Before I started writing and learning about the utility industry, I honestly never really noticed the difference, nor did I actually know the difference, in transmission poles or how they were installed. Now, a year and eight months later, I catch myself scurrying to get my iPhone out while driving down the highway to take pictures of tapered tubular davit arms or dead-end h-frame structures.
Have you ever paid attention to the way a transmission pole was installed? I’m sure if you’re not an engineer or someone in the utility industry your answer would be no, but for those of you who are, have you noticed the foundation method? Do you know the different methods?
Well, in the ASCE 48-11, Design of Steel Transmission Pole Structures, three specific methods used to place a steel transmission pole into the ground are pointed out:
1. Drilled Shaft Foundation with Anchor Bolts
2. Direct-Embedded Foundation
3. Embedded Casing Foundation
There are also other methods such as spread, pile, rock anchor foundations, etc. that can be used for more specific applications. But the two that I want to focus on are drilled shaft foundation (also known as drilled pier foundation), and direct-embedded.
When deciding on which method is best suited, there are some considerations that should be addressed in initial design as well as restrictions to pay attention to. Things like type of structure, importance of structure, allowable foundation movement or rotation and geological conditions are important and shouldn’t be overlooked.
Direct Embedded Poles:
- Tends to be more economical over concrete foundation because it essentially just requires digging a hole, dropping the pole into the ground and then backfilling it with rock, concrete or other specified backfill.
- Typically used for tangent and light angle structures where the overturning moments are smaller.
- As loads get larger, embedding a pole becomes less favorable because they are solely using the pressure of the specified backfill to resist the pole from coming out of the ground.
Drilled Pier Foundation:
- After the hole is dug into the ground, a combination of reinforcing steel and anchor bolts are lowered in place followed by concrete.
- Typically used in medium to heavy angle structures as well as dead-end steel structures.
- The massive weight of the concrete that is in the ground is larger in diameter than the pole, so it can engage more soil, as well as have a greater bending force at the base.
Other things to consider when selecting foundation types include:
- Soil properties
- Foundation loads
- Design limitations
- Equipment availability and accessibility
- Environmental restrictions