Residential Foundation Problems

[RobPE]

I have a client whose house is experiencing foundation problems of some sort. The house is masonry stucco with a slab foundation/perimeter footing with a rear addition. A 0.5 in. wide crack has caused damage to the front living room floor. The tiles are loose in the vicinity of the crack, and there is also stair step cracking in the masonry where the crack intersects the perimeter walls. There are signs of strain in the ceiling, but none in the roof at least yet. There is also cracking and tile damage along the boundary where an addition meets the rear of the original house footprint. In this rear addition, the floor is also bulging noticeably over 2 separate areas approximately 6'x3' that are within a few feet of each other. There are also two other areas with stair step cracking along the masonry joints. The signs of movement are relatively recent according to the owner.

I've checked for obvious signs of water including surface drainiage problems, changes in the meter for periods when water is not in use, and exorbitant water bills. None of these seems to be the problem. The area has been known for problems with highly plastic clays. However, the problems are fairly recent (past 2 yrs for a 50 year old house) which suggests some recent change in conditions such as a water leak. I don't have much experience with these types of problems and would like a little advice on how to pin down the source of the problem before recomending expensive remediation such as helical piers and underpinning. Thanks for the comments.

 

[GeoPaveTraffic]

You seem to have hit the most obvious issues. It might be worthwile to make a hole, coring most likely, in the floor in one of the worst areas and see what the soil is like. Otherwise, based on what you have said, I don't see anything else to try.

 

[rochplayer]

What has the weather been like in the past two years? Do their gutters still work? Has the landscaping changed? Did they plant new flowers/shrubs/roses/etc. near the house and have they been watering them more regularly? Was grading or other construction been done upgradient of the home? Has the grading near the house settled over time so that the slope has finally broken toward the house in the past two years? Water is likely the trigger for the problem, but it may be a subtle source, like a change in habits by the owner.

A 50-yr old house has changed over time and like my knees, things may not work as well as they did when they were new.

 

[VAD]

Some other things that you may wish to focus on:

How old is the addition in relation to original building.

How was the disposition of the foundation system of the addition with respect to that of the original building.

I presume that the living room is far away from the addition

Swelling soils can become active by moisture resulting from evapotranspiration cut off. I have also noted that watering of plants placed next to building foundations on these soils can be the cause as well.

You have to be like a geo-detective searching for all possible clues and eliminating those that do not fit the Bill.

I would also check if waste water drain pipes associated with the new or existing building are leaking.

I would not be surprised if the problem results from subtle changes in the environment under the building footprint.

As GeoPaveTraffic recommended you need to investigate the heaved areas as he suggested.

Good luck and let us know what you find

 

[SCGeo]

In addition to coring on the interior, it might be worthwhile to excavate along the perimeter footing on the outside of the house to verify that the cracking does indeed extend into the foundation and to get an idea of subsurface conditions. A cracked foundation carries a lot of weight legally. It might also be a good idea to obtain some bag samples of sub foundation soils and do some natural moisture and atterberg limits testing.

 

[Mudman57]

The known 'problems with highly plastic clays' suggests to me subsidence being caused by desiccation of the underlying clay soil, rather than a water main/drain leakage issue. Has there been a long period of dry weather? Is there significant vegetation near the house?

You won't know until trial holes are dug to assess the founding conditions. If the soil is dry, you will need deeper boreholes to assess the depth of desiccation. Vegetation management and monitoring is often an option although with a 0.5 inch crack the problem may have gone beyond that stage.

I have seen many properties in London and the South East of England that have been fine for many years (through our major drought of 1976 for example) and then subsided during the 1990's during a less severe dry period.

 

[bushel3]

To add to what Mudman just said,

This year Illinois was really dry. 5th worst drought since late 1800's. I worked on 5 or more jobs last year were trees lowered the water content in the soil and caused the buildings to settle. We did borings on these jobs and water content was usually about 5 percent lower than normally encountered in similar soils. Ie, 18 to 20 percent water content dropped to 13 to 15 percent water content. Prior to this year my company had only seen one similar job where trees dried out the ground in the past 10 years.

 

[RobPE]

Thank you all for the comments. They are very constructive. VAD, the original house footprint is roungly 40'x25' with the 10'x40' addition in the rear done about 10 years ago (original house about 50 years old). The rear addition was essentialy a porch that got converted. The front living room with the large crack is about 30 ft from the area of bulging in the rear addition.

I think a lot of you are onto something that might be the culprit (VAD, rochplayer, mudman57, bushel3). The house is in Tucson, AZ where we've had ongoing drought conditions for the past 5 years so (even for a desert!). This idea - the dessication/ evapotranspiration cutoff problem - may be it. It leads me to a shuddering sense of clarity - there is a city well for the municipal water supply immediately across the street from this house! I wonder if drawdown of the water table due to pumping from the well may be causing subsidence which is in turn causing the damage. This idea is at least partially supported by evidence of cracking in several areas throughout the house. In addition to the 0.5 in. crack and the area of bulging, there are other areas in the house where more minor cracking can be observed. If the damage were due to a leak, vegetation, or surface drainiage problems, the area of damage would be more isolated rather than widespread. Is this correct? I know of heard of one neighbor who has similar problems, but am not sure about the others. Also, I've searched for some of the more blatant problems with the plumbing, vegetation, surface drainiage, etc., and can't find anything that's obvious.

If the damage is due to subsidence from water table drawdown, then any hopes of remediation (piers, underpinning, pipe piles) are very dim. What do you think?

Mudman57, I'm interested in your idea of investigating the depth of dessication. Could you please elaborate on how this is done? In Tucson, the phreatic surface is relatively low. The shallow soils typically have a very low moisture content. Thanks for all the comments and insight!

 

[rochplayer]

RobPE,

Map the cracks both inside the house and look for cracks in the street pavement between the house the water well. If they run perpendicular to the radial distance from the municipal well, your client may have a claim against the city that may provide funds for foundation repair, BUT that all depends on how the surface and subsurface rights are written. Although as an engineer I shudder to suggest it, it may be time to get a lawyer involved, at least so you and the owner understand the legal situation you are working in if the cause of the problem comes from off site.

Subsidence due to draw-down can be repaired, but if the water table is lowered further, additional subsidence may occur. Working with the city, you may be able to set a limit on the cone of depression, so that the water elevation does not decrease further. And you may be able to get funds to fix the problems. The subsidence issues go beyond the house structure - it can also effect all the buried utilities connected to the house.

On the other hand, you also mention "bulging" in some locations - so you may have multiple sources of problems.

 

[cvg]

Rob - subsidence due to groundwater withdrawals is a known problem in both Phoenix and Tucson and is well documented. For example, subsidence near Luke Air Force base in Phoenix over about 50 years caused 17 feet of subsidence and actually caused a drainage channel to begin flowing backwards. Also, note that high pumping and lowering of the groundwater table may result in a subsidence, but generally the subsidence occurs gradually over many months or years after the groundwater was lowered. It usually will cause the entire site to settle and won't cause differential settlement.

A phone call to your city building department can probably confirm the amount and rate of subsidence in your area. However, this doesn't necessarily confirm the differential settlement that you are seeing at your project site which may more related to dessication or water leakage.

 

[Reindeer]

Big doubt that drawdown of aquifer at +-150' is your residential problem. 99/100 times it is soil movement. You need more data. Pits are easiest, cores are easy. Be prepared to be baffled for a long time on where moisture souce is, sometimes several consultants work on the same house before everyone gives up and jacks/reslab.

 

[SafKiwi]

Don't get sucked into the subsidence causator due to draw-down unless there are similar instances in the neighbourhood. The hogging of the floor in the recent addition sounds like your biggest clue. Did the addition need new foundations? If so were they tied into the old ones? What was the increase in the imposed load of the addition. The extra load of the addition combined with increased bulk density of the swelled clays may have caused the diffirential settlement and the slab in the front of the house to 'break its back'. You really need to get samples from directly below the heaved areas, then samples adjacent to the footings all around the house. You say the local clays are known to be reactive - atterburg limits and moisture contents at regular intervals down to 3m from each sample hole might give you some good evidence for differential plastic indeces and moisture contents.
I don't think test pitting is necessary at this stage - simple 50mm dia. hand augering will get the returns (with hand held shear vane at incremental depths) and you can install a few standpipes to monitor groundwater levels - although in clays this is not the best monitoring method but is cost effective. The undrained shear strengths may indicate softening in the subgrade around the new addition.
Most importantly, what does your budget allow?

 

[fndn]

It is probable that if the original footings are not deep enough(24" minimum) that water was not effectively cut off and your plastic clays have swelled. In older homes, 3 or 4 inch thick slabs were common and that is too thin too withstand 150 psf plus heave stresses. Why not as mentioned by others core the slab and test for expansion and atterberg.

Maybe installing new stiffening beams in cored sections of the exisiting slab may solve future bulging concerns.