In the USCS and force of friction

[camanu18]

Hi everyone. I have two questions. The first one: Is there a chance where you have to use 3 simbols? for example: SP - SM - PT...I've read about it, but i'm not sure if that's correct. I think I could happen.

The second question is about the force of friction generated in the interface soil soil against a vertical force that try to raise a block of concrete. How can I calculate this force? I've heard about properties similar to cohesion and angle of friction. I think one is adhesion. Does anyone has a paper or a web page where I can find how to calculate this force?

Thank you so much for your answers,

Manuel Asmat.

 

[camanu18]

It's me, Manuel, again. Just to add something. I'm working in a block of retention for a pile of transmission lines that use a block of retention. In the design of it it's usual that you do not consider the force of friction between soil-soil. I want to consider it to reduce the size of that block in order to reduce costs, but I don't know who to evaluate this force. The block is in traction. I hope this can help understand my problem.

Thank you so much again,

Manuel

 

[jdonville]

Manuel,

I am not quite sure what you are asking, but since you use the word "pile", I am assuming that you have driven pile foundations for your transmission tower.

Check out FHWA HI-97-013 "Design and Construction of Driven Pile Foundations Volume 1" available for download from

http://www.fhwa.dot.gov/bridge/geopub.htm

. Chapter 9 discusses design of pile and pile groups for compressive and uplift resistance. I would tend to be a little careful as the foundation elements for your transmission tower may not all be in uplift at the same time, and may be also subject to lateral (horizontal) loads.

You may also find some guidance in the US Naval Facilities Engineering Command (NAVFAC) Design Manuals 7.1 and 7.2. These can be downloaded from

http://www.vulcanhammer.net

.

Good Luck!

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[jdonville]

camanu18,

About the USCS symbology, it is not possible to produce a 3-symbol classification for the soil, according to the procedures of ASTM D 2487.

Either the soil is a sand with organics OR a peat/organic soil with sand. It depends on the major classification/properties of the soil.

D 2487 indicates that a coarse-grained soil has more than 50% retained on the #200 (0.075mm) sieve. A sand, similarly, has more than 50% of the coarse fraction passing the #4 (4.75mm) sieve. So a soil could, in theory, be classified as a sand by USCS with as little as (50.1) x (50.1) = 25.1% actually being sand sizes by mass.

PT is used to classify a predominantly organic soil composed mainly of fibers and other decomposing organic material. Peat tends to be dark-colored, spongy and have a certain odor.

Organic soils without visible root, fiber, leaf, woody, etc. structure (i.e., residual organic soils) should probably be classified as OL or OH, depending on index properties.

Relatively thin layers of residual soils at/near the ground surface should probably just be described as "Topsoil" and not assigned a group symbol unless tested for grain-size distribution and Atterberg limits.

Please feel free to disagree with me on any of these points.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[camanu18]

Hi Jdonville, first I want to thank for answering me so soon. My question about the 3 symbols clasification for a soil is because I read a document from Corral (1971) where he said that in a very rare case it could happen. Also, he says that if It happens you just have to use the most representative symbols (Just 2). I think It could be added, just for purpose of design, not clasification, select the two symbols that give the designer the worst soil. Just to be conservative. Then, for practical purpose It's not possible and non practical to produce 3 symbols, but in real purposes It could happen. What do you think?

For the second question, I'm talking about "piles" that for example are embedded 2 meters and its total lenght is 15 meters. They could be made of steel, wood, concrete, etc. Some of this ("R" type), need to have a structure, like a cube of concrete that estabilizes the moment of overturning. This are used for high voltage transmission lines (except piles of wood).

This cube of concrete, is joined to the pile by a cable that transmits to the cube a force of traction that tries to get it out of the soil. Here appears a force of friction in the interface soil - soil against this traction. How can I calculate this force?

Thanks a lot for your help,

Regards,

Manuel.

 

[BigH]

The chap may have used the three symbols more of in a descriptive way. Pt (peat) is not part of the USCS classification system per se. Coarse grained soils usually don't see significant organic - maybe in this case it did so he added that the sands had significant organics - hence the Pt. The dual symbol SM-SP is for a very narrow window of the classification of materials (PI between 4 and 7). Anyway, this is the fallacy of using just classification symbols only for descriptions. I like the old fashion way of saying, for example: Compact (medium dense to the Yanks) grey silty sand trace gravel, trace clay, trace organics. In the classification usually used in Canada (Geocon and Golders - back in the 60s) was trace = 0 to 10%; some = 10 to 20%; "-ty" = 20 to 35% and "and" between 35 and 50%. There are other terminologies by different organizations that change these about a bit.

 

[jdonville]

camanu18,

USCS: Like BigH, I prefer to use more descriptive classifications for soils rather than just symbols. The US Army Corps of Engineers explains their (fairly standard) terms in Appendix F of their publication EM-1110-1-1804. It's good stuff - check it out at

http://www.usace.army.mil

.

Concrete Block: What you describe in your last post seems to me to be an anchor block for a guyed monopole. The fact that it's suporting a pole (not a pile) is not particularly relevant to the analysis of the block. What is important is knowing what maximum (design) vertical force will be exerted on the block by the guy cable.

The conservative design approach will likely assume that the sole resistance to vertical load is provided by the self-weight of the anchor block, unless the design code explcitly permits side resistance to be included. I would also imagine that if the design code permits side resistance, then it would also offer guidance on how to model the resistance.

However, as a mental excercise, we can look at possible approaches to determining the long-term side resistance.

Long-term, after the backfill has had time to settle around the block, we can look at the adhesion (in a cohesive soil) or friction (in a cohesionless soil) on the sides of the block and assume that there is no wedge displacement of the surrounding soil to simplify the idea and still stay fairly conservative.

I would think that an approach similar to that for the design of drilled shafts would be more appropriate than the approach for driven piles.

I would probably use a value of skin resistance of 0.7*0.55*su (see FHWA IF-99-025 "Drilled Shafts - Construction Procedures and Design Methods", pp.280, 288) for cohesive soil surrounding the block.

For cohesionless soils, the uplift resistance formulae for drilled shafts are dependent on the soil type and are given on pages 281 and 288, with further discussion in Appendix B.

Alternatively, it could be argued that the correct approach would be to look at the adhesion/friction force determination for backfilled rigid retaining walls. There are many texts on the subject (the NAVFAC manuals cover this subject, and the Canadian Foundation Engineering Manual also covers the subject fairly well), so I see no reason to tread over this ground again.

Note: Take care to check that the tendency of the block to overturn about the corner nearest the top of the tower has an adequate factor of safety. Also, depending on the backfill conditions, it may be prudent to assume that the near-surface backfill material will yield.

Hope this gives you some ideas,

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[dmoler]

The ASTM subcommittee D18.18 was revising the organic classification system for organic soils. I dont know if they have officially submitted the revisions yet. A summary of the proposed changes is at this document, Chapter 2:

http://www.dot.state.fl.us/research...tion of embankment distress at sanders creed'