Bracing system partial collapse – Examined with LEM, NL and FEM analysis in DeepEX
Introduction:
Designing deep excavations requires accounting for various forces that can act on the system, including soil pressure, groundwater, adjacent structures, external loads, seismic activity, and more. While most geotechnical design codes recommend safety factors or reduction factors to ensure that excavation systems can withstand these forces, what happens if a critical support fails, such as a bracing level?
Would the remaining bracing system handle the redistributed stresses? What methods should be used to analyze and mitigate such risks? This article aims to provide valuable insights into addressing these questions and explains how DeepEX, a comprehensive excavation design software, can help engineers evaluate and design safe systems in such scenarios.
Deep Excavations and DeepEX:
DeepEX is a powerful software solution for simulating and designing deep excavation wall and support systems. It offers a variety of analysis methods, including Limit Equilibrium (LEM), Non-Linear (NL) analysis with elastoplastic Winkler springs, and 2D/3D Finite Element (FEM) analysis, enabling engineers to capture and evaluate complex excavation scenarios.
One of the standout features of DeepEX is its ability to model emergency conditions, such as the failure of a bracing level. The software can assess how the system redistributes stresses, calculate new wall moments and shear forces, and analyze changes in support reactions. By allowing users to quickly switch between analysis methods, DeepEX provides flexibility in evaluating these scenarios, helping engineers design safer and more robust excavation systems.
Case Study: Failure of a Middle Strut in a Multi-Braced Excavation Model
In this case study, we modeled a 40-foot deep excavation with AZ 26 sheet pile walls and pipe struts (PP 24x0.5) arranged in three bracing levels. Using DeepEX, we analyzed the excavation at the final construction stage using three different methods—Limit Equilibrium (LEM), Non-Linear (NL), and 2D Finite Element (FEM) analysis. Afterward, we simulated an additional stage in which the middle bracing level failed.
Figure 1: 40ft deep excavation model in DeepEX software
LEM Analysis (LEM):
In LEM, DeepEX calculates stresses on the wall from soil, water, and external loads by forming a net pressure diagram. However, this method does not account for previously developed stresses and displacements from earlier stages—it works independently at each construction stage. The program uses various beam analysis methods (such as CALTRANS, Blum’s or FHWA) to determine wall stresses and support reactions.
Figure 2: Final excavation stage – LEM analysis results in DeepEX
Figure 3: Loss of a bracing level – LEM analysis results in DeepEX
Results:
In the final stage, we observed slight increases in upper and lower support reactions and a small localized increase in wall bending moments after the middle bracing level failed.
The behavior was mainly localized due to the beam analysis method, which treats support locations as hinges, limiting the effect to the former support position.
Non-Linear Analysis (NL):
In NL analysis, DeepEX uses elastoplastic springs to simulate wall-soil interaction and incorporates soil stiffness, behavior, and construction staging effects. Unlike LEM, this method accounts for stresses and displacements from earlier stages. In this analysis, we assumed 50% interface friction between the soil and the wall and set an over-consolidation ratio (OCR) of 3 for undrained clays.
Figure 4: Final excavation stage – NL analysis results in DeepEX
Figure 5: Loss of a bracing level – NL analysis results in DeepEX
Results:
Compared to LEM, the overall system behavior is more realistically captured, as NL analysis reflects the global redistribution of stresses across the wall.
In addition to changes in support reactions and bending moments, NL analysis also provides insights into wall displacements—a key parameter not evaluated by LEM.
Finite Element Analysis (FEM):
FEM analysis accounts for the effects of all construction stages and allows for modeling soil-structure interaction. The soil is represented by a mesh of quadratic triangular elements, and DeepEX automates the stiffness calculations and helps estimate FEM analysis parameters. Different soil models can be applied for each soil type, including Mohr-Coulomb, Soil Hardening, and Cam Clay, among others. Both drained and undrained clay behavior can be considered, along with water flow analysis. For this model, we assumed 50% interface friction between the soil and the wall and used a medium-density mesh for the finite elements.
Figure 6: Final excavation stage – FEM analysis results in DeepEX
Figure 7: Loss of a bracing level – FEM analysis results in DeepEX
Results:
FEM analysis resulted in higher support reactions, moments, and wall displacements compared to the LEM and NL methods. This is due to its consideration of constitutive soil laws and the interaction between the soil and structure.
FEM predicted noticeable movements at the base of the excavation walls, which were not captured in the NL analysis, providing additional insights into the effects of bracing failure in deep excavations.
Conclusion:
The loss of a bracing level in a multi-braced excavation can have significant implications for wall stability, especially in high-stress environments with adjacent structures or high surcharge loads. Each analysis method—LEM, NL, and FEM—offers unique insights into the behavior of the excavation system under such conditions. While LEM provides a simplified approach, NL and FEM offer more detailed and accurate evaluations, especially when considering wall displacements and full soil-structure interaction.
DeepEX proves to be an invaluable tool in this context, providing geotechnical engineers with access to all three methods, each with its own advantages. By quickly simulating different scenarios and evaluating the results across multiple analysis methods, DeepEX allows engineers to design safer and more resilient excavation systems, even in emergency conditions.
If you're not yet using DeepEX, now is the time to explore its capabilities and see how it can help you ensure the safety of your projects while gaining a competitive edge in the industry.
