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This paper presents a generalized theory of soil resistance formulated from some 28 years of research in applied soil mechanics. Soil resistance is described in terms of three stress reactions, developed pressure, shearing resistance and a principal stress difference. Developed pressure represents resistance to being compressed, involving a stress reaction of cubical compression similar to the pressure transmitted by fluids. In soils this stress reaction is associated with but not limited to the soil moisture, while in molecular systems it results from molecular forces of repulsion related to the kinetic energy of the molecules. Shearing resistance associated with rigidity or resistance to displacement in cohesive soils is the product of molecular forces of attraction. Shearing resistance in a soil mass is manifested as perimeter shear, by means of which concentrated loads are distributed to the supporting mass accompanied by a decrease in transmitted pressure. A Principal Stress Difference, commonly known as the unconfined compressive strength, is designated as a separate stress reaction although it contributes to the developed pressure and is a function of shearing resistance in cohesive soils. The variation in perimeter shear and developed pressure controls the manner in which reaction to applied load is developed in different materials. The ratio of these two stress reactions is used to characterize different materials in terms of soil resistance coefficients and ratio between these coefficients. In cohesive soils the transition from the range of elastic behavior to the semifluid behavior of the plastic range is determined by the critical value of this ratio. Volume changes or varying compressibility change the sequence in which the stress reactions are developed and alter the soil resistance curves but do not otherwise change the basic relationships. Resistance in granular materials is referred to briefly. Reactions to both pressure forces and shear are recognized as the result of mechanical arrangement of particles. Nevertheless, it may be shown that confined granular masses react within certain limits in much the same way as cohesive masses.
Housel, William S.
Professor of Civil Engineering, University of Michigan, and Research Consultant, Ann Arbor, Mich.