Drilled Pier

[lovethecold]

I am preparing a report for a guyed tower approximately 500 feet tall. The tower has a downward load of 165kips and a lateral load at the base of 2 kips. I spoke with the engineer and contractor and they prefer to use a mat foundation at frost depth (~7'). We would prefer this as well. However, I am looking to give recommendations for support of the tower using a drilled pier. The problem with this at the site in question is a dense water bearing sand layer about 30 feet below the surface.

During drilling, when our driller first encountered this layer, the sand heaved several feet up into the auger and water rose to approximately 2 feet below the surface almost immediately. The soil above consists of sandy lean clay with average bpf of 8 down to 17 with an average bpf of 20 below this. There is a water bearing sand layer about 4-6 feet (this doesn't appear to be under pressure and didn't cause problems during drilling). The are around the tower location is a slough. The tower is only about 4 feet above the slough and many of the guy anchors will be in the slough.

My question becomes is it even feasible to consider a drilled pier at this location? If so, is it possible to calculate what sort of pressure the dense waterbearing sand is under to know how far above this layer the drilled pier should stop to avoid the sand blowing up into the drilled shaft and creating some most undesirable problems?

I should also say that when drilling at one of the guy anchor locations, the sand blew up into the auger causing the center bit to become stuck in the auger. To remove, the crew had to pull the auger and drill rod together in order to recover them. The sand at the guy anchors begins between 26 and 27 feet.

Thanks for any advice.

 

[aeoliantexan]

A piezometer can be installed in the sand to determine the static water level. This can be done like a monitoring well, with a screened section in the sand (2 to 5 feet is plenty) a 2- to 3-foot bentonite pellet seal just above the sand, and cement-bentonite grout backfilling the rest of the hole. If the sand heaves up into the auger, you will need to wash it out, so it is best if the auger rig has wash boring capabilities also. Alternatively, the hole can be drilled by mud rotary. Fill the riser with water as you install it to help keep the mud from clogging the screen, then pump water down the riser to flush the mud out of the hole before placing the bentonite pellets and grout.

If there are no significant sand layers in the clay, the pier hole can probably be drilled to within a couple of diameters above the sand. Hurry up and place the concrete. The clay is not very strong, so the pier diameter will be large, 5 or 6 feet, mayber greater.

If the water level is not more than a few feet above the ground level, the pier can be installed by slurry displacement techniques. The hole is kept full of mud slurry during drilling and the concrete is tremied to the bottom, displacing the slurry above it. You will need a pier driller who really knows how to use this method.

Driven piles bearing in the dense sand are feasible, but mobilization costs will be high for just three piles.

A 10-or 12-foot square footing at frost depth is simple and probably most economical.

You might consider helical piles and anchors for both the tower support and the guy anchors. They can be installed using a torque motor on a trackhoe, which may facilitate mobilization into the slough. Talk with A.B. Chance Company or Anchor Engineering and Inspection, Denver, among others.

 

[howaboutthis]

composite helical pipe piles-CHPP used for towers. there is some info on them at the maclean-dixie web site with case studies...

http://www.macleanpower.com/products/item.asp?ITEM_ID=2391