drilled pier installation with water at the bottom.

[COEngineeer]

So the county want a representative from the structural firm (me) and geotech at the site when they install these piers. The representative from the geotech. firm was there the whole day. I left after 4 pier installation. My question is, we specify to use (3)-#5 with ties at 48" o.c. We encountered some water in the hole so they decided to stick (3)-#5 without the ties so that you can lower the concrete pump hose all the way to the buttom so you can displace the water out. Is that common practice for residential construction? I was worried because how would we know that the rebars are lined up properly? The geotech guy told me it was ok. I didnt say anything because I felt that the geotech should be more in charge with the piers then I do. Comments?

http://www.krmconsultants.com

 

[civilperson]

What is the shape and dimension of the piers? What is the clear spacing to the ties? Why are ties at 48" spacing? What do these piers support and what is the loading?

 

[COEngineeer]

This is 20 ft drilled pier 10" diameter supporting total load less than 35 Kips or less. The ties are there just there to keep the (3)-#5 apart. These piers penetrate at least 12' into the bedrock.

http://www.krmconsultants.com

 

[ShawnR]

Wow, thats a big hole. You should be fine. It is normal to displace water in a cast in situ piling this way. The biggest problem is going to be sinking the cage through the aggregate. Normaly you would use a grout instead of concrete in this situation. Grout uses more sand instead of aggregate so re-bar cage placement isn't a problem. I think the large diameter of the hole will work to your advantage here. Why cant you place the cage and then put the hose down through the center? This will eliminate your concern for cage placement.

 

[COEngineeer]

so no ties no problem?

http://www.krmconsultants.com

 

[dik]

For a 10" dia pier/pile? could you not use a single #5 (15M) and no ties?... I often use a single 15M for 16" dia piles x19'+ long (no uplift or lateral loads). Rebar, in these environs, comes in 18m lengths...

Dik

 

[fratrowie]

COEngineer im not sure if such loading need the ties or not but i want to correct you that for what i beleive its not the goetech responsibility with regards the ties and rebars placements, your the structural representatives, it yours...hope that technically under such load your pile does not need ties and the rebars lined up properly.


fratrowie[/]

 

[structuralaggie]

We always put ties @ 12" o.c. at a minimum, but I think it is only required in certain seismic areas. See section 1809 in the IBC.

 

[ShawnR]

I just re-read your post. I thought you said 10 feet diameter. LOL

 

[emmgjld]

Assuming these piers are for expansive clays and vertical steel will be in tension.
Most of the Pump Hoses I see are smooth, and the end piece does not have a connection. We have lowered the hose through a steel cage in 10" dia piers and withdrawn it during the tremie operation. Be that as it may, just put a rebar tie at the bottom of the vertical bars and maintain some control at the top. Longer piers may be more troublesome, requiring intermediate tie bars.

 

[kslee1000]

For practical/strength concerns, the ties shall be placed near the top few feet of the pier to provide confinement strength. The ties at the lower part only help to position the vertical bars.

 

[COEngineeer]

Ok, I will know what to do next time. I will tell them t put ties at lower part only next time.

http://www.krmconsultants.com

 

[csd72]

10" diameter x 20 feet long-that is not a pier that is a pile!

 

[genomty]

I'm not totally sure about ties are needed or not that should be responded by the structural engineer, in this case you COEengineer, ACI 318 and ACI 336 could be usefull to answer this question, regarding to the encountered
groundwater, again ACI 336-3R in chapter 4 provide some criteria regarding to dewatering and seepage entering ratio and this should be reviewed by the geotech engineer

 

[csd72]

I would be worried about the slenderness of this thing, check your applicable piling code for guidance.

 

[TDAA]

A ratio of 30 is typically used in the area. This would allow a 25-foot, 10-inch pier. This is pretty standard for the CO area in my experience. Above that length, I would look to a 12-inch for residential.

If the 3 bars were creating space issues, I would have gone to two bars with equivalent area, unless you needed this configuration for anything else such as lateral forces. My guess is the uplift forces provided by the geotech dictated the amount of steel.

As far as who is responsible, it is both of you. It is likely the geotech guy was a technician (especially if he was there all day), and to his knowledge, this installation method has worked fine. Are you sure that he knows what all the issues could be? It was your design. You should be making sure it is being followed. That is why you were asked to be there.

 

[emmgjld]

TDAA brought up several good points.
I want to concentrate on the size of the pier.
In expansive soils, the majority of the pier will, sooner or later, be in tension. The issue of minimum dead load and surface area which will be 'grabbed' by the expansive soils in the upper 1/3 to 1/2 of the length is every bit as important as designing the pier to be in tension. In my experience, Larger pier diameters will experience significantly more uplift force. I keep to 10" (up to 35' long) or even 8" (up to 20') diameter piers for lightly loaded conditions.

My experience has been that smaller pier diameters in expansive clays always work better. The only cases of failure at the pier/stemwall connection, which I am aware of, have been after the piers have moved, moved a lot. I have kept the pier diameters as small as practical, for constructibility, and have experienced very little problems.