Surcharge Pressure on Retaining Walls 8

[cap4000]

I am looking for clarification on line, strip and point load surcharge pressures. Is it when the surcharge is only located inside the soil failure wedge say 60 degrees up from the heel footing (AASHTO way) or is by the traditional Boussineq and Terzaghi Formulas. Both ways yields quite a difference in the "thrust load" on the wall. NAVFAC DM-7 uses the traditional Terzaghi formulas even if the surcharge is well outside the failure wedge zone. Can some explain this big difference in analysis. Thanks in advance.

 

[PEinc]

cap4000,

You said, "It's possible that once the retaining wall is actually installed then backfilled in your scenario, the horizontal soil stresses go from being relaxed and stable say at 45 degrees to now "exicited" and then crushed by the non-yielding retaining wall now built in its way."

I really don't see how buttressing an existing slope with more embankment will increase lateral pressure from the previously existing footing or railroad track. Just as in footing design, more overburden or soil confinement helps improve bearing capacity. The soil beneath the footing is more competent. How would this be any different when adding embankment to an existing slope?

As for the 45 degree slope, I wasn't trying to advocate designing steeply sloped, unreinforced embankments. I'm just telling you what railroads have done for over a hundred years when they are paying for it. When someone else is paying, such as a DOT, the railroads are much more demanding and conservative. It becomes, "Do as I say, not as I do."

 

[cap4000]

PEinc

Check out this incredible pdf.

http://www.info.usda.gov/CED/ftp/CED/tr74.pdf

in particular pdf pg 38 of 152. Could this be the end of this going back and forth?? I doubt it. Your Take.

 

[BigH]

Its this going back and forth that drives the though processes and opens up new approaches, etc. I enjoy the tos and fros. Will look up the site tomorrow.

 

[cap4000]

BigH

Check out the instrumentation actually just done on this amazing retaining wall experiment. The internet is an amazing library tool if you have the time to search things out.

http://www.lrrb.org/pdf/200514pdf.

Note the actual bearing pressures measured under the footing is opposite to what all books indicate on how to design the toe and heel.Unreal.
Your Take.

 

[cap4000]

BigH

Site has moved to this new adress.

http://www.lrrb.org/more.cfm?code=1975

 

[PEinc]

cap4000,

Thanks for the USDA pdf. I looked at Page 38. I agree. However, I believe that the concept is more important than is the actual angle of influence. 40 degrees vs. 45 degrees? - makes little difference.

 

[BigH]

MnDOT report looks interesting (a very brief cursory look at only a few sections) and something that should be followed up especially the bearing pressures on the cantilever wall base slab. Hope some other DOTs or FHWA carry out similar studies with other foundation and similar foundation types to build up sufficient evidence for "text" book or "this study" findings. Thanks.

 

[DRC1]

Wow what a great thread. Just thought I would throw in my 2 cents.
1. If you actually compute wall pressures using Boussinesqu or Terzaghi or similar methods, The point of maximum pressure is located at about the depth equal to the offset of the load. As you move the load back, it a.) lowers the point of maximum pressure on the wall and b.) diminishes the magnitude of the load. So by the time the load is at the intesection of the 45 deg. line to the base of the footing, the maximum pressure is at the base of the wall and is fairly reduced. I have found that most times if you can keep most loads traffic 5 to 7 feet away, the loads do not have a significant effect on the wall (You should verify that for your structure, as I said most times) P.E Inc's comment is valid that surchage loads beyond the failure plane will not significantly impact the wall. The important point to remember is that if the surcharge is significant, it will change the failure plane. I don't know if bousinesque theory would apply to the railroad embankment. The embankment has settled and developed a load suppporting strucure. The new fill will not disturb that strucure. Since there would be no additional movement of the embakment, the load would not transfer to the new fill. Thus it really is not the same media assumed by the model. Fianally, an easy way to do this is to use a Culman's diagram, which with cadd is really fairly quick. This will give you the lateral load and the straight line approximation of the failure plane.
In the end I feel the 40 or 45 or 60 deg. slope is some what emperical, based on what will leave a significant impact. Any load will in theory impact the wall and will impact the full height of the wall. Generally it is best to run the numbers and check it. Doing math is why we all became engineers anyway- Right?

Good Luck

 

[cap4000]

DRC1

I agree. Wow. What an informative exchange of views. Boussinesq is probably rolling around in his 77 year old grave as I think he has by now figured out a differntial equation on how to twist and turn while buried.

 

[jheidt2543]

A very interesting thread, cudos to all! I looked up the two books mentioned and found the following:

White and Prentis book "Underpinning" published in 1931; used first edition in good condition $450 US.

"Retaining Walls" by M.and A. Reimbert; used good condition $54 US.

If you have a copy of the White and Prentis book, treat it well!

 

[cap4000]

jheidt2543

I have both of them in addition to Huntington,Terzaghi and Spanglers earth pressure books and would not sell any of them only because they would be of my main defenses in court if I ever had a legal problem with these tricky soil issues. Another name not as well known is Anthony Goh who has done extensive retaining wall field tests in 1993 and has concluded the active pressure is too low to use in design and the backfill pressure is actually between active and at-rest and not Rankines. The typical safety factor margins used everyday in retaining wall designs is what he claims actually keep them from failing. Scary.

 

[cap4000]

Perhaps EM 1110-2-2502 dated Sept.29,1989 will put this to bed. On page 3-51 the "a" factor on the bottom of the page stops at 0.7. Its possible that no surcharge effects occur at 0.71 and up. Maybe? This seems to verify Reimberts lab results. Merlin Spangler in 1956 is credited with finding the now famous 2 factor controversy done by Boussinesq normally for only solid materials and not soil. To be continued.

 

[miecz]

Some years back I was doing a field inspection at a job site where I had designed an aeration tank that was about 20 ft deep. Roaming the job site, I noticed that the contractor had excavated for another tank and dumped the excavation near the aeration tank. I was looking at a 20 foot hill that began some 10 feet away from the aeration tank. I had not designed that tank for any surcharge. We immediatlely filled the tank with water, and I nervously set about to find "what distance to the surcharge would have no effect on the tank wall."

I based the solution on formulas from a book published in Poland. The interesting result was that the "distance for no effect" was related to the ratio of the height (or intensity) of the surcharge to the height of the wall. Here's my solution.

K=ho/H
R=2K/(1+K)
B=(2-R)tan(phi)-(R/tan(phi))
tan(gamma)=(sqrt(B*B+4)-B)/2
a=H*tan(gamma)

where/
a=clear distance to the surcharge
ho=height of the surcharge material
phi=soil angle of repose
H=height of the cantilever wall