Prediction of the Behavior of Coarse-Grained Soils Using CSM
CSM is applicable to all soils. However, there are some issues about coarse-grained soils that require special considerations. Laboratory test data show that the NCL and CSL lines for coarse-grained soils are not well defi ned as straight lines in (ln p9, e) space (Figure ) compared to those for fi negrained soils. The particulate nature of coarse-grained soils with respect to shape, size, roughness, structural arrangement (packing), particle hardness, and stiffness often leads to localized discontinuities.
Tests using X rays on coarse-grained soils show shear banding (Figure 10.4) and nonuniform distribution of strains, even at low strains (,1%). Averaged stresses and strains normally deduced from measurements in soil test equipment cannot be relied upon to validate CSM. CSM is based on treating soils as continua, with smooth changes in stresses and strains. CSM cannot be used when shear bands occur. Other models (e.g., Coulomb or Mohr–Coulomb) may be more appropriate than CSM. However, the soil within the shear band is generally at critical state, and it is likely to behave as a viscous fl uid.
responses of coarse-grained soils
Overconsolidation ratio and preconsolidation ratio are useful strictly for fi ne-grained soils. There is no standard technique to determine the preconsolidation stress for coarse-grained soils. There have been attempts to defi ne a new state parameter for coarse-grained soils within the CSM framework, with some success. These attempts are beyond the scope of this textbook. Despite the nonlinearity of the NCL and the CSL in (ln p9, e) space for coarse-grained soils, and the diffi culties in determining Ro or OCR, the framework by which CSM describes and integrates strength and deformation is still outstanding for all soils.
Critical State Boundary:
The CSL serves as a boundary separating normally consolidated and lightly overconsolidated soils from heavily overconsolidated soils (Figure). Stress states that lie to the right of the CSL will result in compression and strain-hardening of the soil; stress states that lie to the left of the CSL will result in expansion and strain-softening of the soil. More detailed analysis of how a soil will likely behave.
State boundary for normally
and lightly overconsolidated soils and
heavily overconsolidated soils.