Artesian concerns for Auger Cast Pile

[eric1037]

I have a project that has very soft clay to a depth of about 40 feet with sand below. During drilling, groundwater was encounted in the sand at 40 feet. Upon completion of drilling, groundwater was measured at about 12 feet.

Therefore, I feel an artesian condition exists.

What are the issues, if any, of constructing auger-cast piles in this situation?

I have a couple of concerns:

1. If grout head is lost during auger withdrawl, the grout column would not be enough to hold back the water.

2. After the pile is complete and before the grout has cured, water could force the grout up or out.

Are these concerns valid? Are there other concerns?

Thanks in advance!

 

[Ron]

From your data, you know your "artesian" head....28 feet. consider this in your instructions for the head maintenance for the augered concrete piles. As long as you put sufficient controls on the placement and monitoring, it shouldn't be a problem.

 

[GeoPaveTraffic]

Maintaining head is the key. Typically, 10 feet or so of head is maintained, you might want to specify something closer to 15 for this project. Also, contractors tend to reduce the amount of head in the upper 20 feet or so to avoid wasting grout. I would not allow that practice given the high groundwater pressures.

This will both increase the cost of the piles and the amount of cleanup, but I have seen a test pile fail due to high water (within about 5 feet of the ground surface) and low head near the top of the pile.

 

[BigHarvey]

As a contractor I would never construct CFA piles under artesian conditions. Even if you can construct the pile, you have no warranty that water won't pipe up inside the concrete ( see Tomlinson 1986 ).
A driven pile or a permanently cased drilled piles are the only alternatives in these conditions. Price comparison sometimes leads engineers to specify things which should not be done. Obviously you will always find some hungry contractor that will take a chance !
Another point is the soft clay problem: the european norm will not allow you to construct CFA piles in clay having Cu values lesser than 15 KPa.

 

[eric1037]

I initially recommended helical piers, but the local auger cast contractor called me and said he was going to bid auger cast as an alternate.

I have some reservations, hence the question.

I believe if there is very tight specs, quality control and multiple load tests the risks could be reduced.

However, in my report, I mentioned issues with auger-cast with an artesian condition.

 

[casimmons]

Are you sure the water is aretsian? Maybe it is just taking some time to stablize in the clay layer.

 

[dirtsqueezer]

Just a thought, but couldn't your clay layer be a sort of reverse-acquitude, acting as sort of a cap to pressurized gw below? I would tend to agree with casimonns. It seems possible your gw situation could stabilize over time.

Cheers.