Reduced Lateral Pressure For Back to Back Walls

[RFreund]

If you have a situation where you have 2 walls separated by a distance L say 5'. One wall is a rigid wall possibly a foundation wall and the other wall is flexible wall such as an MSE wall. They are both H=15' tall, with soil between them. Is it reasonable to analyse the lateral soil pressure as follows:

Where ka is evaluated using coulomb's equation.

The force due to the soil pressure on the back of the flexible wall (z=0 is top of wall):
A triangular distribution (from z=0 to 5') =Ps1=ka*gamma*L*L/2 Resultant at z=L/3
A uniform distribution (from 5' to 15') = Ps2=ka*gamma*L*(H-L) Resultant at z=L+H/2

Is this reasonable as apposed to:
A triangular distribution (from 0 to 15') Ps=ka*gamma*H*H/2

Thanks

 

[BigH]

I'd suggest you search this site; this has been discussed quite a bit. I'd look up earth pressures for storage bins - silos.

 

[fattdad]

with the presence of geogrids, the matter seems moot, but I haven't had my coffee. It'd seem to me that as an MSE wall, the 5-ft separation would all be in the reinforced zone and act as a gravity mass (i.e., after all, the block face is not falling).

My approach is to do the following (i.e., related to your OP):

Determine the friction angle
Using a protractor, strike a like from the wall face up to ground surface using a Rankine failure angle (i.e., 45-phi/2 as measured from the vertical).
Find where the line intersects the opposing wall (i.e., as you move down one wall, at some point the Rankine line will strike the other wall).
Below this depth, there will be no further horizontal stress increase.

You'll end up with a triangular top and a rectangular bottom.

Then again, others may have other approaches.

f-d

¡papá gordo ain’t no madre flaca!

 

[msquared48]

That's what I'd do... - as fattdad suggests.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[RFreund]

Thanks, yeah I was asking this question more as an academic one rather than practical situation. Or else yes I would expect the geogrid to extend to the wall. I tried searching the site and found a couple of posts but I was struggling to find exactly what I was looking for.

fattdad - that is pretty similar to what I was describing and along the lines of what I expected.

 

[PEinc]

Isn't less than 5' of geogrid length too short for a 15' high MSE wall? I hope the quoted example of 5' feet is not an actual geogrid length.

http://www.PeirceEngineering.com

 

[RFreund]

This was just to illustrate my question. As I agree with you that would be bad.

 

[RFreund]

I'm debating weather or not to start another thread with this question -

Can you recommend a good reference that discusses offset surcharges and lateral loads from applied surcharges. As applied to retaining walls.

I understand the basics however searching this site it seems that there are a few different methods out there - boussinesq, wetergaard..
It seems Bowle's book is good except it is geared toward a computer program. There is also conduto's book. I have Das but it doesn't go into much depth.

thanks

 

[PSlem]

NAVFAC DM7.2 has effect of point and line loads behind a retaining wall. It is on the

http://www.vulcanhammer.net

site

 

[InDepth]

In general I agree that 5 ft would compromise the stability of the MSE wall.

In practice If I have a situation with two walls seperated I design it for the full load, because you never know if one day the adjacent property decides to backfill one side and bury one of the walls. I've worked on several projects where we found buried retaining wallsc(just cheaper than removing).

 

[InDepth]

Surcharge loads are an art. There are even descrepancies between manuals such as the USACE, USS Sheet Pile, and NAVFAC manuals. Software programers even have their own convuluted methods. The European codes typically are more comprehensive in covering surcharge loads. Unfortunately a lot of the surcharge loads assume elastic conditions, or are empirically based on research that had assumptions long lost.

The main items you have to think about are:
1) Flexibility of your wall
2) The surcharge load magnitude relative to your soil wedge (i.e. Is your failure plain significantly affected by the load, or is it minimal). Global vs local+global effects
3) proximity of the load
4) Soil stiffnes, poisson ratio, ability to arch, plasticity
5) type of load
etc...

Surcharge distributions are judgement calls.

 

[RFreund]

InDepth - Yeah I've done a lot of research in the last few weeks since I originally posted this, thinking that there has to be a standard set of equations for offset surcharges and the back to back wall situation that I previously described. However, it was as if I opened a can of worms because there are many different theories. I'm glad I did the research though as I now have a much better understanding (or lack there of) of the stresses that occur in soil (probably should have payed better attention in my geotech classes).

EIT

 

[BigH]

Geogrids are usually 0.7 to 0.8 x the height of the wall (See BS 8006).

 

[dcarr82775]

Do an old fashioned trial wedge analysis, this will give you the horizontal loading on the wall for an active case. It is a dirt simple calc as well.

Another approach would be to use bin pressures. I think Blodgett (sp?) talks about it in one of his books. You will not get much help in pressure reduction at these H/D ratios is I remember correctly.

As for the point/line surcharge, the Boussinesq method is vert straightforward, but it assumes a rigid wall. In reality most walls are flexible so the pressures given using this method will be rather conservative since they drop once the wall moves.

 

[Geostructsparks]

There are two "real" theoretical solutions, I don't know which is correct, to the original thread.

Its either
1) Having evaluated wedge equilibrium personally, the active pressure for a wedge is proportional to its mass. So if you know it has a regular shape of a right triangle of 45 + phi/2, and you chop off the mass of the back 1/3 because of the adjacent wall, then you only have 2/3 of Ka computed by regular equation. The centroid of loading is still probably the lower third point.

2) However, the back edge of the wedge (pushing against the building wall) does have an appreciable force, which is 1/2 Ka H^2 for a shallower wedge, which this time may be shallow enough it does not interact with the MSE wall on the other side. The building wall would have to resist this force. By force principles, the soil must feel an equal and opposite reaction, e.g. the wall adds that amount of active pressure to the soil.
With enough reflections, any narrow soil wedge will have the full active pressure at the base of the lowest wall.

I believe that the second version is the most correct, design both walls for full-height active pressure. As noted above, this is somewhat moot for a wall with MSE-fill, provided you design the MSE internal stability that way.

 

[ishvaaag]

Last year I was trying the parametrical abilities in autocad 2010 and made one parametric dwg dealing with total active push when soil between two walls, not one of the more beautiful that I did but still curious to see. I find it still works in release 2011.

 

[ishvaaag]

And the page of the book

Apuntes de
Geotecnia, Cimientos y Puentes de Fábrica
Muros y Taludes

José Entrecanales Ibarra y Carlos Lorente de No
ETSICCP, Madrid

It seems a book of the thirties' yet my brother who is an Ingeniero de Caminos bought it when studying the matter in the early seventies'... graphical statics, long lost.

The matter is further developed in 3 more pages surely following more complicated methods per the references at the foot of this page.

 

[ishvaaag]

And what the superseded loads code NBE AE-88 (standing till 2005) descended to precise about the matter