Advanced Settings

Under the Advanced tab in the Project Settings dialog, the following analysis settings can be configured.

Stress used in Settlement Calculation

Two options are available for the stress used in the settlement calculations:

• Use vertical stress only
• Use mean 3D stress

By default, settlement is calculated using only the vertical stress. A more accurate analysis can be performed by using the three-dimensional mean stress in the calculations. The mean stress at any point is the average of the volumetric stress components.

For more information, see sections 3.1 and 4.2 in the Settle3 Theory document.

Secondary Consolidation Parameters

If you are carrying out a time-dependent consolidation analysis, and you are considering secondary consolidation (i.e. creep), then the following parameters can be specified for the secondary consolidation analysis.

• Start of secondary consolidation (% of primary)
• Minimum stress for secondary consolidation (% of initial)
• Reset time when load changes

The start of secondary consolidation is specified with respect to the percent completion of primary consolidation. The default value is 95 percent.

In order to prevent the creep settlement calculation from being initiated at unduly low levels of stress, a minimum loading stress value is also specified, with respect to the initial stress. The default value is 1 percent.

By default, strain due to secondary consolidation (creep) is calculated by:

where e_eop is the void ratio at the end of primary consolidation, t_eop is the time at the end of primary consolidation, t is the current time and t_load is the time at which the first load was applied. So by default, the value of t_load does not change if the load changes.

If the option Reset time when load changes is checked on, then t_load will be reset each time the load changes (if the load is greater than the minimum stress for secondary consolidation set above).

Buoyancy Effect

As settlement occurs, material tends to sink further below the water table elevation. This will increase pore pressure, decrease effective stress, and decrease the amount of settlement at a given point, and is referred to as the buoyancy effect. By default, we ignore this effect in the settlement calculations, as it is usually quite small.

In some cases, the buoyancy effect could be significant. If you wish to include the buoyancy effect in the settlement calculations, then select the Include buoyancy effect checkbox in the Advanced Settings dialog. For further information, see section 5 in the Settle3 Theory document.

Vertical Stress Reduction

As settlement occurs, the height of the soil column above a given point decreases. Assuming that the material density does not change, this will decrease the vertical stress at the point. By default, we ignore this effect in the settlement calculations, as it is usually quite small and difficult to quantify precisely.

In some cases, this effect may be significant (e.g. for models with large settlement). If you wish to include this effect in the settlement calculations, then select the Include vertical stress reduction checkbox in the Advanced Settings dialog. For further information, see section 6 in the Settle3 Theory document.

Settlement Cutoff

This option allows you to specify the minimum amount of loading stress required to induce settlement, as a fraction of the in-situ effective stress at any point. If the loading stress at depth drops below the specified fraction of the in-situ effective stress, then no settlement will occur below this point.

To use this option, select the Use settlement cutoff checkbox and enter the ratio of loading stress/in-situ stress to be used as the cutoff value. By default this option is OFF, which means that settlement can occur at any loading stress.

Note that for rigid loads, because the displacement is kept planar beneath the loading stress, you will not see a change in settlement right underneath the load with this option but rather the distribution at depth where the stress ratio reaches the user-input value will be cut off.

Minimum Settlement Ratio for Subgrade Reaction Modulus

This option allows you to specify the lowest settlement ratio used in the modulus calculations. The ratio is calculated as the settlement at a specific depth divided by the maximum settlement along the query point. If the settlement ratio is less than the minimum the reaction modulus value is set to zero.

Use Average Properties to compute Layered Stresses

If your model includes multiple soil layers with different elastic properties, and you are using one of the following analysis options:

• Westergaard stress computation method, OR
• Rigid load type

then the Use average properties option is applicable. When this checkbox is selected, this ensures that weighted average values of Young's modulus E, and Poisson's ratio, will be used in the stress calculations. The weighting is based on the layer thickness. The Westergaard method only requires Poisson's ratio. The rigid loading calculation uses both modulus and Poisson's ratio.

If this checkbox is NOT selected, and your model has multiple layers, then the modulus and Poisson's ratio of the TOP layer of the model, will be used in the stress calculation (i.e. the elastic properties of the top layer will be considered representative of all soil layers).

This option is only applicable for the circumstances described above. If your model does not fit any of the above criteria, then this option will have no effect on the analysis results.

Update Cv in each Time-Step

When this option is enabled, the effective stress is checked with the preconsolidation pressure continuously through the water dissipation process. By default this option is selected in new files, and it is recommended to keep it on. If this is de-selected, the program checks if a material is over-consolidated or normally consolidated (for the purpose of Cv calculation) only at the beginning of the analysis or when the loading is changed.

Ignore negative effective stresses in settlement calculations

When this option is enabled, both the effective stress from the current and previous stage must be compressive in order for any consolidation strain to be calculated; otherwise, it will set the strain to zero. By default, this option is selected in new files.

Neglect alpha factor for pwp dissipation

When this option is enabled, the alpha factor outlined on Section 4.3.1: Non-uniform material of the Settle3 Theory Manual is set to 1.0. If you use this option you need to use query points with evenly spaced discretization only.

Add field points to load edges

When this option is enabled, if a field point grid is used, grid points are added to the perimeter of the loads. If this option is not enabled, field point grid points are not explicitly added to the edges of the loads.

Poisson ratio for Boussinesq stress computation

Enabling this option allows users to use Poisson's ratio in the project setting to be applied in 3D mean stress computation. Please refer to #4 Settle3 FAQs for details on Poisson's ratio and how it's used in Boussinesq stress computation, and also in our theory manual (Section 3.1.3). Note the following points when using this option:

• You must be using the Boussinesq method.
• You must select 'Use 3D mean stress' in order to use the Poisson's ratio.
• Using this option allows you to use this ratio from the dialog. (if this option is not selected, Settle3 will use the Poisson's ratio from the soil properties dialog).