ABSTRACT
Unsettling features of eq. (2), however, are the functional form of the parameter χ, and the validity of its values preset by Bishop. Is it a linear function of the degree of saturation? Is it a sole function of
1 EFFECTIVE STRESS PRINCIPLE
According to Terzaghi (1942), effective stress refers to a stress quantity (variable) that is “effective” in describing soil’s shear strength and deformation. He described this stress quantity as “that part of the total stress which produces measurable effects such as compaction or an increase of the shearing resistance.” For soil under saturated conditions, Terzaghi defined effective stress as the stress going through soil skeleton, which can be quantified by the difference between the total stress and pore water pressure:
σ σ′ − uw (1)
Up to now, Terzaghi’s effective stress (1) has been widely accepted to be “effective” for describing saturated soil strength and the corresponding deformation prior to failure. Although elasto-plastic theories or stress-strain relations are needed to fully describe the state of stress and strain, quantifying a failure state and deformation prior to failure is often sufficient for many geotechnical engineering designs and analyses. Under saturated conditions, although effective stress in soil is not directly measurable, the total stress and pore water pressure can be measured or quantified. The total stress is generally determined by soil self weight, geometry of earthen structures, and other
the degree of saturation or some other variables? Is it dependent on wetting or drying ( hysteresis)? Is it zero when the soil is dry? An elusive character of the χ parameter is that as soil dries, matric suction (ua − uw) could be very high and the χ could be very small. Since the introduction of eq. (2), much effort has been expended to define the χ parameter. For example, χ has been expressed solely as a function of matric suction normalized by the air-entry pressure (Khalili and Khabbaz, 1998; 2004), only using the degree of saturation (e.g., Bishop, 1959; Houlsby, 1997; Borja; 2004; Karube, 1986; Vanapalli et al., 1996), as a unique function of the effective degree of saturation (e.g., Karube, 1986; Vanapalli et al., 1996; Lu and Likos, 2004; 2006), in addition to other saturation quantities (e.g., Pereira et al., 2011).
