Analyzing excavation stability is critical to prevent unwanted structural failures and maintain project schedules. Numerical Modelling is an essential tool for doing this. However, it is necessary to understand that modeling results should always be compared with engineering judgment. This is particularly true for complex geotechnical problems.
Risk Assessment
First, try to eliminate risks – but if that is not reasonably practicable, minimize them using administrative controls, such as warning signs and suitable personal protective equipment. This includes a safe system of work and regular workplace inspections.
Ensure the excavation is adequately fenced and labeled, including areas with buried services and structures. Keep people and plants away from excavation edges, using physical barriers that can withstand falling loads.
Use shoring designed for specific soil load profiles and excavation depths (e.g., slide-rail shoring) if possible. Consider groundwater’s impact on the excavation’s stability using suitable drainage arrangements. Keep utilities well clear, stringing independently supported flag bunting 1.5 m below the span and consulting the utility owner to get their consent before working near them. Also, consider the risk of workers being struck by overhead or underground services. This can be mitigated by carefully planning the work and identifying all services before digging, stringing flag bunting, and, where necessary, using an approved temporary traffic management plan.
Numerical Modelling in excavation processes provides invaluable insights by simulating and predicting ground behavior, helping engineers anticipate potential challenges and optimize construction designs. By employing numerical modeling techniques like finite element analysis, construction teams can enhance safety, minimize risks, and efficiently plan excavation projects, ultimately leading to more cost-effective and well-executed outcomes.
Stability Assessment
Deep excavations are often needed for construction purposes such as underground utilities, shafts, and tunnels. Such excavations require thorough site investigations, engineering design, groundwater control, and slope stabilization to ensure the integrity of the excavations.
Stability assessments can be conducted using various analytical methods, including numerical modeling. However, the results of these analyses should be interpreted in combination with expert engineering judgment. Moreover, the resulting predictions should be checked with known and physically expected outcomes.
Utilizing slope stability analysis offers a proactive approach to identifying potential slope failures, allowing engineers to implement preventive measures and ensure the safety of construction sites. By accurately assessing and managing slope stability through numerical modeling, construction teams can optimize excavation designs, reduce the risk of landslides, and enhance the overall efficiency and safety of the project.
PCBUs should eliminate risks if reasonably practicable, such as installing warning signs or providing PPE such as hard hats and hearing protectors. If elimination is not possible, then risk should be minimized by implementing administrative controls and engineering controls such as benching and battering excavation sides to a safe angle of repose. In addition, monitoring systems can be deployed to detect deformation and trigger appropriate responses in case of a problem. These are commonly comprised of inclinometers, piezometers, and settlement gauges.
Design Optimization
Numerical models enable engineers to test different scenarios and identify potential stability risks early in the design phase, reducing the need for costly physical prototypes and trial-and-error approaches. They also provide a more accurate understanding of failure mechanisms and allow engineers to optimize designs, minimizing risk.
Using FEA, it is possible to determine the optimal embedment depth for the sheet pile wall and strut profiles to reduce stress concentrations ahead of excavations. This is achieved by modifying the model geometry in several calculation phases until the optimum value is found.
The plan view results shown in Figure 2 demonstrate the effects of the various analysis options used in this study. As expected from the colored square plots, surface effect stress and multiple sewists were zero along the cross-sections corresponding to the tunnels and interconnections.
Risk Mitigation
Numerical models are more accurate, convenient, and less costly for analyzing redistribution stresses in excavation environments than traditional methods. They also provide higher flexibility, improved safety, and time-saving benefits.
For instance, a study on the effects of FEM and FDM models on stress redistribution in longwall panels showed that they modeled well the s1 stresses observed at the URL test tunnel. However, these models do not capture the s3 stress near the excavation edges.
The FDEM model uses a discrete fracture network to simulate faults in brittle rock masses. This enables the simulation of the occurrence of faults in a mined rock mass and the prediction of their effect on stress distribution. Furthermore, it allows the calculation of a much more extensive displacement range compared to a FEM model with the exact computational resolution while retaining accuracy and speed. This approach is, therefore, ideal for assessing stability risks in deep excavations.
