D-Foundations FAQ and known issues

Known issues

Solved in 23.1. Wrong reference to the NEN in the report

As of version 19.1.1, D-Foundations is updated to the latest version of the NEN, NEN 9997-1+C2:2017. However, the report still contains references to the former NEN of 2016 by mistake. This will be fixed in our next release.

Solved in 22.1. Model Bearing Piles (EC7-B): Implementation of the Belgian method not up-to-date (Issue MFO-1734)

In 2022, a new version of the Belgian standard will apply. The changes are so big that it has been decided to no longer support the Belgian standard in D-Foundations and therefore to remove it entirely.

Solved in 22.1. Model Tension Piles (EC&-NL), Verification option (Issue MFO-1668)

When using very low pile loads for the Verification option of the model Tension Piles the program is likely to crash when the report is being generated. This crash is caused by the fact that in those circumstances very small displacements (pile rise) are calculated which in turn crashes the determination of the pile/CPT combination where the displacement is the largest. A simple work around for this problem is to use more realistic values for the pile loads.

Solved in 22.1. Model Tension Piles: when calculating several pile types at once, an unexpected error (Access Violation) may occur. (Issue MFO-1771)

The reason for this error is that the program divides the given pile plan in subgroups to be able to properly calculate all piles with the proper pile group effects. This division is done based on the dimensions of each pile type. When these dimensions differ (too much), this can lead to different pile group divisions. And this is something the program can not handle as all results are grouped per pile type per pile group. The simple solution is to make sure you perform the calculation per pile type.

Solved in 19.1. Model Tension Piles (EC7-NL): At the option "Pile tip levels and net bearing capacity" the pile tip levels as shown in the "Eurocode 7" tab of the Design results are wrong. (Issue MFO-1459)

Instead of presenting the level for which the bearing capacity is reached for all CPT’s at the appropriate decisive case (ksi3 or ksi4), the level that is presented now is, in fact, the very first level encountered during calculation where the bearing capacity is met, regardless of CPT or appropriate decisive case.

Solved in 19.1. Error in the iterative determination of the influence depth (Issue MFO-1374).

According to article 6.5.2.2 (d) and Figure 6.b of the Eurocode NEN 9997-1+C2:2017, the influence depth z_e must be determined using the characteristic value of the effective friction angle. If the determination of the influence depth is performed iteratively, then D-Foundations uses the design value instead of the characteristic value, which is not correct. However, if the determination is not iterative (for example in case of only one layer), then the characteristic value is used, the influence depth is therefore correct.

Frequently Asked Questions

Model Shallow Foudations (EC-7 NL) Determination of calculation Case A

In the determination of the proper calculation case for the undrained as well as the drained situation, Case A (homogenous soil profile) will only be used as such in D-Foundations when the soil profile contains only one cohesive layer. When a profile consists of multiple layers, even if these layers use one identical cohesive material, Case A will not be selected.

Where can I find the Release Notes?

The Release Notes can be found on this page.

Tension Piles (EC7-NL): How does DFoundation determine whether a pile is situated in “clay only”?

D-Foundation determines which layers are alongside the pile and checks their type. For calculations based on fixed pile tip levels, this is no problem. However when a trajectory (or in fact a set of pile tip levels) is used, THE pile tip level required for the determination, is not clear. So for this, DFoundation uses the deepest pile tip level from the trajectory for the determination.

Is CUR 2001-8 (Open tubular steel piles ) part of D-Foundations?

No, CUR 2001-8 is not (yet) part of D-Foundations. It is however on the wish list for a coming version. For now, D-Foundations performs two calculations for this pile type. First, it is considered to be a closed steel pile calculating the bearing capacity based on a closed pile tip and taking the shaft friction only on the outside of the pile. Second, it is considered as an open pile calculating the pile tip capacity on the rim of the pile only and taking the shaft friction from both the inside as well as the outside of the pile. From these two calculated bearing capacities, the lowest value is taken into account. This delivers a fairly accurate result.

Why, when importing a GEF, do I get the message "GEF-CPT-Report 110: First reading does not coincide with pre-excavated depth"

The first data value in the first column is probably greater than zero. To prevent the message from occurring you will have to edit the gef-file (using notepad). Lookup the following headerline in the gef file: #MEASUREMENTVAR= 13, 0.000000, m, pre-excavated depth. Change its value from zero into the value of the first reading.

Bearing Piles (EC7-NL): Additional pore pressures in D-Foundations

At present, the input of additional pore pressure is not possible for the model Bearing Piles (EC7-NL). This is because the standard does not take these pressures into account. However, the additional pressures do in fact influence the negative skin friction as well as the reduction of the CPT-values in cases where an excavation is part of the project.

The effect of an excavation is described in NEN 6743 5.4.3.2 / EC7NL 7.6.2.3

Here it is stated that the CPT-values should be reduced based on the effective vertical stress before and after the excavation. This article does not provide a model for the determination stresses.

Negative skin friction is described in NEN 6743 6.2 / EC7NL 7.3.2.2

The determination of the negative skin friction too is dependent of the effective vertical stress. Here a specific model for the determination of the stresses is given and that model is based purely on a hydrostatic pore pressure.

D-Foundations uses the given specific model for both the cases where effective stress plays its part, this to be consistent. However, when additional pore pressures do occur, the hydrostatic model used can calculate bearing capacities that are either too low or too high. So be sure to choose your phreatic level wisely when dealing with additional pore presures.

Bearing piles (EC7-NL): under "Indication bearing capacity" how do I calculate positive shaft friction?

When in the additional data of the profile the “top of positive friction” is exactly the same as “pile tip level”, the “Indication bearing capacity” will keep the positive shaft friction at zero. When the top of the positive skin differs from the pile tip level, a positive shaft friction is calculated.

Bearing piles (EC7-NL): Why does the sum of the individual settlements not match the total settlement?

For calcualtion of the total settlement under limit state 1B for a stiff construction, the average settlement is calculated. Would the choice have been a flexible construction, the settlement would be the sum of the individual contributions, for which the values are first summed, and only then rounded.

Bearing Piles (EC7-NL): What is a surcharge?

This is the permanent load that will be present next to the pile. In the ‘Bearing Piles’(EC7-NL) model, this is used to calculate the negative skin friction and the subsidence of the pile group (w2d). It is also used as a correction for the excavation reduction, although this reduction never makes a positive contribution; eg. a small excavation and large superimposed load does not result in an increase in the bearing capacity.

Shallow Foundations: Does the maximum stress increase displayed in the output correspond with the 20% limit of Article 6.1 in NEN 6744?

No, the maximum stress increase when calculating the subsidence is the maximum stress increase displayed as a percentage of the foundation pressure. This is not similar to the 20% limit in the norm, as this % applies to the stress increase itself. The maximum stress increase provides an indication of the reliability of the subsidence calculation. The subsidence calculation may be considered reliable if the percentage is at least 80%.

Shallow Foundations: What is the 5% limit when calculating subsidence?

In Article 6.1 of NEN 6744, according to the norm, layers with a stress increase less than 20% do not contribute to the subsidence and do not have to be included in the subsidence calculation. Deltares considers this percentage too high, we only neglect layers having a stress increase below 5%. For this reason the subsidence table contains two columns; following the 20% as well as the 5% criterion. It is up to the user which value set to apply.

Shallow Foundations: Why does adding ‘dummy’ layers not increase the accuracy of the subsidence calculation?

D-Foundations calculates the stress increase in the middle of the layer. A thick layer can have a stress increase in the middle that is less than the defined 20% or 5%, so that according to the norm no settlement needs to be determined. As settlement can occur in the top part of the layer one may overrule this assumption. It is advised to add thin layers at levels where the stress increase is less than 20% or 5%, in most cases one will add ‘dummy’ layers just below the foundation level.

Tension Piles (EC7-NL): Why is the Calculated Tension Force at a given depth related to the trajectory choice?

Check the sampling rate versus depth of your CPT or SPT; very low sampling rates may cause inaccurate results.

Tension Piles (EC7-NL): What is surcharge?

This is the permanent load that will be present next to the pile. In the Tension Piles (EC7-NL) model, the weight of the surcharge is included in the unit weight.

Tension Piles (EC7-NL): The program displays a message that f1 > 10. What does this mean?

When calculating for a pile group, the compaction by surrounding piles can be included (‘Options/Use Compaction’). This compaction is expressed in an augmentation factor of f1 that results in a linear increases of the cone resistance. In reality the value of f1 will usually not exceed 1 to 3. In arithmetical term, there is nothing against f1 > 10; the program simply continues to calculate. In such cases, however, you are advised to check the material parameters, as particularly unrealistic e_min and e_max values can be produced. If necessary, for sand use the values 0.4 and 08 in this case.

Tension Piles (EC7-NL): Why does the top of the tension zone change to a lower value after calculation?

In accordance with EC7-NL, the top of the tension zone may not be higher than 1 m under the surface/excavation level. The program automatically makes this correction.

D-Foundation General: Why is a GEF verified using the CUR verification tool not accepted?

The GEF verification tool of the CUR does not contain all checks to verify if a GEF file meets the requirements of the GEF-CPT-Report definition. For example, according to the GEF-CPT-Report definition: Column 1, the sounding length, must always be in metres; the second column must contain the cone resistance values in Mpa. Though this is checked by D-Foundation , this is not checked by the CUR tool, therefore D-Foundation may reject files that are compliant to the CUR tool verification. To solve this problem one may edit a GEF file using the GEF>PlotTool.