Soil-Structure Interaction -Effects, Analysis and Applications in Design

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The study of soil-structure interaction (SSI) is related to the field of earthquake engineering. It is very important to note that the structural response is mainly due to the soil-structure interaction forces that brings an impact on the structure. This is a form of seismic excitation. A committee of engineering research deals with the study of soil-structure interaction only when these forces brings an appreciable effect on the basement motion when we are comparing it with the free-field ground motion. The free-field ground motion can be defined as the motion recorded on the surface of the soil, without the involvement of the structure. The structural response to an earthquake is highly dependent on the interactions between three linked systems, namely:

1. The structure
2. The Foundation
3. The underlying soil

The soil-structure interaction analysis is the method of evaluating the collective response of the three linked systems mentioned above for a specified ground motion. The soil-structure interaction can be defined as the process in which the response from the soil influences the motion of the structure and the motion of the given structure affects the response from the soil. This is a phenomenon in which the structural displacements and the ground displacements are independent to each other. Soil-structure force are mainly interaction forces that can occur for every structure. But these are not able to change the soil motion in all conditions.

Considerations in Soil-Structure Interaction Effects

A structure, when analyzed by considering its foundation to be rigid, is said to have no soil-structure interaction effects. Now, this case is considered even if the interaction force impacts the foundation. The influence on the soil motion by the interaction forces will depend upon:

• The magnitude of the force
• The flexibility of the soil foundation

The base mat acceleration and the inertia of the structure can be used to estimate the value of interaction forces. The heavier the structure the more is the soil-structure interaction effects for a particular soil site and for a given free-field seismic excitation. Most of the civil structure, whether it is lying on the hard or medium soil does not show any sign of SSI effects. As mentioned above, the SSI effects are more dealt with heavy structures that includes hydraulic structures like dams, nuclear power plants (NPP) reactor buildings. We can conclude that the soil interaction in earthquake engineering study was mainly developed and applied for these fields of construction industry. Another condition considered the soil-structure interaction effects are the soil flexibility. Softer is the soil, more is the chances for the occurrence of SSI effects. This is for a given structure and a site that have a free -field seismic excitation. Note: The product of mass density of the soil and the square of shear wave velocity will give the soil shear module. In practice, the mass density of the soil will vary around 2,0 t/m3. Hence the main characteristic of soil stiffness can be considered to be the shear wave velocity Vs.

If Vs< 300m/s then the soil is considered to be soft

If Vs> 800m/s then the soil is considered to be hard.

If Vs > 1100m/s the soil is considered to be rigid.

Application of Soil Structure Interaction

1. It is used in Heavy structures like hydraulic structures and nuclear structures
2. For those structure where the P delta effects are prominent, the analysis based on soil structure interaction is helpful.
3. The study of SSI has a significant role in the deep - seated foundations, structures supported over soft soil, tall or slender structures which have an average shear velocity of 100m/sec.

Soil-Structure Interaction and Structural Response

Based on conventional theories it has been said that the soil structure interaction has effects that are beneficial for the structural response. Most of the design codes for structures recommends neglecting the effect of SSI in the seismic analysis of the structure. This recommendation is because of the false myth that the SSI brings good response of the structure and hence have chances to increase the safety margins. More flexible structural design can be obtained if we consider the effects of soil structure interaction. This helps in increasing the natural period of the structure. This provides an improved structure when compared to a corresponding rigid structure. Incorporation of SSI effects on the structural design helps in increasing the damping ratio of the structure. This study is limited or neglected for conservative design procedures. The SSI analysis is very complicated in nature. The neglection will reduce the complexity in the analysis of the structures. This means that the myth put forward that the SSI effects are good for structures is not true. In fact, SSI can bring detrimental effects to structures. Neglecting SSI effect can bring unsafe design of the superstructure and the substructure.

What are the effects of Soil Structure Interaction?

The effects of the SSI are more focused on its detrimental effects. As mentioned, even if studies have told that the design based on soil structure interaction increases the time period, increase in time period is not always a beneficial factor. There is elongation of seismic waves when it is on a site of soft soil sediments. This results in the increase of the natural period hence leading to resonance. This happens with a long period vibration. If the natural period increases, the demand for ductility also increases. This may result in permanent deformation and soil failure that will further worsen the structural seismic response. A structure under the action of seismic force (seismic excitation), there is interaction between the soil and foundation which brings changes in the ground motion. The soil structure interaction can have two types of phenomena or effects (As per FEMA P-750, NEHRP). They are:

1. Kinematic Interaction
2. Inertial Interaction
3. Soil Foundation flexibility effects

Kinematic Interaction

The soil displacement caused by the earthquake ground motion is called as the free-field motion. This free field motion is not followed by the foundation that is located on the soil. The kinematic interaction is caused by the inability of the foundation to sink with the free field motion of the ground.

Inertial Interaction

The additional deformation caused in the soil due to the transmission of inertial force to the soil by the superstructure is called as the inertial interaction. When the ground shaking is of low level, the kinematic effect of SSI is more prominent. This results in the lengthening of period and there is increase in the radiation damping. When stronger shaking commences, the radiation damping is limited by the soil modulus degradation in the near field and the soil pile gaping. At this situation, the inertial damping is more prominent. This will hence cause excessive displacements near the ground surface. This will bring damage of the pile foundations. The study and researchers from the past and recent earthquakes show that the overall response of the structure is affected by the:

• Response from the foundation
• Response from the soil

The SSI have become great cause in the collapse of large structures when subjected to earthquake. These include the Hanshin Expressway, in 1995 due to the Kobe earthquake. Following are the factors to which the above-mentioned effects are related to:

Stiffness and Damping of the foundation

When a vibrating structure develops inertia force it give rise to moments, torsion and base shear. These are the forces that brings displacements and rotation in the interface between the soil and the foundation. The formed displacement and the rotation is a result of flexibility lying in the soil and the foundation. This flexibility is the basic reason for whole structural stability. The displacements created results in energy dissipation. This affects the overall system damping. As all these effects are more rooted with the structural inertia it is called as the inertial interaction effects. Variations Existing Between the Free-field motions and the foundation input motions These motions can differ because of the:

• Kinematic Interaction
• Relative displacements between the foundation and free field

The foundation motions are created by the stiff foundation elements that are placed either above or below the ground surface. This is done to have deviation from the free field motion, in the absence of structure and the foundation inertia forms the kinematic Interaction.

Foundation Deformation

The forces and the displacements applied on the foundation elements by the superstructure or the soil medium results in the flexural, axial and shear deformations. These parameters are the demands for which the components of the foundation must be designed. These effects are more significant in the case of foundation like rafts and piles.

Analysis in Soil Structure Interaction

The above-mentioned interactions can be measured by two methods of analysis. They are the:

1. Direct Analysis
2. Substructure Approach

Direct Analysis in Soil Structure Interaction

In this type of analysis, the soil and the structure is used in the same model for analysis. They are analyzed as a complete system. As shown in figure-2 below, the soil system is represented as a continuum. One such example is by the representation of finite elements. The foundation, structural elements, the load transmitting boundaries, the elements at the interface located on the edges of foundation are also included.

Fig.1: Illustration of Direct Analysis of Soil Structure Interaction with the help of Finite elements

This method is rarely used in practice as it involves large computation and is very complex to analyze.

Substructure Approach in Soil Structure Interaction

The soil structure interaction activity is divided into two parts. These are later combined to form a complete solution for the problem. In this approach, a model is generated with certain requirements:

• Free -field motions and the corresponding soil properties is evaluated
• The transfer functions are evaluated to convert the free -field motion to the foundation input motion
• Springs and the dashpots are incorporated. The springs represent the stiffness and the dashpots represent damping at the soil and foundation interface
• Response analysis of the combined structure

The figure-2 below shows the how a general problem A is evaluated. It is divided into two problems A1 and A2 such a way that A= A1 + A2. This is done based on the principle of superposition. Each problem is evaluated separately and the combination of the results will give the final solution.

Fig.2: Splitting of a Problem A by Superposition - Substructure Approach

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