Model: Delft Egg

The Delft-Egg plasticity model uses a generalization of the Modified Cam-Clay yield surface, defined as

$f = q^2 - M^2 \left[ \alpha^2 \, p \left(\frac{2 \alpha}{\alpha+1} p_c -p \right) - \frac{\alpha^2 (\alpha-1)}{\alpha+1} p_c^2 \right] = 0 \quad ( \text{for }p_c > \frac{\alpha}{\alpha+1} )$

$f = q^2 - M^2 \, p \left(\frac{2 \alpha}{\alpha+1} p_c -p \right) = 0 \quad (\text{for } p_c \leq \frac{\alpha}{\alpha+1} )$

where $\alpha \geq 1$ is the shape parameter. For $\alpha = 1$ , this model leads to a Modified Cam-Clay (MCC) type model with an ellipsoidal yield surface. For $\alpha > 1$ , an egg-shaped yield surface is obtained. The additional parameter makes it easier to fit the cap behavior of a broader range of soils and rocks.

Because Delft-Egg is frequently used for hard rocks, GEOS uses a linear model for the elastic response, rather than the hyper-elastic model used for MCC. This is a slight deviation from the original formulation proposed in the reference above. For reservoir applications, the ability to use a simpler linear model was a frequent user request.

Parameters

The supported attributes will be documented soon.

Example

<Constitutive>
   <DelftEgg
    name="DE"
    defaultDensity="2700"
    defaultBulkModulus="10.0e9"
    defaultShearModulus="6.0e9"
    defaultPreConsolidationPressure="-20.0e6"
    defaultShapeParameter="6.5"
    defaultCslSlope="1.2"
    defaultVirginCompressionIndex="0.005"
    defaultRecompressionIndex="0.001"/>
</Constitutive>