Introduction
It is common practice to design the superstructure and infrastructure elements separately. In this simplistic approach, the soil is considered to be an indisplaceable support and its influence on the superstructure is disregarded. Similarly, the loads on the fixed supports are used to design the infrastructure, and the stiffness of the superstructure is disregarded.
The traditional approach can provide satisfactory results, especially in the case of soils with low deformability. There are several situations, however, in which this approach may be mistaken or unfeasible. In these cases, soil-structure interaction (SSE) should be considered, defined by NBR 6122 [1] as structural analysis processes that jointly consider the deformabilities of the infrastructure and superstructure.
The advent of numerical methods and their application in the field of structural analysis has led to more sophisticated approaches to representing soil-structure interaction in a more realistic and accurate way in terms of the behavior of structures. However, representing this interaction can be a very complex task.
Soil is a heterogeneous medium resulting from the decomposition of rocks by the action of physical or chemical agents, whether or not it contains decomposed organic matter, with liquid and gas in the empty spaces between the solid particles. Given its nature, its composition and, consequently, its physical and mechanical properties can be quite variable along the stratigraphic profile and difficult to determine.
Kot has a Civil Engineering team Civil Engineering experience in modeling and analyzing structures where the use of soil-structure interaction is most appropriate, or even indispensable, providing reliable engineering solutions for its clients.
Soil-structure interaction and its applications
The soil-structure interaction mechanism can be used in various types of structures. Some of the applications are
- Foundation analysis;
- Analysis of containment structures;
- Dynamic analysis of structures resting on the ground;
- Analysis of underground structures such as tunnels;
- Path analysis;
- Analysis of structures subjected to significant variable loads, such as silos, bridges, tanks and thickeners;
- Analysis of structures sensitive to deformation or supported on highly deformable soils.
Soil-structure interaction and impacts on the behavior of structures
Incorrect consideration or disregard for the mechanism of soil-structure interaction can have negative consequences from the economic, operational and safety points of view.
The compatibility between the displacements of the soil and the structure can lead, in the event of excessive differential vertical displacements, to the redistribution of forces in the superstructure elements and in the loads that reach the foundation. As a consequence, there are undersized components in the structure, which makes the structure more susceptible to cracks and unexpected deformations throughout its useful life. In critical conditions, the result is the collapse of elements of the structure due to loads not foreseen in the design.
In addition to structural impacts, ground displacements can have operational impacts on structures such as yard machinery. Vertical and horizontal misalignments in the track (Figure 1) result in increased maintenance requirements and can even lead to a loss of machine stability due to the redistribution of loads.
Figure 1: Vertical displacements on a roadway. SOURCE: Kot Collection.
Soil-structure interaction also influences the dynamic response of a structure. Soil stiffness and damping vary according to its composition. The same structure will behave differently depending on the type of soil it is built on. This is especially important when it comes to structures subjected to excitation sources such as mills and crusher bases (Figure 2).
Figure 2: Dynamic analysis of a crusher base supported on soil. SOURCE: Kot Collection.
Soil-structure interaction in design and modeling
The most common approaches to representing soil-structure interaction (SSI) are to apply discrete and continuous models. The approach to be adopted should be defined based on aspects relating to the behavior of the structure to be analyzed, material properties and external loading conditions.
In continuous medium models, the soil mass is considered as a continuous elastic medium and modeled using two- or three-dimensional elements. This method is a conceptual approximation to the physical representation of infinite soil. Although it is a more realistic representation of soil behavior, it is a more complex approach. One application is the modeling and analysis of underground structures such as tunnels, as can be seen in Figure 3.
Figure 3: Model of soil as a continuous medium under tunnel. SOURCE: Kot Collection.
In discrete medium models, soil-structure interaction is represented according to the model proposed by Winkler, by means of a series of independent springs with linear elastic behavior in which the stiffness of these springs is characterized by a constant of proportionality between the applied pressure and the displacement of the soil, called the reaction coefficient [2]. This representation is widely used in the modeling of shallow foundations and pile caps. Figure 4 shows a model of a radier with soil-structure interaction by means of springs
Figure 4: Radier model with soil-structure interaction using springs. SOURCE: Kot Collection.
The representation of soil-structure interaction using springs is the most widely adopted approach due to its simplicity. The main drawback of this model is determining the stiffness coefficient of the springs, since it is the only parameter for idealizing the behaviour of the soil. The stiffness coefficient depends not only on the nature of the soil, but also on the dimensions of the loaded area.
It should be noted that, regardless of the type of approach chosen, the analysis of structures that interface with the soil requires adequate knowledge of the soil's behavior and properties.
Conclusion
Soil-structure interaction (SSE) consists of the compatibility of displacements between the structure and the soil. This mechanism is linked to the behavior and, consequently, the static and dynamic response of a structure to stresses. Soil-structure interaction is a mechanism of great importance for structures in the industrial sector due to its unconventional nature.
The realistic representation of soil can be quite complex, mainly due to the uncertainties associated with soil behavior and properties. Incorrect consideration of soil-structure interaction can lead to compromised operation and safety of a structure.
Kot has a team of qualified professionals in analysis, including ISE (soil-structure interaction), ready to provide reliable and customized engineering solutions. Contact our experts for more information!
Follow our pages on LinkedIn, Facebook e Instagram to keep up with our content.
References
- Brazilian Association of Technical Standards, NBR 6122. Design and execution of foundations, 2022.
- CAMPOS. J. C. Elementos de fundação em concreto. Oficina de Textos, São Paulo, 2015.
