Modelling of retining wall with solid soil elements
Hello, I was wondering if it was possible to model a retaining wall with automatic calculation of earth and the indirect loading from surcharge load with the help of soil as solid elements?
In the attached file I have made two different models for the same retaining wall.
1. Only concrete structure is modelled.
Soil load and surcharge are applied directly on structure.
2. Soil as solid elements and wall is modelled as a "simple retaining wall"
Soil weight and surcharge are applied on soil surface.
γ_soil= 22 kN/m3
Assuming active earth pressure --> K0 = tan2(45-45/2)=0.17
--> v = 0.145
Structures with soil load:
Comparison of Mx' result:
Comment: Result close in wall but differ more in foundation slab.
Questions:
1. How is earth weight inside foundation slab volume handled? Can this volume be excluded?
2. Do the filling below foundation slab replace the deformation properties of the soil below foundation?
3. Is a modelling with two boreholes in close proximity the best way to model the change of soil height?
4. What calculation method for determining reinforcement requirement is recommended from Strusoft? (examining geotechnical capacity separately)
5. What I can see difference in wall thickness is not possible for "simple retaining wall".
Is this a feature available?
Other comment of modelling a retaining wall that I might have missed would be interesting
Hi Kristoffer,
Hello, I was wondering if it was possible to model a retaining wall with automatic calculation of earth and the indirect loading from surcharge load with the help of soil as solid elements?
Yes, see the Geotechnical module handbook pdf of FEM-Design (124 pages, available on the FEM-Design Wiki page) about the Mohr-Coulomb material model in FEM-Design what could help to model the problem mentioned above. See the verification example 5.4, 5.5 and 5.6 in the 3D Soil handbook. The sample FEM-Design files are linked in the document as well.
In the attached file I have made two different models for the same retaining wall...
The precise active earth pressure calculation method regarding the Mohr-Coulomb theory can be found in the Geotechnical module handbook Eq. 3.21-3.22 in case of cohesive/friction soils. In the attached model you adjusted for the 3D solid soil calculation a drained case with a soil which has friction angle and cohesion as well. In the lateral earth pressure calculation in active earth pressure case the cohesion may be considered as well.
Comment: Result close in wall but differ more in foundation slab.
If you check the graph view of Mx you can see that they are very similar (advice: use a slightly denser mesh in case of non-linear soil calculation with 3D solids). The difference comes from the above mentioned consideration of the lateral “active earth pressure”. But in the non-linear soil calculation with 3D solids it comes from the calculation itself following the Mohr-Coulomb material model which is the basics of active/passive earth pressure theory. It is considered in a more adequate way in 3D solid soil calculation because the material model decides, based on the deformations, if the soil is in “active” or “passive” or “rest” earth pressure condition. The material model contains these behaviours, see e.g. Chapter 3.2.2 in the Geotechnical module handbook.
Here, the graph view of the Mx moment is attached. The difference also comes from that the bedding of the foundation directly comes from the soil supporting conditions (different boundary condition) compared with a simple bedding modulus in the non-soil case. Considering these inputs, we can say that the results are very close to each other in your models.
Questions:
1. How is earth weight inside foundation slab volume handled? Can this volume be excluded?
The slab and retaining wall static system consists of “shells” considered by their midplanes, which is necessary because this is the way at the end we can see internal forces in the slabs. Thus, the earth weight inside the foundation cannot be excluded. However if you think that this is an issue, one solution can be that the neglection of the self-weight of the slab, because the 22 kN/m3 weight of the soil is very close to the weight of the concrete.
2. Do the filling below foundation slab replace the deformation properties of the soil below foundation?
If filling volume is defined somewhere, then that filling will replace the “original” soil properties.
3. Is a modelling with two boreholes in close proximity the best way to model the change of soil height?
Yes, the borehole tool is the tool to manage the soil height. You can also use the excavation tool where excavation is necessary.
4. What calculation method for determining reinforcement requirement is recommended from Strusoft? (examining geotechnical capacity separately)
With the internal forces in the slab/wall model calculated in the Analysis tab, you can use the Concrete tab to design according to the EC2 calculations.
5. What I can see difference in wall thickness is not possible for "simple retaining wall".
Is this a feature available?
In the “simple retaining wall” tool the wall can have only constant thickness.
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