Settlement calculation with Diamonds – interview with Dorothee De Pauw

1. Tell us more about your involvement in soil analysis

I am delighted to talk about soil settlements, as this topic is close to my heart. Within BuildSoft, I have personally been involved a lot in the calculations of soil settlement analysis, more specifically the settlement verifications. Starting with some basic soil mechanics and soil settlement knowledge learned at university, I extensively researched design codes, soil parameters, soil investigation, and settlement calculations. It struck me how big the topic ‘soil’ was and how widespread the information and design rules are. Moreover, you can only find a few detailed design examples for soil calculations. My expectations from design codes like Eurocode 7 were high, but I was left wondering about where I could find the design formulas.

2. What are the limitations of Eurocode 7 when doing settlement calculation?

Unfortunately, Eurocode 7 does not provide actual design formulas or recipes for settlement calculation, only possible design strategies and methods. It is stated as ‘Commonly recognized methods for evaluating settlements should be used’. These methods, however, state that you should consider the soil layer profile, include the effect of neighbouring foundations, and calculate both immediate and delayed settlements. These points are essential to know and should be checked with every method you use. Otherwise, your calculations do not comply with Eurocodes. But it is not detailed how you should take all these aspects into account in soil analysis.

This ‘general & open’ philosophy is all over Eurocode 7. Unlike the other Eurocode parts, Eurocode 7 has minimal ready-to-go formulas or ‘recipe’ design rules. In the beginning, it felt somehow ‘disappointing’ to me. I did not understand why there was so little hands-on information, but I learned there is a good reason for that. Geotechnical design is unique. It cannot be compared to other design practices in the construction industry. The commonly used models vary from country to country because geologies are different. Also, many practices are so called 'local traditions'. These differences cannot be harmonized easily.

3. What are some sample methods in Eurocode to model soil for settlement calculation?

There are basically 2 methods: the adjusted elasticity method and the stress-strain method. At BuildSoft, we focus on the stress-strain methods. This method involves computing the stress distribution in the ground due to the loading from the foundation. Calculating the strain in the ground from the stresses is done using stiffness moduli values or other stress-strain relationships determined from laboratory or field tests. Next, you need to integrate the vertical strains to find the settlements. To use the stress-strain method, a sufficient number of points within the ground beneath the foundation should be selected and the stresses and strains computed at these points. Proper soil mechanics knowledge is indispensable, as well as real soil data.

4. What are the most used methods to model in practice soil for settlement calculation?

The most know method for settlement calculation is probably the Winkler model or the spring model. This modelling is usually 1D, and the soil is considered to be an elastic and linear material. The whole soil behavior is represented by one single spring constant, also known as the ‘coefficient of sub-grade reaction’. You only need one parameter and all standard engineering software, including Diamonds, supports this feature. Although this method seems very easy, it is more challenging than you think: if you only use one parameter to represent the soil, you must ensure the value is correct. And that is where most people go wrong: they use values derived from previous experiences, habits or what they have found in books or online. This might work to get an idea of a slab or spread foundation’s behavior, but it will not give accurate settlement calculation results. The Winkler model does not consider the whole soil layer profile (unless you do a Plate load test), the effect of neighbouring foundations or delayed settlements.

You could work (semi-) empirical, analytical formulas for stresses & settlements, combined with field test results. The focus remains on the superstructure, but the overall soil behavior is incorporated in the global analysis. This method can be used for simple hand calculations using a worksheet or can be found in general structural analysis software like Diamonds.

Lastly, specialized soil analysis software uses the Finite Element Method to model the soil’s behavior, considering real soil data from field or laboratory tests. These models can be both 2D and 3D.

5. Global settlement vs. differential settlement: what are the allowable settlements?

When talking about allowable settlement, Eurocodes has distinguished several components such as overall (or global) settlement, relative (or differential) settlement, rotation, tilt, etc.

The most important components are overall settlement and differential settlement. These can be estimated using structural analysis software. Although Eurocode does not provide limit values for each component, publications like Skempton & MacDonald provide good practical indications of limit values to avoid damage.

In my opinion, differential settlement can cause the most damage and cracking in the building. For slab on grade, it certainly is not sufficient to only do settlement calculation in just one reference point of the foundation. You really can go wrong here if you only evaluate the total settlement, especially when the soil behavior changes across the slabs’ dimensions or there is a lot of variation in loading.

6. How can Diamonds help structural engineers when doing soil layer profile analysis?

As we know by now, a complete workflow for calculating settlement is not available in Eurocode 7. So, you must rely on experience or what you have learned at university – which is usually quite limited. You need to put your trust in the calculation methods that you are using. I have learned that many structural engineers do not know how to start with soil settlement calculation. This is because the structural engineering field is already so big, and the geotechnical field is at least as significant. As such, you must invest time and effort in creating your calculation worksheets. From my experience, I know that these sheets can only work for standard slab-on grade geometries, such as rectangular, circular, or square. Any other configuration will not be covered. And that is where we come in: Diamonds has an excellent soil settlement calculation functionality included, based on the actual measured soil parameters on-site and allowing all types of slab geometry.

7. What are the best practices to find manageable models and design formulas given the assumptions that the soil is elastic and non-linear?

On one hand, you can only relate to Eurocode. An example of a semi-empirical method for calculating settlements of spread foundations in coarse soil, using qc from CPT, is provided in part 2 of Eurocode 7. It is important to know that this method is not promoted as the go-to method. Moreover, this method has several limitations, like geometry/shape of the slab and type of loading. The method can be used for hand calculations but is unsuitable for converting to FEM analysis software.

On the other hand, you can do time-consuming research. A third – and probably best – option is to use structural analysis software with built-in settlement calculations that comply with Eurocode.

8. About the calculation model in Diamonds: how to find the proper bedding for the plate corresponding with the soil behavior and all the input parameters from the field test?

The settlement calculation is based on the principle that the slab deflection is equal to the settlement of the soil. Of course, this is how the actual situation will be. The slab is supported by suitably chosen spring bedding, in each mesh point a different spring value. We do not know the values of the springs in advance. They will be determined iteratively based on the introduced soil parameters and in such a way that settlement equals the deflection. In this iterative process, we use well-known soil equations such as Boussinesq formula for distribution of loads in soil and Terzaghi for settlement calculation. This is entirely following the proposed stress-strain method as in Eurocode.

9. Don’t you need to enter a ton of information for doing such a soil layer analysis?

No, you do not have to. The most important value is the compressibility constant C - per soil layer that you want to consider. Afterwards, you need to enter the dry and humid soil weight, usually taken 16 and 20kN/m^3. And that is it. The compressibility constant can be found in the geotechnical investigation ‘cone penetration test’ report (CPT) or calculated from the compressibility index C_c and void ratio e_0. In other words: we use real on-site measured soil data. The complete soil layer profile can be easily imported in Diamonds, so you do not need to enter layer per layer.

10. Share with us a practical situation related to settlement analysis.

Excavations are a widespread practice. They provide additional building space (i. e., parking) below ground level. In addition, you can take advantage of the preloading state of the soil because of the previous soil mass. The preloading stress will significantly reduce the final settlements, both global and differential. When working with excavations, water pressure becomes relevant. Both upwards and horizontal water pressure need to be accounted for.

If there is no (need for an) excavation, preloading can be present due to ground surface loading. Think of an old building to be demolished or a big pile of sand.

Finally, the ground level is not horizontal but inclined in some cases. And most slabs are not 100% rectangular or square. They are irregular-shaped.

With Diamonds, you can model all these practical situations.