I have a situation where a large black oak is growing near the top of an embankment near a sidewalk. A 5' high retainer wall (quite old) recently collapsed. The wall will have to be rebuilt.

What suggestions do you have for backfill material, and why? the exposed root stock goes nearly straight down from the base of the tree to below the sidewalk level. All quite solid, no defects of any sort. Stability is not a concern.

Reply to post by Russ Carlson, on January 25, 1999 at 17:55:11:

Russ,

Couple of questions: Was the tree the cause of the retaining wall collapse? Will the wall be reconstructed in essentially the same position or will it be moved out from the tree? In rebuilding of walls where the wall was moved forward, we have backfilled with a mix of organic material and couse sand (about 15% organic material by volumn). at about 6 to 8 inches from grade we switch to rough-screened topsoil if sod is to be used behind the wall, otherwise, a course gravel may be used. The idea is to maintain aeriation and water movement in the environment where the roots have developed. I have worked with an architect who does restoration work using this method and as long as the wall removal, cleanup, and new construction is executed carefully, the tree has come through well each time.

Its a method, by no means the only one. Im interested in what others are doing as well.

Wayne

Reply to post by Wayne Cahilly, on January 25, 1999 at 17:55:11:

There is an advantage to sandy or gravelly soil for drainage against the wall, HOWEVER, this will encourage more massive root growth against the wall, which is not desirable.
verytreelylew

Reply to post by lewbloch, on January 26, 1999 at 17:17:56:

Why would sand or gravel cause roots to grow lateraly when we don't expect to see roots grow horisontally into well drained soil. If there is no water or nutrition, the roots should not grow there, I don't think.

Put up root barrier fabris if there is a concern about roots pushing the wall over. When I was in England last summer I saw some really old walls with large trees growing on the back side with no apparent root buckling.

tom

Reply to post by Wayne Cahilly, on January 25, 1999 at 17:55:11:

I believe the tree did cause the collapse. The remainder of the wall is leaning outward. An old photo of the house shows a similar wall on the other side of the property- taken over 60 years ago. A low wall was extended since then at 3 feet high, along the embankment.

Behind the wall, There was solid root wood, like an inverted trunk. Very little branching, only one or two roots over 1/2 inch. There was a little fine rooting, but most of the roots were from English ivy growing as a ground cover at the base of the tree.

There is room to move the wall farther out about 1 foot, leaving the sidewalk 4 feet wide. The tree does not need the wall, but there could be soil erosion without it. I would like to see the roots after the remains of the wall are gone and the soil removed.

The trick will be construction a sturdy wall without further damage to the tree. The engineer that drew the initial plan for the new wall has an 18 inch deep footer. He needs to be educated....

The sketch above shows roughly the situation. The wall is semi-circular, about 10 feet in diameter. It protrudes a bit into the sidewalk space. The ground line at the base of the trunk flare is now over air space. The brown area in the sketch represents the approximate location of the root behind the wall. Further back is unknown. The wall has bee pushed out, and being over 60 years, finally let go after heavy rain last week.

Actually, I thought styrofoam "peanuts" would be a good backfill. The roots won't grow into them, and they will compress as the roots expand. Unfortunately, everyone use the biodegradable ones now, so they'd probably just dissolve.....

Reply to post by Russ Carlson, on January 25, 1999 at 17:55:11:

Russ,

I thought about this for a few days, so I hope I'm remembering what was in the rest of the responses and your graphic.

1. Where do those root go after they go down (behind the old wall)? Under the wall and sidewalk (outward radially from trunk) or to the sides (parallel to the wall) and into undisturbed soil area? That goes a long way to addressing whether the deep footing proposed will be OK or not.

2. Assume deep footing is not OK, can you design a grade beam type structure with piers as appropriate? This leads to #3.

3. What will the new wall be made of? Dry stack stone? Mortered masonry (stone or block)? Poured and reinforced concrete? Two issues: A)Structural strength and B) ability to drain.

4. If dry stack it will drain well with anything but impervious back fill but will invite the same problem all over again. Not good structural strength. If masonry will want either weep holes or "French" drains to outsides. If poured and reinforced over grade beam it sould be able to survive pressure from roots, a semicircle is an inherently strong shape. Could have weep holes or pourous back-fill to sides, either way roots won't damage the wall much and Bio-barrier is not necessary.

5. What's the budget for all this? Is the tree worth it?

6. Any other disturbances in forseeable future? EG to rest of sidewalk outboard of wall.

Reply to post by Scott, on January 25, 1999 at 17:55:11:

The engineering problems I have well in hand. Was just looking for suggestions on the backfill. My concern is that A) we don't want excessive fine root growth in this area and B) we want something that will compact with root growth, but not exclude oxygen. But here's the discussioni on your questions.

In order:

1- The roots extend sideways and back into the embankment. Some roots apparently extend under the sidewalk, as heaving is occurring.

2 & 3- Post and beam is undoubtedly necessary. An engineer drew a plan that included an 18 inch deep footer the whole width, but this will obviously not work. He planned a hand-laid stone wall, as before. The masonry contractor suggested a poured reinforced concrete with stone face. This later is probably best combined with a stone face.

4- Drainage will be attended. This is part of the reason for my original question. I expect a crushed stone base behind the wall with weep holes through the wall. Imagine a few vertical aeration pipes through the backfill??? Hmmm...

5- Budget? Non-existent. Well, the owner is "strapped for cash", as she put it (he said, hoping he gets paid). There is a chance the city will help a bit. This black oak is second on the 5-year old list of Delaware's Big Trees, and there may be some support.

6- The sidewalk will be repaired when the wall is done. Future? The tree is in very good condition for 150 years. Needs pruning, but is sound and sturdy. It could be a neighborhood-breaker though. Some neighbors want it gone, as they are afraid it will fall, others want it saved at all costs.

An interesting exercise, from a technical viewpoint. So, what backfill mix do you suggest?

Reply to post by Russ Carlson, on January 29, 1999 at 07:15:33:

The backfill coice is both a structural and arboricultural question. If you go with poured / reinforced / veneered face / weep holes (my choice) then it's probably not a structural issue. (Except maybe some filter fabric to keep the holes from plugging). Especially if the wall is a concave semi-circle facing the tree and pressure from roots / soil water. So my guess is a soil mixture that approximates texture and other characteristics of the rest of the root zone. Don't know that vertical pipes add much. I'd be more concerned at this point about engineering specs for later sidewalk repair. If those roots are going to be hacked at, maybe its better just to excaate the footing now and do a careful root pruning.