Atterberg limits and soil classification

[fa2070]

Hello,

Can anyone please refer me to a layman's primer on soil mechanics? Book..website..posts..whatever..
Preferably I'd like a super-short rule-of-thumb-type guideline that tells me how to identify and classify soils based on their Liquid Limit, Plastic Limit, Plastic Index and fineness (% that passes sieve #200). Also, I would like a method to infer the modulus of subgrade reaction (k) associated with the aforementioned properties.
It happens that many reports contain these properties (or a subset of the Atterberg limits) but don't tell me if the soil is silt -or- clay with/without plastic or compressible characteristics.

I'm a civil engineer and not a soils specialist, and all textbooks on soil mechanics that I have skimmed are lengthy and full of differential equations rendering them mostly impractical.

Thanks in advance.

 

[fattdad]

ASTM D-2487 provides a flow chart for classifying soils based on the percent fines (i.e., passing the 200 sieve) and the Atterberg limits. There are a subset of soil types that require the full sieve set to identify the gradation characteristics that allow the determination of "well-graded" versus "poorly-graded" sands or gravels.

I know of no reliable correlation between soil classification and subgrade modulus (not saying there isn't one), I just know that a soil that classifies as a silty sand or a clayey sand (for example) can have widely varying subgrade moduli. I use CBR values to correlate the subgrade modulus as the prepared sample is typically normalized to a reference density (i.e., 95 percent of the Standard Proctor maximum dry density).

I'd give you a reference book, but will not be in the office for a few days. I'm sure others will chime in.

f-d

¡papá gordo ain’t no madre flaca!

 

[fa2070]

Hello, thanks fattdad for replying.

Definitely ASTM D2487-06 is a must, but, to complement it, what other good book(s) on soil mechanics coupled with foundation design concepts would you recommend ?. Ideally, I'd like a book that introduces soil mechanics from a practical point of view, together with hints and guidelines for approaching foundation design for each type of soil, or family of soils better said. I'd like something practical, a how-to type of book.
I'm looking for something along the lines of:

[soil family][blue] -->[/blue] considerations before approaching foundation design for this soil [blue]-->[/blue] pitfalls to avoid with this soil (things to be aware of) [blue]-->[/blue] Compaction mandatory ? (yes/no)[blue]-->[/blue] lime stabilization needed ? (yes/no) If yes, how to do it. -OR- cement stabilization preferred ? (yes/no) If yes, how to do it. [blue]-->[/blue] recommended compaction procedures for this type of soil [blue]-->[/blue] Atterberg limits and their threshold from/to other types [blue]-->[/blue] average "k" e.g. "the range of k for clays goes from .... to ...." [blue]-->[/blue] average unit weight of this soil [blue]-->[/blue] further information the engineer should ask the soil specialist (if fine calculations are mandatory) for this type of soil.

I understand there's a big gamut between the different types of soils and certainly are not typifiable as I summarized above, but I guess there must be some shortcuts senior engineers use to do quick calculations, mostly for quoting purposes.

As an aside this is what I don't want. I don't want a general purpose (text)book on soil mechanics. I don't want books that delve deep in how the SPT is performed, or Mohr's circle plus all that truckload of theoretical stuff always seen on books on soil mechanics, which, regardless of the authors, seem to have been sharing the same Table of Contents for decades.

Thanks.

 

[BigH]

Robert W. Day wrote a "pocket" geotechnical handbook that is geared towards what you want. For reference, see:

http://www.amazon.com/Geotechnical-...16/ref=pd_sxp_grid_pt_1_2/102-2497192-1104115

 

[fa2070]

Thanks for the reference, BigH.

 

[fattdad]

I'm not aware of a reference book that can satisify your questions. And, I doubt there will be one (well answer ALL of your questions).

Considerations pertaining to foundation design require more than just "soil type". It requires an understanding of the geology within the influence zone of the foundation. The geology will dictate what soil (or rock) characteristics are the most important. Each strata may have a different item of concern (i.e., consolidation characteristics, position with the water table, moduli, strength).

Regarding "pitfalls", there are some general trends, but there is also local experience (and contractor's familiarity).

For a load-bearing fill, as far as I'm concerned, compaction is mandatory.

Lime stabilization (from my experience) is a constructability issue and related to moisture content. If the cut soils are too wet and there is insufficient time to dry, the additon of lime can help adjust the moisture-density relations to allow better workability. It comes with greater expense, but can allow construction to move forward. There is no general relationship that would say, "SM, CL, CH, etc." warrants lime stabilization. Rather, a silty sand (i.e., MH fines and 55 percent sand) with a natural moisture content near the liquid limit with earthwork scheduled for the fall), may really benefit from lime stabilzation.

Recommended compaction proceedures: Again, this is often to be determined by the contractor. There are some general "rules" (i.e., fine grained soils are better compacted using a pad-foot compactor), but often we are better left "seeing" what the contractor has in mind. Our goal is to demonstrate via field density testing that the contractor is doing what is specified. That said, non-plastic sands and silty sands are best compacted using a vibratory smooth drum.

The ASTM I referenced gives threashold values for Atterberg limits for the various soil types.

"Soil Technology and Engineering Properties of Soils", page 139 has a chart that shows general correlation between Unified Soil Classifications and subgrade moduli as well as CBR. I don't know the author of this reference (others may chime in) as I just have a photocopy of the chart.

You may want to check DM-7.1 and DM-7.2 for additinal information.

f-d

¡papá gordo ain’t no madre flaca!

 

[fa2070]

Thank you fattdad for your thorough insight on the subject.

 

[dirtsqueezer]

It just sounds like what you're looking for is a book that summarizes the practice of geotechincal engineering in a nutshell. I reality, those guys go to school for however many years and collect a library of books, each with the different data you're asking for along the way. I think your best source would just be a geotechnical engineer. Take him to lunch, sub him out for a project. He could probably give you those figures in five pages through a copy machine. Just be sure you're buying him/her good beer.

 

[SoilRocks]

fa2070,

Take the advice of dirtsqueezer and take a geotechnical engineer out to lunch. You are about to go down a dangerous path with soil modulus values. Keep this in mind - soil modulus values are NOT soil properties - it is just a model element. THERE IS NO SOIL MODULUS VALUE FOR EACH SOIL TYPE. The soil modulus values you will find listed in textbooks are typically for 1-foot plate load tests (very limited depth of influence - i.e. think tire load). Soil modulus values are used to "model" soil conditions, from simple to complex.

However, congratulations to you, you're way ahead of the crowd by having the curiosity to question numbers reported without any description of how they were derived.

 

[fa2070]

Hi,

You're right dirtsqueezer. I couldn't have said it better.

As a general purpose civil engineer, one doesn't have too much time for trial and error with regard to further education. By trial and error I mean taking courses, seminars, reading publications, books, whatever... with a low signal to noise ratio. I'm not implying that all publications on Geotechnics or Soil Mechanics qualify as such, just that they're mostly targeted at geotechnical engineers. On the other hand I see there's very little targeted at the practising civil engineer that has to get things done, usually within time constraints. For such person that has to deal with the (huge) broad scope of civil engineering a resource that aggregates the (summarized) collective experience in Geotechnics in the form of a knowledge base with FAQ's, DO's and DONT's, a few representative case studies and prescriptive guidance on foundation design + some theory to back all the concepts would be really priceless. Of course, by no means should the job of a geotechnical engineer be taken over by a civil engineer, it's just that not everyone has the luck of having one in the team.
In the meantime I'll give the books recommended by fattdad and BigH a shot.

Thanks.

 

[BigH]

fa2070 - actually, you many times find some good articles of generality in Structure Magazine, CE Magazine and some other structurally oriented magazines. Also, the geo section in the CRC Civil Engineering Handbook is pretty good primer too (as well as Baker's Handbook of Highway Engineering).

 

[dirtdr2]

Just a word of caution in utilizing subgrade modulii. There are different subgrade modulii for different applications, one for vehicular load (derived from CBR relationships) and the other from limiting deflection. The actual modulus will be a function of the nature of the soil, load type, shape of the loaded area, and deflection (total or allowable). As an example for a large uniformly loaded slab (i.e. warehouse) where only a modest amount of settlement or deflection can be tolerated the subgrade modulus can go well below 10pci. Where as use of the CBR charts would indicate values of 75 to 125pci.