A. Significance of Construction Staging
Deep excavations always require staged construction, since even the wall construction can affect the performance.
One of the most common mistakes in deep excavation design practice, is that often only the last construction stage is considered and analyzed. The last stage is not always the most critical. Significant support reactions and displacements can occur in earlier construction stages, and if this is not taken into consideration, it can lead to severe underestimation and under-design of the wall and support structural sections. This can affect the total project cost and lead to severe cost increases and even legal consequences.
In addition, the detailed construction staging can lead to more realistic analysis results, when more advanced stage-dependent methods like the Beam on Elastoplastic Foundations or the Finite Element Method are utilized. In both FEM and NL methods, the first step is to adequately define the initial “at-rest” conditions. Then the full construction staging for excavation, support installation, and backfill operations needs to be defined. The results for each construction stage strongly depend on the previous construction stage, as it considers the prior stage soil deformations, stresses, wall displacements, developed moments, and more.
Figure: Typical Construction Stages in an Anchored Wall Model
B. Construction Stages in DeepEX Software
In DeepEX, we should always start from an initial stage with at-rest conditions (no excavation), and then start adding design stages and perform design steps, trying to simulate the construction procedure (perform excavation up to the first support level, install support level, perform a further excavation or a backfill operation etc.).
The construction stages can be graphically added in any deep excavation model. The DeepEX model wizard can create all the construction stages automatically.
Figure: Construction Stage Options in DeepEX
Additional stages can be added after the final stage, and these can be used to check different soil pressure methods, seismic conditions and more. This can be proven a valuable feature since the designer can check the effect of different scientific methods on the same model.
DeepEX will run the analysis and present the results of each construction stage in tables and graphically on the model area. This way, we can identify the most critical stage, which is not always the last one. All structural items will be designed from the most critical stage results. The recognition of the most critical stage allows us to decide to take actions (edit support locations, structural sections etc.) to achieve the most efficient model optimization.
C. Construction Staging Examples - Models and Result Graphs
Example 1: Cofferdam Design – Construction Stages
Figure: Construction Stages of a Sheet Pile Cofferdam
Example 2: Temporary Excavation Walls and Permanent Structure
Figure: Construction Stages of the Temporary Excavation Wall System and the Permanent Interior Structure (Basement Walls and Slabs).
Example 3: Analysis Results in Different Construction Stages
Figure: Wall Moment and Shear Diagrams, Support Reaction in (a) Stage 3 and (b) Stage 5
Figure: Soil Pressures and Support Reactions (a) Stage 5 – FHWA Pressures and (b) Stage 6 – Peck Apparent Pressures
Figure: Non-Linear Analysis: Wall Moment and Displacement Diagrams, Support Reactions (a) Stage 3 and (b) Stage 5
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