Cracked Basement Wall

[waytsh]

I am looking at a church basement wall which is experiencing horizontal cracking. The block wall supports first floor joists and glulam wood arch frames. Where the frame columns sit on the wall the wall has been built out with a pilaster. The wall is backfilled to a depth of about 6’. The wall is moving inward and according to the owner seems to worsen in the late spring, summer, and early fall. The cracking is isolated to about the last 15' of the long wall. All other portions of the wall seem to be fine with no visible cracking. The entire perimeter if the building is equally backfilled. The only thing I noticed different about the area that is experiencing the cracking is that there are several large trees within about 20' of the wall. The branches of these trees hang well over the roof of the building which indicates that the root system of the trees could easily be extending into the wall. There is going to be a backhoe on site in a couple weeks. While it is there I am planning to have them excavate the wall in the cracked area to see if the roots have reached the wall. The downspouts all appear to be draining water well away from the building. Does anyone have any thoughts or comments? Has anyone seen a root system impact a foundation from a distance like this? The building has been in place for 33 years and the cracks have only started developing in the last few years and have been gradually worsening. This is what leads me to believe it has something to do with the tree roots but maybe I am barking up the wrong tree.

 

[msquared48]

The root system may have clogged the perimeter foundation drain at the base of the wall, locally raising the hydrostatic pressure on the wall.

Also, any downspount lines in the vicinity of the corner may also be compromised.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[waytsh]

Good point about the foundation drains although the original drawings are not showing any. It will be interesting to see what we uncover. The downspout on that corner of the building ia above the surface and drains out to daylight. On that endwall of the building the slope drops off rapidly after about 50' to 75'. The downspout runs out into a gravel covered parking lot and from there it appears the water would quickly drain away from the building. Assuming it and the gutter are not clogged and functioning properly.

Once the area has been excavated what would be a good material to backfill with?

 

[ToadJones]

If you have good gravity footing drains, I'd back filled with the most freely draining material you can get.

In my area we use 57 washed river gravel for this.

I have seen many a house/building in my area with clays so impermeable that water would sit in the clay strata & work its way through foundation walls before reaching the gravel on the footing drain a few feet below. So, avoid placing any poor quality fill over your free-draining material.

 

[waytsh]

I was planning to use a 2B gravel but was unsure of the benefit of using it if there is no footing drain in place. If I discover that there has been no perimeter footing drain installed would it be reasonable to attempt to install one after the fact? Or at least a partial one in the area of the problem?

Depending on what we find I am also entertaining the possibility of constructing a perpendicular masonry wall on the interior to buttress the existing wall.

Has anyone ever heard of or had any experience with a trees root system exerting enough pressure on a basement wall to crack it? I'm assuming that is possible.

 

[MiketheEngineer]

Though rare - I have seen it happen and from your description - it may well be the cause.

Block walls just do not seem to stop roots as well as PC - seems that the roots can "see" the mortar joints and go for them.

I have even seen roots grow through the block basement walls.