Transmission tower foundations are required to stabilize the towers by transferring the structural loads to the underground environment. They must be designed to resist movements such as settlement, uplift, and lateral displacement

 

Protecting underground steel grillage power transmission tower foundations

Underground corrosion of structures supporting power transmission and distribution (T&D) lines is the primary cause of in-service equipment degradation. Each year, utility companies allocate increased corrosion mitigation budgets to refurbish a large population of aging and corroded structures.

Accordingly, effective and economically feasible corrosion mitigation techniques, such as cathodic protection (CP) systems specifically designed for T&D structures, are in great demand.

Widely used conventional CP design methods are based mainly on empirical formulas and designer experience. Such design methods, although very useful, were primarily developed for piping systems and are not optimal for structures with more complex configurations. They fail to incorporate all design factors and often require the use of relatively large safety factors.

To address this issue, an electrochemical simulation tool was developed for designing efficient CP systems for buried components of transmission structures. Grillage-type foundations were selected to highlight some capabilities of the proposed approach because these types of foundations are common in transmission structures, and their geometrical irregularities (e.g., edges, holes, bends, and joints) pose a recognized CP design challenge that requires further investigations.

 

Steel Grillage Foundations

Transmission tower foundations are required to stabilize the towers by transferring the structural loads to the underground environment. They must be designed to resist movements such as settlement, uplift, and lateral displacement.

Among different types of foundations, steel grillage foundations are the preferred choice for four-legged lattice towers when footing conditions allow their application. Grillage foundations include a horizontal grillage base plate constructed from structural steel (usually galvanized angles, beams, and channels), and some vertical members, which are extensions of a tower leg. Common grillage foundation configurations that connect the tower leg to the grillage plate include pyramid supports, a single tubular member, and a single leg stub.

The main advantages of grillage foundations are their low cost and ease of installation. They are entirely shop-fabricated and typically can be purchased with the tower. Furthermore, grillage foundations require minimal installation time and allow immediate tower assembly. They usually do not need a concrete pour, so concrete-related transporting and curing time is not required.2

The major drawback of grillage foundations is the necessity of relatively deep excavations for their installation. Sometimes, due to variations in soil environments along the power line route, tower foundations need to be enlarged by pouring a concrete base around the grillage if actual soil conditions are not as good as those assumed in the original design. In addition, large grillages are difficult to set and require accurate adjustments for tower installation.2

 

Cathodic Protection Model

The primary goal of the proposed CP design tool is to determine detailed distribution of potential and current density (CD) on the surfaces of grillage foundations. Such information allows examination and optimization of the anode bed design so the structure can be sufficiently polarized in accordance with the NACE International criteria for CP.3

 

Conclusions

These simulations confirm that areas with geometric features (corners and edges) located in the vicinity of anodes receive the maximum protective current while flat surfaces, particularly when shielded, are least polarized/protected.

As a result of geometrical complexities, multiple anodes for CP of the grillage foundation are required. Furthermore, in soils with high resistivities, it is necessary to consider a greater number of anodes buried close to the structure (≲2 ft [0.6 m] away) to achieve a good level of protection.

For large grillage foundations, horizontally buried anodes are preferred to protect the horizontal members of the grillage, while vertically buried anodes are recommended for protection of vertical (leg) components. Nonetheless, it is always recommended to provide full CP to critical load-bearing members of the foundation—usually the legs—thus, a combination of vertical and horizontal anodes might be required.

For galvanized structures, the equilibrium potential of the structures gradually shifts toward electropositive values as the zinc layer is consumed and corrosion progresses into the steel substrate.

Accordingly, the design of CP systems for two identical galvanized foundations in the same soil environment depends on their age and the quality of the remaining galvanized coating.

Highly distributed anodes improve the performance of a CP system, but the higher construction costs should be considered.  (materialsperformance)