PHYSICAL PARAMETERS The essential parameters that need to be determined are the parameters entering the constitutive models described in Section 3.2. In addition, the natural frequency of the soil profile proves to be very important for the analyses: it represents a good indicator for the validation of the site modelling (geometry, properties), at least in the linear range, and is useful to constrain the range of possible variation of the soils characteristics. The parameters entering the constitutive relationships depend on the adopted model for the soil. For viscoelastic models, elastic (small strain) characteristics and variation of these characteristics with shear strain are required. In view of the large uncertainties involved in soil-structure interaction problems, it is sufficiently accurate to consider an isotropic material, thereby reducing the numbers of moduli to 2: the shear and the bulk moduli, or equivalently the S-wave and P- velocities. Energy dissipation is represented by the loss coefficients (or damping ratios) associated to each modulus. Practically, due to the difficulty in damping measurements and to the scatter in the results, a single value of the loss coefficient is usually considered applicable to both moduli. These small strain characteristics depend mainly on the soil density and past and present state of stresses. The nonlinear shear strain-shear stress behaviour is characterized by the variation with shear strain of the secant shear modulus and loss coefficient (Figure 38); these curves are mainly influenced by the present state of stresses and the soil plasticity index. For 1D-nonlinear models, in addition to the previous parameters, the maximum shear strength that can be developed under simple shear loading is required (equation (3.12)); for dry soils or drained conditions, it can be computed from the knowledge of the strength parameters, friction angle and cohesion and of the at rest earth pressure coefficient The latter parameter is ’ c’, K0. the most difficult one to measure directly and is usually estimated from empirical correlation based on the friction angle and overconsolidation ratio of the soil. (3.12) For saturated soils under undrained conditions, the maximum shear strength is the cyclic undrained shear strength of the soil. For the 3D-nonlinear models the same parameters as for the 1D-nonlinear models are needed but, these models also involve a large number of additional parameters which do not all have a physical meaning; these parameters are variables used to calibrate the constitutive “hidden”
PHYSICAL PARAMETERS The essential parameters that need to be determined are the parameters entering the constitutive models described in Section 3.2. In addition, the natural frequency of the soil profile proves to be very important for the analyses: it represents a good indicator for the validation of the site modelling (geometry, properties), at least in the linear range, and is useful to constrain the range of possible variation of the soils characteristics. The parameters entering the constitutive relationships depend on the adopted model for the soil. For viscoelastic models, elastic (small strain) characteristics and variation of these characteristics with shear strain are required. In view of the large uncertainties involved in soil-structure interaction problems, it is sufficiently accurate to consider an isotropic material, thereby reducing the numbers of moduli to 2: the shear and the bulk moduli, or equivalently the S-wave and velocities. Energy dissipation is represented by the loss coefficients (or damping ratios) associated to each modulus. Practically, due to the difficulty in damping measurements and to the scatter in the results, a single value of the loss coefficient is usually considered applicable to both moduli. These small strain characteristics depend mainly on the soil density and past and present state of stresses. The nonlinear shear strain-shear stress behaviour is characterized by the variation with shear strain of the secant shear modulus and loss coefficient (Figure 38); these curves are mainly influenced by the present state of stresses and the soil plasticity index. For 1D-nonlinear models, in addition to the previous parameters, the maximum shear strength that can be developed under simple shear loading is required (equation (3.12)); for dry soils or drained conditions, it can be computed from the knowledge of the strength parameters, friction angle ’ and cohesion c’, and of the at rest earth pressure coefficient K0. The latter parameter is the most difficult one to measure directly and is usually estimated from empirical correlation based on the friction angle and overconsolidation ratio of the soil. (3.12) For saturated soils under undrained conditions, the maximum shear strength is the cyclic undrained shear strength of the soil. For the 3D-nonlinear models the same parameters as for the 1D-nonlinear models are needed but, these models also involve a large number of additional parameters which do not all have a physical meaning; these parameters are “hidden” variables used to calibrate the constitutive P-
CHAPTERS_1-7__15_Dec_2016_ver_507.docx