The  Helical Pile Temporary Support

 

One of the most challenging and creative solutions for steel lattice towers involves completely disconnecting and removing one of the four main components of the tower: the post leg. In some cases, a partial post leg replacement is the only viable approach to ensure the extended structural integrity of the tower. The structure relies on all four post legs and foundations for its stability, so the challenge is keeping stability once a leg is disconnected. A temporary support is therefore required to ensure the tower stability is not compromised during restoration. 

Heavy corrosion at the groundline is a common issue requiring post leg replacements, which affects both the foundation and post leg. Another common issue are members becoming mechanically damaged by right-of-way impacts (Figure 1). Prompt action is required in these cases to reduce the potential of structural failure.

Different lattice tower types utilize different foundations. For example, tangent structure foundations may consist of steel grillage members embedded directly in the soil, while larger structures like river-crossing and heavy angle strain towers usually require reinforced concrete foundations with steel base plates. In all cases, a disconnected post leg must be supported temporarily to ensure proper load transfer while the new post leg replacement or splice is installed. Temporary support will vary in their design capacity and geometrically with the type and orientation of the tower.

The temporary support for a tower with steel grillage foundations, shown in Figure 2, offers safe conditions for replacing damaged components. Most steel lattice structures built over 50 years ago will have a steel grillage foundation.  With these types of foundations, the post leg and diagonal members are bolted to a bent spade plate connected to the steel angle grillage members below grade. The number of grillage members may vary from one to four, but a four-legged setup is most commonly seen. If a post leg is damaged beyond simply splicing the member with new steel, the continuity with the steel foundation is likely compromised and a temporary support is required. Figure 2 shows the tripod-shaped support system designed to stabilize the structure and minimize deflections during reconstruction. This 3D support system is composed of three parts: steel helical piles, support columns, and top splice. The top splice will connect the system together and safely transfers the leg load to the ground through the support columns that are attached to the soil with steel helical piles. 

 

Designing and Installing a Temporary Support

To design the temporary support:

  1. A proper design load must be determined to correctly size the elements and connections. To obtain this design load, a construction or maintenance load combination must be calculated in conjunction with basic tower parameters like height of the tower, line angle, tower location, and conductor types. 
  2. Soil conditions are evaluated to determine how the helical piles will perform within the tower’s environment. Common properties to observe include depth of the water table, presence of sand or clays, soil rigidity, and differential soil layers. For project locations with unknown soil conditions, or heavily damaged structures, a site visit may be necessary. 

Once the parameters are evaluated, the temporary support is installed with these steps:

  1. The helical piles are designed for the appropriate helical shaft and plate sizes. 
  2. Minimum embedment depths are calculated for instances of tension or pullout from net uplift due to wind loads.
  3. Piles are drilled into the soil at a battered angle, adjacent to the damaged lattice tower leg or foundation. This angle minimizes local bending near the cap plate or top of the helical pile and allows the pile to act axially in tension or compression.  
  4. Following the base installation, the superstructure of the support is properly oriented so all three columns are adjoined at the defined connection support point, usually 10 feet above the groundline. Connections between components consist of bolted and threaded rod connections to properly transfer all loads without the use of field welding. As pictured in Figure 3, all components are working in unison to support the tower as a portion of the post leg is removed. 

 

Benefits

Temporary supports provide many benefits to utilities, including:

  • In most cases, the temporary support and the post leg splice or replacement can be completed within one working day in the field. This is an extremely efficient option that saves time and resources by preventing the need for a crane or a mechanically operated system to temporarily support the lattice tower. 
  • Temporary support provides versatility with tough geographical landscapes like valley-crossing towers with varying soil conditions. 
  • This process is streamlined for varying structure types, including larger towers consisting of large post legs. 
  • Previously, utilities may have been forced to close out a line for an extended time due to safety concerns, but this can now be minimized or prevented with an expedited restoration with the aid of a temporary support. 

New temporary supports are under continuous development and optimization by the Osmose Engineering team, with the goal that every steel pole and lattice tower type can be successfully supported and restored in the safest and most efficient way.

For more information, contact your local Osmose professional or email steel@osmose.com.