breakout strength of anchors in rock.

[struct_eeyore]

Hi all - I've been trying to find a reference online about the breakout/pull out strength of anchors in rock and have yet to come up with something useful - so hoping someone can help. I'm working on a piling project in the Florida Keys with very shallow limestone bedrock. (I've yet to receive the geotech report - am structural). The contractor is suggesting to use pin-pile/augercast combo, sch 40 tube driven into the bedrock, and have concrete poured around. With this pile cantilevering off the bottom, my withdrawal load on the tube is ~44k, which I do not think will be matched by the friction between the limestone and pipe (maybe someone can enlighten me here). My thought is to set the vertical reinforcing of the pile in grout in the bedrock - naturally need to know how deep. Thanks in advance.

 

[MTNClimber]

I'm not fully understanding the pile system. Can you drive a schedule 40 tube into limestone? How do you get concrete to the outside of it after its been driven? I'm definitely missing something here. Maybe a sketch would help.

You can get high resistance in limestone if it's in good shape. Typical grout-to-ground values for preliminary design of micropiles in limestone (fresh-moderate fracturing, little to no weathering) is 21.6-43.2 ksf per Sabatini, et al. 2005.

If the structure is not too important then you may be able to run with those values with a large factor of safety applied (depending on local codes). Otherwise you should run a load test on a test pile to determine if your assumption is correct or to find out the true skin friction of the pile is by running it to failure.

What are the piles supporting?

 

[BridgeSmith]

For situations where we require substantial uplift resistance and flexibility (to accommodate thermal movement of the integral abutment we're holding down) , we have used drilled shafts with embedded steel H piles, where the drilled shaft is grouted only to a level 10-15' below the abutment cap. We sometimes use temporary casing to facilitate backfilling above the drilled shaft with sand or gravel. We wouldn't consider permanent casing shells, since the friction values would be severely reduced.

 

[MTNClimber]

Interesting HotRod. Are the H piles only 10-15' long?

Maybe that's similar to what the contractor here is proposing to do? Drill a hole, drop in the steel tube, and then pour concrete which I've seen a bunch but I couldn't tell by the OP's description.

 

[oldestguy]

Much will depend on the condition and bedding of the bedrock. I agree with above. It would seem to me that at least one pull-out rest should be done before dedicating how to finish the job. Even then a healthy safety factor would be required. Such a test should have reactions well beyond a cone of fracture. Don't forget the unit weight of rock is the submerged unit weight.

 

[struct_eeyore]

MTNClimber - I probably should not have called it a pile. As it stands, the tube is only being driven into the limestone 4-5' - there is no soil above it, just water. Concrete pier will be poured around. The pipe is the anchor if you will. ( I have issues with this, hence my question ). The 21.6-43.2 ksf values you mention are shear bond strength values?

 

[MTNClimber]

I understand now. Those values would not apply to your situation. Those skin friction values would be if the concrete was in contact with the limestone the entire length of the pile, ie pile is drilled, steel is installed and then grouted. I don't know of any publications that provide skin friction values for steel to bedrock. Maybe the geotechnical engineer on the project has an idea from experience with the limestone in that area.

I would guess that your best bet would to run a PDA or a full static load test which always makes the schedule and ordering of material difficult.

Also, 4-5' doesn't seem like it would be enough embedment for lateral loads and possibly uplift.

 

[struct_eeyore]

It looked shoddy to me from the start. I'm getting a bending withdrawal force of 47k just from wave action at base flood. What do you think of my original idea of taking rebars and setting them 3'-4' in non-shrink grout in the bedrock?

 

[BridgeSmith]

The piles go the full depth. We just omit the concrete at the top so that the abutment can move (translate) when the superstructure expands and contracts with temperature changes, without excessive restraint or broken concrete.

 

[BridgeSmith]

"Maybe that's similar to what the contractor here is proposing to do? Drill a hole, drop in the steel tube, and then pour concrete..."

I would think that would be more difficult than using an H-pile. You'd have to pour concrete inside and outside of the tube, wouldn't you? I would think the H-pile would be cheaper per pound also.

 

[BridgeSmith]

Having the piles subjected to bending is different than if they are subjected to axial uplift (which I thought was the loading). What you describe sounds like what we would consider a soldier pile retaining wall. Drilling the hole, dropping in a pile and grouting the hole, would probably work.

A pipe pile could work also in that instance, if most of the bottom was notched out to allow the concrete to be pumped through the pipe and out under the bottom of the pipe to grout the annular space. The difficulty I foresee is verifying that the whole annular space gets grouted, without pumping an excessive amount of concrete just to be sure.

For bending resistance in one direction, pound for pound the H-pile gives you more strength, while the pipe gives you the same strength in any direction, higher than the H-pile's weak direction bending capacity, but lower than in its strong direction.

 

[PEinc]

Where's the sketch? What size pipe? How do you drive a schedule 40 pipe 4' - 5' into bedrock without damaging the thin-walled pipe? Will the 44 kip tension load be exactly in line with the pipe so that there is no combined axial load with bending?

http://www.PeirceEngineering.com

 

[MTNClimber]

"Maybe that's similar to what the contractor here is proposing to do? Drill a hole, drop in the steel tube, and then pour concrete..."

I would think that would be more difficult than using an H-pile. You'd have to pour concrete inside and outside of the tube, wouldn't you? I would think the H-pile would be cheaper per pound also.

H-piles aren't typically used in waterfront applications except for maybe a deadman along a seawall. Since there isn't a weak axis of a pipe pile, they are usually preferred when in the water. It seems like the issue here is the ability to obtain enough embedment. In my waterfront experience, typically there is overburden material that you can drive a steel pipe pile and then drill a rock socket within the pipe, install a center bar or center pipe pile and tremie grout or concrete. Usually that's for highly loaded piles (piers for coast guard, shipping ports, ect.) without enough embedment for uplift or bending. In this case, I'm not sure if you can just open hole from the mudline and then drop in your steel and tremie. I would defer to an experienced contractor on that. I'm also still confused how you would drive a pile into bedrock. I've been able to drive an H-pile 7-10 feet into weathered shale but it wasn't on purpose. I suppose you if you could seat the steel pipe a foot or two into the limestone then you could do it if the water is protected from wave action. Once again, a contractor's input would be good.

Structee - I'm not sure if what you are saying is a good idea. I would have to know more about the loading, what this anchor is holding, and have some sketches of the system.

 

[struct_eeyore]

Sorry about the confusion. The pile is not purely driven, but will be pre-augered. I will try to post a sketch soon.

 

[BridgeSmith]

Thanks for the info, MTNClimber. I didn't realize that the loading parallel to a seawall would be that significant.

As far as driving H-piles in shale, from what I've seen the depth to refusal depends on how weathered it is. In unweathered shale we'd normally expect to only get 3' to 4' into it before refusal, but highly weathered shale can be pretty soft.