Soil Reports 1

[theclipper]

I always have trouble assuming an angle of friction for soil given a thorough soil boring log, and feeling comfortable with it. Without a good feeling about that, its difficult for me to use Terzaghi's Bearing Capacity Formulas.

Can anyone offer advice on how to develop an accurate angle of friction from a soil report?

 

[BigH]

Clipper - Sorry, I don't want to be "first" - call me Coward. But, one thing to remember; allowable bearing pressures are almost (usually) governed by the amount of settlement (total, differential, immediate) that a foundation can undergo and not by bearing capacity. There is another recent thread on this topic although it might be hidden by the "liku liku" nature of the threads ("liku liku" is Indonesian for "meandering&quot.

 

[VAD]

I agree with BigH. However, I am a bit surprised that the angle of friction would not be mentioned in the report or net or allowable bearing pressure. If this information is absent then it tells me that you perhaps just wanted a hole to be drilled without any analysis. This is an assumption. If otherwise you need to ask your geotechnical company for the information you require.

Cheers

 

[PEinc]

I read many, many, geotechnical reports in the course of my work. Rarely does any report contain any actual tested values for unit weight, friction angle, cohesion. Usually, if any values are "recommended", they are the typical, usually conservative, 120 to 130 PCF, 30 degrees, c = 0, ka = 0.33, kp = 3.0 which many geotechs seem to pull out of the air. This tells me that soil property testing was not done and that all someone (owner, structural engineer?) wanted was, indeed, a cheap hole in the ground to find rock, water, or general soil types. Most reports I read just present the boring logs. (Then the project engineer wants to argue with me about the values I assumed.)

 

[VAD]

Indeed, many of the values "recommended" are generally determined from the soil type and appreciation of the density, strength etc based on correlations. If SPT testing was done along with pocket pens then the typical report would often have parametric values based on established correlations tempered by local experience. To undertake testing other than moisture content and the odd Atterberg limits would cause the price to be high and often this would result in the geotech not been given the project. It is always a battle to get the appropriate testing done as clients often feel that the information is commonly known. In fact some geotechs use past information to develop their drilling program and also to provide values based on past experiences.

The only time that one seems to carry out any testing is when the project is complex.

Personally, if I were to undertake a project for which foundation recommendations were required, I will look at all possible types that are commonly used in the area in question and hence my report will provide the necessary values. There has to be a reason for undertaking an investigation and on that basis the resulting report is developed. It is always good to discuss what you will be providing as an outcome of the investigation so that the Client, Structural Engineer etc know what they will be getting and that they are satisfied.

Having said that there is of course, what some one referred to as the "fast food" Geotechnical Engineering outfits. Unfortunately, many clients want cheap info and hence they receive what they pay for. Many clients often phone for prices and whoever quotes the cheapeast gets the job and they never say what they really want. These are often not satisfied with the end result.

A little story - one of many. I was once called by a house owner asking if I could stamp a drawing of plans for an unheated deck which was to be an extension to her house. I was promptly told that her huband was a draftsman and had already drawn up the plans with 10 ft length of piles etc and she just wanted someone to stamp same. I told her that I could not participate in this type of exercise and gave my reason, which I think is obvious to all. She then asked if I knew of any other companies that would do such work. I promptly replied that I did not know of any other company. The real culprit was of course her husband the draftsman. Very often, the same goes for geotech engineering business.

It is interesting that if we go to a medical doctor we would sit quietly and let him take all the samples to carry out his testing and evaluation. Does anyone stop to think about this process. Yes it goes to the lab and the lab tech puts at the side the limits and yes when it is returned the Doctor goes through and comments on the likely problem when such and such result does not fall within the established limits. However, we are very obedient at that time. If he said he wanted a million dollars we would reach into our pockets immediately. Why, we are afraid of the unknown and that if we did not do such tests we may die. But for geotech engineering everyone is an expert because there is often the layman's opinion that there woulfd be no loss of life and the geotech is making a big fuss. Hence, he too is an expert. Too many of such experts around. There are many quack doctors who are equally as good as the one that have graduated. Do we tolerate them today?

I guess I am straying too far from the topic.

Cheers

 

[theclipper]

Thanks for all your input fellas!!!

I guess what I really was asking is as follows:

I am a structural engineer working for an A/E firm that designs many buildings of varying size and use. Our office standard is to to always get the owner to pay for SOIL BORINGS and then to have the structural department design the foundations if possible. "If possible" means that the building loads and soil conditions warrant a safe assumption that typical concrete foundation wall/footer construction can be utilized. If any more advanced foundation system is required we would look outside the firm for help.

So that is where I am (and believe me...I realize this is not an ideal situation!!!)- A structural engineer fairly verse in concrete design looking at a SOIL BORING LOG (not a soil report)and trying to decide weather to assume the soil is 2000 psf or 5000 psf bearing so I can design the footings accordingly. I am given soil descriptions and blow counts. That is where my original question came from....an attempt to use Terzaghi's Equations to derive bearing capacity, which require the soils' angle of friction.

Basically I think I'm just looking for advise on sizing foundation members with the only info being known is the SPT blow count. I've been doing this in an "ultra conservative" fasion for a long time now and I was just wondering if any of you soil gurus could help me improve my far from perfect system.

Any direction (and i'm sure criticism) is welcome!!

Thanks in advance fellas.

 

[PEinc]

VAD, you are right on target with your comments. I just love it when someone calls and says that they "just need a stamp" but don't need any engineering. That's like calling a doctor and asking him or her to just send over a bunch of prescription medicines without seeing the patient.

People do this to engineers because too many engineers let them!

 

[BigH]

theclipper: What you need to understand is that instead of "get(ting) the owner to pay for SOIL BORINGS and then to have the structural department design the foundations if possible" , your A/E firm should get the owner to hire a competent geotechnical engineering firm (not a drilling outfit) and have a comprehensive geotechnical report - factual information and interpretative report - issued to you so that your structural department can design the foundations. The specialist geotechnical engineer will provide valuable comments on the type of foundations that you should be considering for the struture at hand (shallow or deep foundations), the allowable bearing pressure (or allowable pile loads), the anticipated settlements for your foundation design along with comments/recommendations on safe excavation slopes if in cuts, required shoring, etc. Then, and only then, should your structural department enter the fray.

As a side-note: I am in the process now of putting together an abstract for a possible paper to the Indian Roads Congress Conference later this year. Many owners/designers do not understand more than rudimentary and begrudging role that a geotech plays. For this paper, I have chosen the title: “The Geotechnical Engineer: Do We Know Their Role? Do We Know Their Worth?”. From my current draft, I am putting forth the following:

"geotechs do more than just drill holes, print out nice logs of the borehole findings and present pretty pictures of laboratory tests."

"In the developing countries, we find that, in many cases, the geotechnical engineer and his role is one of an after-thought – it not fully or even sufficiently understood. The Geotech, more or less, is thrust into the role of borehole driller and factual data presenter. This is the job of a drilling contractor – not a geotechnical engineer. What a waste of precious talent. The true worth of the geotech does not emerge."

theclipper - get good solid geotechnical advice; your life will be far easier!!



wonderful comments by VAD and PEinc once again! Oh, to be a doctor and get all the tests you want. Life and death = tests without "?"; for us, whose small budgets can only permit the bare necessities, I quote Richard Handy: "Virtually every structure is supported by soil or rock. Those that aren’t either fly, float, or fall over.” Many seem simply satisfied with the falling over!(?)

 

[SoilRocks]

To theClipper:

You are pretty brave. So many things can go wrong with soil, even when making conservative assumptions. I'm surprised your firm (and your insurance company) takes that risk. Most structurals I know won't touch geotechnical work with a 10-foot pole. They want guidance on EVERYTHING! Here's what I would suggest along the lines of what BigH said above: Why don't you prepare a standard requirements list in the form of a RFP (Request For Proposal), that you would provide to the owner. The owner would use that to solicit geotechnical proposals. The requirements would request the information you think you would need on the project. Example:

1) Allowable Bearing Pressure for shallow footing assuming
__?__ kip column load;

2) Estimated total and differential settlement between foundation elements;

3) etc...

The list could/should be modified for each project - and don't overdo it (e.g. don't ask for pile information for a project where they aren't expected to be needed). Extra information that you don't need costs the owner money. I occasionally see these standard lists of requirements from structurals, and I don't particularly like them (they're a bit cookie-cutter). But, it's better than getting two borings and no other information!

Good Luck.

 

[Focht3]

Great thread, guys!

to theclipper:

You have received very solid advice from a group of experienced engineers - both geotechnical and structural engineers. Your employer really needs to rethink how they do business. It's damn foolish to expect a structural engineer to go so far out on a limb to evaluate allowable bearing pressure from boring logs alone. Hell, I've got a Master's degree in geotechnical engineering and over 20 years of experience and I won't do that!

When you develop the RFP, be sure you make it one that is a "performance" - type, and not a prescription-type. In other words, ask for allowable bearing pressures, depth of footings, allowable wall pressures, estimated footing settlements, etc. - the things you need. But don't specify the number of borings, test types, number of tests, etc. Do ask for an indication of the number of borings and expected tests to be performed. After all, the guy who only does one shallow boring and no lab tests will almost certainly be the cheapest, but won't give you what you really need.

It sounds as though our advice may cause you to "swim upstream" for awhile. Stay with it - you will find that your designs have fewer problems over time.

 

[jgrek]

to theclipper,

A lot of good comments especially by SoilRocks about the 'performance specs' for geotechnical work.

However, no one has yet adressed your initial question. I think you are looking for calculation aids and useful reference sources that may help you solve your problem. Books, geotechnical reference texts, suseful formulae, Journal Articles or softwares that can assist you in your calculations.

Possibly the equivalent of Gaylord's Structural Engineering H'Book for Soils.

 

[BigH]

jgrek - yeah, but he is only getting the boring log. Obviously theclipper is not comfortable with looking at a boring log and assigning the values to the various formulii that, I am sure, he is well acquainted with. The thrust of most of our comments is why would he take this on himself. Isn't one of the tenets of being a professional engineer the self-evaluation of doing only the work of which you are capable (read that, comfortable)?? I am a geotechnical engineer - I can look up the formula for designing a steel column to steel floor beam connection but I wouldn't feel comfortable since the last time I looked at this was 28 years ago in university. I leave that to you and theclipper.

 

[jgrek]

BigH we are in agreement and your points well taken--

However as engineers we must continually learn and push our knowledge boundaries, especially as young engineers.

Sometimes reviewing the calculation procedures for a engineering issue can be a sobering experience that demonstrates one's lack of knowledge. It is also a great learning opportunity.

We must differentiate between undertaking calculations and sign-off. I do think that 'theclipper' should seek engineering consultation on the work, but if he is a dedicated and sound structural engineer he could soon become an expert.

 

[Ron]

theclipper...as stated by others, you might want to get your firm to reconsider its way of doing business. In some states, you would be practicing outside your stated area of expertise to do both Geotechnical and Structural Engineering, unless you could justify extensive experience in both.

I would suggest you get with a good geotechnical engineer in your area and go over the elements of a competent geotechnical report. If you need further help, contact the Association of Soil and Foundation Engineers (ASFE). They have publications that will help you.

Yes, often there are "recommended" values given in the report for physical and design parameters. Even if not specifically tested for the parameter, the geotechnical engineer's judgment as to the proper value to use should take precedent. They usually don't provide such recommendations lightly.....they are usually based on competent experience and past testing.