Retaining Wall to Support Utility Pole

[ktwannabe]

We are working on a site development project where the proposed parking lot will require a 4 foot cut into a short hillside approximately 5 feet away from an existing utility pole. We anticpate installing a retaining wall to support the cut and maintain the stability of the pole. The information we recieved from the utility company indicates the pole is embedded 9.5 feet below grade and has a moment at the ground surface of 286 kip-ft due to wind and ice loading on the pole and wires. The pole is approximately 75 feet high above the ground with 3 wires that are approximately 50 to 60 feet above the ground. The moment seems high to us and we can't get the existing condition to be stable for overturning. The overburden is a dry, residual sandy silt weathered from shale that varies from a friction angle of 34 degrees to 38 degrees based on SPT correlations. The overburden is approximately 7 feet thick and is underlain by a moderately weathered shale. My question is whether the moment seems to high based on a wind speed of 60 mph and if its not how do I calculate what loading it will impart to the retaining wall?

 

[dicksewerrat]

I wouldn't argue with the utility's number but use them to get the number you need. If anything happens to the pole while you are playing around, removing soils, causing ground vibrations and putting up the new wall, they will come after you. Ask the utility for a price to place a new pole at that location with the new ground level. It will probably be less expensive than your wall.

 

[PEinc]

The way I understand it, pole loads and moments are not only based on wind load. I believe that the loads also come from the wires (weight, ice, temperature forces, etc.), especially if one or more wires become unattached from one side of the pole. Do you know what direction the pole forces and moments are acting (parallel or perpendicular) with respect to the face of your wall?

 

[cvg]

can you or the power company install a deadman on the pole to counteract some of the moment?

 

[eric1037]

That overturning moment doesn't sound excessive to me. I used to be a structural engineer for a power consultant. They are designed for tension, ice, wind, and broken wires on one side.

 

[Focht3]

The loads look right, but the embedment looks to be short. I assume this is a tangent structure - in a straight line between two other poles. The loads are very likely transverse to the power lines.

I disagree that replacing the pole will be cheaper than the wall. You will have lots of expenses beyond the cost of the new pole and insulators.

Use a cantilever drilled shaft wall for your retaining structure, with "braces" beneath the new pavement that bear on additional piers beneath the new pavement. The piers for the drilled shaft wall will need to be nearly tangent with one another - and heavily reinforced. You can construct the drilled shaft wall one pier at a time, minimizing the risk to the existing pole. After the drilled shaft wall is built, you can excavate to grade, then install some additional (larger) piers in front of the new wall and tie the drilled shaft wall and the "extra" piers together with stiff concrete beams that are also cast below the final pavement grade.

I've used this approach in some challenging areas - to support a 12 inch 190 psig natural gas line on the property of the Galleria Mall in Houston, for example. It can be done safely, fairly quickly - and relatively cheaply.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[cap4000]

I have never done this load calculation before but as a suggestion where to start here goes. Start with a "idealized" pole/beam design with a 84.5 foot (75+9.5) span length. Place the known loads on the pole exactly where they occur with their directions. As for your reactions assume the soil as a PUSH & PULL Reactive Force at the 2/3rds point of rotation. Use 0.67 x 9.5feet from the top down as this pivot point. Try to use "simple statics" and derive reaction point loads imparted from the pole on to the soil/ new retaining wall. As for your moment,a 6 kip load acting 50 feet up is 300k-ft which looks reasonable to me. Be very careful excavating in front of the pole as your soil support structure may fail. No matter what you do proceed with extreme caution. A concrete deadman may ultimately be required. Good Luck.