When to Use Orthotropic Plates for Secant Pile Walls in PLAXIS 3D


Application PLAXIS 3D
Version All supported versions
Date created 11 September 2026
Date modified 11 September 2026
Original author Vasileios Basas - Technical Support Group
Keywords PLAXIS, PLAXIS 3D, secant pile wall, orthotropic plate, isotropic plate, plate stiffness, retaining wall, hard/soft, hard/firm, hard/hard

1. Introduction

Secant pile walls are commonly used retaining systems in which reinforced concrete secondary piles interlock with primary piles of lower stiffness, typically plain concrete or soil-cement, to form a continuous and water-tight retaining structure. CIRIA C760 Appendix A3.5 describes hard/soft, hard/firm, and hard/hard secant wall variants.

Typical secant pile wall with alternating primary and secondary piles

Figure 1. Typical secant pile wall geometry showing alternating primary and secondary piles and key geometric parameters.

In parts of the secant wall literature, secondary and primary piles are also referred to as male and female piles, respectively. This article uses secondary pile and primary pile terminology throughout because it is clearer and avoids ambiguity when discussing hard/soft, hard/firm, and hard/hard walls.

In PLAXIS 3D, secant pile walls are often represented using plate elements. A conventional approach is to assign an isotropic plate stiffness based on the flexural rigidity of the reinforced secondary piles, consistent with the type of expression given in CIRIA C760 Table 4.2 for soil-structure interaction analysis. This can be appropriate for hard/hard walls and for preliminary assessments, but it can overestimate bending stiffness in the horizontal direction for hard/soft and hard/firm walls where a strong stiffness contrast exists between the pile types.

Conceptual comparison of isotropic and orthotropic plate behaviour

Figure 2. Conceptual difference between isotropic and orthotropic plate behaviour in PLAXIS 3D.

2. Why an Isotropic Plate May Be Insufficient

A PLAXIS 3D plate resists bending in two local directions. If a single isotropic Young's modulus is used, the same stiffness governs both directions. For a secant wall this may be too restrictive because the wall's vertical and horizontal bending mechanisms are not equivalent:

For hard/soft and hard/firm walls, calibrating one isotropic plate material to the secondary-pile vertical bending stiffness may therefore overestimate the wall stiffness in the horizontal direction. An orthotropic plate provides a practical way to assign different stiffnesses in the two principal plate directions while still using a continuous structural element.

Important modelling note: There is no unique equivalent orthotropic plate for a secant pile wall. The wall is a discrete periodic structure, not a true continuum. Any equivalent plate representation requires engineering assumptions about how the discrete pile geometry is smeared into continuous stiffness parameters.

3. Scope and Limitations

The orthotropic plate approach is intended as an engineering equivalent-continuum method for global 3D modelling. It aims to preserve the dominant flexural behaviour of the wall while keeping the model practical for routine PLAXIS 3D analyses. Users should apply the approach with the following limitations in mind:

Limitation Consequence
No unique equivalent plate Different homogenisation or averaging assumptions can produce different equivalent stiffnesses. The values used in this workflow are a defensible engineering estimate, not a unique solution.
EA/EI inconsistency PLAXIS plate stiffness depends on both EA = E·d and EI = E·d3/12. For a selected equivalent thickness, one E value cannot match axial and bending stiffness exactly. The method prioritises bending stiffness.
Local pile forces are not reproduced A smeared plate cannot provide individual pile moments, shear forces, or interlock forces.
Shear stiffness is uncertain G12, G13, and G23 cannot be derived robustly without formal homogenisation. Engineering estimates and sensitivity checks are recommended.
Composite action is assumed The approach assumes sufficient interaction between adjacent piles to transfer actions. If interlock is limited mainly to compressive contact, horizontal stiffness may be lower.

4. When the Approach Is Useful

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Application Recommended? Comment
Global excavation deformation, SLS Yes Primary use case; directional stiffness can influence wall deflection and ground settlement in 3D models.
Preliminary and parametric design Yes Efficient for sensitivity studies on the influence of wall stiffness.
3D corners, re-entrant geometry, changes in alignment Yes Horizontal stiffness can influence load redistribution along the wall.
ULS bending moments and prop forces With caution Global force distribution may be useful, but individual pile design forces should not be extracted from a smeared plate model.
Local pile forces and interlock behaviour No Use explicit pile modelling, embedded beams, or volume elements where pile-level response is required.
Detailed structural design of the piles No Section design requires pile-level actions, not smeared plate output.
Hard/hard secant walls Usually not necessary When both pile types have similar stiffness, the conventional isotropic approach is usually sufficient.

5. Alternative Modelling Approaches

Before adopting the orthotropic plate approach, users should consider whether simpler or more rigorous alternatives are more appropriate for their purpose:

Approach Advantages Limitations Typical use
Isotropic plate Simple, established, familiar to practitioners. No directional stiffness; may overestimate horizontal stiffness for hard/soft or hard/firm walls. Preliminary design, hard/hard walls, 2D benchmarking.
Orthotropic plate Captures directional stiffness contrast while retaining a practical continuous wall element. Engineering approximation; shear parameters uncertain; local pile forces unavailable. 3D global analysis, corners, irregular geometry, sensitivity studies.
Embedded beam elements Represents discrete pile stiffness more explicitly. Higher modelling effort; interaction and spacing need careful definition. Detailed 3D analysis where pile-level forces are useful.
Volume elements Highest fidelity and local stress representation. Computationally expensive and mesh-intensive. Research, forensic checks, validation of simplified approaches.

6. Recommended Decision Workflow

  1. Start with the modelling objective. If the purpose is global deformation or force redistribution in 3D, a smeared plate may be appropriate. If pile-level actions are required, use a more explicit modelling approach.
  2. Identify the secant wall type. For hard/hard walls the isotropic C760-based approach is normally sufficient. For hard/firm and hard/soft walls, orthotropic stiffness may be important.
  3. Check whether 3D effects matter. Orthotropic stiffness is most relevant near corners, re-entrant geometry, changes in wall alignment, openings, or nonuniform supports.
  4. Estimate and assess the directional parameters. Use the companion calculation workflow to estimate E1 and E2, then perform sensitivity studies when stiffness contrast or shear response is important.
  5. Verify the local plate axes. Before interpreting results, verify the local plate axes in PLAXIS 3D. Direction 1 and direction 2 must correspond to the intended vertical and horizontal directions, or the stiffnesses must be swapped or the plate orientation corrected.

Companion calculation article: The companion article provides the detailed equations for estimating E1, E2, ν12, G12, G13, and G23 for PLAXIS 3D input, together with a worked hard/firm secant-wall example. Open the companion calculation article.

7. References

 

See also