Friction pile with steel casing installation problem

[DCHAZEN]

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

 

[Ron]

I'm not sure what your entire soil profile is, but if it's like the description you gave, this would likely be a good application for Augercast pile. The only issue is the porosity of the surrounding soil (cobbles). If high grout loss is expected, that might be the reason for the casing specification.

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.

 

[ishvaaag]

I think that an exterior grouted with mortar cements for most cases will exceed the allowed shear (and maybe friction itself) of an installed casing out of the prongs in the surrounding ground, and your problem seems to agree most with the general case than with the exceptions. So I would tend to agree in this case more with the contractor than the geotech. Even better of course if you create physical indentations adding welded rebar outside.

 

[BigH]

It is a bridge - a big one? or a small one? I don't see why, unless there is geological evidence to the contrary why you couldn't found the bridge on spread foundations. I've been involved with many on that. Refusal at 2.5 ft? Seems ideal. Of course, there may be reasons that the geotech knows and we don't . . . yet.

 

[DCHAZEN]

I do not know why they chose not to use spread footings- unfortunately it is to late in the game to change the design that much. The cassings are purchased, fabricated, coated and ready for mobilization to the jobsite, the drill rig is here because it was mobilized for other cassions on the job. After a little more research I discovered that the cassings were added to prevent the cobbles from caving into the hole during and after drilling.

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

 

[Ron]

DCHAZEN...if it were only another 5 or 10 feet to a bearing stratum, I can't imagine that the geotech wouldn't have carried the shafts on down, so assuming there is no bearing stratum down there, overdrilling and placing concrete would have no benefit. Further, fiber reinforced concrete wouldn't help in that situation, with or without a bearing stratum.

 

[DCHAZEN]

I dug into the bridge and geotech's structual calculations. They are using 1 KSF as the allowable skin friction. And 4 KSF for the allowable end bearing. Does anybody know if there is a standard of values for skin friction with differnt types of soils, or does the engineer calculate the skin friction value for each application based on the geotech report? They do not mention where they got their value in the calcs.

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.

 

[Ron]

DCHAZEN....the skin friction depends on soil conditions (different soils have different friction characteristics, depth (friction is generally greater with depth because the normal force on the pile/shaft is greater, and the interface material (in your case a steel casing, preferably uncoated and roughened).

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.

 

[DCHAZEN]

Thanks for all of the help. I will right all of this up and see what the geotech thinks.

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

 

[Ron]

Almost all piles, drilled shafts, caissons or whatever deep foundation moniker you want to apply resist loads by a combination of skin friction and end bearing. End bearing is preferred because of the predictability of performance; however, many foundations resist load in mostly skin friction, which is fine if the conditions are predictable.

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.

 

[BigH]

To make a point on "skin friction/adhesion" and end-bearing. This has been discussed in another thread - but skin friction is developed ahead of end-bearing - in other words, it takes smaller movements to fully mobilize skin friction than end bearing. So even with design for both, the working load, for the most part is actually the part carrying the load and most of the end-bearing can be, in many cases, the safety factor. Look at load transfer curves from top to bottom of pile.

 

[Zambo]

Why can't they drive the casing with a vibro? I assume cranes are on site so they just have to hire the vibro for a week. Then they can excavate with the auger.

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.

 

[Zambo]

that should have been 30" for a 24" casing - not very good with feet and inches

 

[BigH]

We understood! We are used to hearing English spoken in America with some 40 to 50 accents!

 

[dirtydude]

I would have to rule out augercast as a viable option with the amount of cobbles you were mentioning...perhaps micropiles would have been a better application here.

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.

 

[Zambo]

Perhaps I'm missing something here. I assume the full depth casing is being used to support the excavation during construction of the pile.

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.

 

[DRC1]

it seems yo ar drilling a 20 foot cassion to rock. If that is the case, and a 30 inch cassion works, why not just go full depth with the 30? also if you are bearing on soil, a case can be made for skin and end bearing.On rock however, you probably will not get a lot of settlement, so the skin friction may not engage. But if the rock is of any decent quality, you should be able to get far more than 4 ksf out of the rock. Local building codes generally have presumptive bearing capacities that can be used. How deep do the cobbles go and how dense are they? drilling trough cobbles and boulders is difficult and time consuming. I agree with dirty dude that augercast is not the way to go. I would also look at mini piles or if the cobbles are not too bad, driven heavy H-Piles. What sort of load do these cassions take? Final note, GRL does have high strain Dynamic testing for piles with its apple system.