Bored-In-Pile In Artesian Condition

[kxa]

Does any body have any experience with the situation when there is a high head (5 to 10 feet) of water pressure in the drill hole. The pressure is being balanced by a slurry mix as the concrete is being placed down about 100'.

My question is really about the soil and cement interaction since there is high pore water pressure. Can there be a good bond between the soil and the pile to provide the needed friction or because of the high water pressure, slurry and the liquid cement mix, the contact does not develope.

 

[rochplayer]

So, to clarify, the phreatic surface of the groundwater is 5 to 10-ft above the existing ground surface?

What type of slurry are they using? Hopefully polymer because mineral slurries can cause caking on the side walls and reduction of side friction.

So, assuming phreatic surface is 10-ft above existing grade, then the pore pressures at the bottom of your shaft is approximately 110' x 62.4 pcf = 6864 psf. Unit weight of concrete ~145 pcf, so 145 pcf x 100' = 14500 psf total stress as shaft bottom or ~7600 psf effective stress. You are removing soil and replacing it with concrete. Assuming 120 pcf for the soil, you are increasing the effective stress at the bottom of the shaft from ~5100 psf to ~7600 psf, or ~50%.

In a nutshell, if your contractor is using good construction techniques (excavating using polymer slurry to prevent side collapse and bottom heave, tremmie concrete from the bottom up, maintaining sufficient slurry head over the concrete during the pour, etc.) you should not have problem getting sufficient bond.

 

[BigHarvey]

you will certainly have major problems to guarantee the stability of the excavation no matter which drilling fluid you are using.
To guarantee the stability of your pile, your working platform must be at least 1 to 1.5 m above piezometric level.
Trying to increase the density of the drilling fluid is not an easy thing.
You generally see your excavation collapse after mud treatment before concreting.
The safest solution even if it is not economical is to have a temporary or permanent steel ( depending on the type of drilling rig you can have in your area )casing installed to prevent any collapse.
I totally disagree with rochplayer regarding drilling fluid : there are procedures for drilling fluids, no matter if it's polymer or bentonite, if you don't follow them, you just end up with a bad product. A bentonitic mud correctly used will not reduce your friction ( at least not mora and not less than with polymer)

 

[rochplayer]

BigHarvey, I agree with you that following correct construction procedures when using slurry is vital for a good product (see my email). Mineral slurries are not allowed in Washington and other jurisdictions, but are allowed in Virginia and others, if approved. Mineral slurries have inherent short-comings that synthetic slurries do not have, but they may be less-expensive.

I also agree that using a temporary steel casing is the better option in the case presented in this thread.

 

[kxa]

Temporary steel casing is being used and withdrawn as the final cement grout is being pushed into the hole. The withdrawal rate is every two feet while the volume of cement and the pressure is being measured. At the end the top 10'-20' of the casing will be left permanently for lateral loads such as seismic. The bond between the cement pile and the soil is a concern.

 

[rochplayer]

It sounds like you are using standard construction practice. Concrete and soil bond should be good.

 

[BigHarvey]

Rochplayer
we had an alert in europe for the use of mineral slurries ( environmental concern ) but hopefully we can still use them ( strong lobbyng of the profession ). I use both type of slurries ( bentonite and polymer )but I can tell you that sometimes polymers just won't do it. In very open ground, the polymer mud just goes in the ground due to lack of fines . In this case we use bentonite as an additive to the polymer ( 5 to 7 kg/m3 )in order to bring to the mud the necessary fines to build the minimum cake ensuring stability of the excavation.

 

[rochplayer]

BigHarvey
We have had the same issue with polymer slurries (excessive loss into the formation), so generally it drives us to temporary casing the if the loss becomes unacceptable.

Mineral slurries are falling out of favor here both due to performance concerns (caking, which I agree can be overcome by a good contractor - Aye, there's the rub!) and environmental issues (perceived GW contamination, slurry runoff, disposal of used slurry, etc.). Environmental regulations is the driving factor, though.

One of the nice features of polymer slurries is that it is possible to "break" the polymers and return the slurry to water with insoluble salts settling out through additives, making it easier to dispose of.