Engineering Guide: Design a shallow footing in Clayey soils using the GeoMechanica Desktop.

From the soil report to the selection of the dimensions of the foundation

This Tutorial shows how GeoMechanica Desktop can empower geotechnical engineers to perform fast, reliable calculations, allowing them to focus on engineering rather than worrying about the computations. Different Modules of the GeoMechanica Desktop were used during the process, from data visualization to bearing capacity and settlement calculations.

1. The Problem

A shallow foundation use needs to be validated for the soil conditions shown here. The foundation is to support a column load of 500 KN. The factor of safety against bearing capacity failure should be 3.0. Settlement should be less than 2.5cm.

2. Soil report

Data from the soil report is compiled from the soil report and plotted using the Field Canvas module. This allows for more visual interpretation of the soil layers and how each layer would influence the design. The plot is shown in Figure 1.  The water table was found at a depth of 2.3 meters below ground level. Note that the modulus of elasticity was calculated from constrained modulus values with an assumed Poisson’s ratio of 0.25.

It is clear from Figure 1 that the 4m clay layer would be a concern due to the expected compressibility. It has a low modulus of elasticity. The optimal depth of placing the foundation would be 2m below ground level. It cannot be placed below the compressible layer.  

3. Design Parameters

Design Parameters are usually taken from the bottom of the footing to a depth of 1.5B, where B is the footing width. Since B is not known yet, several cases are considered for the design parameters and are summarized in Table 1. Not much difference in design parameters is expected.

Figure 1: Soil Profile Showing various soil properties.

Table 1: Design parameters for the footing at different assumed widths

B	1.5B	φ	Ε
m	m	Degrees	MPa
1	1.5	32	33
2	3	32	33
3	4.5	31	32

4. Bearing capacity

Bearing capacity was calculated using the bearing capacity module. This module uses Meyerhof’s general bearing capacity equation. It can take into account eccentricity, water table effects, and inclined loading. The user only needs to plug in the foundation and soil data. The program uses advanced neural networks that can obtain the needed factors from the various design charts related to the equation used. A snapshot of the program is shown in Figure 2. The values of the bearing capacity at different foundation widths are shown in Table 2, and the corresponding factors of safety are shown in Table 3 and plotted in Figure 3.

Figure 2: A snapshot shows using the Bearing Capacity Module

Table 2: Values of the allowable bearing capacity of the foundation

	L
B	1	1.5	2	3
0.5	1587	1476	1420	1364
1	1880	1677	1576	1475
2			1923	1744

Table 3: Factor of safety of footing against bearing capacity failure

	L
B	1	1.5	2	3
0.5	1.587	2.214	2.84	4.092
1	3.76	5.031	6.304	8.85
2			15.384	20.928

Figure 3: Plot of the factor of safety of footing at different lengths.

The column load is 500 KN. Table 2 shows that using a width of 1m and a length of 1m is adequate. One could conclude that the ultimate limit state is satisfied, but the values used here assume a drained friction angle. This wouldn’t make a difference in the sandy layers, but for the clay layer, a check on the undrained factor of safety using the undrained shear strength is needed. This would make the problem multilayer. There are several theories on how to handle layers of sand over saturated clay. A basic check is to calculate the increase in effective stress at the clay layer and check if the resulting shear stresses are larger or smaller than the undrained shear strength. This can be easily done using “The Effective and Added Stresses at Center” Module. The module uses solutions for loading on an elastic half-space to determine the stresses at different depths in the ground. Advanced Physics Informed Neural Networks look up the needed factors to determine these stresses. A snapshot of the program analyzing the selected footing configuration is shown in Figure 4. You can find the maximum shear value, q, in the table from the module. It was found to be 26 kPa (see figure 5), which is about half of the available undrained cohesion at 60 kPa. This is a rough estimate. So far, the ultimate limit state is satisfied. Still, the serviceability limit state should be checked.

5. Settlement check

Geotechnical Desktop offers 3 modules for calculating the settlement of foundations. Two for elastic and immediate settlement, and one for consolidation settlement. Since the drained moduli of all soil layers were given in the report, the consolidation settlement or long-term settlement can be calculated using the elastic modules and the drained modulus. The difference between the two modules is that one assumes a uniform modulus of elasticity, while the other assumes Gibson soil. For the first module, an average modulus of elasticity taken from beneath the footing to a depth of 5B can be used. The depth averaging and the layers considered are shown in Figure 6. This depth of 5B is mentioned in many geotechnical codes because this is where the added stresses become closer to zero or very small compared to the soil's effective stress, as shown in Figure 6. The average modulus of elasticity can then be calculated as

(1)

Using the equation, the modulus is found to be 31.64 MPa.

Figure 4: Window of effective stress module with calculations of stresses due to footing load

Figure 5: value of maximum shear at the clay layer

Figure 6: Zone of influence below the foundation for settlement calculations.

A snippet of the module is shown in Figure 7. The settlement of the footing was 8.4mm, which is acceptable. Note that the depth to the rigid layer was chosen very large because it was assumed to be infinite. Another approach is to set it to 5B, which would then give a settlement of 7.6mm.

Figure 7: A screenshot of the Elastic settlement module.

The second module, which assumes Gibson soils can also be used. The assumption of the Gibson soil needs to be checked; in this case, it is valid as shown in Figure 8. The soil modulus of elasticity presents a clear pattern of increasing with depth. Linear increase parameters are shown in Figure 8 and used in the settlement module. A snippet of using the settlement of foundations on the Gibson soils module is shown in Figure 9. The settlement was found to be 12mm, which is 42% higher than just averaging the layers. However, it is still within reasonable limits. One could also check with the depth of the rigid layer and see how it affects the settlement, and if it does affect the result, the depth of the rigid layer needs to be specified in the soil report. In this case, it isn’t much. Overall, the foundation width and length are sufficient and don’t need any further increase to reduce the settlement.

Figure 8: showing a clear pattern of increasing modulus of elasticity with depth. Also shown is the linear increase slope and initial values.

Figure 9: A snippet of the Settlement of foundations on the Gibson soil module

6. Summary of the modules used

As shown, the geotechnical desktop app contains multiple modules that aid geotechnical engineers in designing foundations and choosing the right geometry for the foundation's load. A prototype design for complex problems can be achieved within minutes. There are also multiple other modules for piles and seepage problems, with more modules coming. Table 4 shows the Geotechnical desktop modules used in this tutorial and the purpose of each module.  

Table 4: Geotechnical Desktop Modules used in this tutorial, and what they were used for

7. Conclusion

A foundation design using the GeoMechanica Geotechnical Desktop app was presented here. Several modules were used for different purposes. The bearing capacity was calculated using the bearing capacity module. Based on these calculations, the foundation width and length were chosen to satisfy a factor of safety against bearing capacity failure of 3.0. After calculating the bearing capacity, the serviceability limit state also needed to be satisfied.  A target allowable foundation settlement was set at 25mm. Calculation of the settlement of the selected foundation was done using two methods. The first assumes a uniform soil with an average elastic modulus for the soil layers beneath the footing. The second method was by assuming a linearly increasing soil modulus, also known as Gibson soil. Both methods gave a settlement value that is below the allowable settlement. The assumption of Gibson soil gave a higher settlement value than assuming a uniform soil, but still below the allowable settlement. Other modules were used in this tutorial, which can also be used by geotechnical engineers for data visualization and representation of soil layers.