Concrete light pole base

[tebodm]

A local contractor has contacted us to review the proposed foundation design for some high mast sports light poles. The concrete poles are 95' tall and will be embedded 15' into the ground as per the manufacturer. The poles were to be set in a 4' diameter hole and filled with concrete. The butt diameter of the poles are 29". The existing soil conditions did not permit a nice smooth 4' dia. shaft due to high ground water and lots of cobbles, so the contractor now has a much larger hole that they will dewater and fill with concrete by tremie method. The question I have is how long does the pole need to be supported by the crane before the concrete has enough strength to keep it erect and plumb?Thanks in advance.

 

[civilperson]

The crane may be replaced by guy wires after concrete achieves 50% of F'c. Some sort of bracing is needed for duration of cure unless loads are very small, (wind and conductor tension).

 

[csd72]

Does the pole go all the way down to the bottom of the hole?

If so then the concrete is really only there to spread the load into a wider area of soil and is soon much higher in strength than the surrounding soil.

 

[tebodm]

Yes the pole does go all the way down to the bottom of the hole. The pole manufacturer said the excavation could be backfilled with compacted fill, but due to the size of the excavation and the high ground water the contractor has elected to place concrete instead. They have 4 poles total to set and was hoping to do more than one per day. I'm thinking that the concrete needs to set for 6-8 hours before removing the crane.

 

[concretemasonry]

tebodm -

What do you plan to schedule for the wind and weather for the period your pole base cures/sets up?

It seems it would be cheaper to use Type III cement in the concrete to get an early set up and cure. This will save the contractor money so he can move the crane to othe poles.

Your foundation base is already much more than adequate the the original 48" with 9" of concrete cover.

At 6-8 hours, you have enough stability and could eliminate any cables/guy wires.

Dick

 

[tebodm]

concretemasonry-
I wish I could control the weather for the contractor.

I've never used/spec'd the Type III cement so I am a little leary at this point because the contractor says that at least one of the holes is going to be very large in diameter by the time they get it cleaned up from the cave-in and estimates approximately 20+ yards of concrete. How will the large mass behave with the high early strength concrete? Also can that be used in a tremie placement? Thanks for the comments.

 

[hokie66]

Agree with concrete masonry. Using high early cement is appropriate for this application, and won't cost much in the scheme of things. The contractor should be able to do one early in the morning, then one late in the afternoon.

 

[BigH]

I've a question. Who approved the work method of the contractor? Who is going to pay for the significant oversize of the hole? There are methods that could have been used to prevent the significant caving - might have required sacrificial casing (or sonotubes?). If the contractor chose the work methods, I wouldn't be paying for anything more than the 4.5 yd3 or so of concrete needed to fill the design hole.

 

[csd72]

Cranes are expensive, concrete is cheap.

Tell them to use the high early strength concrete and then dont accept any claims for extras.

 

[tebodm]

We weren't the design professional for the original project. The architect's spec's left the foundation design up to the contractor and the pole manufacturer washed their hands of it by stating that if the holes and soil conditions differ from "ideal" than the foundation is by the contractor. I don't see how people get away with this. Since when does a contractor have initials after their name and can design stuff like this. Not sure who will be paying for the extra work/concrete.

Thanks for everyone's input. I'll go with the type III and hopefully the contractor can get two done a day.

 

[csd72]

There has to be a PE that signs off on these things. If it is no-one else then it sounds like it is you. make sure someone pays you for your time and then previde a spec. The concrete is not really working very hard it just needs to be in a reasonably solid state when the crane lets go.

The usual expectation in this situation is that the contractor will employ their own PE to design this.

 

[oldrunner]

Before you go any further, you should require that a geotechnical engineer become involved in this project. He should determine what kind of a foundation should really be installed for this pole. He should be able to obtain the overturning forces from the Pole Manufacture, assuming that the local wind conditions are the same or less than what the pole has been designed for. Either the geotech can provide the structural design or he can provide a structural engineer with the design data – i.e. allowable passive pressure (from saturated founding material!) as well as where the passive pressure starts (below the ground line) and the soil bearing data.

 

[itdepends]

Surely it'd be easier to use a casing to prevent the caving rather than using gobs of concrete? Maybe a concrete soakwell or similar would be suitable?