Perform Better Geotechnical Numerical Analysis: Part 1

In the field of civil engineering, geotechnical numerical analysis plays a crucial role in understanding and predicting the behavior of soil and rock structures. It enables engineers to simulate real-world scenarios and make informed decisions regarding the design and construction of geotechnical projects. In this blog post series, we will explore the various aspects of performing better geotechnical numerical analysis. 

In this first part, we will delve into

1.  Analysis planning, 
2. Geometry simplification, 
3. Plane strain and axisymmetric assumptions, 
4. Locating boundaries, 
5. Fixities
6.  Axes of symmetry.        Let's begin!

• Analysis Planning:

Analysis planning involves several crucial steps to ensure the success and accuracy of the numerical analysis. Here are some key points to consider:

1. Justifying Numerical Analysis: Before opting for numerical analysis, it's essential to assess whether it is necessary for the technical problem at hand. Numerical analysis can be time-consuming and expensive compared to conventional methods. Justify its use based on the complexity and specific requirements of the project.
2. Defining Aims and Objectives: Clearly document and communicate the aims and objectives of the numerical analysis. These goals will serve as a reference throughout the project, ensuring all stakeholders are aligned.
3.  Gathering Information: Gather comprehensive information about ground conditions, historical activities on the site, existing structures, proposed structures and infrastructure, and construction methods. This information will guide the decision-making process and help simulate accurate ground conditions.
4.  Site Investigation: Conduct a thorough site investigation in phases to obtain input parameters for the analysis. This investigation will provide valuable data for modeling the constituents or modules accurately.
5. Software Selection: Understand the strengths and weaknesses of the numerical analysis software you choose. This will help you avoid errors and ensure that the software operates effectively on your computer.
6. Reporting: Ensure that the outputs of your analysis are presented in a clear and informative manner, meeting the needs of other designers and decision-makers who will utilize the results.

 

• Geometry Simplification:

Simplifying complex real-world geometry is crucial to create manageable and representative analysis models. Consider the following points:

1. Volume of Ground: When creating a geotechnical analysis model, it is important to include a significant volume of the ground relevant to the problem. Determine how much of the surrounding area should be incorporated into the model.
2. Simplifying Structure Geometry: The geometry of the structure being analyzed can be simplified as long as it does not significantly affect the desired outputs. Remove unnecessary details while ensuring the model remains representative.
3. Locating Boundaries: When simplifying the external environment, locate the boundaries of the model. Exclude features that are far enough from the area of interest, eliminating their impact on the analysis results.
4. Including Surrounding Structures: If predicting the settlement of adjacent structures is an objective, include these structures in the model. Focus on modeling the foundation structures rather than the entire buildings.
5. Balance Accuracy and Simplicity: Simplification should be performed based on the specific aims of the analysis. Determine the level of detail required to achieve sufficiently accurate outputs.

 

• Plane Strain and Axisymmetric Assumptions:

To simplify the analysis process, geotechnical numerical analysis often utilizes plane strain and axisymmetric assumptions. Consider the following points:

1. Plane Strain Assumption: This assumption assumes that all strains occur within the plane of analysis, with no strain in any direction away from the plane. It simplifies the analysis process but requires accurate specification of input parameters.
2. Two-Dimensional Axisymmetric Assumption: The two-dimensional axisymmetric assumption is applicable when a structure exhibits circular symmetry. In this assumption, strains occur within  a plane and the structure exhibits rotational symmetry around an axis. It is commonly applied to problems involving cylindrical structures such as tunnels, pipelines, and shafts.
3. Locating Boundaries: Clearly define the boundaries of the analysis model. This includes identifying the extent of the soil or rock mass being considered and determining how far the boundaries need to be from the area of interest. Properly locating the boundaries ensures that the model captures the relevant behavior and reduces computational efforts.
4. Fixities: Fixities are constraints applied to the model to represent the actual boundary conditions. They simulate the interaction between the structure and the surrounding soil or rock mass. Fixities can include fixed supports, pinned supports, rollers, and other types of restraints. Determine the appropriate fixities based on the actual conditions and behavior of the system.
5. Axes of Symmetry: In cases where the geometry or loading conditions exhibit symmetry, identifying the axes of symmetry can significantly simplify the analysis. Symmetry reduces the computational effort by modeling only a portion of the structure or system, taking advantage of the repetitive nature of the problem.
6. Boundary Conditions: Along with fixities, boundary conditions need to be defined accurately. Boundary conditions represent the applied loads, displacements, or pressures on the boundaries of the analysis model. Consider the actual loading and interaction between the structure and the ground to determine the appropriate boundary conditions.
7. Soil-Structure Interaction: Geotechnical numerical analysis should account for the interaction between the soil or rock and the structure. This interaction can be simulated using interface elements or contact elements, depending on the type of problem and the behavior being analyzed. Proper modeling of soil-structure interaction enhances the accuracy of the analysis results.

 

• Model Verification and Sensitivity Analysis:

Once the analysis model is developed, it is crucial to perform model verification and sensitivity analysis. These steps ensure the reliability of the results and help identify the critical parameters influencing the behavior of the system.

1. Model Verification: Verify the analysis model by comparing the numerical results with analytical solutions, empirical data, or field measurements. This process validates the accuracy of the model and ensures that it adequately represents the real-world behavior.
2. Sensitivity Analysis: Conduct sensitivity analysis to evaluate the influence of different input parameters on the analysis results. Vary one parameter at a time while keeping others constant and observe the corresponding changes in the outputs. This analysis helps identify the parameters that have the most significant impact on the system's behavior.
3. Iterative Process: Geotechnical numerical analysis often requires an iterative process of refining the model and adjusting the input parameters to achieve accurate results. It may involve multiple iterations of modeling, analysis, and validation until the desired level of accuracy and reliability is achieved.

 

Let's illustrate the concepts discussed in the  explanation with an example.

Example: Analysis of a Tunnel Excavation

Suppose you are a geotechnical engineer tasked with analyzing the behavior of a tunnel excavation in a cohesive soil. Here's how you can apply the concepts covered:

Analysis Planning:

Justifying Numerical Analysis: Determine if numerical analysis is necessary for accurately predicting the tunnel excavation's behavior, considering factors such as complex ground conditions or potential stability issues.

Defining Aims and Objectives: Clearly state the objectives of the analysis, such as assessing the stability of the tunnel, predicting deformations, or evaluating support requirements.

Gathering Information: Collect data about the site's geological properties, groundwater conditions, historical activities, and construction methods used for similar projects.

Site Investigation: Conduct a thorough site investigation, including soil sampling and laboratory testing, to obtain input parameters like soil strength, stiffness, and permeability.

Software Selection: Choose appropriate geotechnical analysis software that can handle the complex nature of tunnel excavation analysis.

Reporting: Plan how to present the analysis results in a clear and informative manner, ensuring that the findings can be easily understood by stakeholders.

Geometry Simplification:

Volume of Ground: Determine the extent of the ground to be included in the analysis model. It should encompass a sufficient volume around the tunnel to capture the relevant behavior.

Simplifying Structure Geometry: Simplify the tunnel's geometry while retaining its essential features. For example, represent the tunnel as a circular cross-section with appropriate dimensions and exclude non-critical details.

Locating Boundaries: Clearly define the boundaries of the analysis model, ensuring that they are sufficiently far from the tunnel to eliminate their influence on the analysis results.

Including Surrounding Structures: If adjacent structures' settlement is of interest like a pile foundation ,buried conduit  , include simplified representations of these structures, focusing on modeling their foundations rather than the entire buildings.

Balance Accuracy and Simplicity: Adjust the level of detail in the model to balance computational efficiency and accuracy based on the specific aims of the analysis.

Plane Strain and Axisymmetric Assumptions:

Plane Strain Assumption: Assuming plane strain, consider that strains occur within the plane of analysis without any strain in the direction perpendicular to the plane. Apply this assumption to simplify the analysis, but ensure accurate input parameter specification for the soil behavior.

Two-Dimensional Axisymmetric Assumption: If the tunnel exhibits circular symmetry, utilize the two-dimensional axisymmetric assumption. This simplification assumes rotational symmetry around the tunnel's central axis, reducing the analysis to a two-dimensional plane. It is appropriate for cylindrical tunnel structures.

Locating Boundaries, Fixities, and Axes of Symmetry:

Locating Boundaries: Determine the extent of the soil or rock mass to be included in the analysis model. Set the boundaries far enough from the tunnel to capture the relevant behavior but exclude non-essential regions.

Fixities: Define appropriate fixities to represent the interaction between the tunnel and the surrounding soil. For example, fix the tunnel's base to simulate the support provided by the excavated material or include rock bolts to represent additional reinforcement.

Axes of Symmetry: Identify the axes of symmetry if the tunnel geometry or loading conditions exhibit symmetry. This reduces the computational effort by modeling only a portion of the structure, taking advantage of the repetitive nature of the problem.

Boundary Conditions: Accurately define boundary conditions by considering the actual loading and interaction between the tunnel and the ground. Apply appropriate loads, displacements, or pressures on the boundaries based on the specific scenario.

Model Verification and Sensitivity Analysis:

Model Verification: Validate the analysis model by comparing the numerical results with analytical solutions, empirical data, or field measurements. Verify that the model adequately represents the real-world behavior of the tunnel excavation.

Sensitivity Analysis: Conduct sensitivity analysis by varying one input parameter at a time while keeping others constant. Observe the resulting changes in the analysis outputs, such as tunnel deformations or support forces. Identify the parameters that significantly influence the system's behavior.

Iterative Process: Refine the analysis model iteratively, adjusting the input parameters based on the sensitivity analysis results and comparing the outcomes with the observed behavior. Repeat the modeling, analysis, and validation process until the desired level of accuracy and reliability is achieved.

By following these steps, you can effectively apply geotechnical numerical analysis to assess the stability, deformations, and support requirements of a tunnel excavation in cohesive soil. The concepts discussed in this example can be adapted to other geotechnical analysis scenarios as well.

Remember, this example is simplified and provided for illustrative purposes. In practical applications, additional considerations and complexities may arise, requiring further analysis and expertise.

 

Conclusion:

In this first part of the series on performing better geotechnical numerical analysis, we discussed the importance of analysis planning, geometry simplification, plane strain and axisymmetric assumptions, locating boundaries, fixities, and axes of symmetry. These steps are crucial for developing accurate and efficient analysis models. In the next part, we will explore the selection and calibration of constitutive models, meshing techniques, and numerical analysis methods. Stay tuned for more insights into improving geotechnical numerical analysis.