Friction pile with steel casing installation problem
Friction pile with steel casing installation problem
(OP)
BACKGROUND:
We are in the early stages of construction of a design build power plant in Southern California. Part of the project includes a small bridge with drilled caissons supporting the abutments. In the area where the caissons are to be installed the geotechnical report shows that augers were refused at 2.5' below grade. The boring logs also show that gabbro and dark dioritic cobbles with sitly to clayey fine to coarse sand were the majority soil components. The caissons are also directly adjacent to a protected water way and the geometery of the site eliminates the possiblity of using pile driving equipment- the caissons must be drilled.
The caisson design calls for a 24" steel cassing with rebar welded to the inside of the top 5' of the cassing to connect it to the bridge abutment.
The caissing will then be filled with 4500 PSI concrete. The design also calls for an epoxy coating on the top 5' of the caisson to resist corresive properties of the top soil.
THE PROBLEM:
The drilling subcontractor cannot install a 24" cassing in a hole drilled with their 24" auger because the large amount of cobbles in the soil will result in a hole smaller than 24" at multiple depths and locations throughout the bore.
They propose to drill a 30" hole, install the 24" cassing, and grout or concrete around the outside as well as inside of the cassing after installation.
The geotechnical engineer does not like this solution because he believes that it will signifigantly decrease the skin friction and therefore the bearing capacity of the pile.
Additionally it was suggested to weld rebar to the outside of the cassing to increase skin friction- this solution is still being evaluated.
QUESTIONS TO THE FORUM:
Any ideas or solutions to consider?
As a young engineer I do not understand how concreting or grouting outside the cassing could decrease the skin friction- the cassing will be bearing against almost exclusively rock and I would think concrete would increase contact area between the rock and cassing increasing skin friction. Help me to understand this mechanism.
Also- this is beside the point because the current design must be made to work- but I would think this would have been an excellent application for an auger cast pile. Just for my general knowledge any other design ideas would be appreciated.
Thanks
We are in the early stages of construction of a design build power plant in Southern California. Part of the project includes a small bridge with drilled caissons supporting the abutments. In the area where the caissons are to be installed the geotechnical report shows that augers were refused at 2.5' below grade. The boring logs also show that gabbro and dark dioritic cobbles with sitly to clayey fine to coarse sand were the majority soil components. The caissons are also directly adjacent to a protected water way and the geometery of the site eliminates the possiblity of using pile driving equipment- the caissons must be drilled.
The caisson design calls for a 24" steel cassing with rebar welded to the inside of the top 5' of the cassing to connect it to the bridge abutment.
The caissing will then be filled with 4500 PSI concrete. The design also calls for an epoxy coating on the top 5' of the caisson to resist corresive properties of the top soil.
THE PROBLEM:
The drilling subcontractor cannot install a 24" cassing in a hole drilled with their 24" auger because the large amount of cobbles in the soil will result in a hole smaller than 24" at multiple depths and locations throughout the bore.
They propose to drill a 30" hole, install the 24" cassing, and grout or concrete around the outside as well as inside of the cassing after installation.
The geotechnical engineer does not like this solution because he believes that it will signifigantly decrease the skin friction and therefore the bearing capacity of the pile.
Additionally it was suggested to weld rebar to the outside of the cassing to increase skin friction- this solution is still being evaluated.
QUESTIONS TO THE FORUM:
Any ideas or solutions to consider?
As a young engineer I do not understand how concreting or grouting outside the cassing could decrease the skin friction- the cassing will be bearing against almost exclusively rock and I would think concrete would increase contact area between the rock and cassing increasing skin friction. Help me to understand this mechanism.
Also- this is beside the point because the current design must be made to work- but I would think this would have been an excellent application for an auger cast pile. Just for my general knowledge any other design ideas would be appreciated.
Thanks





RE: Friction pile with steel casing installation problem
As for the backgrouting of the casing, the friction loss shouldn't be significant and might even be better than the casing driven into cobbles, since it reduces the voids against the casing and molds around the cobbles to increase the friction.
RE: Friction pile with steel casing installation problem
RE: Friction pile with steel casing installation problem
RE: Friction pile with steel casing installation problem
At this point we are trying to figure out how to build the design or modifiy it with minimal impacts to cost and schedule.
Thanks for all of the input I really appreciate it.
Another question to the forum:
What would the effects be of drill 5' to 10' below the end of the cassing and pouring a fiber mesh concrete "footing" for the cassing and pile to bear on. Would this have positive, negligible, or negative impacts on bearing capacity. I am only looking for conceptual answers if you can provide one with the minimal amount of information provided.
One problem I can see with this solution right out of the gate is being able to drill to that depth. From the bore log the drilling contractor is anticipating 3 days of drilling per 20' caisson.
Let me know
RE: Friction pile with steel casing installation problem
RE: Friction pile with steel casing installation problem
Also the way that I see it these piles relie on skin and end bearing capacity. I mentioned over drilling and pouring concrete in an attempt to increase end bearing- I should have made that statement before. I mentioned fiber mesh concrete because when I talked to the geotech he was concerned about the concrete shrinking and cracking around the cassing and I thought the fiber mesh would help minimize that- should have stated that too.
I guess what I am getting at is there any viable way to increase end bearing enough to offset any losses in skin friction? Or would it be better to pursue increasing skin friction? Right now I think that welding rebar to the outside of the pile is the most practical solution if we can make the math prove the design.
I really appreciate all of the comments and particapation.
Ron... Thanks for all of the responses- I have learned more today researching this problem than I did in during an entire foundations course in school.
Thanks a bunch.
RE: Friction pile with steel casing installation problem
If you could "bell" the overdrilled section, you could increase the end bearing, but short of that, not likely.
4ksf is not much end bearing...only about 18-20 kips. For a 24" dia. drilled shaft or Augercast pile we'd expect easily to get 75 to 100 tons capacity, so your skin friction is carrying most of the load.
The fiber reinforced concrete won't help the shrinkage that much. Make sure the grout that's used for backgrouting has as little water as practicable to reduce shrinkage. Use a high-range water reducing admixture (superplasticizer) to get the flow that you need. Use the largest coarse aggregate that you can get into the grout (usually for grout, that would be a #89 stone).
Welding rebar on the side of the casing is probably the best advantage you can get.
RE: Friction pile with steel casing installation problem
One more question, since the only dumb question is one not asked. My understanding is that this is a friction pile, but most of the reading I have done sounds like most friction piles are driven. In your last post you said a "drilled shaft" pile, making it sound like a drilled shaft and friction pile are to seperate things.
I suppose the answer to this question is that my piles are a drilled shaft friction pile. This entire problem arises because we are trying to rely on the friction between the casing and soil after the shaft has been drilled to 30" and the casing is only 24". Bottom line given our soil conditions we probably should have utilized an auger cast or other pile type, or a differnt system of install.
Does that last paragraph sound legitimate? Thanks again
RE: Friction pile with steel casing installation problem
I agree that other systems would have likely been better and easier to install. Unless grout loss is an issue, you could still switch to Augercast at this point.
RE: Friction pile with steel casing installation problem
RE: Friction pile with steel casing installation problem
I read something about the geometry of the site not allowing access for a piling rig, but I would expect a vibro would be as easy to set up as an auger.
The problem with excavating a 30' hole to fit a 24' casing is that I expect the excavation will fail and this is where I doubt the ability of the pile to work in friction.
RE: Friction pile with steel casing installation problem
RE: Friction pile with steel casing installation problem
RE: Friction pile with steel casing installation problem
In regards to the shaft, I agree with what was said in that there should have been no loss of friction with the overdrilled hole, but actually improved side friction with the physical elements and the backgrouting of the annulus.
RE: Friction pile with steel casing installation problem
Now it is proposed to drill a larger hole and then install the casing. So what will be supporting the excavation until the casing is installed? Is bentonite proposed?
If no bentonite then the excavation will fail and the soil surrounding the pile will move to some extent towards the excavation. This will change the soil condition from that indicated by the BH logs and will not be the same as used in the design.
So it seems to me that bentonite would be required, in which case maybe the casing isn't needed.
RE: Friction pile with steel casing installation problem