Surcharge Pressure on Retaining Walls
Surcharge Pressure on Retaining Walls
(OP)
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.





RE: Surcharge Pressure on Retaining Walls
RE: Surcharge Pressure on Retaining Walls
RE: Surcharge Pressure on Retaining Walls
RE: Surcharge Pressure on Retaining Walls
Can you briefly explain why 45 degrees is the typical angle used for a horizontal zone of disturbance when underpinning a structure and yet the surcharging effects on a concrete wall extent well beyond this 45 degree angle. Most good soils are at 30/60 with yet some even at 26/64 failure zone angles. I guess it has do with the rigidty effects of the concrete wall versus had the soil been its place. Your take please. Thanks
RE: Surcharge Pressure on Retaining Walls
RE: Surcharge Pressure on Retaining Walls
The active failure plane is frequently said to be at 45+(phi/2)above horizontal. Therefore the soil that wants to fail is above a 45 degree plane. I have designed many retaining walls where the existing footings behind the wall were beyond the active failure plane and where I did not consider the footing's surcharge effect on the wall. Now this doesn't necessarily apply to all situations, but it has worked successfully many times in my experience. You really need to make a judgement call based on the soil type, building type, wall type, etc. I've seen different influence lines used by engineers when looking at the influence of one footing on another or on a wall. I've seen 3H:1V (18.4 degrees), 2H:1V (26.6 degrees), 1H:1V (45 degrees), and also 0.577H:1V (60 degrees). They all work sometimes. They all fail sometimes. Look at each situation carefully.
RE: Surcharge Pressure on Retaining Walls
Thanks for excellent advise as usual. I guess the operative word here is "sometimes". For me Soils Engineering can be very gratifiying work and can be very scary at the same time as it relates to this particular issue. Have you ever seen the book "Retaining Walls" by the Canadians M.and A. Reimbert. They have done extensive lab work and testing and they insist that if the surcharge is outside the failure plane it or actually on the failure line it cannot "thrust" the wall.
RE: Surcharge Pressure on Retaining Walls
Soil mechanics (geotechnical engineering) still involves as much "art" as it does hard engineering rules. Frequently the "art" is more important than the rules. With all of the new computer programs and design manuals, the "art" part is being forgotten. Soil mechanics is fast becoming "plug & chug" just like structural engineering.
RE: Surcharge Pressure on Retaining Walls
Another very good book that I have is authored by White and Prentis called "Underpinning" published in 1931 then again in 1950. In it on page 21 they clearly say that 1H and 1V is safely assumed and that its more like 2V and 1H as the actual failure/fracture plane of any good soil.
RE: Surcharge Pressure on Retaining Walls
RE: Surcharge Pressure on Retaining Walls
RE: Surcharge Pressure on Retaining Walls
I got that chart, but that assumes no retaining wall structure and just plain soil suuroundings. That I think is the major problem. Once you introduce a rigid concrete wall or even sheet piling, call it now the "elastic half space" with 1/2 the soil missing, things all of sudden change dramatically on the wall. Both Spangler and Terzaghi have since verified the Boussinesq incredible theory of 120 years ago. None the less a great chart to study. Thanks.
RE: Surcharge Pressure on Retaining Walls
There is also some debate, now, about the multiplication of "2" on rigid wall - you can see Bowles (5th edition) for a debate on this subject. Anyway, I think it is quite common practice (and well founded) to forget any pressures on the wall if your footing is outside the 45 degree line.
RE: Surcharge Pressure on Retaining Walls
I totally agree with you about the 2 factor and somewhat less with 45 degree angle. However, my surchage would only 2 to 3 meters away on a 12 foot high wall. I have 2 retaining wall programs that use the Boussineq non-yielding wall formulas used in NAVFAC DM 7-1 & 2 and is in contradiction to what you are are essentially saying. Basically, beyond the failure plane of say 50 degrees you still get a decent surcharge on the wall. The other tricky issue is when the surcharge is actually placed and how compacted is the soil or was at some point. I don't think you can apply those bulb charts when a retaining wall is the main reason for this post. For settlement issues or vertical pressures in a soil. Yes. Your Take Please.
RE: Surcharge Pressure on Retaining Walls
As for the bulb charts - they still represent the pressure (both vertical and horizontal (but not the same chart)) - and your pressures on the wall will be resulting from the change in stress states under the footing.
I'll do some more checking - I don't use computer programs on this, sorry, I'm an old bugger!
RE: Surcharge Pressure on Retaining Walls
I appreciate your effort in trying to explain to me a very difficult surcharge loading condition to understand. Forget everything thats been posted so far, the main issue is once you have passed the failure plane say at 60 degrees does a surcharge actually occur on the wall?? I have seen it credibly both ways, but I am going with the big guns Boussinesq, Terzaghi, Teng, and Spangler all who say surcharge effects do occur for the full height of the wall(bell shaped diagram) with the resultant at about 0.5H "WAY BEYOND" the failure plane and even including beyond past 45 degrees. Thanks Again.
RE: Surcharge Pressure on Retaining Walls
RE: Surcharge Pressure on Retaining Walls
Consider this: We've probably all have seen railroad tracks sitting on embankments with steep slopes (often about 45 degrees) made usually from relatively poor soils. If you decided to build, and then backfill, a new retaining wall at the base (toe) of the existing slope (and didn't add any more tracks), would the new wall be subjected to a railroad surcharge? I doubt it. However, according to many analyses, including Boussinesq, there would be a lateral surcharge on the new wall due to the track. If there wasn't any dirt there to begin with, and if the slope was originally stable, how can a new wall built beyond the toe of slope be subjected to a surcherge? I know what the equations tell us, but sometimes we have to factor in a little common sense too.
RE: Surcharge Pressure on Retaining Walls
I love your common sense approach, however, if I end up in a lawsuit and court over a collapsed embankment at 45 degrees with workers killed on the job, common sense tells me I am going to loss the case hands down, for sure my business will fold up and may be go to jail for NOT applying what is widely accepted and proven engineering practices. The one thing to keep in mind here is that these Bossinesq formulas are based on linear differental equations which is far from common sense. Its 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. Does this qualify as common sense? Your Witness rather Your Take Please.
RE: Surcharge Pressure on Retaining Walls
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."
RE: Surcharge Pressure on Retaining Walls
Check out this incredible pdf.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.
RE: Surcharge Pressure on Retaining Walls
RE: Surcharge Pressure on Retaining Walls
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. 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.
RE: Surcharge Pressure on Retaining Walls
Site has moved to this new adress.http://www.lrrb.org/more.cfm?code=1975
RE: Surcharge Pressure on Retaining Walls
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.
RE: Surcharge Pressure on Retaining Walls
RE: Surcharge Pressure on Retaining Walls
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
RE: Surcharge Pressure on Retaining Walls
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.
RE: Surcharge Pressure on Retaining Walls
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!
RE: Surcharge Pressure on Retaining Walls
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.
RE: Surcharge Pressure on Retaining Walls
RE: Surcharge Pressure on Retaining Walls
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