Why is it important to consider detailed construction stages in FEM?
In Geotechnical Finite Element Analysis (FEA), the simulation of construction stages is very important, particularly when analyzing deep excavations or subsurface structures. Here's an overview of the significance of construction stages in geotechnical FEA and why simulating them accurately is critical:
Realistic Ground Behavior: Simulating construction stages allows you to realistically represent the evolving ground conditions as excavation progresses. Deep excavations often involve changes in soil properties, groundwater levels, and support systems. Accurate representation of these changes is vital for understanding how they impact the stability of the excavation.
Safety and Stability Assessment: Construction stages analysis helps assess the safety and stability of deep excavations at different points during the digging process. By considering the effects of excavation depth, soil removal, and shoring installation, engineers can identify potential issues, such as excessive ground movements, deformation, or instability, and take corrective measures before they pose risks.
Load Redistribution: As soil is excavated, loads are redistributed within the surrounding ground. Analyzing construction stages allows engineers to track these load redistributions and evaluate their impact on nearby structures, utilities, and neighboring soil. It's crucial for avoiding ground settlements or damage to adjacent assets.
Groundwater Effects: The groundwater table can significantly influence deep excavations. Simulating construction stages helps in modeling the changing groundwater conditions as the excavation deepens. Accurate representation of groundwater levels is essential for assessing pore pressure changes, seepage, and the potential for soil liquefaction.
Material Properties: The properties of soils can change with time and excavation depth. For instance, soil may become more compacted or exhibit different shear strength characteristics at varying depths. Simulating these material property changes ensures that the analysis accounts for the evolving behavior of the soil.
Support Systems: Temporary and permanent support systems, such as retaining walls, bracing, or soil nails, may be installed during different stages of excavation. Properly modeling these support systems and their installation sequences is crucial for evaluating their effectiveness in maintaining excavation stability.
Cost and Schedule Optimization: Analyzing construction stages can lead to more efficient excavation sequencing, which can help optimize construction costs and schedules. It provides insights into critical excavation phases that may require additional support or monitoring.
It is generally important to simulate construction stages in FEA as detailed as possible to obtain accurate results. However, the level of detail and complexity in the simulation should be balanced with practical considerations, such as computational resources and analysis time. Engineers should make informed decisions about which construction stages require detailed simulation and which can be simplified without significantly affecting the results.
A case study – Examine a 35’ excavation with tiebacks
In the following paragraphs we compare 2 identical deep excavation models, simulating a 35’ (10.6m) deep excavation. In the first case, we examined all intermediate construction stages, including initial conditions, cantilever excavation, installation of support levels, and subsequent excavation stages (see Figure 1 below). In the second case, we omitted the intermediate excavation stages (Stages 1, 3, 5 and 7). The following figures show the calculated wall moment, shear and displacement diagrams, as well as the support reactions for the equivalent stages of each case, illustrating the difference in the results. The model simulation and analysis has been performed with the tools of our DeepEX – shoring design software, utilizing our Finite Element Analysis engine, DeepFEM.
Figure 1: Considered construction stages for each examined case in DeepEX software
Figure 2: Wall displacement & moment graphs, support reactions for each case – Stage 2
Figure 3: Wall displacement & moment graphs, support reactions for each case – Stage 6
Figure 4: Wall displacement & moment graphs, support reactions for each case – Stage 8
Figure 5: FEM settlement shadings for each case, Stage 9
In the comparative analysis of a 35-feet deep excavation using two distinct modeling approaches, our findings underscore the crucial role of simulating all construction stages in geotechnical Finite Element Analysis (FEA). In Case 1 the FEA results exhibited significantly larger wall displacements, moments, and support reactions when compared to Case 2, which omitted these intermediate stages.
These differences in results can be attributed to several factors. In Case 1, the comprehensive modeling captured the evolving ground conditions and support system adjustments at each construction phase. This allowed for a more accurate representation of how the excavation process influenced soil behavior and structural responses. In contrast, Case 2, by omitting intermediate stages, simplified the analysis, neglecting the intricacies of the excavation process and the varying loads on the support system. As a result, it tended to underestimate the potential displacements, moments, and reactions.
The findings of our study underscore the importance of simulating all construction stages in deep excavation analysis. Our software, DeepEX, proves to be an invaluable tool in this regard, empowering users to swiftly create and modify construction stages, ensuring a higher level of certainty when designing underground structures. With the capability to comprehensively model the excavation process, DeepEX not only enhances the accuracy of geotechnical FEA but also supports engineers in optimizing designs and ensuring the safety and stability of deep excavation projects.
In some cases, simplifications or idealizations may be acceptable if they do not compromise the accuracy of the final analysis. Ultimately, the decision on the level of detail for construction stages should be based on engineering judgment and the specific goals of the analysis, such as safety assessment, design optimization, or performance evaluation. In these conditions, DeepEX stands as a valuable asset for informed decision-making and robust underground structure design.