Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.


Dear all, I have a question for

Dear all, I have a question for

Dear all,

I have a question for weak rock. To ask this, I will illustrate with an example.
Assume that our uniaxial compression strength is 3000 kPa. (Average of many, that do not scatter much.) RQD is %30. We are looking for its bearing capacity.

There are three possibilities for me to calculate it:

*Rock Bearing Capacity
First, I will reduce the UCS with a factor. I have one reference, that says for RQD b/w 25-50, reduce the lab result with 0.20. Ok. Now our design UCS is 3000 x 0.2 = 600 kPa.
Now, Canadian Design Manual recommends that qall=KspxUCS where UCS is the sound rock's UCS (in our case, we reduce and can use it.) Ksp changes between 0.1 and 0.4. For this RQD, I will choose 0.2. So qall = 600 x 0.2 = 120 kPa is the allowable bearing capacity.

*Clay Like Behavior.

UCS = 3000 kPa. Desing UCS = 3000 x 0.2 = 600 kPa. qult = 5.14 x 600 = 3000 kPa. Using FS=3, qall=1000 kPa. (Do not mind 5.14, Nc can be calculated for foundation dimension. )

Using Hoek-Brown method, c=75 Ø=17 can be found via RocData. Terzaghi Bearing Capacity equation Nc=12.34, Nq=4.77, Ngama=3.53.
If we do not ignore, Ngama, for depth of foundation = 0, bearing capacity with FS=3 is 600 kPa. If we ignore, 482 kPa.

Now we have reached a conclusion with three different bearing capacity:
  • Rock Bearing = 120 kPa
  • Clay Like = 1000 kPa
  • Hoek Brown = 600 kPa.
WHY ROCK BEARING CAPACITY GIVES RESULT THIS LOW? I should reduce the UCS definetely. I do not know what to do and think.

RE: Dear all, I have a question for

Your UCS is obviously not applicable since it tested the stronger part, not the stuff lost during coring. I wouldn't apply Terzaghi references that were for soil. Rock description also is needed, such as "granite", etc. and bedding direction, fractures, etc.

RE: Dear all, I have a question for


Please.. My question is if you have these, what would you do? Rock is claystone. At least 70% of the projects are designed without the things you have asked. If you think the question cannot be answered without these info, please pass this topic

RE: Dear all, I have a question for

Well, you answered part, but being sassy here won't get you anywhere.

RE: Dear all, I have a question for

Oldest Guy,

I admire your experience. And I login everyday to see what others ask and what people answer. Almost at every single topic, your answer is like these. You never answer to question but bring the argument to somewhere else, which may be important in general theory, but totally irrelavant to the answer.

To be honest, I format my questions just thinking of you. Everytime I ask something, I think to myself, oldestguy now will come and ask these, comment on this, and argument will go somewhere else, I will be left without a reply.

Do you understand? I asked something about SPT correlations and you gave a speech about corrections and how important they are. I ask a question in an exam format so for you to make best estimate. Answer is as always.

Anyway, I definetely do not ask you these questions. There are some people here that can improve me, but not you.

RE: Dear all, I have a question for

I had some input which i was considering adding to the topic, some references of Peck on soft rock bearing capacity, information about whether the rock was classed as intact or not based on the with of your foundation etc.

I will not post them here though as your comments above (and on other threads you have started) go completely against the ethos of this forum. This forum is intended to provide general guidance on every day problems engineers encounter. An engineer stating that they need more information to provide input is completely fine. If you dont have the information all you have to do is state that. This forum is not a last resort are somewhere that people will jump at the chance to do your work for you.

There is an old saying 'manners cost nothing'.....

RE: Dear all, I have a question for

Since you are not asking me to answer, I'll describe to others what you should have done. You are free to delete your post and comments. They don't help you.

What commonly is found at these sites are questions containing limited data. What I have found, through experience working, is that one never knows ahead of time how important some trivial item may become. Thus, as complete a set of data in resolving any problem is always required. Take this post, the title alone says noting of the question. Included should have been, at the least: location of the job,local geology information, local building codes applicable as to bearing, a complete boring log which describes all things found, including percent rock recovery, method and equipment details of sampling rock, elevation of proposed foundation with respect to rock surface and load to be applied.

RE: Dear all, I have a question for

Thank you for your answer oldestguy. I do not have these information. Because it was a hypo question. If you cant give any answer with given inputs, I thank you for time.

RE: Dear all, I have a question for

Eire, without answering your rude comments on making people doing my work, I will answer you. I have those charts by Peck. I dont like to use common capacities based on RQD. Do you have any comment on three method given above? Like this is wrong, this seems correct etc

RE: Dear all, I have a question for

One for from OG. Take a look at your other posts, as well as this one. I can well imagine your eagerness to get answers to questions. I only hope the way you are creating a reputation here is not the way you work and associate with others. We can't change you, but, in time these ways will affect your future negatively. I've run into a few engineers with somewhat the same attitudes and they suffered the consequences by losing their jobs.

RE: Dear all, I have a question for

Dear bdbd, we apply some correction factors to UCS values for the design according to some references as you said. But firstly you should check if this correction ıs necesseary to calculate bearing capacity according to canadian manual or not.

There are some simple and conservative approachs to estimate allowable bearing capacity by RQD or UCS. Anyway i just checked the BS8084 (refered at EC7)and have seen some charts to estimate bearing capacity referring to dıscontinuite of rock mass, UCS and rock type. According to data you have given, the charts show the allowable bearing capacity about 0.5 MPa.

In my opinion, the using of terzaghi's formulas or considering clay like bahaviour to estimate capacity is not realistic since these equations were derived in consideration of general shear failure behaviour. However the bearing capacity failure on rock material is more relavent to compresibilty of rock material.

Dear oldestguy,
i would also like to know what is the effect of bedding direction to the capacity of ground. Is there any avaliable equations which refers to these datas?

RE: Dear all, I have a question for

Secant: No specific equation, unless failure prediction is along those planes. I general, getting a full description of the rock has to be taken into account when using any method that is tied to shearing strength. We can't just go blindly not looking at situations that may not show up in specific tests or other methodology. Engineering opinion sometimes can't be expressed as a number.

RE: Dear all, I have a question for

Now, if I may - "weak rock", etc. is indelibly better than 120 kPa - which would be equivalent to an undrained shear strength of 60 kPa (if it were, say, a soil) - and this is judged to be a stiff (low end) soil.

That said - if you are talking claystone/shale, etc., unless you use triple tube core barrels and have a good driller, you are very unlikely in the upper reaches of the formation to get RQD's greater than about 40%. You would like find some grinding of the core - which implies that the core was better than you retrieved (grinding -you can see little indents on one core and a little point on the other side). Given experience in, say, Queenston shale, you should be able, easily, to have a bearing pressure permitted in the range of 500 kPa at the worse. Bedding shouldn't make much difference in a horizontally bedded claystone. As bearing is typically compressive, the beds wouldn't "delaminate". I have recently seen vertically bedded shales and this might be a bit more along the lines of being a bit more conservative in approach.

In harder rocks, assume, as a worse case, the rock formation is a gravel/cobble model - would you accept 120 kPa? No, unless there were large clays seams. Many harder rocks might have surface "damage" due to glaciation movement, etc - but again, I would surmise, as a minimum, a bearing pressure in excess of 500 kPa would be possible - unless of course, you find fracture zones that could slide on each other (the infilling of angled fractures, etc. You could pick up Coates' mining book (Canadian) that he wrote back in the 60s on rock.

As to other points, be respectful, tactful . . . the purpose as I see in many forums (or forii??) is to provide guidance on your exploration of a problem . . . to get you (or any one of us) a little tidbit to ponder as we make up our own minds. A person is a prisoner of his own experience and that is why I fully enjoy seeing posts by oldestguy, Ron, cvg, et. al., because their experience becomes, in part, now my experience. You may not get a "concrete" answer to all or even most of your questions - but if you read very carefully, you will gain some valuable insights.

RE: Dear all, I have a question for

Thank you all.

RE: Dear all, I have a question for

In such cases of weak rocks i always use BS8004 1986, Appendix A and Fig 1. I found is very helpful whenever i encountered a situation like that of yours.

RE: Dear all, I have a question for

bdbd, sorry to jump in, and maybe not the straight answer to your question but, what type of structure are you placing on this weak rock? In some locations in my area, we have mudstone bedrock (shale) at shallow depths and although we normally got unconfined compression strengths on the order of 1~5 MPa, I report 300 kPa for allowable bearing pressure for shallow foundations since this is the typical published values for bedrock in my area. If you don't have a heavy building structure, structural engineers will be more than happy if they have an allowable bearing pressure of, say for example 500 kPa as BigH mentioned. I think that your reported allowable bearing pressure should be also in line with typical values used in your area for that type of rock and also consider the type of the planned structure. By the way, if shale is shallow and exposed to drying-wetting cycles, you have to be careful of potential slaking issues (erosion). Typical practice in my area is to pour a 50mm layer of lean concrete on top of the shale to protect it against slaking.

Finally, I have learned A LOT since I started reading/participating in this forum and really consider very valuable each of the answers from the fellow members here, so I am 100% with BigH, please be thankful and respectful. Engineering is a learning process that never stop and you are lucky that experienced engineers can take the time to read and provide input to your questions...

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close