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Surcharge Pressure on Retaining Walls8

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

If I understand correctly, the answer to your questionn and probably your confusion results from the problem that you are trying to analize.  If you want to know the stress distribution on a concrete wall for a surcharge then a Boussinesq type of analysis may be appropriate.  Remember that if the wall is un-yielding then the pressure may be double that of Boussinesq.  If you are trying to analize the stability of the wall then think of the surcharge as part of a slope stability analysis where the slip surface includes the surcharge.  Remember that earth pressure analysis on a retaining wall is simply a special case of slope stability.

RE: Surcharge Pressure on Retaining Walls

(OP)
I am only looking for the stress distribution on the wall. Say its unyielding is it really double or appropriate?? The other main thing is the distance off the wall and when the surcharge load actually has no effect on the wall.

RE: Surcharge Pressure on Retaining Walls

It is commom to assume that there is no lateral pressure on a wall if the vertical surcharge is located beyond the failure plane or some similat line of influence.  However, not all plan reviewer or agencies will accept not applying some load.  For instance, most railroads want the designer to use a Boussinesq analysis for the surcharge load.  With Boussinesq, you enter an offset distance to where the surcharge begins.  The farther away the surcharge, the less the resulting lateral pressure.  For whom are you designing this wall?  What is their surcharge requirement?

RE: Surcharge Pressure on Retaining Walls

(OP)
PEinc

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

Cap4000 is closest to the truth,  the influence of the pressure distribution occurs as some depth equal to the friction angle minus 45 degrees from the vertical.  Who knows for sure!   60 degrees is a reasonable number because 45+27=67 is a resonable number.  These theories are provided for guidance only.  As engineers we must think outside the box.  What if this particular theory doesn't work in this case.  Look at the range of possibilities. Will your design work for all of those possibilities.  If you don't design for the range you will be burnt at the stake.  If you want to design outside the range then specifically state that this is a nonconcervative design and that if they want assurances then the design would be different.

RE: Surcharge Pressure on Retaining Walls

cap4000,

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

(OP)
PEinc

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

Thanks, cap4000.  I have not seen the Canadian book, but it sounds like something I'd like to read and possibly quote as a reference in my many battles with plan reviewers.

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

(OP)
PEinc,

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

There's not much that's been written about underpinning that wasn't written by White.  Most newer books just reprint what he wrote.

RE: Surcharge Pressure on Retaining Walls

I tend to agree that if the footing is outside the active zone - defined as the sliding plane, then you would not need to consider surcharge on the wall.  If you look at a Bousinesque pressure bulb for vertical pressures (and take the horizontal pressure as 1/2 - for discussion purposes - you would not see anything significant on the wall - for a footing to affect something that is that far away would require a very large footing and a very high wall and a very high bearing pressure.  You could always do a graphical method replacing the fooing pressure by equivelent soil height and see if the lateral force is affected.

RE: Surcharge Pressure on Retaining Walls

(OP)
BigH

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

Well, I am not so sure - I can't see how your wall will see such low pressures that would be, say at a nominal distance of 6 to 7 m away with a wall height less than 8 m - the pressure distribution is so low horizontally that I don't see how there would be much interaction.  I always did the calculations if my footings were close to the wall which, by your comments (" 1/2 the soil missing " ) it appears that you are putting forth.  Draw the whole situation up by pencil and paper to scale - put on the bousinesq bulbs and you will see.  Actually you probably will find the horizontal pressure bulbs in Poulos and Davis' Elastic Solutions for Soil and Rock.

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

(OP)
Big H

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

You say you get a "decent" surcharge on the wall - how "decent" is decent? (say a percentage of the total lateral earth pressure)  How is the pressure distributed?  Is it mainly in the lower half of the wall - or lower 1/3 or 1/4?  If you have your surchard 2m from the wall - a 4m high wall - then your angle from the horizontal is about 75degrees (contradicts your initial post of being outside the 60deg angle) - this is within the active zone of 45+30/2, say, or 60 degrees and definitely within the 45deg zone.

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

(OP)
Big H

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

Can't disagree with your approach - YOU have to feel comfortable with what you are using.  I still would, though, like to know what the calculated surcharge loading on the wall is - based on your calculations (and the geometrics of all the forces, etc) - just to let me understand how much loading (compared to active pressures) there really is. Please advise. . . . and good luck.  It would be nice if you could instrument your wall for confirmation - and to prove/disprove the various thoughts.

RE: Surcharge Pressure on Retaining Walls

cap4000,

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

(OP)
PEinc

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

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."

RE: Surcharge Pressure on Retaining Walls

(OP)
PEinc

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

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.

RE: Surcharge Pressure on Retaining Walls

(OP)
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. 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.

RE: Surcharge Pressure on Retaining Walls

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.

RE: Surcharge Pressure on Retaining Walls

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.

RE: Surcharge Pressure on Retaining Walls

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

RE: Surcharge Pressure on Retaining Walls

(OP)
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.

RE: Surcharge Pressure on Retaining Walls

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!

RE: Surcharge Pressure on Retaining Walls

(OP)
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.

RE: Surcharge Pressure on Retaining Walls

(OP)
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.

RE: Surcharge Pressure on Retaining Walls

3
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

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