Piles driven into bedrock
Piles driven into bedrock
(OP)
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.
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.





RE: Piles driven into bedrock
RE: Piles driven into bedrock
RE: Piles driven into bedrock
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.
RE: Piles driven into bedrock
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.
RE: Piles driven into bedrock
RE: Piles driven into bedrock
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.
RE: Piles driven into bedrock
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.
RE: Piles driven into bedrock
RE: Piles driven into bedrock
RE: Piles driven into bedrock
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.
RE: Piles driven into bedrock
RE: Piles driven into bedrock
RE: Piles driven into bedrock
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.
RE: Piles driven into bedrock
RE: Piles driven into bedrock
Dik
RE: Piles driven into bedrock
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.
RE: Piles driven into bedrock
All comments greatly appreciated.