Cantilever Soldier Pile Wall Design
- Oct 13, 2023
- 2 min read
Updated: Jun 4
Cantilever soldier pile walls are commonly used for shallow temporary excavations where sufficient passive resistance can be mobilized below the excavation level and groundwater is not present above the final subgrade. If groundwater is encountered, dewatering or an alternative watertight support system may be required (Terzaghi et al., 1996).
These systems are typically practical for excavations up to 10 ft to 15 ft (3 m to 5 m), although greater depths may be possible in favorable soils with stiff soldier piles. In most cases, displacement control becomes the governing design issue before structural capacity is reached (FHWA, 1999).
Design Method
The most common preliminary design approach is the free earth support method, originally developed from classical earth pressure theory and beam-on-elastic-foundation concepts (Terzaghi, 1943; Peck et al., 1974).
In this method, active earth pressures are applied above the excavation grade on the retained side, while passive resistance is mobilized below the excavation level.
The required embedment is obtained by satisfying moment equilibrium about a point below the excavation. However, because the free earth method does not fully satisfy shear equilibrium, the calculated embedment is usually increased. Common practice is to increase the theoretical embedment by approximately 20% to 40%, followed by additional project-specific safety requirements (NAVFAC DM-7, 1982).
For soldier pile walls, the analysis should also account for three-dimensional arching effects between piles. Below the excavation level, passive resistance is mobilized around discrete pile elements rather than along a continuous wall. Therefore, pile spacing, flange width, drilled shaft diameter, and soil arching assumptions can significantly influence the calculated embedment and pile forces.
Wall Displacements
Cantilever soldier pile walls can experience relatively large lateral movements because no anchors or struts are present to restrain the wall. Wall flexibility increases rapidly with excavation depth, especially in softer soils.
For displacement estimates, some transportation agencies and empirical design approaches use a semi-empirical virtual fixity point, commonly taken at approximately 25% of the required embedment below excavation grade (FHWA, 1999). Although this adjustment is not rigorously derived from theory, it often produces wall deflections that correlate more closely with measured field performance.
Structural Design
The soldier pile section is selected based on the maximum bending moment obtained from the lateral earth pressure analysis.
For steel soldier piles using allowable stress design, the required section modulus may be estimated as:
For reinforced concrete soldier piles or drilled shafts, structural capacity should be checked according to the applicable reinforced concrete design code, considering bending, shear, and axial interaction requirements.
Modeling Recommendations
Reliable cantilever soldier pile wall analysis should consider:
Active and passive earth pressure assumptions
Groundwater position and possible need for dewatering
Pile spacing and 3D arching effects
Embedment increase beyond theoretical equilibrium depth
Wall stiffness and displacement limitations
Surcharge loads near the excavation
Structural bending and shear capacity
Serviceability performance in nearby structures
Advanced numerical tools such as the DeepEX platform can combine limit equilibrium methods, nonlinear soil springs, and staged excavation analysis to evaluate both stability and wall deformation behavior.
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