Settlement after 53 years 1

[mjr6550]

I recently looked at a 53 year old one story masony house. The rear half of the house is built over a basement and the front half over a crawl space. The basement foundation walls show little sign of settlement, but the crawl space foundation walls have settled significantly-approx. 3 to 5 inches. It appears that the settlement has occurred over a long time and is ongoing. I was able to dig several small holes at openings in the crawl space floor slab. The soil appears to be very loose granular fill. There are bits of broken glass, bricks, etc. in the fill. I was able to dig down abount one foot by hand (could have dug further). At that point I could easily push a scewdriver into the ground up to the handle. The house is located in Philadelphia. Records suggest that the house may be located near a buried stream or in an area where significant fill was added. Some adjacent structures also show signs of significant structural distress resulting from settlement.
My questions are:
1. Shouldn't the soil have consolidated over this length of time?
2. Is piping likely to be occurring?
3. Any suggested soils tests that could be performed? (value of the house is relatively low and therefore owners budget is small).
4. Any thoughts on stabilizing the foundation using helical piers? other ideas?
Thanks

 

[PEinc]

I would try to get a few soil borings and /or test holes deeper than 1 foot. The house could have been built on some old fill (wood, ash, other organics) which is deteriorating. I was called about a Philadelphia townhouse that was settling. I got the owner to have a couple of borings made. They showed wood, water, voids, etc. beneath the house. The house was built over an older, filled-in basement. The fill appeared to be the demolition material from a pre-existing building. After years, the fill material decayed enough to cause the house to settle.

Helical piers may not be a viable option if there are filled-in obstructions beneath the building. Wood, concrete, steel and other types of fill could make it hard or impossible to install the helical pires. Conventional pit underpinning might be a better stabilization method. But you still need borings.

 

[oldestguy]

PEinc appears to be the way to go.

However, in my 52 years in the business this usually is caused by sitting on coal ashes. I have never seen it happen that the settlement stopped. So, you probably can look forward to it continuing for the forseeable future.

By passing the stuff is the cure.

 

[Ron]

I would bet on the debris as PEinc noted. That's fairly common. Coal ash would likely be isolated (though it could be isolated to your location!); whereas, debris can and usually is at any location.

Also, the screwdriver test isn't valid for granular soils. It doesn't surprise me that you could push it in all the way. Even compacted granular soils won't fully resist a screwdriver (that's a pretty high stress on the soil..example...a 1/4" flat blade screwdriver with 10 lbs of force on the handle would exert many thousands of lbs/sf on the soil, thus you failed the soil in bearing with your point load)

 

[GeoPaveTraffic]

I agree with PEinc.

 

[mjr6550]

Thank you all for the replies. PEINC, since you appear to be in the Philadelphia area, do you have any leads on who would do soil borings for a small job. I tried before to find someone for another job and did not have much luck. Also, I assume that SPT would be most important, along with looking at the soil profile and water level. Any other testing you would recommend?

 

[oldestguy]

One more thought.

Another cause of this kind of setlementis shrinkage of some clay soils.

The usual clues are the presence of fast growing trees nearby, taking moisture out.

Should the test borings show "hard clay",and no significant junk material, well then shrinkage is the probable cause. This effect can extend a deep as 20 feet in some areas.

Correction can take many forms, but interesting enough removing the tree, or watering the ground can make a big difference, without any fancy underpinning, etc. It is not an easy fix otherwise.

For test borings, look in the yellow pages under that subject or foundation exploration, etc.

 

[epongra2]

What about the possibility of subsurface water causing the settlment-related distress? I have seen this on several occasions. The structure has been fine for many years, then a water line bursts or a slow irrigiation line leak develops. Even water migration from a new off-site source needs to be looked at in some cases.

May want to have the homeowner check for water lines or have a plumber do so.

 

[BigH]

mjr6550 - might check out Geotech in Maple Shade NJ. They do/have done a lot of work in Phillyin these kind of conditions.

 

[PEinc]

Oldestguy,

Philadelphia really doesn't have shrinking clay soils. Usually, we have 1. silts and silty sands over sands and gravels or 2. silts over decomposed mica schist and harder schist.

mjr6550,

I'll try to answer your last question tomorrow. Couldn't get to my office today. All of the river bridges to my office were flooded & closed today. Massive traffic jams and closed roads along the Schuylkill River, Perkiomen Creek, and French Creek. A really bad day!

For borings in the Philadelphia area, I'd recommend Geosystems Consultants in Fort Washington or Schnabel Engineering in West Chester. Unfortunately, Geosystems got flooded badly today. Their office is right at the PA Tpk Fort Washington Interchange.

 

[BigH]

forgot to mention - I did one job over north of Philly in Conshehawken (sorry about the poor spelling). Did boreholes and found one thing; did test pits and opened eyes to a lot of infilled basements as has been alluded to earlier. A test pit or two might even be more appropriate than a boring - but the geotech can decide that one.

 

[PEinc]

BigH, you really get around!

 

[PEinc]

Conshohocken.

 

[oldestguy]

mjr6550

BigH hit on a good idea. With all these people flooded out, an easy start with a small back-hoe will tell you (anybody with some eyes) what is there.

Test borings these days, in their hurry for making footage, can miss seeing important things, due to hollow stem augers. I'd vote for test pits below the depth of the foundations of the shallower part.

Not like in the old days with the Gow method (bet many of you never heard of that method started by Raymond).

BigH Ever visit B.K. Hough lab in Ithaca?

 

[mjr6550]

I like test pits, but it was hard to even dig down about a foot or so by hand at the crawl space floor level because the soil kept caving in. To go several feet below the crawl space footing level I would need a test pit 5 to 8 feet deep. The sides would have to slope back at a shallow angle making a pretty wide hole. Otherwise, the hassle of shoring. Anybody have any experience with test pits in sandy soil?

 

[Ron]

mjr6550...I am in a sandy soil area and we use test pits (observation pits...) routinely. If the soil is very dry, sloughing is common and becomes a problem, since the strata and horizons are not stable to view. Most of the time that isn't an issue since our water table is relatively high and capillarity keeps some moisture in the soil to near the surface.

You might consider flooding (or at least significantly wetting) the area to be observed just prior to the excavation. This will help keep the sandy soils together for viewing.

 

[BigH]

oldestguy - I'm going to play ignorant on the lab name! We had a soils lab at Cornell Hollister Hall and a pretty neat structural lab over in the mechanical and T&AM building (forgot name at present) that could do some big beam testing. I graduated there in 75 with ME(c) and haven't been back since 1989 or so - would like to but can't get it on the agenda.

 

[oldestguy]

BigH

Knowing you also are a Cornellian, thought you might have run into this fellow, a former professor of Soil Mechanics who then left to run a geotech business. He had built a number of the Universal soil testing machines that were there in my time. They were housed in temporary building overlooking Beebe Lake. Nice memories.