Piles driven into bedrock 2

[DataAB]

I have two geotechnical reports (prepared by different geotechnical engineering companies) pertaining to two different sites, each with similar soil profiles. The soil profile at both sites consists of clay overlying a weak sandstone bedrock which has layers of shale/siltstone interbedded in the sandstone.

Both reports provide recommendations for driven pipe piles (the locations are remote and both owners have requested recommendations for the use of driven pipe piles) but their skin friction values are very different. Both geotechnical engineers recommend driving pipe piles into predrilled holes with hole diameters equal to 95% of the pipe OD. However, one geotechincal engineer provides allowable skin frition values of 100 kPa in the bedrock, whereas the other geotech engineer recommends an allowable skin friction value of 40 kPa.

I am curious as to how these values were arrived at. I have found literature on the skin friction (or unit rock socket side shear capacity) to use for drilled concrete piles in bedrock, but not pre-drilled and then driven piles. Is there any load test data for this type of installation? Is there any design literature?

Comments appreciated.

 

[msucog]

when i design on a site and look at several borings across the site, the numbers can be widely varying. you have two different sites (and two different profiles whether they appear similar to you or not), two different subsurface explorations, and two different geotechs. and keep in mind you're dealing with dirt...most of which is unseen/unsampled/untested. i would not try to compare such scenarios unless i were the geotech on those projects.

 

[DRC1]

There are many different methods of selecting skin friction values. Results can vary widely. Driving through the bedrock , even with a preagured hole will be difficlt. Wht happens if the pile fetches before the bottom of the hole ? will these be post grouted? If so why not do a drilled pile, cased to rock and the pressure grouted? Or if you are not grouting due to remote access, simply drive to bearing on the rock and adjust the pile spacing based on capacity?

 

[DataAB]

msucog

Thankyou for your response. However, my question was regarding "how these values were arrived at". I realize that the allowable friction values are different. There is obviously a reason for the difference (or else someone made a mistake). What I want to know is..... why are they different? i.e. what is the criteria used for determining the skin friction values?

Let's consider just one location, and for arguments sake, say that the bedrock is sandstone with the following characteristics:
* rock quality designation of "good"
* strength classification of Grade R3 (i.e. medium strong)
* an unconfined compressive strength of 15 MPa (obtained from performing compressive strength tests on intact rock cores)

If a client requested allowable skin friction values for a driven pile (initially pre-drilled to 95% of the pipe OD) into bedrock (with the characteristics noted above), how would you go about determining that value?

I have found literature on the ultimate skin friction values to be used for drilled cast-in-place reinforced concrete piles in bedrock (based on the unconfined compressive strength of the rock), but nothing on piles driven into pre-drilled holes in bedrock.

Comments or suggestions appreciated.

 

[DataAB]

DRC1,

Thankyou for your response.

To answer your questions...... My preference would be to use a drilled cast-in-place concrete pile but no concrete is available (i.e. location is remote). Reaching refusal prior to reaching the pre-drill depth is a concern. The owner wants to use driven pipe piles. The piles will be subject to substantial uplift due to wind.

 

[PEinc]

Maybe the answer is to grout the rock sockets with flowable grout mixed on site (not ready mixed concrete). Can you get bagged cement and a mixer delivered to the site? The grout could be a neat water cement mixture or a sand cement water mixture.

 

[DataAB]

PEinc,

Grouted-in pipe piles may be an option. I would have to check with the geotechs (it wasn't included in the recommendations).

That being said ...... how much clearance is required between the outside of the pipe and the surface of the drilled hole, when grout is used? Would the grout have to be pumped in? Which is preferable, neat grout or sand grout, for the application I would be looking at?

Thanks.

 

[PEinc]

Very little clearance is required to grout-in a steel pipe to a rock socket. I believe less than an inch.

Yes, the grout would need to be pumped in from the bottom up or possibly tremie poured from the bottom up. You would need to get grout inside and outside of the pipe pile in the rock socket length. You could set the pipe pile into the drilled rock socket right after the grout is placed.

Neat cement grout is most often used for ground anchors and micropiles. However, if you have a large quantity of grout to place, you could add sand. The grout strength is usually around 5000 psi.

 

[PEinc]

Basically, this would be a rock socketed micropile with only the steel supporting the compressive load unless you filled the entire pile and annular space with grout.

 

[GeoDocinFL]

Due to the multitude of methods available to estimate unit skin friction, you are probably better off just asked the Geotechs that published the reports what methods they used.

 

[gonethesun]

I think there are several reasons why these values can be different, despite similar soil borings. The engineers in each case are likely using different factors of safety and references for steel/sandstone friction. The nature of the project might also tend to make one engineer more conservative in their recommendations than the other.

Every driven pipe pile I've done was concrete filled; I presume these would be as well (the dead weight would have some benefit as regards your uplift concern). Concrete could be mixed on-site, despite the remote location. Quality of mixture wouldn't need to be a great concern, as long as you were not counting upon it for compressive strength.

For example, in applications where I've done pipe piles to depths of 110 feet, the dead weight of the piles and concrete was more than enough for seismic up-lift concerns.

I'm unsure how you would go about grouting a pipe pile driven into a pre-drilled rock-socket that is 95% of the diameter of the pile. Then again, depending upon the depth or length of the pipe pile, I'm not even entirely sure about the end of the driven pile ending up in the pre-drilled socket! Piles of any length tend not to be perfectly plumb, and pipe piles tend to sort of spiral or twist as they are driven.

In any case, if this is a critical application, I presume you would do uplift testing to verify the capacity of the pipe pile, regardless of what skin friction factors you select for design.

 

[ianlander]

Why not consider using hollow bar type systems to drill the hole with ? You can then grout simultaneously ensuring you have correct grout coverage and also correct location of the pile in the bedrock.

 

[DRC1]

The rock you describe is fairly compenet. I have not heard of the procedure you mention and do not beleive it is commonly done. I would be hesitant to assign friction values in this case as it would depend on how many factors were effected during driving. You will probably need to drive a heavy wall pipe with a ring. Since you are shaving the side of the rock, you may need to drive open end to provide relief for the spoils. Your best bet is to do a test pile program. Since this is somewhat of an unorthodox proceedyre, it will be hard to predict performance with out it. The variance in your recomendations seems to indicate that.

 

[born2drill]

ianlander:

Are you talking about something like CTS Titan, MAI, Belloli, etc? If the rock is competent I doubt you'll be able to advance a hollow bar into it, even if the bar supplier says otherwise.

 

[ianlander]

born2drill - Yes that is exactly the type of system I was talking about. I have seen the MAI bar used with a Tungsten Carbide bit (from the same source I believe) drilled using rotary percussive action into strong rock with success.

 

[dik]

How deep is the clay layer and what is the consistency of this?

Dik

 

[ishvaaag]

Imagine for a moment you only would have weak sandstone. Hence for friction at the interface you might look for friction in a steel-weak sandstone interface. I have some AASHTO old codes that I use for technical info (ASD 1992 and LRFD 1994). LRFD gives a summary of procedures on HOW to estimate side resistance qs in MPa in Sands, so it may serve you as guide. The classical approach in cohesionless soils is to apply to the lateral pressure on the pile at the depth a friction coefficient tan(small delta) where small delta uses to be less than the soil friction angle phi. However the lateral pressure for nailed piles in weak rock is uncertain. ASD indicates something around a 0.30 friction factor or small delta=17 deg.

Those providing the values may well have made an engineering judgement on the amount of adhesion of clays -if any- and friction from weak rock present, less any "oiling" effect of loose clay powder plus maybe water on the extant rock (being pushed along with the nailing process). Just a doubt on this might justifiy a 2.5 times difference in estimated value, and there's also the question of the standing value of pressure around the pile helping to develop friction.

 

[DataAB]

Thanks ishvaaag (and everyone else).

All comments greatly appreciated.