Tree Roots Behind Retaing Wall 1

[SlideRuleEra]

For several years, I have been watching an old concrete block retaining wall slowly fail. The wall is about 5 feet high and has rotated about it's base; it is now leaning at an angle of 15 degrees or so.
Several fair size, healty oak trees are behind the wall (within 10 feet or so). Are the tree roots, part of the problem (causing the wall to fail) or part of the "solution" (helping to retain the soil & minimize lateral pressure on the wall)?

 

[cvg]

you don't provide enough information to determine the cause, but it is very likely that the roots are extending underneath the wall and pushing it over.

 

[kkoloj]

My guess would be part of the problem. You might want to check a few "tree books" (i.e., ornamental horticulture books) to see what the root structure of an oak tree is. For example, it the roots are generally only a foot or two deep, then the roots may be providing an added force to the top of the wall? You would also have to check how far the canopy (branches above ground) extends from the trunk. It is my understanding that a tree's roots extend from the trunk approximately the same distance as the branches above ground (although I don't know if this is true or not). If the branches above ground extend to the affected retaining wall, then the roots would be near the back of the retaining wall. Hope this helps.

 

[PSlem]

It's an old rumor that the roots mirror the tree. Look at any that blow over in a storm and you see a very small root ball. I would guess the ground above the wall is at a slope. Slope creep is pushing a lot of 2' and 3' CMU walls over here in GA.

Engineering is the art of not constructing...of doing that well with one dollar what any bungler can do well with two after a fashion.

 

[Riggly]

I think the tree roots are more the problem rather than reducing the problem. Similar to water expanding when frozen, the growth of the roots expands with an 'unstoppable' force. I have seen on several occasions where roots have forced the way through walls,resulting in unsightly cracking and failure.
However, you should also look at other possible defects or inadequate design of the wall. Example, is it poorly drained? I also seen several brick walls in resential areas rotating about their base with no large trees in close proximity.

 

[eric1037]

It's probably a combination of poorly drained soil and tree roots. The roots wouldn't extend into that soil area if it had proper drainage (or at least not as vigorously).

 

[GeotechGuy]

This response is under the assumption that the wall is leaning over a great portion of it's length. Tree roots would create isolated point loads on the retaining wall, creating possible bulging in small areas. I would suspect that this is a masonry wall lacking any reinforcement material behind it (i.e. Geogrid) to support the backfill material. The only cure would be to cut behind it and construct it back properly (long-term solution), remove the material directly behind it and straighten it out (cheap and short-term solution). Nailing the backfill now will not straighten out the wall without reconstruction so I would use step 1.

 

[ChrisMcLean]

Surely the sheer weight of the tree(s) should be considered?

Also, Oak trees have a large root structure that easily extends 6-7m from the trunk of the tree for even a relatively immature tree. A lot of these roots are shallow feeder roots quite close to the surface of the ground. It is worth considering the shrinkage/heave potential of the soils behind the wall - Oak trees are notoriously thirsty and will cause quite large effects in shrinkable soils at the distance described in the initial post and could be implicated in the observed movement of the retaining wall.

 

[jheidt2543]

I tend to agree with GeotechGuy's comments. I would also add that here in the upper midwest freeze/thaw cycles are also a cause of retaining wall failures. Each cycle the wall is pushed out by the expanding frozen ground, but there is little or no rebound to the wall's original position. The cycle is repeated each year with the wall tipping more and more until it ultimatly falls over. Good drainage details when constructing the wall avoids this.

 

[SlideRuleEra]

Thanks to all for your comments. ChrisMcLean - A good point about the weight of the tree itself, it has got to go somewhere.

 

[jdmm]

Something that hasn't been mentioned is the effect of wall friction. I have found that the overturning of walls with time is generally due to a low initial factor of safety. Frequently the wall will have little or no footing but will stand up anyway due to, I believe, friction on the back of the wall that tends to decrease with time resulting in overturning and ultimate collapse of the wall.

 

[BigH]

SlideRuleEra didn’t put forth the likely type of soils encountered beneath or behind the retwall – so I thought I might put forth another possible contributor for consideration – that of the effects of tree roots on swelling and shrinkage of expansive soils due to seasonal moisture changes. There was a classic paper by Bozozuk (Bozozuk, M (1962), “Soil shrinkage damages shallow foundation at Ottawa, Canada,” Eng. J. Canada, 45, pp 33-37). I believe that he also described in another case a situation about how trees roots caused a building and its separately constructed stairwell to pull apart and then go back with the seasonal moisture changes. I did a search on yahoo.com using the phrase “tree roots and swelling soils” and got quite a few hits the url’s of which are below to a few of them. The third one, in particular, addresses the effects on a retaining wall. I think that you might find the “nrc” publications to be of interest too since this is the organization for which Bozozuk worked. In the dry season, the roots will suck out the water from the clayey soil in which it is rooted. In the wet season the soil will rehydrate and swell. This movement, amplified in some manner, may be one of the reasons the masonry wall has deteriorated over a number of years. It is an interesting topic that I studied a bit years ago – but with age (I remember the slip-stick only too well) sometimes things go a bit fuzzy!!
Try:

http://www.irc.cnrc.gc/cbd/cbd184e.html

http://www.irc.cnrc.gc/cbd/cbd084e.html

http://www.fairfax.va.us/dpwes/publications/marineclay.htm

http://www.irc.cnrc.gc/cbd/cbd062e.html

http://www.irc.cnrc.gc/cbd/cbd148e.html

http://www.aevoid.com/surevoid_web/soil_maps/intro4.html