How to take an excavation into account

An excavation means that the soil was consolidated, resulting in less settlement. By default, Diamonds will not take the excavation is not taken into account (= safest approach). It’s up to you to configure the necessary settings to ensure that any existing excavation is charged correctly. This article outlines what you need to do.

You can find a comprehensive course on soil calculations in Diamonds on our e-learning platform (available in Dutch).

Necessary adjustments

1. Enter the soil layers starting from the original soil level.
   Diamonds calculates for you how many metres of soil you will excavate, based on the Y-coordinate of your foundations and the position of ground level.
2. Define the ground water level.
   The groundwater level is only used to determine if Diamonds has to calculate with the dry or wet selfweight of the soil.
   It’s always up to you to define water and soil pressures.

3. The recompression index A should be larger than the compression index C for sand, loam and slay soils.

4. Set the drainage coefficient CC to 0% for all soil layers.
   This coefficient is a measure of how well the soil can remember the preload due to the excavation. 100% means that the soil has completely forgotten about the excavation. For example: you dig a well in prehistoric times and start building in it today. That ground has long forgotten that you ever dug a well there. 0% means that the soil can fully remember the excavation. For example: you dig a well today and start building in it tomorrow.

5. At some point settlement calculations must stop. Eurocode allows 2 standards:
   • Or you use the stress tolerance (see step 9).
     We do not recommend this option as much because it sometimes causes iteration problems.
   • Or you adjust the layer thickness of the bottom soil layer in such a way that the total package of soil layers is as deep as once to twice the width of the foundation (EN 1997-1-1 §6.6.2 (7)).
     For example: a soil layer profile of 7m deep is entered using soil layers that are 0.2m thick. The width of the foundation plate is 10m.
     Assume we conservatively want to calculate with twice the width of the foundation, then the soil layer profile must be 20 m deep in total. So the bottom layer must be 14m thick.
     Don’t forget to uncheck the option ‘Bottom layer extends infinitely deep’ (see step 8).
     Leave the stress tolerance on 0% (see step 9).

6. Define the reference load group / combination.
   A table with soil layers is not suitable for FEM-calculations. So the soil layers are translated into functions. Each mesh node in the foundation will have it’s own function, depending on the loads present in that mesh node… That’s why you have to choose a reference combination. The loads in that reference combination will determine the behaviour of the soil function in that mesh node.
   Soil functions in Diamonds, can only be trained to do one thing: either understand how compressed soil works, or either understand how soil under tension works.
   
   By consequence, if you need the settlement in combinations that contains mainely vertical downward loads, you should take a reference combination that contains mainely vertical loads.
   If your model also contains combinations that mainely consist of vertical upward loads (water pressue) then you should redo the calculation a second time, using a reference combination that also contains mainely vertical upward loads.
   
   We generally opt for a quasi-permanent load combination SLS QP1 to start with and see how far it gets us.

Optional adjustments

7. Has the soil ever been préloaded with a load different from the escavation? For example: an old building that was demolished or a pile of sand that was removed.
   Then define the overconsolidation ratio OCR > 1

8. Uncheck the option Bottom layer continuous into infinity.
   By default Diamonds will assume that the bottom layers continuous to a depth of 200m. In some cases this gives conservative results. When you uncheck this option, Diamonds will stop the calculations after the last layer. This actually means implying there is a very stiff material (like rock) below the last soil layer.

9. In step 5, we saw two ways to stop the calculations. If you choose the method with the stress tolerance, you can apply it here.

10. Check the option Apply removed ground at interface.
    This option handles how the excavation should be taken into account.
    The option is ticked off, then Diamonds assumes that the ground will be removed in different steps, leading to a reduction of stress, under as well as next to the excavation.
    The option is ticked on, then Diamonds assumes that all removed ground is taken away at once, leading to a larger prestress at the excavation level.