Is it possible to adjust the impact loads for deadman walls?
- May 19
- 3 min read
In shoring design, systems such as stepped excavations or closely spaced deadman walls function as an integrated unit. The analysis must account for the critical interaction between these walls. When the walls are in close proximity, the passive resistance generated by the rear wall influences the active pressures acting on the front wall. The confined soil zone between them cannot fully mobilize the theoretical passive capacity. A portion of this passive pressure is transferred as an additional impact load on the active side of the front wall. Accurately modelling this load transfer is essential for achieving a safe and optimized design, and DeepEX software provides advanced options to simulate this complex soil-structure interaction effectively.
Default Software Behaviour (Simplified Approach)
By default, the software calculates the full passive pressure (Kp) on the back wall. A portion of this pressure is then applied as an additional active-side load on the front wall. The magnitude of this transferred load is automatically reduced by a factor based on the distance between the walls. The closer the walls are, the greater the transferred load.
Real-World Behaviour vs. Model It is important to recognize that this is a simplified model. The passive pressure on the back wall is rarely fully mobilized to its theoretical Kp value; the actual earth pressure is often closer to an "at-rest" (Ko) condition. Consequently, the default calculation often yields a conservative result.
User Control: The Impact Load Adjustment Factor
To provide control over this approximation, you can adjust the Impact Load Adjustment Factor. This factor directly scales the influence of the back wall's pressure on the front wall:
Factor = 0: No interaction. The walls are analysed independently, with no pressure transfer from the back wall to the front wall.
Factor = 1: Full interaction based on the model's logic. The full theoretical Kp pressure from the back wall is considered, with the distance-based reduction applied as described above.
The effect of this adjustment is illustrated in Figure 2. The figure presents the results of an example analysis, contrasting a scenario with a factor of 0 with one using a factor of 0.665. This comparison clearly shows the impact of the adjustment factor on the calculated pressures and moments.
The user can select a value between 0 and 1 to fine-tune the analysis based on your engineering judgment and project-specific conditions. A value less than 1.0 will reduce the conservatism of the default calculation.
Conclusion
In summary, accurately modelling the interaction between closely spaced walls in stepped excavations or deadman systems is crucial for a safe and efficient design. DeepEX provides a practical, simplified method for this by transferring a portion of the back wall's passive pressure to the front wall as an impact load.
The key takeaway is the Impact Load Adjustment Factor, which gives the engineer direct control over this interaction. Understanding that the default calculation can be conservative is essential. By using engineering judgment, informed by factors such as wall spacing, soil properties, and the calculated factor of safety of the back wall, you can calibrate this factor to achieve a more realistic and optimized design.
Ultimately, this feature ensures that the analysis reflects the complex soil-structure interaction, bridging the gap between simplified theoretical models and real-world behaviour. Always validate the chosen factor by reviewing the resulting bending moments, shear forces, and factors of safety to ensure a robust final design.
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