Understanding Wall-to-Wall Interaction in Deep Excavation Systems
- deepexcavation
- May 28
- 4 min read
Passive Pressure Interference and Impact Load Modeling in DeepEX
In deep excavation projects where multiple retaining walls are used—such as in stepped excavations, tieback walls with deadman anchors, or multi-level shoring systems—engineers must account for the complex interaction between these structural elements. A critical phenomenon in such systems is the interaction between the passive pressure zone of a back (deadman) wall and the active pressure zone of a front retaining wall.
This interaction can significantly alter the load distribution on the front wall and affect the safety and performance of the excavation support system. Understanding this behavior, and knowing how to model it effectively in software like DeepEX, is essential for safe and efficient design.
Figure 1: A-Frame deadman wall model in DeepEX software
Why Does Interaction Between Walls Occur?
In geotechnical design, retaining walls are supported by a combination of active, passive, and neutral earth pressures. When two walls are constructed in proximity—as is the case with a front wall supported by a rear deadman wall—the soil between them becomes a shared zone of influence. This overlap causes several effects:
The passive resistance zone of the back wall (deadman) requires room to mobilize full strength. This typically involves a wedge of soil in front of the deadman wall being displaced.
If the front wall is located within or near this passive wedge, it blocks or interferes with the soil's ability to move and develop passive resistance fully.
As a result, not all of the passive pressure behind the deadman wall is developed, and some of the "unrealized" resistance is effectively transferred forward, increasing the load that the front wall must support.
This manifests as an "impact load" on the front wall—essentially an artificial increase in the active earth pressure due to back wall interference.
The closer the walls are, or the taller the back wall is, the greater the likelihood and magnitude of this interaction.
Figure 2: Deadman system in DeepEX – LEM analysis active pressures with impact load
How DeepEX Models This Interaction
1. Limit Equilibrium Method (LEM) and LEM+Nonlinear (NL)
In DeepEX, the interaction between two walls is considered when using LEM or LEM+NL analysis methods. When the back wall's passive resistance cannot be fully mobilized, the software calculates an additional impact load on the front wall. This load is based on:
Geometrical factors (wall spacing, wall height)
Soil strength and pressure conditions
User-defined impact adjustment factors
This load is superimposed onto the active pressure diagram of the front wall, increasing the total demand on the wall and its supports (e.g., struts or anchors). This is a simplified but practical way to approximate the interference between walls.
However, this approach does not inherently reduce the impact load based on the actual passive capacity of the back wall. In reality, the deadman wall will resist part of the load based on its embedment and passive soil strength. DeepEX provides a tool to refine this:
Impact Load Adjustment Factor (ILAF)
Users can manually adjust the additional load by applying a semi-empirical reduction factor.
A common approach is to calculate the Factor of Safety for Passive Resistance (FS_passive) of the back wall and use at most:
ILAF= 1/FS_passive
For example, if the FS_passive = 2.0, then 50% of the calculated impact load would be transferred to the front wall.
This adjustment brings the analysis closer to physical behavior while maintaining the efficiency of LEM.
Figure 3: Impact load adjustment factor in DeepEX
2. Finite Element Method (FEM) in DeepEX
For more advanced and realistic modeling, DeepEX also includes a Finite Element Method (FEM) module, which is capable of automatically and continuously simulating the interaction between walls through the full soil continuum.
In FEM:
The soil mass is modeled as a continuous deformable medium, discretized into elements connected by soil constitutive models.
Both walls are part of the global equilibrium system.
Soil-structure interaction is captured in full, including:
Overlapping pressure zones
Redistribution of stresses
Soil blockages and wall stiffness effects
The software inherently determines how much passive pressure is developed at the back wall and how much, if any, contributes to added loading on the front wall.
Figure 4: Deadman wall system – FEM analysis in DeepEX software
Unlike LEM, no manual adjustment is necessary in FEM. The interaction effect is automatically accounted for based on the geometry, soil properties, wall configuration, and loading conditions.
This method provides higher fidelity results and is especially beneficial when:
Wall spacing is tight
Soil behavior is non-linear
Load-deformation response is critical (e.g., in urban excavations)
Summary and Best Practices
Analysis Method | Interaction Modeled? | Automatic Load Transfer? | Manual Adjustment Needed? |
LEM / LEM+NL | Yes (impact load applied) | No | Yes (via ILAF) |
FEM | Yes (continuously modeled) | Yes | No |
In complex excavation systems, particularly where deadman walls or multiple rows of supports are used, understanding the interaction between walls is not optional—it is fundamental. Engineers must consider whether simplified approaches like LEM are sufficient or if detailed FEM modeling is warranted.
DeepEX empowers engineers with tools for both approaches, giving flexibility and control based on project needs, available data, and design objectives.
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