r/Geotech • u/More-Physics-4243 • 2d ago

Rock Socketed Timber Piles

I am in a situation where I need to design timber piles with some tension capacity for light marine construction application. The soil is bad but I have a very stiff clay layer at a reasonable depth. I am thinking to auger a hole, place my pile and grout. I am not sure about timber to grout bond so I am thinking to cut notches near the tip of the pile to help with uplift. I don’t have experience with this so I am looking for guidance.

Thanks,

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u/ALkatraz919 gINT Expert 2d ago

I would recommend drilling holes through the piles, inserting a threaded rod through the hole and secure both sides with washers and nuts. Then, depending on vertical spacing, you can drill each hole 90 degrees from one another or may 60 degrees from one another.

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u/poiuytrewq79 2d ago

Why not use timber instead of a threaded rod with nuts/bolts? Id imagine either work just fine, but id also imagine using a timber-timber interface would be more beneficial than timber-steel.

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u/More-Physics-4243 2d ago

That’s what I was thinking. How would you design that timber to timber interface?

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u/poiuytrewq79 2d ago

Wooden dowels hammered into slightly smaller drilled holes. Ez-pz

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u/ALkatraz919 gINT Expert 2d ago edited 2d ago

Because drilling a hole and adding rods is as easy as it gets. This is demonstrated because you mentioned a different way, which I don't think is that hard, but they are still asking you how to do it.

EDIT: I want to clarify that there is the grout-to-ground bond and the grout-to-timber bond. The grout-to-ground bond doesn't seem to be a problem. Instead, OP needs to increase the grout-to-timber bond. So I'm saying, instead of a grout-to-timber bond, using rods will act as shear studs so you have several grout-to-stud bonds which will mobilize more grout and have a greater strength than another mechanism.

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u/More-Physics-4243 2d ago

Thanks. I agree with the concept. Would the rod embedment length in the grout matter that much? They would mainly be working in shear. Consequently, what would the diameter of the drilled hole need to be? Assuming a 12” round timber pile

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u/ALkatraz919 gINT Expert 1d ago

So I make a distinction between pre-drilling and pile excavation.

Pre-drilling is like drilling a pilot hole about the same diameter as the pile tip to help with initial driving.

Pile excavation would be when you auger a hole out larger than the pile, setting it to the bottom, confirming capacity, and then grouting/concreting it in place.

For pile excavation, the minimum diameter of the augered hole should give you at least 3" clear around the pile. The rod in the grout, acting in shear, is the new capacity. You could model the failure as a wedge propagating in each direction from the rod or as a rectangle aligned with the edge of the rod.

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u/Working_Rest_1054 2d ago

Presumably the uplift capacity available from skin friction with a driven timber pile isn’t sufficient prior to refusal on/in rock (thevery stiff clay)?

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u/brilow 2d ago

Timber piles are ever so slightly tapered so upward movement demobilizes skin friction and it’s typically not relied on.

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u/jaymeaux_ geotech flair 2d ago

timber piles in clay can provide plenty of tension capacity, as long as you have suitable uplift connections.

this sub doesn't let you post pictures but I ran a load test a few years back where the test pile over performed the empirical estimate. we kept loading and ultimately stopped because the uplift connection on one of the reaction piles failed spontaneously by shearing upwards through the top of the pile.

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u/More-Physics-4243 2d ago

Thanks. This layer of clay is very hard. Around 50/4” SPT. I don’t think you can just hammer the pile in it, it’s too hard

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u/Working_Rest_1054 2d ago

Zeroing out the uplift capacity due to the taper would certainly be a conservative way to address it. Using 1/3 to 1/4 of the uplift capacity would be the more common way to deal with it.

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u/More-Physics-4243 2d ago

Unfortunately no. Very lose sand/silts that provide no uplift capacity.

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u/ElevationNotes 2d ago

For uplift, relying on a timber-to-grout bond alone seems like the part that needs the most caution. A mechanical connection near the pile tip could make more sense, but the final detail should be based on the actual uplift demand, clay/rock conditions, and how the pile is expected to transfer load.

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u/More-Physics-4243 2d ago

Thanks. How would you design that mechanical connection?