Cantilever retaining wall with narrow site cut
Cantilever retaining wall with narrow site cut
(OP)
Hi all!
I've got a project here where a basement retaining wall is being loaded with only 2-3 feet width of backfill. The site is on hard limestone so the cut is nearly vertical. The geo report provides active and at rest EFP. Since this is not a standard retaining condition, I'm wondering if these EFP can be reduced based on my situation.
If so, is it appropriate to use a different angle of internal friction or some other rational method? Can this be done without the geo recalculating the EFP for me? I'm just wondering for the future if this can be done without geotechnical intervention.
My guesses are that either the EFP is lower, or the EFP remains the same and the load resembles a triangular distribution that is "capped" off at some point, resulting in a triangular load to a uniform load.
I've got a project here where a basement retaining wall is being loaded with only 2-3 feet width of backfill. The site is on hard limestone so the cut is nearly vertical. The geo report provides active and at rest EFP. Since this is not a standard retaining condition, I'm wondering if these EFP can be reduced based on my situation.
If so, is it appropriate to use a different angle of internal friction or some other rational method? Can this be done without the geo recalculating the EFP for me? I'm just wondering for the future if this can be done without geotechnical intervention.
My guesses are that either the EFP is lower, or the EFP remains the same and the load resembles a triangular distribution that is "capped" off at some point, resulting in a triangular load to a uniform load.






RE: Cantilever retaining wall with narrow site cut
We often have imperfect or missing geotech data on walls that we do. In such cases, we will assume a k_active of .5, which is pretty high. We'll also keep the bearing pressure under 1,500psf. We then generate a design and compare to standard designs published by TxDot. (I think Florida and California have published designs too.)
Link
http://www.dot.state.tx.us/insdtdot/orgchart/cmd/c...
If you take that approach, you can save the geotech money, but you might end up with an overly conservative design.
RE: Cantilever retaining wall with narrow site cut
Unfortunately, no. The EFP is calculated based on the granular fill's unit weight and angle of repose. These two properties do not change with horizontal thickness. The EFP calcs "transform" the effective properties of a "heavy" granular solid into a "light" liquid. This "light" liquid now is assumed to act like a "real" liquid, say, water.
Consider your project, if the substance being retained was actually water (instead of granular fill), the hydrostatic pressure on the wall would be exactly the same even if the horizontal distance was 2 feet or 2 inches or 2 miles. The same reasoning applies for an "equivalent fluid".
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RE: Cantilever retaining wall with narrow site cut
If water, then yes, width behind wall is unimportant unless seismic is considered.
However, the very simplified equivalent fluid pressure analogy for soil is no longer valid for thin backfill strips (column of soil). Total backfill weight and friction are relevant.
Having said that, I would proceed with caution and conservatism. Walls and backfill are never wished into place. Consider the saturated condition and potential pore water pressure, and how the backfill is placed and/or compacted, and the resulting lateral pressures may be similar to conventional construction.
For low volume, you may be better off backfilling with 4 foot lifts of CLSM ("CDF" / 2-sack slurry).
RE: Cantilever retaining wall with narrow site cut
It seems to me that this "wedge" has been altered thus we need to alter our design method.
RE: Cantilever retaining wall with narrow site cut
RE: Cantilever retaining wall with narrow site cut
Unfortunately there is a lot of basement wall and fill, so this would not be a cost effective option. This wall is being constructed now. There is a PVC drain pipe and a Miradrain waterproofing system. And I did end up designing with the full EFP in the end just to be safe.
I just wanted to bring this up for discussion in case this situation comes up again.
RE: Cantilever retaining wall with narrow site cut
ATSE - I won't dispute that there are other methods, besides EFP, to compute the force on a retaining wall. But the OP is working with EFP values. Since that is what is available, the simplifications and limitations that go with an EFP approach can not be ignored. If the granular backfill is going to be "transformed" to an equivalent fluid, then the equations that apply to true fluids must be followed.
sail3 - Absolutely. For a horizontal gap that is continuous from top to bottom, a measurement of 1/8" (or less) will put full hydrostatic pressure on the wall. Here is an example:
Assume water weighs 62.4 lb/ft3
Water pressure (lb/ft2) depends on ONLY one variable, the water depth. For a 10' high wall, pressure is 624 lb/ft2.
The force per foot of wall length (from water alone) is depends only on the water depth. For the 10' high wall that is Pw = 1/2 (62.4 lb/ft3) (10 ft.)2 = 3120 lb / foot of wall length.
Note that nowhere does the horizontal distance (1/8", per your question) enter into the equations.
The combined force on the wall from the water's hydrostatic pressure plus the soil behind the wall can get more complicated... the soil may be submerged instead of being "dry".
bhiggins - I've marked up your drawing (see below). If the wall was suddenly removed, the granular fill outlined in red is what would collapse. Note that this occurs along the "typical soil failure plane". Soil properties define that plane, not the horizontal dimension.
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RE: Cantilever retaining wall with narrow site cut
RE: Cantilever retaining wall with narrow site cut
If 24" (horizontal) is "ok" but 1/64" (horizontal) is not... exactly where does the change happen? The equations would have to define that situation, but they don't. Horizontal distance is not a factor.
In "real life", I do accept that a continuous 1/64" wide gap that is 10' (or so) tall is unlikely. In that case, it's not the theory or the equations that are wrong, it's just that other issues override them.
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RE: Cantilever retaining wall with narrow site cut
RE: Cantilever retaining wall with narrow site cut
RE: Cantilever retaining wall with narrow site cut
SRA - The thrust on the wall is a function of the vertical load behind the wall e.g. consider a surcharge at the top of the wall. A 1/64" gap filled with a column of water cannot produce the same thurst as say as a 6 foot gap fIlled with a column of water, assuming the limestone is impermeable, the thrust would be magnitudes greater that the weight of the column. I would suggest the equations are assuming the depth behind the wall is considered part of a continuous medium which would be typical, so maybe not applicable in this case.
RE: Cantilever retaining wall with narrow site cut
http://www.eng-tips.com/viewthread.cfm?qid=197530
RE: Cantilever retaining wall with narrow site cut
RE: Cantilever retaining wall with narrow site cut
For water at 62.4 lb/ft3, I get 3120 lb/foot of wall length in both cases.
If there are other equations for special cases of water pressure I would like to know what they are.
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RE: Cantilever retaining wall with narrow site cut
RE: Cantilever retaining wall with narrow site cut
RE: Cantilever retaining wall with narrow site cut
RE: Cantilever retaining wall with narrow site cut
Do you only have equivalent fluid pressures from the geotech report? Pretty ordinary report if so.
If you have some geotechnical parameters such as friction angle, cohesion and density, there are ways to calculate a reduced force such as doing a Coulomb wedge analysis and making an estimate of the height that the resultant force acts on the wall. Taking SlideRuleEra's red-line picture a few posts above, you can see that a full active wedge isn't available so the load on the wall is reduced.
Alternatively (or also), there are equations for pressure on silos and bins that take account of the narrow width compared to height.
On the hydrostatic pressure vs width question:
It only depends on fluid density and depth.
However, consider a very narrow pool of water with a limited volume of water (ie doen't get topped up constantly). When the wall moves, the width increases and the water depth reduces...
RE: Cantilever retaining wall with narrow site cut
RE: Cantilever retaining wall with narrow site cut
- The truly hydrostatic lateral load on the wall arising from ground water is independent of the horizontal extent of the retained mass. Just like fluids class.
- The equivalent fluid pressure load on the wall arising from soil shear failure is very much dependent on the horizontal extent of the retained mass. Less horizontal extent means less load in a way that may be complicated by friction against the wall, increasing shear strength with overburden depth etc. The stuff Okiryu pointed us too leaves little room for debate on this in my opinion.
- The behavior of a shear-weak material like soil is not the same as the behavior of a shear-less fluid like water. The equivalent fluid pressure business is just a convenient device used for calculation, not an endorsement for soil masses being characterized as fluids.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cantilever retaining wall with narrow site cut
I had a similar wall project just last week involving a wall near to ledge and spent a lot of time pondering this situation of a narrow strip of soil behind the wall and the loads imparted on it. My understanding is the failure plane leading to the wall will be greatly changed by the presence of the unmoving vertical surface. Thus, the soil will slide not along it's typical failure shear plane but some different plane, which reduces the load it puts on the wall.
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RE: Cantilever retaining wall with narrow site cut
1. Depends on two, and only two variables, the unit weight of the soil and the soil's angle of repose.
2. The one, and only one correct way to use this EFP is to consider the soil to be an ideal liquid that performs as an ideal liquid.
3. Based on Item 2 (above), when using EFP the horizontal dimension of the soil is not a factor.
Don't believe it... here are three authoritative sources from past and present to confirm it.
From: "Highway Bridge Superstructure Engineering: LRFD Approach to Design & Analysis", Taly, Published 2014, Page 313
From: "Handbook of Building Construction", Hool and Johnson, Published 1920
From: "US Steel Sheet Piling Design Extracts", Published c. 1932
Thanks to jayrod12 and hokie66 for the vote of confidence. This subject is very important, the load from the soil is real (not a statistical probability, like wind loads). Making erroneous assumptions increases risk.
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RE: Cantilever retaining wall with narrow site cut
1. How to use equivalent fluid pressure?
If I were being unkind, I'd say throw it and anything that mentions it in the bin. As SlideRuleEra's source says in section 3.22.4: "The equivalent-fluid method may be used where the Rankine earth pressure is applicable." Rankine coefficients are so simple to derive and use there is no reason to start talking about mythical fluids. If, however, you choose to use EFP and don't know how the EFP numbers were derived, best stick to what SlideRuleEra says.
2. Is EFP conservative in this case?
Maybe so. Use one of the other methods mentioned in this thread or linked threads and compare to Rankine. I don't think you'll end up with a hydrostatic lateral pressure distribution though, so it won't back-convert to an EFP.
RE: Cantilever retaining wall with narrow site cut
Historical Hydrostatic Paradox Experiment
There comes a point when other forces will no longer be negligible compared to gravity, as stated above:
When the diameter of your water column approaches its limit (single molecules of water stacked vertically) then atomic and molecular forces, surface tension, and capillary rise will likely play a large role, and can not be ignored. Luckily this is outside the scope of civil engineering.
RE: Cantilever retaining wall with narrow site cut
I'm far for being able to give a proper answer, because i'd never researched to deep on something like this. But i'll give you the following images, just to note that there are several ways to estimate the backfill pressure depending on differents assumptions and also the coloumb scheme (with infinite width substrate) which match the equivalent "water-pressure" (only when the substrate is width enough).