Masonry Wall Cracks 2

[YuleMsee]

Did a site visit for an 18month old building since exterior renders were done but 4 year old since masonry walling was done experiencing masonry wall cracks. The building sits on very hard rock, 1m below the ground. A year ago, the neighbour off gridline "F" starting excavation his basement, the excavation was 3m below this buildings foundation. Because of the impervious rock, water levels are very high, and now the neighbour has to constantly pump water out of his basement sumps.
The cracks appeared at the beginning of this year and grown in size until I was called for an opinion last week.
The structural layouts are thus,
Foundation layout

Typical floor layout

Pdf files that are clearer have also been uploaded
Cracks starting at window openings.
Cracks along Gridline "10 / D-F"

Cracks along Gridline "6 / D-F"

The cracks are wider at the window corner about 0.5mm tapering to 0 crack towards columns on gridline "D".
An analysis of the two strap beam tied foundation shows the two pad foundations along gridline "F" (ground bearing pressure 241KN/m2) are more heavily loaded than those on gridline "D" (ground bearing pressure 179KN/m2) due to the foundation sizes not being balanced as calculated from expected column loads. Tho. non of them are anywhere near the safe bearing capacity of the rock they sit on (approximately 400KN/m2)
Another crack occurs on the parapet wall above roof slab along gridline "10 / D-F". The parapet walls are 4 courses of natural stone block masonry sitting on the edge beam with a cast in situ top coping.

An analysis of the edge beam carrying the parapet wall shows its too stiff to suffer from deflection.

The window cracks are definitely differential settlement cracks, but the stiff underlying rock is giving me pause, could the excavation / draining water be a culprit or aggravate the issue?

 

[Lomarandil]

If the building footings bear on a substantial profile of rock, differential settlement (either due to unequal loading or the neighbor's excavation) is not likely the culprit in my opinion.

Unless by some circumstance there's a more permeable/soft layer below the rock, which is settling or being drawn down by the neighbor's excavation and dewatering.

I would be more willing to suspect cracking due to out-of-plane loading, assuming the masonry is unreinforced infill. That would definitely fit the behavior and construction of the parapet.

 

[JAE]

A couple of thoughts on this:

1. You stated: "An analysis of the edge beam carrying the parapet wall shows its too stiff to suffer from deflection.". There's no such thing as an infinitely stiff beam. The beams along grids 6 and 10 (between D and F) are parallel to the joists and may have SOME deflection that could explain, at least, the horizontal parapet cracking.

2. I see no expansion or control joints anywhere - though your window diagonal cracks don't appear to be caused by wall shrinkage/movements.

3. Diagonal cracks at the two insert walls both appear to show a drop down settlement of some type at the two outer columns (F-6 and F-10). This settlement could be the result of a rotation in the corner footings due to flexibility in the two strap beams. Even though the footings are on "rock" there could be some type of rotational response in the footing on the rock due to flex in the strap beam and this could possibly explain the diagonal cracks above.

Your deflection analysis of these beams should use an effective moment of inertia assuming some cracking at static load conditions.

Last question: did the footings get cast directly on a hard competent rock or was there some type of leveling bed of crushed rock, sand, or earth used between rock and footing?

 

[YuleMsee]

@JAE,
Your point 3 is somethimg to ponder on, I had not checked that strap beam but I noticed its not as deep as I expected from similar projects I've done before. I'll ask for a thorough review of walls running along gridline 4 & 11 / D-F to see whether there is any cracks, tho they are interior partion walls so no openings for a crack to originate from
On point 1 you are right, I'll definitely do a thorough analysis but from my quick analysis and zero floor loading it passes the stiffness test
On point 3 there are no movement joints, tho from the distances between columns and the 200mm THK. walls I don't expect that to be the cause.
On point 4, no idea, no construction records / photos exist.

@Lomarandil
On point 2, there exists soft material below the rock, but the rock is about 15-20m thick as observed at a nearby quarry, I think that makes the rock quite stiff. The neighbour didn't reach that layer, so no effect on his dewatering, but could the deeper excavation on one side reduce rock stiffness enough?
On point 1, its tricky, since there are no cracks on span Gridline 10 / A-B, that is mirror image to 10 / D-F,

 

[hokie66]

Based on your description and the excellent responses you have received, I suspect that the excavation has contributed to a bit of settlement, leading to the masonry cracking. It takes very little in plane movement to cause cracks in brittle masonry, particularly if unreinforced. JAE's suspicion of some type of leveling material being used between the rock and footings rings true. Detailed footing excavation in rock is difficult and tedious.

 

[GeorgeTheCivilEngineer]

Definitely cracks in the masonry and not the render?

 

[1503-44]

Long straight cracks and window corner cracks would indicate probably not render related.
My render cracks look like spider webs.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[GeorgeTheCivilEngineer]

Might be a case of completely forgetting to add additional fiberglass reinforcement at the corners of the windows.

It was the horizontal one which made me want to ensure it really was the masonry. A horizontal crack like that is unusual on new masonry construction. I can't quite think why it would occur.

 

[1503-44]

I'd chip out a bit of that horizontal crack to see how far it goes into the wall and if it has cracked past the rendering. Might just be a case of render and wall having different thermal expansion coefficients. You probably need to know how significant it is in any case.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."