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. NOTE:
Mean stress is only used in the calculation of Immediate Settlement. The mean stress is not used in the calculation of Consolidation Settlement or Secondary Settlement because the relationship between strain and mean stress is not clearly defined for non-linear materials.
Mean stress is also applicable for calculations of excess pore pressure.
For more information, see sections 3.1 and 4.2 in the Settle3D 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 eeop is the void ratio at the end of primary consolidation, teop is the time at the end of primary consolidation, t is the current time and tload is the time at which the first load was applied. So by default, the value of tload does not change if the load changes.
If the option Reset time when load changes is checked on, then tload 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 Settle3D 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 Settle3D 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.
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.
NOTE: this option was introduced in Settle3D version 2.004. By default, the option is turned on, for new files. If you read in a file which was saved using a program version prior to 2.004, then this checkbox will be turned off, to ensure consistency with previously calculated results. If you have old files with multiple layers and rigid loading, you may wish to turn this option on, and re-compute results.