Understanding the Family of Soil-Water Characteristic Curves

D.G. Fredlund, H. Rahardjo, M.D. Fredlund

Dans les comptes rendus d’articles de la conférence: GeoEdmonton 2018: 71st Canadian Geotechnical Conference; 13th joint with IAH-CNC

Session: Soil Mechanics and Foundations (Advanced Soil Mechanics)

ABSTRACT: Soil-Water Characteristic Curves, SWCCs, are a family of volume-mass variables versus soil suction. The SWCC relations are hysteretic in nature and consideration can be given to a number of volume-mass variables. Consequently, there can be many families of curves which are all referred to as soil-water characteristic curves (or water retention curves). It is the intent of this paper to define and describe the multitude of variables associated with the term SWCC.

RÉSUMÉ: -masse par rapport à la succion matricielle du sol. Les relations CRE sont de nature hystérétique et on peut considérer un certain nombre de variables volume-masse. Par conséquent, il peut y avoir de nombreuses familles de courbes qui sont toutes appelées courbes caractéristiques sol-eau (ou courbes de rétention d'eau). L'objectif de cet article est de définir et de décrire la multitude de variables associées au terme CRE. Introduction Research in unsaturated soil mechanics over the past few decades has witnessed an ever-increasing reference to the role of the soil-water characteristic curve, SWCC (Fredlund, 2006). The SWCC has been referred to as the key to the implementation of unsaturated soil mechanics in geotechnical engineering practice (Fredlund, 2015). Along with the many references to the SWCC, there has also been considerable confusion about exactly what is meant by the term, SWCC. The SWCC term has been used in diverse ways without a clear definition as to its meaning. SWCC is really a family of relationships with two bounding curves and an infinite number of scanning curves (Klute, 1965; Pham, 2005). The relationships involve various volume-mass soil property values versus soil suction. Soil suction ranges from low values in the order of 0.01 kPa to a maximum value of 1,000,000 kPa (i.e., 7 orders of magnitude). As reported by Klute (1965, 1986), the bounding curves may represent the: i.) initial drying curve, ii.) main drying curve, or iii.) main wetting curve. The result is a family of curves because the drying and wetting processes are hysteretic (Pham et al, 2003, 2005). The term SWCC is also sometimes referred to as a unique relationship and as any relationship between the amount of water in a soil and soil suction. Klute (1965, 1986) also noted that the amount of water in the soil may be quantified in terms of: i.) gravimetric water content, ii.) degree of saturation or, iii.) volumetric water content. Soil suction may also mean matric suction, total suction or osmotic suction along with arbitrary boundaries between the components of soil suction. Variations in the water content and soil suction designations can lead to considerably different interpretations of laboratory test data (Fredlund and Rahardjo, 1993). Some of the confusion related to the SWCC terminology is the result of attempting to import past experience and technology from soil physics (and other agriculture-related disciplines) into geotechnical engineering. There is much valuable information to be gleaned from past research in agriculture; however, care and caution needs to be exercised when moving between the two disciplines. The objectives of this paper are as follows: i.) to define and thereby attempt to standardize appropriate terminologies most suitable for usage in geotechnical engineering practice when referring to the SWCC family of curves, and, ii.) to illustrate the application of use of laboratory measured gravimetric water content soil-water characteristic curve, SWCC, and a shrinkage curve for the estimation of a variety of SWCC relationships (Fredlund and Zhang, 2013). Hydraulic property functions for an unsaturated soil are used to illustrate the application of the family of SWCCs in this paper (Fredlund, 2002). The authors are the first to realize that it is difficult to obtain a complete consensus on such a broad range of soil property relationships where past terminologies have been generated in different countries and for

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