Software: FLAC: Pre-Release 6.0

CY Soil Model

The Cysoil model is a simple strain hardening/softening constitutive model for soils, characterized by a frictional Mohr-Coulomb shear failure envelope (zero cohesion) and a non-linear volumetric cap. The model can be customized to fit different characteristics of soil behavior through the selection of hardening laws. For example:

1. a cap hardening power law captures the volumetric behavior observed in isotropic compaction tests. Soil stiffness usually increases in a non-linear fashion as a function of isotropic pressure. The cap hardening law accounts for this behavior. The Cysoil model is a modification of the FLAC Double-Yield (DY) model, and replaces the plane cap of the DY model with an elliptic cap that provides a better representation of the effect of the cap shape on the coefficient of lateral earth pressure.

2. a friction hardening law reproduces the hyperbolic stress-strain behavior observed in drained triaxial tests. Soils, when subjected to deviatoric loading, usually exhibit a decrease in stiffness accompanied by irreversible deformation. In many cases, the plot of deviatoric stress versus axial strain obtained in a drained triaxial test may be approximated by a hyperbola. A friction hardening law captures this behavior in the Cysoil model. This behavior is similar to that produced by the Duncan-Chang model. However, the Cysoil model is formulated in the theory of hardening plasticity and, thus, does not have a difficulty with simulating loading/unloading that can result in non-physical behavior with Duncan-Chang.

3. a compaction/dilation law models the irrecoverable volumetric strain taking place as a result of monotonic soil shearing. When tested under drained triaxial conditions, soils generally exhibit shear induced volume changes that are strongly dependent on soil density. Typically, there is a tendency for the soil to contract under small shear strains, and to dilate under larger strains, unless it is very loose. In particular when fluid fills the pores, it is this tendency of the soil skeleton to contract and dilate that controls its liquefaction response. Also, the shear stress - shear strain response of loose soils may exhibit a softening response under undrained conditions. It is the existence of a peak in shear strength that may lead to instability during a monotonic load-controlled process. Shear induced volume changes can be accounted for in the Cysoil model by means of the dilation hardening/softening law.

A comparison of FLAC with the Cysoil model to a benchmark exercise: a triple anchored excavation wall in sand (see Schweiger, 2002), is provided to illustrate the applicability of the Cysoil model. The friction-hardening and cap-hardening laws are used to fit the model to the soil characteristics described for the benchmark test. The Cysoil model is also calibrated to fit the behavior of the Plaxis "Hardening-Soil" model that has also been compared to the benchmark exercise (Plaxis Material Models manual, 2002). The problem conditions for the exercise are shown in Figure 1. The Cysoil results for wall deflection are compared to measurements in Figure 2, and the results for surface settlement are compared to the Hardening-Soil model results in Figure 3.