Recurring settlement

[PaulLevy]

I am located in northern Alberta, Canada. I have been approached by someone with a peculiar problem on his property. Over the course of the winter, the front of his property subsides, anywhere from 10 to 30cm. This settlement then starts to disappear in spring following the thaw, and by mid-summer the ground has completely recovered. Started happening approximately five years ago. This past winter he tried to keep the snow off the yard, and the settlement was less pronounced (~10cm instead of the typical ~30cm). House was constructed in 1996. The location of the lot services doesn't coincide with all of the area of settlement, so I'm hesitant to suggest that's the cause. He feels it may be related to the neighbor's concrete driveway (which is adjacent to the settlement) being poured on bare clay instead of gravel, and is reportedly pinned to the house. While the driveway does show some cracking, nothing suggests to me there is significant heave there, and I find it unlikely a concrete slab would cause a settlement.

Any suggestions? I'm rather stumped by this one.

 

[cvg]

google "expansive clay"

 

[oldestguy]

It is more likely that the house has been raised in winter by frost heave action. While it may appear that the front has settled, that may be just a relative thing. Frost heaving usually means all three of these things are needed. Below freezing of the soil, a source of water (can come up from 5 feet depth via capillary action) and one needs frost susceptible soil.

 

[oldestguy]

The area that has heaved may not be the house. What is the "settlement" measured against? Could be the nearby driveway? That reference may have been heaving. Taking snow off the yard likely meant that area froze more than usual, thus more heaving to imply "less settlement".

 

[PaulLevy]

The ground had settled in reference to the adjacent driveway, sidewalk and streetlight. It's (thankfully) not in an area near the house. Worth noting we have primarily clay soils here, with the odd sand lens. While aware of frost heave, I hadn't realized it could be so uniform. The house doesn't appear to have any damage inside or out that I would expect to see from the heave being greater under various parts of the house than others.

 

[oldestguy]

If it is expansive clay, then check for trees there. I find it hard to believe they will be drawing water in the winter. Usually settlement from trees taking water is in the dry part of the summer. That amount of movement is much more than what I've seen from trees even drying the ground to 15 feet or more.

 

[PaulLevy]

There is a tree in the yard, but it's not a terribly large one. I agree it's counter-intuitive for the settlement to be occurring in winter. A couple photos:

The grass is cut fairly short on the right, and the ground seems to be a similar elevation to the driveway and sidewalk. I do find it odd no adjacent properties exhibit the same behavior, though I suppose if there was imported material (the white mark on the sidewalk aligns with the CC for the property) it might explain different material being present. Any theories as to why expansive clay (if that's what it is) would only start doing this 5 years ago, when it would have been placed 19 years ago?

 

[oldestguy]

You have a good elevation reference "bench mark" right in the yard. It is that metal power or light pole. It probably does not heave in winter, although we do know in farther north areas (periodontist) that these sort of things get gradually "jacked" out of the ground in time. If that pole and the nearby ground stay the same, probably no heave or settling of that low area, but other area nearby, probably yes. What was the reference data for the measurements you have?

Those trees do not look sufficient large to be of consequence for affecting expansive soil.

That CC trench might have been backfilled with clean sand. That material does not heave.

Looking at the two pictures, I'd say the sidewalk is higher in the winter with respect to that pole. And the low area only slightly up.

 

[PaulLevy]

The reference the person was using for measurement would have been the front sidewalk, and those were reported/estimated measurements, I unfortunately don't have any hard data for the elevation differential. Service trenches here are typically backfilled with sand around the pipe zone only, so there's probably ~2.5 metres of clay over the service pipes. Typically the trench is backfilled with the same material that was excavated, but it's possible there might have been imported material when the lots was rough graded.

 

[oldestguy]

While we don't gamble here, I bettcha that when you drive the lower volume roads in your area in the winter that you see sags in the road at large culverts. There, the soil on either approach has heaved, but the culvert does not heave much if at all. Same situation at your house.

Where snow is present, that serves as a form of insulation. So, paved and areas clear of snow will heave more than those covered with snow. So removing snow from the site of "sag" results in less "sag". That is explained by you allowing that "sag" to heave more than the nearby snow covered area, but heaving some, appearing to be less sag. So, the "settled" or "sagged" areas really just have not moved, while uncovered driveway and walks heave in winter. In Wisconsin we have that same situation.

How do I collect on the bet???

 

[PaulLevy]

We don't really have too many roadways here in town with large culverts, but it's something I will keep an eye out for in the future. The logic behind your explanation is sound, certainly the most feasible I've had to date.

Two small items that are keeping me from buy-in: no other yard in the neighborhood seems to exhibit the same behavior, and this yard didn't exhibit it for at least 5 years (current owner has had the place for 10).

 

[oldestguy]

Hi Paul: Frost heaving is rather interesting. I can't really figure out why differences occur. Specifically at my place, the soil is quite sandy so frost action is less. However, I have one slab that heaved noticeably against the garage opening one year (in the 5 I have been here). No heaving noticed since. I "mud-jacked" a nearby settled walk way slab (due to former owner installing a drain) up a little more than needed, anticipating it would match that driveway heave the next year. No heave. However, it may be they both heaved together!!!.

Trying to figure what went on during construction as to soil conditions at other lots, is unlikely to be of use here. Backfills to trenches Amy or may not be done per specifications

Anyhow use that light pole for a reference and see what happens in the future. A carpenter's level along with a straight board should suffice.

 

[PaulLevy]

Thanks so very much for all your input. I have access to GPS survey equipment, so I may use that as well as a rod and level with the light pole to monitor over the coming winter and spring, enabling me to get some hard data.

 

[oldestguy]

One more from OG. Since we know frost heave takes three things for it to occur, what of the three can vary? Water availability and temperature down there. I'd put more opinion in varying water availability, but of course we know weather and that snow insulation can vary. So the history of the site heaving maybe is tied to both those variables. I do know that weather here in Wisconsin has significantly varied from year to year, such as snow depths and temperatures. Maybe same deal at the subject site.

The rise of water from below with respect to what may come from, above can be quite variable. While the use of the term capillary rise for part of it is not totally right (formation of frost lenses is tied to mineralogy of the soil more so than capillary rise), that's good enough for us here. My measurements of heaving, etc. show that a water table within 5 feet of grade has an effect, but not much if it is lower. That also is a variable from year to year.

 

[Ron]

PaulLevy...it might be counterintuitive for settlement to occur in winter, but think of this.....

With snow on the ground and not melting, there is little liquid water for uptake in the tree. The tree needs water even though its growth might be dormant as there is dehumidifying with the cold. Therefore, the tree takes as much moisture from the soil as it can.....resulting in loss of volume in clayey soils and, thus, settlement.

 

[oldestguy]

Aw common now Ron. That spindly tree drawing water from so far away in winter no less??? I think back of the many settlements I've investigated and none have come about in winter up here. July brings about most of the complaints. Do a Google search for "tree moisture demands in seasons". This one has some pretty good data.

http://treephys.oxfordjournals.org/content/16/1-2/263.full.pdf

Here is a funny story. A friend of mine used to work in a nursing home in Lansing, Iowa. One day we took a trip back for him to see his old buddies. I noticed two of the rooms were unoccupied. The manager indicated he was contacting a geotech firm in Iowa to come and see what is wrong, since the floors had settled and the exterior wall was cracked.
I noted a pretty large tree sitting just outside the wall and related how watering the ground alongside the old high school auditorium at Menash, Wisconsin brought back that settled wall and closed the cracks. Trees outside were drawing water. All noted in summer, by the way. That's one of several I've seen work.

A year later we made the visit again and I see the rooms were occupied and the tree still there. Asking the manager what the geotech firm did to fix it. He said he didn't call them, but decided to try watering the tree. Sure enuff, it did the job and one of my competitors lost a job. I shudda billed him.

 

[killswitchengage]

Interesting , Oldestguy and Ron taught us a new thing today : watch for dem trees