Rock RQD versus Alpha E Values 3

[cap4000]

Does anyone have the values correlating the RQD and Alpha E values for Soldier Piles in Rock Sockets. Any values would be appreciated. Thanks in Advance.

 

[jdonville]

cap,

AlphaE is a function of RQD = 0.0231*(RQD)-1.32 >= 0.15

The reference in HB-17 is (Gardner, 1987). A fuller citation is: Gardner, WS (1987). "Design of drilled piers in the Atlantic Piedmont." in 'Foundations & Excavations in Decomposed Rock of the Piedmont Province', GSP 9, ASCE

I have wanted to track down the derivation of AlphaE, especially as the limit of 0.15 is reached when RQD is less than or equal to about 64%, which limits the utility of the function dramatically.

I am sure that the Linda Hall Library could get you a copy of this paper.

Jeff

Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.

 

[cap4000]

Nice Formula. I have the Failure and Breakage of Rock Book 8th Symposium on Rock Mechanics published in 1967 and on page 269 it has jacking tests performed that confirms your formula. The RQD value of 64% is critical. Below 25% rock is treated as a soil. Carter and Kulhawy describes this a little further depending on the joint width. Tight versus open. They go as low as 0.10. Thanks.

 

[jdonville]

cap,

Thanks for the reference.

Jeff

 

[cap4000]

Another great reference to get for Soldier Piles in Rock is the 30th Annual Conference on deep foundations published by the Deep Foundations Institute in Sept. 2005. In it on pages 479 thru 484 it clearly rationalizes a design used on a New York City job. The 2 PE's, PhD's are Aly Mohammad and Dongi Yue who both finally clarify how a rock socket should be designed.

 

[ItsOnlyDirt]

Hi

I'm amazed you use the RQD system with such precision. It is after all an empirical parameter dependent upon a number of things but most importantly quality of the driller who obtained the core and of the person who logged the core. Whether RQD is 63% or 65% will make very little difference to your actual design. I am not too familiar with alpha e but if it is indeed related to RQD as suggested in the formula there is obviously even more room for error and thus as usual, engineering judgement is far more useful than simple number crunching.

 

[cap4000]

JCGSEWARD

I have the ASCE Rock Foundations Book #16 by the US Army Corps of Engineers. In it on page 28 they relate the RQD value to the in-situ rock modulus of deformation. The RQD has been used for over 40 years. I have read where massive column footings have settled over 6 inches on rock because the frequency of the joints was unknown. I agree a few percentage points is not a big deal but you have to start somewhere. Thanks for the logging and drilling tip. Alpha E I believe is AASHTO's terminology.

 

[ItsOnlyDirt]

I totally agree its a good starting point but its easy to get hung up on trying to find a number to use, double it, then half it and see what difference it makes, you might find the problem is in the structural design rather than the ground. Settlement should be fairly instant too I would think. I'm based in the UK so I'm not too up on the US Army Corps stuff.

Happy Crunchin' !

 

[BigH]

Snaps to JCGSEWARD! 64% is a little to "nifty" - 65% would have been taken as what it is - approximate. Good points on drilling - especially in shale - it is hard to get RQD when half your core is ground due to the fracturing in a zone. Driller's expertise is essential; and telling a machine break to a natural break is difficult in many instances as the wash water during drilling has a tendency of eroding some of the machine break line in softer rocks. I just gave a seminar on geotechnical engineering to some Indonesian engineers and showed them the nice "line" you get in text books for, say, Bjerrum's vane correction factor - and then the line and data points from which it was derived! Similar to the 64% in a recent thread on perolation and permeability/soil type when a perc rate of 288 was quoted from some general source. Judgment is paramount still and one should review some of Terzaghi's comments on it.

Thanks for the references guys!

 

[jdonville]

All,

Apparently, the alphaE equation as presented above correlates well with the "best fit" Bienawski's Modulus Reduction Ratio - as shown in the LPILE Plus 5.0 documentation.

Anyone have a source for this figure showing best fit line and scatter data?

Jeff

 

[cap4000]

jdonville

I have the 2001 softcover book by Reese and Van Impe called "Single Piles and Pile Groups Under Lateral Loading" and on page 104 it has all the scatter and best fit line.

 

[Panars]

You can also find the figure in Reese's article in ASCE Journal of Geotechnical and Geoenvironmental Engineering, November 1997 Analysis of Laterally Loaded Piles in Weak Rock.

 

[jdonville]

cap4000,

I have a copy of that book - I should have thought to look there as IIRC, Ensoft is Reese's consultancy company. Doh!

Panars,

As usual, you're the goods!

Jeff

 

[oldestguy]

Question?

Agree RQD is depndent on driller for good quality of work, but also his equipment. Are we talking E cores,AW, NX, BX, single tube or double tube or wire line?

Any one of these mentioned in those texts?

I'd guess thre might be as much as 50 percent error (or difference) between smaller and larger core diameters and methods.

 

[fattdad]

I may have something to offer to this discussion or maybe I'll just learn a few things - not sure yet. Is AlphaE intended to describe the rock-mass modulus, what I'd call Em? If so, Mike Duncan and myself made an interesting plot that incorporates Bieniawski's equation: Em=2RMR-100 into other sets of data. If anybody wants a pdf emailed to them send me a message.

Regarding RQD, it is universally considered to be an "N" sized core measurement (i.e., 2-in diameter). I'm fairly sure that it is actually a two-times the diameter measurement, which for 2-in core would be the total length of 4-in or larger pieces, excluding drilling induced fractures. However for "A" sized core if you based it on 4-in pieces, you'd understate the RQD.

I hate to sound ignorent on the AlphaE matter, just don't have that stored in the grey matter. I do have a thesis that has lots of information on rock mass properties.

f-d

 

[cap4000]

fattdad

I just finished a rock socket H Pile job with a RQD of 85 to 95%. My lab used a 2-1/8 inch NX core. I beleive AlphaE is based on the ratio's of rock field modulus over rock modulus for a perfect intact lab specimen. I use the Fred Kulhawy Chart to come up with the actual rock cohesion or residual shear value reductions with my actual RQD value. This lab counted the rock that remained in the hole for its RQD value as it was a very hard sound schist/gneiss with mica trace. Based on the grey water I saw coming up and out of the drilled holes it looked like the RQD was going to be very high anyway.

 

[fattdad]

I would encourage you to look at Bieniawski's 19778 publication on "Determining Rock Mass Deformability: Experience from Case Studies", published by Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 15, pp. 237-247 (call your local interlibrary loan). For an RQD of 90 the Efield/Elab ratio would be about 0.8. That said, none of this relates to the ultimate strength of the rock mass (i.e., the actual rock mass cohesion/residual shear strength). For that I'd refer you to Bieniaswki's 1974 publication on "Geomechanics Classification of Rock Masses and its Application to Tunneling", Proc. 3rd Cong. ISRM, Denver, Vol. 2 p. 27.

One way we're getting into trouble with this discussion is an understanding of your ultimate goal. If it's end-bearing on an h-pile into rock you may just be looking at ultimate strength with a safety factor. If it's slope stability, tunneling or something else more detail in the assissment is warranted.

For a better understanding of these things, I'd start with the 1974 publication by Bieniawski and his "Geomechnics Classification".

Now that I've said all of this Kulhawy is a good reference also.

f-d

 

[cap4000]

fattdad

I used 0.70 for a HP Soldier Pile/Timber Timber Lagging Design holding back 8 to 10 foot of Lean Clay approx 175 feet long. AASHTO provides a nice formula for the HP embedment depth based on the residual shear value. Thanks for the tip.