Rock Dowels vs. Tensioned Rock Ancors for Uplift

[moe333]

An 80-foot high wireless internet tower is to be founded on hard, sound granite on the top of a mountain in Southern California. A potion of the uplift capacity (against wind and seismic) will be provided by either resin grouted dowels or with pre-tensioned resin grouted anchors. The dowel installation would be easier, but there are some advantages to having a pre-tensioned anchor (smaller deformation). Assumming the deformation from a dowel is tolerable, is there any other reason a tensioned anchor should be used rather than a dowel?

Also, does anyone know of a minimum borehole depth for the anchor/dowel in this type of rock (regardless of the required bonded anchor/dowel length)?

 

[Focht3]

Where in the L.A. basin?

I worked on the wind farm in San Gorgonio Pass in 1990/1991 - the granite varies in quality for many different reasons. The drilled piers were designed to maintain at least three shaft diameters in "fresh" rock. But that was a lateral load problem, not a shaft pullout -

What will the loads be like? Hole size? Previous experience with the proposed anchors? Give us some details...



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?"

 

[moe333]

The site is in the Julian area in east San Diego County. The anchors/dowels will be mainly for uplift and will be tied into a base slab. The required capacity is not known yet as some of the capacity will be from the dead weight of the slab. My guess is there will be 4 anchors/dowels with working loads of about 20 kips each. The rock is hard, sound, not weathered at all. Boreholes will probably less than 4-inches.

 

[GeoPaveTraffic]

Provided the deformation of the dowel is acceptable, there is no practical difference between them. As for the minimum length of the anchor; I would make them at least 10 feet and longer if the normal joint pattern of the rock is more than 10 feet. The goal should be to make sure that once you are in sound rock that the dowel/anchor is utilizing at least two blocks of material.

 

[BigH]

GeoPaveTraffic has good comment - I too, would err on the side of being "long" - why? Yes, it is not necessarily good engineering but when you get the rigs up there to install the anchors, etc., the drilling of extra length as far as cost and the cost of the anchor is not that great. Take the extra length. If, of course, you are using many anchors, then I would be more critical of the design length. Rather be pound wise than penny foolish!

 

[clayR]

Suggest you look at PTI recommendations for pretensioned rock and soil anchors. It will answer most of your questions for both active or passive anchors.

General Rules
Drill hole size for resin grout is limited to something slightly larger than the bar. Say 1.25"-1.5" hole for 1" or #8 bar. Because of this, longer resin grouted anchors are not pratical because sectional percussion drill rods require couplings of larger diameter thus increasing the drill hole size. Therefore you are limited to the stroke of you drill, without using special techniques.
Resin anchors also limit the corrosion protection choices.
As BigH and Geo said, 10 ft should be a minimum. For pretensioned bar anchors this is needed in the free stressing length alone, then you have to add the bonded length.
A dowel on the other hand can be designed for pullout resistance providing you have taken minimum depth into account. With a dowel you will have a shorter anchorage, and no blockouts or sleeves as you would with a prestressed slab anchors. Granted, you could tension the anchors to the rock by using a another plate.
I dont agree that a dowel will perform equally. By pretensioning the anchor, the steel will not be subjected to cyclic loading, as the pretension load would be mobilized before the anchor moves. Granted, we are talking about small loads.

 

[moe333]

Thanks to everyone for the comments. I can appreciate the minimum 10-foot length, the bore hole diameter and corrosion protection issues, and the advantage of pre-tensioning to mitigate the effects of cyclic loads.