Deadman Walls - Adjustment of Impact Load
In this article we will present an example of a deadman sheet pile wall, analyzed with different analysis methods - Limit Equilibrium, Non-Linear and Finite Element Analysis. We will investigate how the position of the deadman wall affects the additional surcharge that is transfered from the passive side of the deadman to the active side of the front wall as an impact load, and we will present an empirical method to reduce this impact load, using an adjustment factor.
A. Introduction - Project Description
This design example presents a case with a deadman wall suystem, supporting a 15ft excavation. Two sand soil layers are considered, and the water table is originally set to El: 0. The front sheet pile wall (AZ 17) is tied to the deadman wall (sheet piles AZ 17), with the use of a tierod row (#18, Grade 60 bars @ 5ft spacing).
The model will be created and analyzed with the tools of DeepEX - shoring design software. In DeepEX, we can create all construction stages. The software presents extensive results for each construction stage, allowing us to review the full project design procedure, and optimize the model in the most effective way.
The following figures present the project parameters, as well as, the modeling of the construction stages.
B. Deadman Wall Analysis Results
DeepEX can perform full structural and geotechnical design ot all project items (walls and supports). The calculated results (soil and water pressures, wall moment, shear and displacement diagrams, support reactions, structural and geotechnical check ratios and more) can be presented both in tables and graphically on the model area. DeepEX can analyze and present results for all construction stages, allowing the designer to review the full project history and take important descisions for the most efficient model optimization.
In this example we have analyzed the deadman wall model, with all analysis methods available in DeepEX software - Limit Equilibrium Analysis, Non-Linear Analysis (Soil Springs) - Finite Element Analysis. The following images present some of the calculated graphs.
C. Deadman Wall - Impact Load & Adjustment Factor
In deadman wall systems, a part of the passive pressures of the back, deadman wall is applied on the front wall active pressures as an additional impact load. This effect is capcured in both Limit Equilibrium Analysis and the combination method (Limit Equilibrium+Non-Linear). This impact load is affected by the deadman wall depth and by the distance between the deadman and the front wall.
In DeepEX, if we reduce the distance between the walls, we can see that the active pressures on the front wall are greatly increased, affecting all other calculated results. The following image presents the soil pressure diagrams for two different deadman positions:
The impact load can be reduced if we increase the distance between the deadman and front wall, or reduce the deadman wall depth. Also, we can modify the impact load adjustment factor. There is an empirical method to define the adjustment factor, that dictates us to review the passive resistance factor of the deadman wall, and use as adjustment factor the value 1/FSpassive.
In this example, we can see that a passive safety factor of 6.3 is calculated for the deadman wall. In this case, we could use an adjustment safety factor of 0.15. The following images present the procedure to estimate and apply the adjustment factor in the software, and the calculated results when the factor is used.
D. Conclusion - Results Comparison
The following image summarizes the results from all examined scenarios and analysis methods. From the results it is obvious that when the optimized adjusment factor is used in Limit Equilibrium and the combination method (Limit Equilibrium and Non-Linear Analysis), then the results can be comparable to the ones retrieved from the Finite Element Analysis, where this effect is automatically adjusted from the finite elements. We can understand that the adjustment of the impact loads is essential, for the software to achieve realistic results.
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