Trees and retaining walls (a real live RE post!)

I’m living on flag lot that is much higher up than the property in front of me. There are retaining walls between us and the front neighbors, two of them - the upper one is about 2’ [corrected from 4-5’] tall with a fence on top, then 2’ forward of that is another 4’ retaining wall. Both wood. The soil is clay.

These neighbors are having a heck of a time with their retaining walls because apparently our property is always having runoff go onto their property. In general, most of their gripes have been about the people who lived here before us, but this time, it’s on us - we apparently had a broken sprinkler pipe which likely caused a 2" crack in the clay of the lower retaining wall and seriously damaged this wall - it’s obviously leaning forward multiple inches. Probably the leak started 2 years ago (when the sprinklers seemed like they weren’t spraying as much), but I’m guessing it got bad in the last 2 months based on a what appears to be a 33% increase in our water usage and the fact that they are now seeing water actively flow out the bottom of the lower retaining wall about an hour after our sprinklers go off (and the sprinklers were only running for 10 minutes).

So I have two questions:

  1. Aside from calling someone to fix the sprinklers, is there anything I should have a landscaper (or structural engineer or…) come out and look at? Should I be worried about a sinkhole somewhere??? Because water coming out 8’ down makes me wonder what large abyss opened up under my lawn.

  2. The husband is complaining about a row of italian cypress trees that are along the top retaining wall. These trees are about 2-3’ from the wall, but he’s concerned that the roots will expand and press out against the top wall and cause it to push out as well. (He’s also concerned about the weight, but I calculated that and it’s something like 30-50 pounds per tree which is nothing compared to the dirt). I’m concerned that if I remove the trees, there will be nothing at all keeping the clay from just turning into an amorphous blob in the next rain and pushing on the wall. What do you think? Maybe we’re both right? Are there trees that dig down, but not out? Should I replace them with bushes? I really wanted to plant fruit tress there to be honest, but I could do rose bushes or raspberry bushes.

  3. Anything else I should know about things like this?

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Yes, a true real estate question. :grinning: Maybe @Elt1 can share his wisdom on this.

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Hard to tell without pictures. But trees on top of a retaining wall can cause issues from roots and watering. The bigger they get the bigger the problem

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The problem with trees next to a retaining wall is that the roots fill up the drainage area necessary to keep the water flowing away from the said retaining wall. The water then sits against the wall and is much, much heavier than the soil or the tree in this case. And don’t underestimate the power of tree roots. I had a Wisteria vine break down a concrete wall before I took it out.

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I got a Flume water sensor earlier this year after my friend experienced a slow water leak that ruined his entire hardwood floor. I suggest you get something similar to monitor for any leaks in the future,

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Shallow rooted trees on top of retaining wall is trouble. This combined with poor drainage can cause the entire structure to fall. A properly built retaining wall can hold the dirt with no issues.

So what should I put up there?

Can you post some pics?

Rock landscaping

Leaning forward retaining wall is trouble. Need some pics. It needs to lean inwards with lots of gravel underneath with drainage to move the water away from the wall.

Lower retaining wall (bottom right of picture)- should be completely straight, but you see it bending forward:

Upper retaining wall neighbor side(Ok, I guess it’s only about 2 feet tall). The dark area on the bottom left of the photo may be the crack in the clay that got wet from the leak:

My side:

Left of the trees:

Right of the line of trees:

I guess what I’m hearing is that I should definitely pull out the trees. Any reason not to plant rose bushes or raspberry bushes? Everything is on drip tape, so unless I get another leak, it’s not a lot of water and rose bushes at least tend to be tolerant of low-watering.

Alternately, if the water weight not the tree weight is the issue, how about dwarf trees in planters?

Retaining walls made of wood? Shouldn’t it be concrete slabs?

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They are on a budget.

The lower wall is failing (bottom right post is an indication), and this needs to be repaired. It is better to get some pro help to fix it. For a quick fix: 1) Drainage should be reviewed / patched, 2) Falling portion needs to be fixed.

It may be just easier to re-do the lower wall properly.

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How much should it cost to build a cement or stone retaining wall? (Specifically the bottom one)

If you have a sloped terrain, and the retaining wall is structurally holding a lot of dirt + you are in a landslide risk area, I would first check with an engineer to assess what is needed. Otherwise an experienced landscape sub-contractor should be able to make repairs. In terms of cost, it is really based on what needs to be done (sorry it not very helpful) and you may get a variety of quotes.

9 years ago, the number for a wood wall was $25/sqft e.g. a 50 ft long wall, 4 ft high would cost 50x4x25 = $5000. That’s with posts 50% exposed , 50% in concrete.

One of those walls failed after 3 years. The pressure treated lumber doesn’t have enough chemical in it nowadays.

For small landscaping walls, I like these stackable blocks, but it is difficult to put a fence on them:

Definitely don’t bury a wood anchor as shown in that sketch. Maybe treated wood and wrap it with something like Tyvek. Proper is a steel cable and a concrete anchor. Multiple cables to one anchor.

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Here is a picture of an anchor, before the concrete is poured:

And after the concrete was poured, 3/8" steel cables, each enclosed by PVC pipe, then tied to the concrete block. The cables were finally tightened with the nut on the left.

Garbage truck drives there, no problem.