Timber piles embedded in concrete 1

[Robertofg]

I have a dock project that requires the use of 12" timber (greenheart) piles with relatively high (7 ton) uplift capacity. The subsurface conditions consist mainly of a thin layer of sand (5 feet or so) underlain by weak fragmented limestone. In order to attain the required uplift capacity we are considering preaugering a socket in the limestone, filling it with grout and subsequently inserting the pile (preforming the timber piles). Does anybody know how to estimate the slip resistance between the timber pile and the concrete socket.

 

[SlideRuleEra]

I don't know the answer to your question. However an old cost-effective "trick" to avoid the problem is to drive the piles "upside-down" (butt down, tip up). Then you have the natural taper of the timber tending to greatly increase uplift skin friction. Depending on driving conditions and other potential loading cases, maybe the preaugering & grouting are not needed or can be minimized.

http://www.SlideRuleEra.net

 

[eric1037]

SlideRuleEra:

Could there be a problem with skin friction if the diameter at the bottom is larger than the diameter at the top? I envision there being a "void" or at least disturbed soil adjacent to the pile because a larger diameter was driven.

I suppose if it is granular soil, the soil would soon enclose around the pile, but it seems like the confining pressures would be less.

 

[SlideRuleEra]

eric1037 - For piling on land, I understand your concerns. I have made the assumption that since these piling are greenheart, and for a dock, that they will be driven through water. In that case the granular soil should "close-in" around the driven pile quickly. Overall I would expect pile capacity to be similar to a pile that was either jetted or driven with a vibratory hammer under the same conditions.

One temporary application is to employ this "trick" when timber piles are used as spuds in deep water to hold a barge in place. In the right soils, sufficient skin friction develops quickly enough (in a few minutes) to keep buoyancy forces from making the pile "pop" out of the ground.

http://www.SlideRuleEra.net

 

[cdh61]

Robertofg,

You could drill a hole through the timber pile and place a rebar in it and cast it into the grout at the toe. Now your timber pile uplift force will also be resisted by the shearing of the timber and rebar. I have used these type of detail in pole frame buildings. I do not have the drawing in front of me, but the hole should be at least 4 inches from base of timber pile, and the use of a 25M should be sufficent. Check to see if a 3 ft grout plug would be fine, as now you have additional resistence from the grout plug via friction, which is more than the timber pile / grout resistence.

Regards,

 

[BigH]

SRE - I've not had to do it yet, but I have heard of using the upsidedown method. As you say, for most soils in water, the sides will close up about the pile and the uplift will not only be by skin friction but by a minimal bearing (reverse of Nordland's formulations). Good point.

 

[Robertofg]

Thanks for the input. It is a docking project and we have been considering driving the piles upside down as you suggested. The reason I was trying to stay away from this solution is because the project calls for various different unsupported pile lenghts which will produce various total pile lengths and consequently, the top of the piles will vary in diameter which will make the construction of the actual deck more difficult and look a little odd. It should not be a big deal but it would look better if they were installed with their butt up. The problem is not so much the driving because the fragmented limestone is not that hard, it is more that the structural engineer is demanding a significant uplift capacity (7 tons) which will require driving the piles approximately 25 to 30 feet into the rock which starts right at the dredged surface. If you add the embedment to approximately 25 feet of unsupported length we end up with very large piles. In addition, the rock has some hard lenses that may impede driving the piles deep enough to attain the required uplift capacity. Hence, we are looking at the concrete socket solution or a rock socket with significant compression capacity but very limited uplift resistance.

 

[jdonville]

Robertofg,

Could you attach a rebar cage to the tip of the piles to provide resistance with the grout/concrete? These could be submerged below the top of the grout/concrete so that they don't show. They could be epoxy-coated if corrosion is a concern. Sorry, marine environments are pretty much outside my current experience.

Hope this helps.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[dmoler]

We usually drive some piles battered (inclined) for extra uplift support. They form somewhat of an upside down V shape and design. They are braced or fixed to the pile cap, transferring the uplift vertical load from one side to the battered pile on the other side. You might also want to look at some bracing on the sides for added support/load distribution.

 

[eric1037]

dmoler:

Good advice. I gave you a star. For some reason, I don't immediately consider battered piles, except for lateral loading. I need to work on that mental block of mine.

 

[Robertofg]

I hadn't though of it that way. Unfortunately, the owner is very capricious, as usual, and is not very open to modifications in their conceptual design section. They would not want to have cross braces for lateral support above the water line because it looks too "woody" and stuff along those lines, so I do not think they will be open to battering the piles. Nevertheless, it is worth a trying. Thanks for the advise.

 

[dmoler]

I finally got a star!! Thank you!

I would like to clarify one thing that doesnt look right the way I wrote it. When I say transfer from one side to the other, I mean the vertical load is transferred to the 'top' half circumference face of the pile pushing on the rock. The failure mechanism would include more of the shear strength of the rock and less of the bond of pile to rock.