Foundation Settlement In Clay Following an Earthquake?

[phamENG]

I inspected a house recently with some significant drywall cracking. House is about 40 years old. The cracks are indicative of differential settlement, and I confirmed by analyzing the existing framing. The deformed shape of the house doesn't make sense with my analysis unless I assume a displacement in my support. So I'm confident that the issue is in the foundation, but I'm trying to make sense of the timing.

As I said, the house is about 40 years old, built in 1979. The current residents are the original owners who commissioned the house to be built. They said that they have had no cracking of any kind until about 2014. That's when these cracks appeared, and have grown progressively worse. In the last 2 years one of the doors from which the cracks emanate has jammed and is completely inoperable. I've seen a lot of settlement issues, but for a house to have no noticeable movement for 35 years and then suddenly start settling seems a bit odd.

There have been some changes over time, most relevant was the addition of a porch roof and elevated patio, but that was done in the late 90s. In 2011, we experienced a once-in-a-generation seismic event. This is eastern Virginia, so that just means we felt it. The sort of thing that the average Californian wouldn't notice, and more than a few people just thought they were having a case of vertigo. We're about 140 miles away from the epicenter.

We have clayey soils - that and muck. Typical depths to marine sediments that a pile can bear in are around 30 to 40 feet. Given the proximity of this house to a river, it's probably closer to 60ft. What is the likelihood that a minor seismic event like this could trigger additional consolidation in clay soils? Given the the fact that these events are so rare, I haven't taken much time to study the goetechnical ramifications, so I appreciate any help you guys can give.

 

[dik]

Could the clay be highly plastic and has the area been subjected to long term drought? Could that be the cause of the differential movement? Just thought, if the clay is near saltwater, often clays often have a (de)flocculated structure(?) and can be a little unstable.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

Thanks, dik. I don't think that's it. This has been a progressive thing for the last 8 years, not a seasonal issue. We've had our ups and downs, but I'm pretty sure we're trending pretty close to average over that time. I have seen that, though, with cracks opening in a drought and then closing again after the first heavy rain.

We do get pockets of flocculated clays around here - had one in my backyard that swallowed a contractor's bobcat up to its windshield - it's possible that that could be an issue here if there's a pocket within the influence of the footing in question.

 

[dik]

With highly plastic clays it's not likely seasonal. They hold their moisture so well, shrinkage could occur over decades. Flocculated clays are particularly bad during seismic events, I understand.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

There was a building under construction near Portage and Main, in Winnipeg several decades back and they accidentall broke a 10" watermain. Within a short time the excavation filled and only the 'flapper' on the exhaust of a D-9 Cat was visable.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[geotechguy1]

Clays can undergo a process called cyclic softening which can result in reductions in shear strength and stiffness leading to settlement. Whether that PGA is enough to trigger anything I'm not sure.

 

[dik]

Catch...

https://geologismiki.gr/cptrobertso...9/evaluation-of-cyclic-softening-in-clays.pdf

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[EireChch]

Seismic loading does not result in consolidation of CLAY but possibly cyclic softening. But that would have occured within a very short period after an event, not over a period of years.

As this is over a period of years I would most certainly say its a moisture content issue. Plot average rainfall over the last 40 years and see how it looks against your damage.

I worked in an area of over consolidated CLAY where houses that were stable for 20-40 years all of a sudden started moving. We often never got to the bottom as to why...

 

[1503-44]

For what its worth, I'm definitely in on the long term, climate change related, shrinkage theory. Do you have PI data on the clay? Marine clays are usually quite high.

A black swan to a turkey is a white swan to the butcher.

 

[HTURKAK]

If the cracks started after about 2014 , ( when the bldg was 35 yrs old,) and your first impression is differential settlement, most probably moisture content changed or local washing of the clay..

IMO ,probably the sanitary and/or water supply plumbing ( under the bldg ) broken and leaking ...

I have faced a similar problem 15-20 yrs ago.. When a pilot excavation just near the bldg carried out, it was visible that the clay around asbestos cementos pipe was soaked and when the bottom of pipe excavated, the cement pipe at bottom section fully eroded/ dissolved and was like a crescent .

 

[Eric C.]

I'm with the change in moisture crowd.
What type of foundation does this home have? Continuous footing, slab on ground, basement, etc.?
Does the grading adjacent to the home provide for drainage away from the home?
Are the downspouts directing roof discharge away from the home?
Are there any visible displacements on the exterior claddings?

 

[phamENG]

Thanks everyone. I'll have to dig into possible moisture content changes in the soil and see if I can find anything there.

House is on shallow spread footings. Continuous around the original perimeter to support brick veneer, but the footing in question became an "interior" footing when the back porch was added. So no runoff issues in the immediate vicinity. Not to say there isn't something going on close enough to impact it, though.

I considered water supply or sanitary, but it's on the opposite side of the house from any buried pressurized lines and the sanitary lines are still above grade in this area.

Yes, there are some cracks in the brick and some signs of previous repair, but nothing extraordinary.

 

[1503-44]

" the footing in question became an "interior" footing when the back porch was added."

Could be when the drying of the underlying soil started.

A black swan to a turkey is a white swan to the butcher.