Soil models

There is no interaction between the subsoil and the foundation for the Simple Assumption model (Linear Contact Pressure method - method 1). Therefore, the soil data are not required in this method (only groundwater G_w and foundation level T_f are required). When soil properties are required to be defined for calculation method 1 (Linear Contact Pressure method), the following Dialog box of Figure 21 appears.

If the water table is located above the foundation, the foundation will be exposed to additional negative pressure. In the Dialog box of Figure 21 define the groundwater depth under the ground surface G_w in order to take the effect of groundwater pressure in the analysis. 

Figure 21 "Soil properties" Dialog box (method 1)

For the two methods of Constant and Variable Modulus of Subgrade Reaction (methods 2 and 3), when the modulus of subgrade reaction is required to be defined by the user, soil properties, in this case, will be the modulus of subgrade reaction k_s besides its coordinates (x, y) in the global system and groundwater depth under the ground surface G_w. If the nonlinear analysis is required, the ultimate bearing capacity of the soil q_ult must be defined (Figure 22).

 Figure 22 "Soil properties" Dialog box (methods 2 and 3) 

When soil properties are required to be defined for calculation method 2 (Modulus of subgrade reaction is determined from Half-Space) and calculation method 5 (Isotropic Elastic Half-Space), the following Dialog box appears.

In the Dialog box, define the settlement reduction factor α, Poisson’s ratio of the soil ν_s, groundwater depth under the ground surface G_w and the modulus of compressibility of the soil E_s. If the nonlinear analysis is required, the angle of internal friction φ and the cohesion c of the soil must be defined.

 Figure 23 "Soil properties" Dialog box (methods 2 and 5)

Based on experience, the real consolidation settlements are different from those calculated. Therefore, the settlement s may be multiplied by a factor α according to the German standard DIN 4019. According to the German standard DIN 4019, the following reduction factors α can be applied:

- Sand and silt	α = 0.66
- Normally and slightly overconsolidated clay	α = 1.0
- Heavily overconsolidated clay	α = 0.5-1.0

For rigid and elastic rafts, it is convenient to determine the flexibility coefficient of the interior node at the characteristic point of the loaded area on that node. For a flexible foundation, it is real to determine the flexibility coefficient of the interior node at that node.

It is possible to determine the flexibility coefficient of the interior node due to a uniform load at that node (Figure 24):

• at the characteristic point of the loaded area, where rigid settlement is equal to the flexible settlement
• at the midpoint of the loaded area, where maximum settlement occurs
• at the interior node on the loaded area

The earlier version of ELPLA determines flexibility coefficients for both interior and exterior nodes by assuming uniform loaded areas on these nodes. This assumption requires the principle of superposition for determining the flexibility coefficients. Now it is possible, optionally to convert the loaded areas on exterior nodes to point loads, Figure 24. In this way, the program does not follow the principle of superposition in the analysis, making it much faster than the old analysis. The new method of analysis is consequently faster and more efficient for problems that contain a large finite element mesh.

If the distance between two nodes is too large, the settlement of a node due to a load on the other will be small enough to be neglected. To reduce the time required for determining the flexibility coefficients for great rafts, a limit distance between node i and j for determining the flexibility coefficient c (i, j) may be defined.

Figure 24 "Calculations parameters of flexibility coefficients" Dialog box (methods 2 and 5)

The bearing capacity factors used to determine the ultimate bearing capacity can optionally be defined according to different codes and authors. These factors are required to carry out the nonlinear analysis of the soil. The bearing capacity factors are defined according to (Figure 25):

Figure 25 "Bearing Capacity factors" Dialog box (methods 2 and 5)

A layered soil model is used in the analysis of the calculation methods shown in Table ‎1. When soil properties are required for one of the calculation methods shown in this Table, the following Soil Properties Tab View appears (Figure 26).

 

Table 1 Numerical calculation methods (Layered soil model)

Method No.	Method
2	Constant modulus of subgrade reaction Winkler's model, Modulus of subgrade reaction is determined from soil layers
3	Variable modulus of subgrade reaction Winkler's model, Modulus of subgrade reaction is determined from soil layers
4	Modification of modulus of subgrade reaction by iteration Winkler's model/ Continuum model
6	Modulus of compressibility method for an elastic raft on layered soil medium Solving system of linear equations by iteration Layered soil medium ‑ Continuum model
7	Modulus of compressibility method for an elastic raft on layered soil medium Solving system of linear equations by elimination Layered soil medium ‑ Continuum model
8	Modulus of compressibility method for a rigid raft on layered soil medium Layered soil medium ‑ Continuum model
9	Modulus of compressibility method for a flexible foundation on layered soil medium Layered soil medium ‑ Continuum model

Figure 26 "Soil properties" Tab view