A model used to simulate clay materials. The general relationship between volume change and pressure in clay ground can be expressed using the concept of normal consolidation line and over-consolidation line. The over-consolidation line is also called the dwelling line and the stress increase (load) follows along over-consolidation line to the normal consolidation line. Passing the intersection with additional stress increase makes the stress state move down along the normal consolidation line. This has similar characteristics to the stress-strain curve of an elasto-hardening plastic model. Hence, the initial linear elastic region of the over-consolidation line can be corresponded to the hardening plastic region of the normal consolidation line.

<Volume-Pressure vs Stress-Strain relationship>

To use the Modified Cam Clay model, the initial void ratio, initial stress and initial Pre-consolidation pressure need to be defined. The Pre-consolidation pressure can be directly entered, or calculated from the initial stress and Over Consolidation Ratio (OCR). When both the OCR and Pre-consolidation pressure are entered, the Pre-consolidation pressure is preferentially used.

The main nonlinear parameters of the Modified Cam Clay model are as follows.

The material properties of the ground are generally obtained from the 1-dimensional consolidation test and the Compression index  and Swelling index  can be obtained from the void ratio ( ) - graph. The Compression index and Swelling index are related to the Normal consolidation line slope and Over-consolidation line slope by the following equations.

The Critical state line slope can be assumed from the effective shear resistance angle (shear resistance angle from drained test).

  :   Interior friction angle from triaxial compression test

The OCR value can be used to calculate the stress distribution of the in-situ state from the current loaded stress distribution. The stress for each depth is calculated from the entered OCR and because the ground surface stress can be underestimated than the actual initial stress, the Pc (Pre-consolidation pressure) can be directly defined. When both the OCR and Pre-consolidation pressure are entered, the Pre-consolidation pressure is preferentially used in the analysis.

When the Pc is entered, the internal solver studies whether the Pc and in-situ stress state satisfy the yield function. If it does not, the Pc is recalculated.

Allowable Tensile Stress

MCC material models fundamentally do not allow tensile stress in the failure criteria (stress-strain relationship). However, various conditions can generate tensile stress, such as the heaving of neighboring ground due to embankment load during consolidation or uplift due to excavation. To overcome the material model limits and increase the applicability, analysis on tensile stress within the 'allowable tensile stress' range can be conducted.

The size of the allowable tensile stress is not specified, and requires repeated analysis to input a larger value than the tensile stress created from the overburden load (embankment) or failure behavior. Hence, the allowable tensile stress value needs to be set, to prevent divergence and halting of analysis results due to tensile failure during analysis.

Hence, the allowable tensile stress value needs to be set, to prevent divergence and halting of analysis results due to tensile failure during analysis.

However, when directly entering the pc (pre-consolidation load), the allowable tensile stress cannot surpass the pc value. When defining using the OCR, the pc value is automatically calculated internally by considering the size of the input allowable tensile stress.