Basic Principles in Soil Mechanics

Abstract

Soil is classified primarily into two categories; namely granular materials and cohesive soils. The former consists of gravel, sand, and cohesionless silts (Fig. 1.1), while the latter is clayey soil (Fig. 1.2). The mechanical properties of granular soils are governed by the grain-to-grain contact as well as friction. Hence, the magnitude of contact force and the geometrical nature of grain packing play major roles. It should be recalled that the magnitude of contact pressure per unit area of soil is called the effective stress, which is the most important concept in modern soil mechanics. In clayey soils, in contrast, chemical and electrical interactions among clay particles are important as well. Hence, shear strength is activated even at zero effective stress when the past stress-strain history allows it. Liquid limit (LL) and plastic limit (PL), which are called the Atterberg limits in combination, are two measures to evaluate the magnitudes of interparticle actions in addition to effective stress. The plasticity index, PI or Ip, is defined by PI = LL − PL. Generally speaking, PI of most clays lies in the range of 40–80 with such an exception of 400 or more of the sodium smectite (bentonite) group. As per 2006, the Japanese soil mechanic practice does not put “%” to the number of PI.