ molecular mass of water.
Introduction Expansive soils that consist of swelling clay minerals pose a major challenge to engineers and are considered as geological hazards throughout the world [1]. As such, characterization of these soils, and the minerals responsible for their swelling behavior, becomes important [2–4]. Determination of swelling pressure is also necessary for designing suitable foundation systems [5–8]. Many tests and methods have been developed or modified for estimating the shrink-swell potential of soils; they can be broadly classified as “indirect methods,” which involve application of soil properties and Manuscript received 21 Jan. 2003; accepted for publication 12 Jan. 2005; published July 2005. 1 Former Post Graduate Student, Department of Civil Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai-400076, India.vksthakur@ rediffmail.com 2 Professor, Department of Civil Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai-400076,
[email protected]
classification schemes such as plasticity index [9,10], activity ratio [11], and cation exchange capacity [12], and as “direct methods,” which provide actual physical measurement of swelling, such as the free-swell test [13], expansion test [14], California bearing ratio test [14], and coefficient of linear extensibility test [15]. Also, various researchers have shown that the shrink-swell behavior of soils can be predicted by examining a combination of their physical, chemical, and mineralogical soil properties [3,16–18]. However, most of these methods are time-consuming and tedious [18–25]. New methods that are easy to perform and yield rapid results have been developed [26,27]. Several researchers have demonstrated, in the past, the importance of suction and its measurement for various civil engineering projects [28–35]. These studies indicate that suction influences the overall behavior of a soil to a great extent. However, the biggest handicap in measuring soil suction is that most of the techniques, methods, and devices currently used are not suitable for measuring suctions higher than a few hundreds of kPa [27,36,37]. This calls for employing a suction-measuring device, such as a dew point R potentiameter, WP4 , which can be used for measuring extremely high suction, of the order of tens of MPa, quickly [38]. However, not many studies that utilize this device have been reported in the literature, except for the classification of expansive soils [26,39]. With this in view, attempts were made to correlate the swelling pressures of minerals with their suction values. Conventional oedometer tests, following the guidelines provided by ASTM [13], were conducted to obtain swelling pressure, while suction was R3 measured using a dew point potentiameter, WP4 . The results obtained are used to correlate suction with swelling pressure, and the correlation is validated using data reported in the literature [35]. The study demonstrates the usefulness of the methodology for the quick determination of the swelling pressure of clay minerals. As swelling soils are constituted of clay minerals, this correlation is
R 3 WP4 is a registered trade name of Decagon Devices, Inc., Pullman, Washington.
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240 JOURNAL OF TESTING AND EVALUATION TABLE 1—Physical properties of the minerals used in the present study.
Property
Bentonite
Montmorillonite
Color Specific gravity Liquid limit (%) Plastic limit (%) Plasticity index (%) Free swell index (%) Specific surface area (m2 /g) (@150◦ C for 60 min)
Yellowish brown 2.76 227 65 162 264 48
Dark brown 2.78 411 71 340 410 310
TABLE 3—Swelling pressure for various samples.
Mineral Bentonite
Montmorillonite
Sample
w (%)
PS (kPa)
B1 B2 B3 B4 B5 M1 M2 M3 M4 M5
13.6 30.1 40.3 60.5 67.4 11.4 28.8 55.3 72.3 98.3
194.26 114.80 99.22 44.69 27.69 454.17 369.26 201.77 169.83 113.73
useful for determining the swelling pressure of expansive soils as well. Experimental Investigations Properties of the Minerals Used Montmorillonite and bentonite minerals in dry powder form have been chosen for the present study; their physical properties are depicted in Table 1. The specific gravity of these minerals was obR tained with the help of an Ultra Pycnometer 1000 [40]. These minerals contain particles of size
