SOIL MECHANICS 189 stress is very large Similarly, a soil which has a relatively high specific volume and the grains are relatively loosely packed will dilate if the effective stresses are very small Sediments at great depth deform plastically Nearsurface soils often behave in a brittle manner and crack Relative Density, or Liquidity Index, on its own is not sufficient to predict the behaviour on subsequent shearing; the effective stress must be taken into account as well Strength of Soil Figure States and state parameters Behaviour of Soil During Shearing Density or Liquidity Index These may be combined into a state parameter Figure 6A, which is similar to Figure 5B, shows the state of an overconsolidated sample at X and the normal compression line All samples with states on the broken line through X parallel to the NCL will behave in a similar way These states can be described by a stress state parameter Ss given by Ss ¼ s0x s0e ½15 where s0e is the equivalent stress on the NCL at the same specific volume as that at X The state parameter describes the distance of the state from the NCL If the swelling index Cs is small, Ss is approximately equal to the overconsolidation ratio, R0 The concept of state is of fundamental importance for soils which are both frictional and which change volume during loading, as it combines both relative density and stress into a single parameter Figure 7A shows a block of soil with a constant normal effective stress s0 subjected to an increasing shear stress t0 The soil is drained and it distorts with a shear strain g and a volumetric strain ev If the soil is undrained, there are no volume changes but the pore pressures change The block of soil represents conditions inside a slip zone in the slope illustrated in Figure 7B or in a foundation illustrated in Figure 7C If the slope is created by excavation or erosion the normal stress decreases and, since soil strength is frictional, it will weaken, whereas below the loaded foundation the normal stress increases and the soil becomes stronger The behaviour of soil initially loose and initially dense of critical is illustrated in Figure The loose soil (marked L) compresses during shearing even Dense and Loose States After shearing, soils reach ultimate or critical states in which they continue to distort at constant state (i.e., at constant stress and volume) The relationship between specific volume and effective stress gives a critical state line (CSL) parallel to the normal compression line, as shown in Figure 6(B) The critical state line is given by v ẳ vc Cc log s0 ẵ16 Soil states which are above the CSL are known as ‘loose of critical’ and the soil will compress on shearing Soil states which are below the CSL are known as ‘dense of critical’ and the soil will dilate on shearing The CSL separates regions of fundamentally different behaviour of the same soil A soil which has a relatively low specific volume and the grains are relatively closely packed will compress if the effective Figure Shearing of soil