Remedial work to retaining wall

[engstructural]

I have been looking at a wall already built. The lower 3.0m high section is a gravity boulder retaining wall with a 2.7m high anchor block and geotextile wall built of the top of it. The anchor wall seems to be adequate but I cannot prove the adequacy of the lower gravity wall when taking into account the total retained height if 5.7m (I added the additional load as an increased surcharge to the lower wall.)

1. Where we have space in front of the wall would it be adequate to place fill material compacted in layers to the top of the boulder wall with a slope down of 3:1?

2. If this is done I assume the lower wall can be ignored, is this correct?

3. Where the is only a 600mm gap between the wall and the property what is the best way to strengthen the wall (re-build/Sheet piles/ Bored piles)?

Thanks for any comments.

 

[engstructural]

Sketch Details

 

[kslee1000]

What's wrong with the existing wall?
What was the lateral soil pressure diagram used in your analysis?
Do you have info on backfill material and drainage system?
Are there weep holes, do they work? Groundwater?

 

[kslee1000]

I hate to see such beautiful (boulder) wall coverd by the boring concrete. Try to safe it the best you can.

 

[engstructural]

When analysised the lower section of wall has in-adequate strength and slides.

The backfilled material is granular (stone)

Personally i have no issue with boulder walls if they work by cannot certify one which I cannot prove the adequacy of. I believe a planning issue means the wall will have to be covered anyway.

 

[kslee1000]

You didn't convince me, since you haven't mention any obvious physical problems with this wall.

I would assume the upper anchor wall was a geo-grid system, for which, if installed properly, does not rely upon the lower wall for lateral stability. In another word, there is no horizontal thrust on top of the lower wall due to the 2,7m fill. The only effect from the top is the 2.7m soil weight that shall be treated as surcharge to the lower wall. If this is the way in the analysis, then you got a case.

Since geo-grid wall is capable of self-support, can you remove a few boulders at scattered locations, and provide keys/ties behind?

I don't feel the concrete solution is more friendlier. But, it's only my opinon, no plan to take over.

 

[csd72]

Have you taken the proportion of soil weight directly over the wall to help in overturning?

diagonal ground anchors and a concrete strip to the front of the base would resist the sliding.

 

[engstructural]

Given that the guy who built the wall in the first place is the owner and will also be doing the remedial work I would like to keep it as simple as possible.

As kslee says the top wall is stable and we are only applying a surcharge load from this to the lower wall, oue code calls for a 10kN/m2 surcharge on the surface which I have also taken as this will be a yard / driveway area. The resultant surcharge is approx. 60kN/m2.

I agree the props to the front of the wall would resist sliding but the wall is also failing in bending (I have not assumed the top of the 3m section is proped or tied by the geo-grids as I am unsure of how good the connecting is.

The main section of wall has to be covered anyway therefore If the fill material can be utilised to support the wall this would be ideal. (what kind of fill should be used and how should it be placed).

The problem at the side is the limited space - I don't think they can get access to the adjacent property to install ties and would not have space for props. Any thoughts?

 

[kslee1000]

eng:

After re-stare at your sketch and your original post, I realize you wanted to place fill in front of the wall. Given there is ample space (8m) to make a good slopping fill, I think you might succeed by doing that. However, I wouldn't ignore the wall. You can do an analysis on the wall, with all previous loads, and the fill in front. It's not easy to evaluate the effects of soil with limit (horizontal) depth, but at least, you may gain some shear friction at the base. Further more, I would provide soil anchors near the top of the lower wall to take out some lateral load, and prevent free bending as well. I might consider fat clay is suitable for this application (check me on this, I could be wrong). Don't forget to extend existing weep holes, and have positive drainage in front of the new toe.

Another way is to construct a "L" shaped reinf-concrete wall, leg pointing the slope. You can soil nail the base (slant backward) if needed, and anchor the top as mentioned above. don't forget to provide anchorage of some kind to tie the boulder wall integrally with the new concrete wall.

All and all, you need to throw some number on the existing lower wall, assume it interacts with all other things around it. Please keep in mind, it is not a piece of cold stone sitting still.

Good luck. Let us know the result/decision, once it is done.