Pile design in soft rock

[LibraTech]

Hi All,

I have found a paper by R.K Rowe called "A design method for drilled piers in soft rock" from a Canadian Geotechnical Journal. This paper addressed methods on how to design with low RQD rock and tight settlement criteria (duplicating the conditions that I have), but is based on case studies with static loads.

These piles will be subjected to cyclic loads only.

Will this paper and the suggested design method still apply ?

Any other suggestions and/or papers to address cyclic loads ?

 

[soiledup]

Oh no, not 'soft rock' again.

I asked my principal engineer what the definition of 'soft rock' was? Thinking about strength and RQD etc etc.

He said 'Bryan Adams and Meatloaf'

Weak Rock,,, always 'weak' rock.

 

[BigH]

soiledup - you can imagine my consternation too when I see soft sand!

 

[genomty]

LibraTech:

you can try the technical report IF-99-025 "Drilled shaft: construction and design methods" published by FHWA or the EM-1110-2908 "Rock Foundations" by the U.S. Army Corp of Engineers.

A comment on always "weak" rock, as in soil clasification there are several methods for instance USCS or AASHTO, in rock classification there is also some guidelines to clasify rocks, I mean, I'm not geologist but if I can remmeber well, in fact the term soft rock already exist and is used to clasify rocks in field according to its hardeness, the criteria is something like

very soft- can be deformed by hand
soft - can be scratched by finger nail
Moderately hard- can be easily scratched with a knife
hard - can be scratched with dificulty with a knife
very hard- can not be scratched with a knife


Now "weak" rock seems to relative for me, for insatnce someone who is only familiar with a region where igneous rocks are the outcropping rocks, is acostum to deal with "strong" rocks or may be heavy or hard rocks? Lets say this rocks have RQD values higher than 90 and compressive strengths higher than 7000 psi, wow! that's a "strong" rock for someone who live in a reagion where sedimentary rocks outcrop, let say shale with RQD values ranging from 30 to 60 and compressive streghts of abouth 2800 psi that seem to a "weak" rock, well to avoid those interpretation clasification methods give the following descrptions

The first rock may be clasified as:

Basalt, dark gray,of excelent quality (based on RQD) slighlty weathered,hard or very hard (based on Hardeness), fine grained, vesicular perhaps, massive I'll say, moderatly to slightly fractured. Perhaphs this rock could be referred as sound or intact rock not as hard or strong rock

The second rock can be calsified as:

Shale, greenish gray, very poor to fair quality (again RQD), maybe moderately to slightly weathered, soft or mederately hard (Hardeness too),very fine grained, medium to thick bedded, highly to moderately fractured, dipping, moderately to highly weathered joints, in this case this rock usaully is referred as weathered, fractured or soft rock

In the other hand, soft sand it is in fact an abomination

 

[epongra2]

With all due respect to the soils folks on these boards, rocks are often described as "soft". A good reference for this and other rock questions / issues is the USBR's "Engineering Geology Field Manual", which is available for download online at

http://www.usbr.gov/pmts/geology/geoman.html.

While I'm at it, shale is by definition "very fine grained". Also, it is fissile or very thinly beddded / laminated, so it can not be "medium to thick bedded". Mudrock or claystone might be a better term for this rock.

 

[soiledup]

Soft - firm - stiff - very stiff are terms used to describe the consistency / strength of cohesive material - clays.

Loose - medium dense - dense - very dense - are terms to described the density of granular sediments.

Very weak - weak - moderately weak - moderately strong - strong - very strong - extremely strong are terms used to describe the strength of rocks.

All these classification systems can be empirically determined or determine in the field by simple hand tests with basic tools ie hands, hammers, pegs etc etc. I have worked across the globe (not just the UK) and have found these terms to be the most widely used adopted methods of describing strength / density. They absolutely should not be interchangable. It is my opinion that the terms 'soft' when describing a rock is an older term that has long since lost it's meaning. New Eurocodes (eugh - sorry), which are being adopted in the far east and Australasia define them better than I ever could.

Sorry for the rant, but this is the 21st century - lets all start singing from the same sheet.

 

[Patgeotech]

I could not resist to respond to soiledups classification system comments (as I disagree)- as a geotechnical engineer that has also worked in different parts of the world, I am afraid that when it comes down to writing a competent geotechnical report, one needs to adapt to what system a particular country may use so that it is understandable to all local professional parties that may read the report - the consequences of something not being understood properly can lead to all sort of problems and finger pointing (often at the geotech engineer). My feeling is that the 21st century geotech engineer needs to be adaptable to the conditions and environments that he/she works in (have many arrows for ones bow), especially as our field of expertise is becoming 'specialised' in many aspects. As long as there is consistency, common sense and adaptability in a geotech engineers working life, there should be no problems for the 21st century geotech guy. It is a pity that this cannot be taught but has to be learned in the field (sometimes the hard way).

 

[soiledup]

Patgeotech - So hang on a minute, as long as a local contractor understands that there is a band of 'strong clay' or 'very loose limestone' is it okay to endorse?
As part of our remit as professionals I continually try to update contractors to the most recent standards. It's called continual professional development. A competent report will have references, but more importantly appendices describing the fieldworks / and insitu testing undertaken and can even be read as a stand alone document. I know mine are.

The irony being that I kinda agree with you anyway!

 

[LibraTech]

Thank you for the contributions, although it did not do any good in solving the problem, however, and I do not want to start a run-away-fire here, Patgeotech is quite correct in making reference to other parts of the world and the classification and identification system used there.

For interest, below find a URL of a guideline document for Soil and Rock logging, that defines "Soft Rock" quite well without making reference to the by Bryan Adams and Meatloaf.

http://www.geotechnicaldivision.co.za/pdf/guidelines_for_profiling.pdf

So if someone can try and answer the question AFTER downloading and reading the pdf and is sure we're talking about the same rock, that would be great.

 

[soiledup]

I can pick holes in the new Euro Standards even though they are very good. I have only just glanced over the South African Standard quoted and can see any number of ambiguities. Firstly I know Tim Spink (as in Spink and Norbury BS5930) was supposed to be trying to eradicate the use of light or dark in British standards as they are ambiguous. Colours should only be described with a colour chart. When someone says light grey 'mudstone' does the 'light ' refer to it's weight? You can see where a layman contractor might get confused.
Then they're using slightly moist, moist, wet. How is this determined?
We use Standards for a reason. They are called standards for a reason. Lets try to lose the ambiguities as this is where errors creep in.
I'm ranting again now arn't I? Sorry.
I still kinda agree with Patgeotech anyway.

Sorry about all that,,,,what was the question again???