Retaining Wall - Active pressure with rock face within failure plane 2

[danyul]

I have a solid basalt rock face and want to build a retaining wall in front of it... there will be soil placed between the wall and rock slope.

assuming the rock slope starts at the bottom-back of the wall and is steeper then the active wedge how do you calculate the active pressure? I assume it will be less than the full active wedge.

I know there are the charts for negative backfill but that is not the same thing correct?

Thanks

 

[fattdad]

dgillette: I'd like to comment to your thought, but may not be fluent enough on the subject of soil arching. To me when soil pressures fail to increase in proportion to Z*density*(some factor), arching is to blame. I think Spangler's equations get to the root of this.

While my sketch of "Case A" shows Rankine failure planes headed toward the wall, you could also show these failure wedges headed toward the rock. After all the rock face will "feel" an equal and opposite horizontal pressure, eh? If you were to draw these opposing failure planes, at the bottom of the backfill you'd have some triangle that would be formed. To me (again, I may be wrong), that is some form of arching in the graphical sense.

I like these topics as it requires engineering judgement and creativity. I'd like to know your thoughts on how you'd derive the earth pressures and if you feel some scaler or alternate solution is needed to throttle up the loads to account for some "arching" affects. This would be a new concept to me.

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

Do you have Winterkorn and Fang? In the 1975 edition, it's in Chapter 5 by Arpad Kezdi. On page 216, see "Pressure of Sand Located Between Two Rough Vertical Walls." The newer work by Handy, mentioned by aeoliantexan, could give different results - I have thus far been too lazy to dig through the journals to track it down. Regardless, the lateral force from arching is roughly analogous to the very large normal force that an arch dam puts on the walls of the canyon, in addition to the shear forces that keep the dam from moving downstream.

DRG

 

[fattdad]

I am familiar with the arch-dam analogy (I've actually worked on a few arch dams in Alaska many years ago). Haven't looked at your references and I don't have Winterkorn and Fang. I'd like to learn more and have always solved the problem as shown herein. When I was in grad school (this was over 20 years ago, so I may have lost some of the details), Duncan lectured on arching related to buried conduits - actually with some interesting guidance for design of culvert shapes. I think much of this is also shown by Spangler (others too I'm sure). There is a distinction in vertical force for pipes constructed in trenches v. embankment conditions. It's the trench sidewalls and the arching of the backfill to either sidewall that affects these vertical forces. This can only imply that for the vertical force to be something other than gamma*Z there must be some increased shear/horizontal forces acting on the sidewall.

I may be mixing concepts here, but if I recall correctly, log spirial earth pressures account for friction along the backfill-to-retaining-wall interface. Maybe there is some design method to look at the O.P.'s concern using log spirial. Something to ponder, eh?

f-d

¡papá gordo ain’t no madre flaca!

 

[escrowe]

Arching is critical to design of pier walls for landslide stabilization, specifically pier spacing.
Many pier walls have been constructed in California (for instance). Day discusses some of the design issues.

Sect. 16.12

http://books.google.com/books?id=65...X&oi=book_result&resnum=2&ct=result#PPT507,M1

 

[fattdad]

The pier wall reference (above) reminds me of the lateral pile capacity term "Cp", typified by phi/2, as this directly relates to arching also, and a very similar concept as used for pier spacing.

f-d

¡papá gordo ain’t no madre flaca!

 

[Mumbo]

Soil Engineering by Spangler and Handy, 4th Edition has exactly what you are looking for regarding a rock face. It is not covered in earlier editions though.

Full earth pressure usually occurs to a depth of approx. 2 x space between wall and rock face and then levels off assuming a certain friction between the backfill and rock face.

Note: Rock is never where it is supposed to be and this calculations is totally dependent on the space between wall and rock. If you assume 3 feet and it is actually 5 feet, the pressure calculation is off by quite a bit.

Note 2: some of this may not apply real well in a seismic zone where the pressures may increase due to a reduction in frictional factors.

 

[fattdad]

Just to correct an error in my earlier posting: Cp is typified by phi/10.

f-d

¡papá gordo ain’t no madre flaca!

 

[palmahouse]

Why don't you draw the flatest failure plane that could form - assuming linear surface, assign strength parameters to it, and run a simple wedge analysis. Calculate the unbalanced force required to give you a safety factor of one. Then, distribute that unbalanced load over the wall as a triangular distribution. This method is pure soil mechanics and assumes no friction between the backfill and the back of wall plus the backfill and the basalt (hence, it is conservative). I did this before and came up with 15 to 20 pcf equivalent fluid pressure (which was great - much less structure required). That wall has been up for 15 years, is 12 feet high, and there is no distress.

If you want to model in the friction between the backfill and the back of wall (but still ingnore the friction between the backfill and the basalt), you could use Coulomb for the prescribed failure plane. Esentially, Coulomb is the same as doing the simple wedge analysis, but adds in the boundary friction.

If you are not comfortable with that - there is a design procedure for "facia walls." I don't have it handy - you could try a google search. I think the facial wall procedure will give you a similar result anyway.

 

[aeoliantexan]

The Handy reference is "ASCE J. Geotech .Eng. 111(3), PP 302-316:.

If you Google "fascia walls" you can get to a preview of Geotechnical Engineering by Handy and Spangler, Page 544, which presents the same solution. Handy did use Spangler's theory for conduits in trenches in developing his derivation.

I would be cautious in using the low lateral earth pressures thus derived in designing a retaining wall. It's not that the theory is not sound; it just might not cover all the things that can happen to the wall. My mentor used to tell me never to design a retaining wall that could not support water, at least at a safety factor of 1.0. He had looked at a building on a daylight basement that had been shoved downhill when water got into the backfill. The soil was loess, which can stand vertically, and the water table was probably at least 40 feet deep.

I believe that Handy got interested in the subject after consulting on a tall stone wall that toppled, flattening a car and occupants. The wall was constructed close to a vertical rock face. I don't know the details.

 

[palmahouse]

aeoliantexan,
Your mentor seems very wise. However, if you think your wall backfill will become undrained, the horizontal stresses on it will actually be higer than that imposed by the unit weight of water (check your soil mechanics on this - the horizontal stresses will be the boyant weight of the backfill times the coeficient of lateral earth pressure PLUS the unit weight of water). If anything, and you want to be conservative, I would design for a F.S. of 1 using this undrained loading.

I would suggest, however, that you should just focus on providing adequate drainage behind the wall so the undrained stress conditions will likey never occur. This means granular backfill, an adequately wide collector pipe, and discharge to a suitable location that cannot get flooded or comprimised.

Perhaps in your mentor's case, the failure occured because of poor drainage, or there was no drainage combined with a design that could not accomodate undrained conditions.

Of course, basement walls that extend below the water table should be designed for the undrained conditions I describe. The funny thing is, lots of old walls in San Francisco are below water and waterproofed, have hardly any reinforcement, and have never failed, even during earthquakes. Arching apparently happens.

 

[aeoliantexan]

Palmahouse, I have no quarrel with anything you said. Certainly the soil will add some pressure to the water pressure. I just wanted to contrast the very real 62 pcf water pressure with the calculated 25+/- pcf soil pressure.

Yes, the failure I mentioned probably involved roof runoff going directly into an uncompacted silty clay backfill during a storm and no perimeter drain. That would have been common local construction at the time, because of the deep water table. As it became more common to recompact 2 or 3 feet of the loess under the building to prevent collapse settlements, we learned that any water that entered the backfill was likely to become trapped there and began to provide perimeter drains no matter how dry the site was.

I can't explain the San Francisco walls you described, unless they are load-bearing walls that can take quite a bit of bending moment due to the axial load.

 

[jbenway]

You may want to check out "Earth Pressure on Retaining Walls near Rock Faces" in the Journal of Geotechnical Engineering Vol. 113, No. 6, pages 586-599.