Understanding Strut–Waler Loads in Excavation and Tunneling Projects
Sepco Consulting Engineers provides licensed structural engineering services across Toronto and the Greater Toronto Area — including North York, Scarborough, Markham, Richmond Hill, Vaughan, Mississauga, Brampton, Etobicoke, and surrounding regions.
A Comprehensive Look at How Hidden Forces Shape the Design of Temporary Support Systems
1. Introduction
Deep excavations for high-rise buildings, underground parking structures, and increasingly for tunneling and subway line construction demand advanced temporary support systems to control soil movement and maintain stability.
Whether constructing a 100-foot-deep basement in an urban core or a metro station box beneath a busy street, the challenge is the same: resist enormous lateral earth and groundwater pressures safely and efficiently.
Among the various systems used, internally braced excavations—built with combinations of walers and struts—remain a mainstay of temporary support engineering.
Though conceptually simple, these systems involve complex load interactions. The strut–waler connection, in particular, must not only transfer massive compressive loads but also withstand bending, shear, torsion, and even occasional tension due to real-world effects like wall deflection, temperature changes, and excavation sequencing.
This paper explores these load effects in depth—identifying where they come from, how they behave, and how to design connections that perform reliably in both deep excavation and tunneling applications.
2. Anatomy of a Braced Excavation System
A braced excavation resists soil and water pressure through internal framing rather than external anchors. Its main components include:
Retaining Wall: Diaphragm walls, secant piles, or soldier pile systems that resist soil and groundwater pressure.
Walers: Horizontal steel beams or box sections that collect and distribute loads between struts along the wall face.
Struts: Large compression members—often pipes or box sections—that span between opposing walers, installed progressively as the excavation deepens.
Connections: Interfaces between each strut and its waler, responsible for safely transferring the compressive and secondary loads.
In tunneling and subway construction, similar principles apply. Struts (or temporary ribs) brace excavation walls or tunnel linings against inward soil pressure. However, the geometry is often curved or irregular, introducing additional three-dimensional effects in the connections.
3. The Ideal Load Path — and Why Reality Deviates
In an ideal design:
Lateral soil pressure acts uniformly on the retaining wall.
The wall transmits this pressure evenly to the waler.
The waler transfers balanced reactions to the struts.
The struts work in pure compression, pushing equally on both sides.
In reality, however, construction tolerances, soil variability, temperature shifts, and wall movement introduce secondary forces.
The strut–waler connection becomes the focal point of these imperfections, experiencing shear, bending, torsion, and tension superimposed on the dominant compression.
4. Primary and Secondary Loads on the Connection
| Load Type | Direction | Typical Source | Design Relevance |
|---|---|---|---|
| Axial Compression | Along strut axis | Earth pressure |  Critical |
| Shear | Parallel to waler face | Misalignment, wall drift |  Moderate |
| Moment (Bending) | About strut–waler interface | Eccentricity, wall rotation |  Moderate |
| Tension | Opposite to compression | Thermal or rebound effects | Occasional |
| Torsion | About waler axis | Off-center loading |  Low–Moderate |
4.1 Axial Compression — The Dominant Force
Typical strut compression loads in deep excavations (30 m / 100 ft) range from 3,000–8,000 kN.
This load transfers through the connection into the waler via bearing plates and gussets. Even minor eccentricities can amplify stress dramatically.
4.2 Shear Forces — Small but Persistent
Shear arises from small alignment errors, wall drift, or friction effects.
Usually 1–5% of the axial force, i.e., 50–250 kN for a 5,000 kN strut—still large enough to warrant explicit checking for bolt and bearing capacity.
4.3 Bending Moments — The Hidden Amplifier
Any offset between the strut’s load axis and waler centroid produces bending:
M=P×e
For a 5,000 kN strut:
e = 50 mm → M = 250 kN·m
e = 100 mm → M = 500 kN·m
This moment can induce local yielding or prying in the connection plate if not properly stiffened.
4.4 Tension — The Unexpected Reversal
Though designed for compression, tension can occur due to:
Thermal contraction (steel shortening in cold weather).
Wall rebound after unloading or groundwater drawdown.
Uneven excavation or differential wall stiffness.
Buckling or eccentric rotation during strut adjustment.
These reversals are often small (≤3% of axial load) but can cause fatigue or bolt uplift if ignored.
4.5 Torsion — The Subtle Twister
Torsion occurs when loads act off-center, particularly in multi-waler or asymmetrical configurations.
Although usually 5–20% of the concurrent bending moment, it can lead to waler twisting and local plate rotation.
5. Load Effects Unique to Tunneling and Subway Excavations
In tunneling, several additional effects arise:
Elliptical Geometry:
Nonlinear load paths create combined axial and bending stresses.Sequential Excavation (NATM/SEM):
Changing boundary conditions cause cyclic load reversals on temporary struts.Ground Loss or Over break:
Uneven soil support alters pressure distribution on the waler system.Dynamic and Fatigue Effects:
Subway vibration or adjacent construction induces small cyclic loads.Environmental Influences:
High humidity accelerates corrosion, reducing friction in bolted joints.
These effects make tunneling connections inherently more three-dimensional and time-dependent than in open excavations.
6. Real-World Factors Amplifying Connection Forces
| Factor | Mechanism | Impact on Connection |
|---|---|---|
| Fabrication Tolerances | Hole offset, misaligned plates | Shear, bending |
| Erection Tolerances | Rotation during setup | Eccentric load |
| Thermal Variation | Expansion/contraction | Axial change, tension |
| Soil Behavior | Creep or rebound | Moment, tension |
| Groundwater Fluctuation | Pressure changes | Wall rebound |
| Differential Settlement | Uneven displacement | Shear + bending |
| Strut Buckling | Local instability | Eccentric loading |
| Bolt Slip or Relaxation | Loss of preload | Increased shear demand |
| Corrosion | Friction reduction | Connection slip |
Many of these effects represent deviations in the assumed load path, where forces redistribute through stiffness, eccentricity, and unintended restraint rather than following the idealized design intent.
7. Design Implications
7.1 Design Philosophy
A robust connection must:
Carry high compressive load safely, and
Accommodate inevitable secondary effects like rotation and thermal movement.
Assuming purely axial behavior is unrealistic; good design embraces controlled flexibility.
7.2 Load Combination Guidelines
Approximate design relationships for preliminary checks:
Where Nc= compressive axial force and e= eccentricity (50–150 mm typical).
7.3 Detailing Recommendations
Align centroids of strut and waler.
Use double walers to reduce torsion.
Provide gussets and stiffeners at load transfer zones.
Design bolts/welds for small tension reversals.
Use slip-critical bolts or preloaded fasteners.
Allow limited movement (slotted holes or spherical seats).
Check bearing and block shear capacities.
Account for temperature and erection sequence in design.
Verify field installation quality.
Monitor during excavation with strain gauges or load cells.
8. Industry Case Insights
Documented case studies from major deep excavation and tunneling projects worldwide have highlighted the sensitivity of strut–waler connections to secondary load effects. Several recurring patterns emerge:
Temperature-Induced Load Reversals:
Projects in colder climates have reported measurable tension spikes in upper-level struts during seasonal temperature drops. Designers mitigated these effects using slotted connections or re-torqued bolts to accommodate thermal movement.Eccentric Loading from Wall Movement:
In excavations through soft clays, differential wall movement has caused eccentric bearing on walers, leading to localized plate deformation or weld cracking. Retrofit stiffeners and additional seat plates were often added to restore stiffness and load uniformity.Cyclic Load Reversal in Sequential Excavations:
In sequential or staged tunneling (e.g., SEM/NATM methods), temporary struts have experienced alternating compression and tension due to progressive excavation. Semi-flexible pinned connections and thicker gusset details proved effective in accommodating repeated cycles without fatigue damage.
These documented experiences reinforce that even when designed as compression-only components, strut–waler connections must tolerate small reversals and eccentricities without distress. The success of such systems depends not only on design precision but also on careful monitoring and adaptability during construction.
9. Construction and Monitoring Best Practices
Preload Verification: Calibrate jacks and record strut loads during installation.
Temperature Tracking: Monitor steel temperature and account for seasonal effects.
Deflection Monitoring: Use inclinometers and total stations to measure wall and strut movements.
Inspection: Watch for weld cracks, bolt slip, or bearing plate deformation.
Adjustment: Re-tension or shim as needed if large reversals occur.
Removal Sequence: Follow engineered staging to prevent unbalanced loads.
10. Digital Modeling and Real-Time Feedback
Finite element modeling (FEM) can predict soil–structure interaction and strut forces across excavation stages.
However, the connection zone remains highly localized and nonlinear—better modeled with spring or hinge approximations.
Modern digital twin platforms now integrate sensor data (load cells, inclinometers) to refine connection response, enabling adaptive safety margins and material optimization.
11. Sustainability and Reuse
Steel struts and walers are often reused across multiple projects.
Repeated load cycling—especially with tension reversals—can affect bolt threads and weld fatigue life.
Routine inspection, reconditioning, and certification before reuse are essential.
Designing modular, bolted connections enhances deconstructability and aligns with circular construction goals.
12. Conclusion
The strut–waler connection may be a small part of the overall shoring system, but it carries enormous structural responsibility.
In both deep excavations and tunneling projects, its performance governs the safety of workers, the stability of the excavation, and the protection of adjacent infrastructure.
Real-world performance depends not just on axial strength but on acknowledging and accommodating imperfection—temperature shifts, wall rebound, alignment tolerances, and construction sequence.
By treating the connection as a multi-force, adaptable component rather than a purely axial joint, engineers can ensure resilient, predictable, and sustainable excavation support systems.