Rozszerzenie Analiza geotechniczna umożliwia dokładne określenie warunków gruntowych i przeprowadzanie analiz geotechnicznych dla brył zbudowanych z materiałów gruntowych. When this add-on is activated, you can enter and model a soil solid directly in RFEM 6. All the parameters required for the calculation are automatically determined from the material data you enter in the program. Therefore, it is very important to understand the soil material models you are working with and the material parameters you assign to the program.
The add-on itself provides various material laws or material models suitable for simulating soil behavior. The available material models for the material type "Soil” are listed in the “Material model” section from the "New/Edit material" dialog shown below.
This text will focus on the Modified Hardening Soil model. First, the theoretical background is explained so that you understand the required input parameters for the program shown later in the text.
Podstawy teoretyczne
The stress-strain response of the Modified Hardening Soil model for both oedometric and triaxial conditions is shown in Image 2. The figures in the image show how the stiffness dependence is considered by the model. When the stress level changes, the slope changes, which also leads to a change in soil stiffness.
The figures also show that the stiffness depends on the load path as well. To be more specific, the soil shows a different behavior under primary loading than under unloading and reloading: an elasto-plastic behavior under the former and a linear elastic behavior under the latter.
Formulas 1 and 2 given below show how the dependence of stiffness on the stress level is considered. The parameters such as reference value of stiffness (Eref), reference stress (pref), cohesion (c), and friction angle (φ) are usually provided in the geotechnical report. There is also an exponent that controls the dependence on the stress level and contains values between 0.5 and 1, depending on which soil material you are working with (for example, the usual value for sand is 0.5 and for clay, 1.0). In this way, an associated stiffness can be calculated for each stress level.
Another important parameter to consider is the material constant CSH. In Image 2b) you can understand the importance of this constant that controls the shear hardening rate. More specifically, the material constant determines how quickly the ultimate shear surface defined by the Mohr-Coulomb failure criterion is reached. The smaller CSH, the closer to the failure surface at the same plastic shear strain.
The yield surfaces in the p-q space and in the principal stress space are shown in Image 3. The modified hardening soil model is an elasto-plastic model with isotropic double hardening, and it uses the Mohr-column failure criterion. It shows plastic strains before reaching failure. The plastic deformation then consists of two components: shear plastic deformations and volumetric plastic deformation.
Application in Geotechnical Analysis Add-on
Knowing the theoretical background of the model, we can now see how the model is integrated into the Geotechnical Analysis add-on by creating a new soil material.
Soil material can be created in RFEM 6 in the usual manner of creating materials in the program, i.e. by opening the “New Material” dialog box and assigning the material properties. In this case, it is important to select the material type "Soil" so that you can select the Modified Hardening Soil from the drop-down menu of material models, as shown in Image 4. The basic properties of the material that you must enter in the dialog box are shown in the same image. Please note that for soil type materials, two values must be defined for the specific weight depending on the water content: the specific weight of the "drained" soil and of the soil “saturated” with water.
By selecting Modified Hardening Soil as the material model, a corresponding tab appears in which you can assign the specific parameters for this material model discussed in the previous chapter.
These include the following:
| φ | Kąt tarcia |
| C | Kohezja |
| ψ | Kąt dylatancji |
| pref | Naprężenie wzorcowe |
| m | Power for stress dependency |
| Eoed,ref | Reference value of the oedometric tangent modulus for initial loading |
| CSH | Material constant for control of shear hardening |
| Eur,ref | Reference value of the secant modulus for unloading and reloading |
| ν | Poisson's ratio for unloading and reloading |
| pPOP/ηPOP | Preconsolidation stress / Preconsolidation ratio |
Please note that you can request the values of these parameters to be included in the geotechnical report. The values are often known from experience as well. However, it may be necessary then to validate the behavior of the material defined in RFEM by calculating a soil test and comparing it with results from laboratory tests or from the literature.
After entering the values, the corresponding material model is used, and the new material is created. Note that you can also define other materials in this way to assign them as soil layers in boreholes. The data from the boreholes can then be used to create the soil massif. This procedure is explained in more detail in KB 1699.