Both the CSA and NESC (and likely others) state that the foundation of the pole must be able to withstand the loads transferred to it by the pole. Neither provide much guidance however as to how to evaluate this requirement. To that end we offer some discussion.
The pole experiences various vertical loads plus lateral forces and moments from possibly any direction. The Soil Foundation is an important part of the restraining mechanism for the structure. Without it, the pole would simply fall over.
From a modeling perspective, almost all Pole Line Design software tools evaluate the pole structure without regard to its foundation. For those software tools that do consider the foundation's ability to support the pole, the required Soil Reactions are calculated assuming that the Soil Foundation is very rigid. This is generally accepted as providing a good estimate of the required loads for the Foundation to withstand and be tested for its ability to provide support.
The following are the major reactions that should be evaluated as part of the Foundation strength requirement:
- Vertical Load Capacity: The soil directly beneathe the pole's butt must not fail under Vertical Load.
- Lateral Load Capacity: The soil must hold the pole vertical while it is holding the required amount of lateral forces and moments at the groundline.
- Uplift Capacity: In cases when the pole is subjected to a net upwards vertical force, the soil's ability to restrain the pole must be evaluated.
Each reaction test will be discussed separately so as to describe how these calculations can be performed.
Vertical Load Capacity
Poles can be subjected to large vertical loads under heavy guying conditions. This net vertical force onto the pole can be significant enough to cause the soil to fail. By failure we mean that the soil failed to restrict the pole from moving under the required load. The common definition of soil failure is when the soil "gives" more than 1 inch (25mm).
There are several methodologies and theories available to evaluate this requirement. The most available come from evaluation of shallow footings.
Lateral Load Capacity
Particularly for Unguyed poles but others as well, there can be significant groundline moments and forces required for the soil to support. Since soil as a material has certain properties such as cohesion, shear strength, friction, it is reasonable to expect that strong groundline moments may required deeper setting depths or stronger soil.
Here are the most common approaches to evaluating lateral soil capacity, in order of increasing accuracy:
- Is Depth of Setting equal to or better than 10% of the pole's length plus two feet (0.61m)?
- RUS Bulletin 1724E-200, chapter 12. Tests categorize soils as good, average and poor.
- Effective Stress Analysis methods such as those proposed by Broms and Flemming. Uses soil parameters such as soil friction angle and undrained shear strength.
- P - Y curves approach corelating pole deflections to corresponding soil reactions at all depths.
- Finite Element Analysis techniques.
Anyone that has ever seen the stub of a pole being pulled out of the ground by a truck with a boom winch, knows that the truck is going to need to work hard in order to pull it out of the ground. Oftentimes the ground next to the pole stub is loosened first in order to make it possible in the first place.
There are two well know mechanisms that help keep a pole from being lifted out of the ground:
- Skin Resistance: The friction of the soil against the side of the pole below ground level.
- Anchoring Effect: Since most poles are tapered, the difference between the pole's butt diameter and that at groundline serves to help anchor the pole in the ground.
Both of these effects increase along the depth of the pole and vary with soil properties and the depth of the water table.
For poles with a Push Brace attached, it is very possible that the pole's weight and that of its attachments may not be enough to overcome the Push Brace's upward force that is generates. Another scenario of vertical uplift can occur when a pole line's elevation changes quickly, possibly combined with low temperatures. In both cases it is important to understand if the pole will stay in the ground or pull out and fall over.
There are few Pole Line Design software tools in existence that consider all of the above aspects of Foundation strength. It is therefore proposed that it is most appropriate to ask software tools to clarify IF they do ANY of these calculations, and if they do, what technique is used.
Coupled with this, if any tests are performed, what Soil parameters are used and what Safety Factors are applied to the results. Here is a form that can be filled in by suppliers.