Foundation Heave

[solex]

There has been increased foundation movement of homes in the Kansas City area in the last year. In many homes it appears to be from heave because of interior doors out of line. There has been as much as 2-inches of differential movement on homes less than 1-year old. There has been a record drought for the past 1-1/2 years which does not make since regarding the heave assumption. Does anyone have any ideas about what is causing this problem.

 

[SirAl]

A lot of unknowns at this point so it is difficult to assess from a geotechnical perspective. Would the majority of these homes have attached garages and would these garages be founded on gradebeams supported by piles vs. footings in the remaining house area? Are the subject houses in close proximity to one-another and/or constructed within a new development. What are the primary soil types and conditions as well as water table of the areas affected.

 

[Grouser]

Are the soils susceptable to shrinkage or expansion? What is the shrinkage limit of typical soils? Shrinkage can cause differential movement just as expansion can. How is phenomenon located geographically? One subdivision? Old stream/river beds? What are the foundation types affected?

 

[VAD]

Movement of water in the vapour stage can also cause seemingly dry soils to gain moisture. Have you checked on lawn watering and planters etc next to homes. This watering can have some effect. The moisture contents in relation to Atterberg limits may give some indication of site conditions.

Grouse and Sir Al have some valid opinions as well

Interesting problem that needs keen site observation.


Cheers

 

[DRC1]

This is just a theroy, but It may be possible that given the fact you have a record drought and a large number of homes in the area experincing some settlement, that the water table has dropped significantly, and soft or loose soils that were submrged are no longer. When the soil is submerged, the water will carry a portion of the weight. when the water table drops the load on the soil increases and the soil settles due to the additional load. Again, this is just a theroy based only these two observations. a more complete study should be done to check it.
Hope your drought breaks soon

 

[ashjun]

DRC1 Your guess may be very right.

Anyone, any possible suggestions for the remedial procedures?

 

[Focht3]

DRC1 and ashjun:
Nah, forget the "increased overburden" theory. The house doesn't weigh that much; settlement would depend on the subsurface profile, not on the house weight. As a consequence, you'd get essentially random patterns of settlement over a fairly large area. [For what it's worth, I evaluated the settlement patterns due to seven new water supply wells as a part of a study in 1986(?) for the City of Houston. Drawdown of 100 feet at the well was only causing a fraction of an inch of settlement in a year at a distance of 200 feet from the well head. Big deal.]

solex:
I have a few suggestions, and a question.

Suggestions

Elevation Data. Have a "deep", stable benchmark put in close to a group of the "problem" homes. Have "true" elevation surveys (not "relative" elevation surveys!) done with a maximum survey closure error of less than 0.004 foot. Yes, that's damn tight. But you do want to figure this out, right?

Sewer System. Have the sanitary sewer systems checked for leaks. A small break - even a the top of the pipe - can cause a lot of damage given the right conditions. And look for joints that were not glued. Locate the plumbing breaks as accurately as possible. Leaking plumbing can keep the house from going up and down as a unit with seasonal moisture changes; the leak keeps one area continually wet, so you get really bad damage when it's hot and dry. Things get better in the winter and spring when it's cooler and wetter.

Be sure to have the lines electronically traced and a scaled drawing prepared. Don't trust the plumber's intuition about where the plumbing lines were placed - they're significantly in error at least 20 to 30 percent of the time. And the original drawings probably won't help - plumbers customarily ignore them in housing construction.

If you find breaks, you will probably want the plumber to clean and flush the lines thoroughly, then run a camera through the lines and videotape what he sees. Be sure he uses a modern system that can see underwater - one brand that will do this is a "See Snake." Study the videotape carefully. (And beware the deadly "brown trout"!)

Geotechnical Data. If you find plumbing leaks, you will need a geotechnical study. You will need at least five borings to depths of 8 to 10 feet (unless your geotechnical consultant says that 10 feet isn't deep enough): one near the center of the house away from plumbing lines, one at the highest point in the house, one within three feet of each leak, one near the lowest area of the perimeter of the house, and one in the yard. Take undisturbed soil samples every foot - run moisture contents, unit dry weights (densities) and Atterberg limits on each sample. Calculate the Liquidity Index - plot results for each boring with depth. Compare and contrast the profiles. Plot the moisture contents and unit dry weights on a "zero air voids" graph; look for patterns. This investigation will be quite expensive. But it works.

Question:
Have you had restrictions on lawn watering during this period? This would aggravate the problem - with or without a plumbing leak.

 

[ashjun]

Focht3:
You are very right. The increase in overburden due to the increase in effective unit weight of the soil below would not be so great as to cause significant settlement even if the ground water table level had gone down by over a meter (I just did some rough calculations to verify that).

Your view on the subject (increase in the saturation level of the soil below) is almost opposite of what was suggested, but can be correct too. In geotechnical engineering there is no straight answer.

 

[Focht3]

Thanks, ashjun. The big problem is there was not truly "fixed" reference point used, so the middle may be "up" as much or more than the perimeter is "down". This type of forensic evaluation is tough - and expensive.