D-Settlement - FAQ and Known Issues

Known Issues

Solved in 20.1.2 Some “Other Loads” (Tank, Rectangular and Circular loads) are not visible in the Input tab of the View Input window (MSL-2136)

When a load is entered as Other Load, it is not visible in the Input tab of the main View Input window. The load is visible in the Top View tab.

This concerns only the Tank, Rectangular and Circular Loads, not the Trapeziform and Uniform Loads.

Solved in 20.1.2 Incorrect functioning of a sequence of temporary non-uniform loads (MSL-2135)

In the Non-Uniform Loads window it is possible to define a temporary load by marking the option End Time. When a sequence of temporary loads are defined, the program is not always able to determine correctly where the new surface level is. This leads to an incorrect determination of the load, and therefore to incorrect effective stresses and settlements.

Note that if a unique temporary load is used as last load, then the determination of the load is correct.

Workaround: To define a temporary load which is not the last load, do not use the option End time, but model the removal of the load with an opposite load (same dimensions but opposite direction and negative unit weight). Refer to tutorial 4 in the user manual for an example.

Unsolved Vertical drains combined with a not-horizontal phreatic line lead to unexpected high excess pore pressures.

Solved in 18.2.2 For Darcy model, the change in effective stress due to soil submerging is not correctly calculated. (Issue MSL-1976)

For the determination of the change in effective stress due to soil submerging, the current position of the phreatic line is needed. D-Settlement should use the original phreatic line position shifted by the settlement at this position, but use the settlement at the current depth which is incorrect.

There is no workaround to avoid this issue, however if the soil is completely saturated, this issue has no influence.

Note that this issue was corrected in version (see MSL-1875 in the Release Notes 18.1.1.3), but unfortunately not on a correct way.

Solved in 18.2 Water Loads are available again, but only for Terzaghi consolidation model. Note that when using Water Loads, no water flow between the start and end situations should take place because it will not be modelled. (Issue MSL-1704)

It is observed that the results of calculations with D-Settlement (version 16.1) will lead to erroneous results if the option ‘water loads’ is being used in combination with the Darcy approach. This combination is therefore disabled in D-Settlement 18.1. At this very moment we are working to solve this problem.

Besides the Darcy approach, it is possible to calculate the option ‘water loads’ with the classical Terzaghi approach. If this option is being used in version 16.1 the schematization should meet certain requirements in order to get the right prediction. When the following script is being adopted, the right prediction will be the result of the calculations when the ‘water loads’ option in combination with the Terzaghi approach is being used.

1. Add a layer boundary at the level of the ground water level
2. Set the layers above the ground water level to undrained
3. Set the boundary condition at the top to undrained
4. Set the cv value of the layers above the ground water level to a high value (for example 1)

Step 1 ensures that only the soil beneath the soil water level is regarded as ‘wet’ during the calculations. Steps 2 and 3 ensure that only one-sided drainage is being adopted during the calculations. The high value of the cv is needed in order to ensure that the contribution of the cv of layers above the ground water level to the cv, which is being used during the calculations, is minimal.

The combination ‘water loads’ with Terzaghi has been available again in D-Settlement 18.2. When the combination with Darcy will be available again is currently unknown.

Frequently Asked Questions (FAQ)

When performing a calculation with vertical drains combined with drained layer(s), I get unexpected results

For the combination drained layers with vertical drains, the hydrostatic pore pressure distribution cannot be correctly calculated by the program. The warning message “Drained soil cannot be used in combination with dewatering of vertical drains” is therefore displayed in the Start Calculation window.

Why does defining a layer as Drained not lead to the same results as defining a layer as Undrained with a high permeability?

When a layer is defined as Drained and has layers above and below which are not drained, this means that the water can flow on the horizontal side so the excess pore pressure is nil. When a layer is not defined as Drained but with a large permeability and has layers above and below which are not drained, this means that the water cannot flow on the horizontal side so the excess pore pressure is equal to the excess pore pressure of the layer above and below.

Why NEN-Koppejan method is not ideal for unloading/reloading?

In case of unloading (e.g. ground improvement, load removal, temporary dewatering, and gradual submerging) with NEN-Koppejan model, D-Settlement uses two unloading parameters: Ap and As. NEN-Koppejan has always been difficult with multiple steps because of the superposition principle which is used. But certainly if unloading occurs, because the unloading parameters must be carefully chosen. If high unloading parameters are used (i.e. As is much larger than Cs’), then the unloading will yield to almost no effect, but the reloading will have an effect (primary and secular). However if you unload without reloading then the use of high parameters is correct.

Why is the end time as displayed in the Fit window not the same as the inputted end time presented in the Calculations Options window?

If the checkbox ”Use end time for Fit calculation” in the Calculation Options window is not marked, only a part of the settlement curve is shown when the time-settlement curve is displayed by clicking the Show Current button from the Fit window: namely until the last settlement measurement + 100 days. This choice was made to get settlements that are more realistic when the option Submerging is used. Indeed, with Terzaghi and with the combination Darcy + NEN-Koppejan, the program first looks where the phreatic line is situated at the final situation. The non-uniform loading is then corrected and next a new calculation is performed. A fit calculation with the final situation the last measurement + 100 days gives settlements that are more realistic than a fit calculation with the inputted end time (by default 10000 days). The fitted ground parameters are therefore determined in a better way. However, the user can choose to use the inputted end time for the Fit calculation by marking the checkbox ”Use end time for Fit calculation” in the Calculation Options window.

My calculation results in the error message "Error occurred during computation of excess head". What should I do?

D-Settlement could not perform the calculation. This is probably due to the large variations in the vertical consolidation coefficients Cv. (or in the vertical permeabilities depending on the consolidation model). Perhaps some materials still have (by inadvertence) their default value of 1m2/s. If you enter an appropriate and realistic value, then the calculation will go well. Nevertheless, if you deliberately entered a high consolidation coefficient (to simulate a drained behaviour), set the material on ” Drained”.

NEN-Koppejan/Terzaghi: Why do settlements calculated using version 7 differ from the results of version 8?

When using NEN-Koppejan/Terzaghi in combination with Water Loads the results may differ. The difference is caused by a different approach; in version 7 the Water Load and the Non Uniform Load are connected; in effect a settlement raises the Water Load, this may have consequences for the Non Uniform Load can submerge. In version 8 the Non Uniform Load is disconnected from the Water Load. Settlement does not have influence on the choice to use wet or dry unit weight based on the water load. This approach was chosen to solve problems encountered in modeling vertical drainage under version 7.

Why does the Phreatic Level defined under Water Loads NOT influence the choice of either wet or dry unit weights of the Non Uniform Loads?

The Phreatic Level as defined under Water Loads is applied to the materials; it has no influence on the choice of either wet or dry unit weight as defined in the Non Uniform Loads. The decision to apply dry or wet weight of the Non Uniform Load is defined on the initial phreatic line as defined under Geometry.

Why is the end settlement as displayed in the Fit window not equal to the end settlement as calculated under the option "Use fit parameters"?

There are two possible reasons:

• In Fit for Settlement Plate, the parameters are applied only on the materials that were selected for the fit. In the standard calculation with the start calculation option “use fit parameters”, the parameters are applied on all materials.
  When the settlements plate is not placed on the surface, the following can lead to misunderstandings: with the Show Current button from the Fit window the Time-History window will be opened at the depth of the settlement plate. The Time-History window (in the Results menu) that can be opened after a standard calculation, with or without fit parameters, opens with the results on the surface.
• In Fit for Settlement Plate, the end time is the last settlement measurement + 100 days whereas for a standard calculation it is the inputted end time. For more information, read de content of the faq “Why is the end time as displayed in the Fit window not the same as the inputted end time given in the Calculations Options window?”.

Does the submerging option apply to soil layers or non uniform loads?

From version 8 of D-Settlement the ‘Submerging’ option applies to the non-uniform loads as well as soil layers. Until version 7 the ‘Submerging’ option only applies to the non-uniform loads, not to soil layers.

How can I prepare settlement plate data?

The software cannot read for instance Excel files. Preparation of the data into a proper format is relatively easy: Place the settlement plate data in two columns: Time (in days) in the first column and settlement (in meters) in the second column. The spreadsheet can not have any other data or text.
Save the file in text format with Tab as separator.
Pay attention that the decimal separator is a period, not comma.
Open this file in a text editor and add the following header on top:

Days Settlement
[Begin measurements]

Save the file using *.slm as the file extension.
This file can be opened with the File Open button in the Fit for Settlement window.

What models do support vertical drainage?

Vertical drains can be applied with all models (NEN-Bjerrum/Isotache and NEN-Koppejan). However, Isotache gives by far the best results, as NEN-Koppejan does not react accurately. On the other hand, the NEN-Bjerrum is more versatile. It is now the same as Isotache with linear strain.

Why do vertical drains not always result in extra settlements?

Often vertical drains lead to larger settlements as the effective stresses increase due to the lower pore pressures. However, the water level in the drains can increase compared to the initial situation. During the consolidation, the drains drain off the excess pore pressure. After the consolidation, the pore pressures has increased somewhat; the effective stresses may therefore diminish. This may result in a final settlement that is less than in the situation without drains.

Why does the settlement in a certain vertical using the “Submerging” option depend on the location of other verticals?

The weight reduction in the load columns as a result of “Submerging” depends on the settlement. The settlement for each load column is found through linear interpolation between the verticals. In general, several verticals will therefore give another settlement image than a single vertical. This means that the weight reduction of the load columns will be different as a result of the underwater subsidence, which again results in another settlement.

Why are the final settlements in time-settlement plots sometimes different from the final settlements in the report?

Before version 6.7 in the time-settlement plots, the course of consolidation is taken into account. If consolidation has not ended after the number of days entered in ‘End of settlement calculation’ (in ‘Calculation Options’), the final settlements in the plots are less than the final settlements in the report.
N.B. From version 6.7 on the final settlement in the report does indeed take consolidation into account and the values in the report and in the time-settlement diagram are identical.

NEN-Koppejan: What is the meaning of natural strain?

Natural strain is derived from the NEN-Koppejan method and prevents the occurrence of settlements greater than the layer thickness. The settlement calculated with NEN-Koppejan is transformed through z_natural = h * ( 1- exp(-z_koppejan/h)), with h being the layer thickness of the package. If z_koppejan/h (therefore strain) = 0 then z_natural is also 0. And if z_koppejan/h goes to infinity, the z_natural goes to h. It is preferred to have this option on.

How can I apply a uniform load of 3m height, and after 100 days remove the upper 1m?

Place at time t=0 a uniform load of 3m height with a determined volume weight – for example, 16 kN/m3. Add at time t=100 a new load with 1m height with a negative volume weight of the same magnitude, in this case -16 kN/m3.

Why do I have problems reading DXF exported files in Autocad?

The option to export pictures in DXF format was developed for, and tested with, AutoCad 14. In more recent releases of AutoCad something has changed in relation to the import of DXF files. This means that these versions do not read our DXF files properly (detected by us in versions 2005 and 2006). The majority of viewers, for instance “Bentley view”, do not experience problems with our exported DXF files.

Where can I find the Release Notes?

The Release Notes can be found in our download portal.