stabilization of gravity retaining wall

[lebeuil]

Here's a simple problem. An existing 30 year old masonry/concrete gravity wall needs stabilizing and I'm considering the use of passive soil nails or anchors. The wall is 3.7m high and probably no more than 600-700mm wide. It is proposed to use 3 rows of nail or anchors. I believe there are two ways of looking at the problem.

1. The wall has some inherent structural capacity and the horizontal spacing between the supports (anchors or soil nails) can be quite large i.e. the soil at the back of the wall is not "reinforced".

2. The structural capacity of the wall is minimal and the stabilization works should be based on a genuine 'soil nailed' application i.e. the wall becomes in effect simply a facing element.

My questions are as follows:

For first scenario, what is the earth pressure distribution at the back of the wall? Prior to stabilisation are we dealing with active or at rest conditions? Following the installation of the anchors, again what can of pressure distribution should be assumed (the Engineer has requested that passive resistance should be ignored but with full hydrostactic pressure on the active side). Once the pressure distribution is determined, how do you work out the bending moment/shear forces in the multi-anchored wall? When dealing with embedded flexible structures, the soil-structure interaction dictates the design. But here we're dealing with a "semi-rigid" structure with no passive pressure in front. Can this be analysed a continuous beam with no restraint at the toe? This would seem quite an onerous constraint. Or is the pressure distribution closer to those obtained for strutted excavation?

For the second scenario. When I check the methodology used in design, reference is always made to the vertical spacing of the nails when checking the internal stability of the "reinforced" block of soil. Surely there must be a limiting horizontal spacing so that one indeed deals with a coherent block of soil? There must also still be some bending and shearing within the facing (albeit small). Does this need considering?

Many thanks for (hopefully) your comments

 

[ishvaaag]

1. Get the dimensions of the wall

2. If in bad state, work on it through injection and reinforcement to make it apt for structural use.

3. Ascertain for the usable state the allowable compressive and tensile stress (may be distinguishing for flexural tensile stress). Use reduced allowable stresses, maybe 50 to 75% of those for new masonry walls for a target additional life of over 30 years.

4. Peck, Hanson, Thornburn recommend use the same pressure distribution than for walls propped by struts; or else use something finer you find in some more modern resource such a paper on the subject.

5. Be it case 1 or 2 of your definition, make a thick plate analysis for the pressures behind the wall, and check against allowables, till the stresses are acceptable.

6. On execution, apply the tensile forces in the anchors gradually to avoid overstressing locally your wall.

 

[Robert168]

A question came to my mind with regard to this problem.

This is a masonry/conctete gravity wall. If the integrity and strength of the wall is reduced due to aging or other factors, what needs to be done is to reinforce the wall itself by any means.

My question is: If the soil properties and hydrogeological conditions in question remain unchanged, is it necessary to add soil nails or anchors to stabilize this wall.

Any commnents from member experts are appreciated.

 

[ishvaaag]

Certainly if the wall was once (maybe even to present standards) able to sustain just by gravity the push, then you are right in that just restoring its structural integrity and capacity would be a right path to proceed.

Respect that deterioration happens there's no the lesser doub, and some 100 years old mortars have deteriorated to a quite weak agglomerate, this not being even in contact with water from the soil. Brick also suffers deterioration with age, erosion, formation of ice, cracks, chemical attack, chemical spalling, increase porosity from drying and washing etc.

 

[Focht3]

Where is your site?

What are the soil conditions?

Please tell us what you know about the existing condition of the wall. (Foundation type & size, reinforcement, material types and properties, etc.)

Why does the wall need to be "stabilized"? What aspect of its performance is substandard?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[lebeuil]

Thanks all for your comments.

The wall needs stabilizing because it is anticipated that its height will be increased in the future (although the ground level on the active side will remain the same) and because it is thought that the original design did not account for full hydrostatic pressure. The material at the back of the wall is granular (phi=28, c=0)

It looks like the Engineer is now considering Case 2 where we should assume that the wall has very little structural capacity and that passive soil nails at close vertical/horizontal spacing will be used. Rather than analysing this as a retaining wall, would it not be better to treat this as a slope stability problem and consider circular and wedge slip planes at the back of the wall. However, this type of analysis does not tell you what the pressure distribution is. How do you convert this in order to work out the shear/bending of the facing. The original question as to what maximum spacing that can be considered still remain a mystery though!

Thanks

 

[clayR]

How could you possibly design for an unknown increased height?


You could treat the stone as a temporary facing, install permanent nails and build a permanent shotcrete facing. Carve the shotcrete to look like the existing rock wall.

 

[ishvaaag]

The maximum spacing will depend on the strength, or allowable strength, if you want. Of course there are unknowns, but there are rational technical solutions proposed by people like Peck unlikely to give bad advice. And to analyze a surface made of plates for pressures orthogonal to the plates is not at all difficult in FEM's world. This analysis will indicate through the resulting stresses if the separation is reasonable, big, or scarce.

So if there is no way to be 100% sure of what the situation is going to be, there are technical solutions that provide parctical ways to tackle with your problem. If only for a start, maybe trying twice the thickness is something maybe closer to your case 2 but starting to be 1. This because that consistent masonry is expected to sustain 45º surfaces in shear as well, so for this 2t thickness you may even think for a moment most bending be scarcely relevant.

But if someone by error imparts excessive tying forces it will distroy the wall through local punching (more likely than bending, even if this is also feasible).

and what clayR says is also entirely true. You need have in mind entirely clear the future life of your wall, and to what it amounts in design terms respect a proposed refurbishment. Where unclear, one must ask the deciding party (be it some senior engineer, or the commissioner) what are the requirements of the work at hand, for it is not feasible to project in good logic for something the use and requirements of which remain to be defined.

 

[Robert168]

Is it possible to reduce the hydrostatic pressure by installing draining pipes?

 

[lebeuil]

Thanks all again for your comments.

Robert 168 is absolutely right in saying that drainage pipes would certainly help a great deal. Unfortunately, the back of the wall is actually private gardens and at this stage the Engineer does not want a solution that will require works in these areas(although as mentioned earlier, the wall will be increased by approximately 1m and will definitely require access through the gardens!). Also, although we will allow for many new weepholes, the Engineer does not want to rely on their effectiveness in the long term.

Ishwaaag. Any reference regarding Peck's works?

 

[Focht3]

Peck, Hanson & Thornburn wrote a textbook together - Foundation Engineering (1974) John Wiley & Sons (ISBN: 0471675857), about U.S. $102 from Barnes & Noble.

Peck, R. B. (1969) "State of the Art Report, Deep Excavations and Tunneling in Soft Ground," Seventh International Conference on Soil Mechanics and Foundation Engineering, Mexico City, Mexico. Peck discusses pressures against braced excavations, with experience from Chicago, Houston, etc. It's a "must read" for anyone who designs earth retention structures.





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[jheidt2543]

lebeuil,

Reading your Nov. 14 response got me thinking along the following line: If the gardens are on the backside of the retaining wall (I take that to be the high side?), then is it possible to core drain holes in the face of the wall at 6' or 8' intervals and push perforated drain pipe into the fill behind the wall. A two or three inch diameter PVC drain pipe, wrapped with filter fabric could be pushed through a 4" diameter steel pipe sleeve for a few feet, then remove the steel sleeve. At eight foot on center you could drain 4' each side of the pipe, spacing would depend on the permiablity of the backfill soil.

Not an easy job, but it might be one way of getting drainage.