Connection Detail for Square Precast Concrete Piles in Tension

[structure_engineer]

Fellow Engineers,

First of all, let me wish you all happy holidays and enjoy your time off.

I am looking for suggestions to connect square precast concrete piles to steel pipe rack base plate, using a Sonotube to cast the pedestals on top of the precast concrete piles. My concern is for tension or uplift. What is the preferred detail to use to ensure a good connection for the uplift due to high wind in the Gulf coast region (over 160 mph)? The same goes to the concrete pile cap. For the concrete pile cap, it would be easier as the precast concrete pile is embedded into the pile cap? Could be as simple as breaking up the concrete in the precast concrete, and bent the rebars at an angle and dowelled into the pile cap? Or if you have a better suggestion that would be great. I was leaning toward using auger cast concrete piles but based on the input from the contractor, the location is not good for auger cast concrete piles as the auger would get stuck by tree stumps and there are a lot of the dead trees in the region. Your input and time are most appreciated. Cheers.

 

[jayrod12]

Generally speaking we have the contractor cut the pile off, whilst also retaining the strands of the pile and design the pile cap height as required to fully develop the exposed tendons in the pile cap. If necessary, we drill and epoxy dowels into the pile but when you start running the numbers on that, it gets pretty onerous pretty quickly.

 

[SlideRuleEra]

...connect square precast concrete piles... using a Sonotube to cast the pedestals on top of the precast concrete piles. My concern is for tension or uplift. What is the preferred detail to use to ensure a good connection for the uplift due to high wind...

Some DOTs use rebar cast to project from the top of the concrete pile. An alternate is to have mechanical rebar splices embedded in the top of the pile so projecting rebar can be added after driving.

The following details are from a Virginia DOT drawing:

As a former bridge contractor, I've driven piles with projecting rebar, it takes special pile driving equipment. Use the mechanical splices so conventional driving equipment can be used.

 

[structure_engineer]

SlideRuleEra,

Excellent suggestions. For precast concrete piles, you often get this question: "What if it hit refusal prior to reaching the desired depth?" For example, if you specified 90' deep pile and at 75' you can't drive any deeper as the pile hits a hard layer of soil? In such case, what do you do?

 

[SlideRuleEra]

What if it hit refusal prior to reaching the desired depth? For example, if you specified 90' deep pile and at 75' you can't drive any deeper as the pile hits a hard layer of soil? In such case, what do you do?

You need minimum specified penetration (ideally past the pile's point of fixity), but once you have that, stop driving. Of course, your example goes far beyond any reasonable minimum penetration requirement.

In my area (coastal SC) their are "thin" layers of sedimentary limestone marl. The art of pile driving is alive and well with those conditions. Sometimes a pile hits the marl and "stops" briefly (maybe just a few seconds). A decision has to be made in real time whether to quit driving or continue. If you are wrong, the pile may resume moving and go far beyond planned depth.

I have worked, as a Contractor and later as an Owner, with Engineers who insisted on needlessly driving to a specified pile tip elevation. This often turns out to be a recipe for "trouble"... for the Contractor, Engineer, and Owner. (As a Contractor, I could not do anything about that... as an Owner, took steps to make that problem go away.)

 

[AskTooMuch]

You need to drive down more even if you hit "refusal" in cases where you haven't reach the minimum depth required for lateral capacity.

 

[Brad805]

"For example, if you specified 90' deep pile and at 75' you can't drive any deeper as the pile hits a hard layer of soil? In such case, what do you do?" Talk to the geotech.

 

[SlideRuleEra]

You need minimum specified penetration (ideally past the pile's point of fixity), but once you have that, stop driving.

One of the steps I took to prevent delays in pile driving was to have our specs written (by the consulting geotech) to include minimum pile penetration. The minimum penetration is adequate for any anticipated loading condition, lateral load, compression (including friction piles), or uplift.

Our electric generating stations could have several thousand driven piling. The last thing we need was halting pile driving, pay the pile driver an hourly standby cost, while trying to resolve an issue with a certain pile that did not reach nominal pile tip elevation. With a specified minimum penetration, the decision to "stop short, for cause" can be made accurately and quickly in the field.

I learned how well this works as a bridge contractor, many state DOTs do basically the same thing.