Cause of Foundation Settlement?

[mjr6550]

I recently looked at a foundation settlement problem at a 40-50 year old house. The house is located at the bottom of a hill and an approximately 6-foot diameter storm water drain passes through the side yard about 25 feet from the side foundation walls. Soil is silt/clay. The only history I have on the house is that the foundation walls were painted after as fire occurred in 1995. Since that time (and possibly before) there has been excessive settlement at the side of the foundation closest to the storm water drain. Based upon limited exploration I believe I hit a layer of weathered rock approx. 5 feet below the basement floor slab. Since the foundation is supporting the house by arching over the settled area there is not much load on the foundation. Erosion or piping seems like a possibility, but I did not think this would be likely with this soil. Of course, owners have little money to spend to evaluate and repair. I have attached a few photos. Any thoughts would be greatly appreciated.

 

[ishvaaag]

As you suggest either repeated losses from the storm drain or even subsurface rainwater draining maybe the cause. In Calatayud, Zaragoza, Spain, there was a case quite famous where a footing resulted to be upon an around 1000 m3 formed cave by water filtration in 20 years (have not the report but will look for it if public). There there are silty clays, and also quite concentrated suburface drain of rainwater, enough to inundate cellars with bad waterproofing. Not exactly your case, but along the whole 25000 people village these circumnstances concurr. The less cohesive part and even the exposed clay may be slowly being filtered maybe even along the storm drain itself, externally or whatever if cracked. If these things were the cause, only between the causes a repair of some cracked storm drain seems a likely partially successful repair. If subsurface currents are the cause, now drain paths may be well established and it is unlikely one can affect effectively all them. The only successful likely course of repair seems to be supporting the repaired and modified foundation at so strong and low a level that the incidence of the loss of material can be almost entirely diminished. This is unlikely within the budget constraints you indicate.
If a ruin is not enforced by the council, with a proper assesment, whilst you should recommend any measures laeding to prevent any sudden loss of life at the property. Maybe you can make use of the strong points towards which the arcing action discharges for placement of some micropiles themselves to support a new frame holding everything above whilst the central settlement is dealt with. Then the central part can be injected with liquid grout at the dry season, too dear everything, anyway. Also you can support a new slab on the ground on foams of enough resistance to compression etc.

 

[PSlem]

Could you have hit a trash pit with your probing and the water used to put out the 1995 fire washed the soil arching over it into the voids?

 

[oldestguy]

There was a similar situation in my town recently. The storm drain was owned by the highway department and they replaced it free.

Perhaps the owner of the storm drain can be contacted and asked to be responsible for the problem and the fix.

I'd suspect that the storm drain is corrugated steel having lost its galvanizing.

Can you get inside the pipe to view it?

 

[mjr6550]

Oldestguy
I would like it to be due to the storm drain, but have not been able to tie it in yet. The owner contacted the municipality and was ignored so far. The drain is concrete. I walked the drain. Some small signs of infiltration at joints, but not much. There is significant deterioration right below the street inlet, but the pipe is still intact. Some Photos attached.

 

[msquared48]

I can see settlement in the slab at the CMU wall, but the CMU wall seems that it is being pushed to the inside, and it is ungrouted and unreinforced for such a lateral load. CMU walls need to be fully grouted below the ground line by current code.

I am wondering if the storm drain is broken allowing the ground to become saturated and increasing the lateral soil pressure, causing the ewall to fail laterally. With clays, if they were dry over a long period and became saturated, they can expand - hence even more lateral pressure.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[cvg]

you need to answer a few questions to get to the bottom of this such as:

when was the drain installed and was it before or after the settlement started?

does the fire have anything to do with settlement?

is the soil mostly silt or mostly clay? is it plastic? how does the addition of moisture to the soil affect the strength?

Is the foundation acceptable or under-designed? What is the quality of the house construction?

is the storm drain pipe surcharged (does it flow under pressure), does it flow constantly, how much flow?

could exfiltration through the pipe joints be introducing water near the bottom of your footings? are the pipe joints gasketed?

how about general drainage around the house, is their any concentrated drainage towards the sinking side of the house? where does the water from on top of the hill drain, towards the house?

what level is the groundwater?

any other utilities such as waterlines or sewers?

any settlement over the storm drain itself? how about excessive vegetation over the storm drain?

any excessive large vegetation near the settled side of the house?

 

[msucog]

here's my guess...trashpit or area of loose backfill left near the lateral limits of the pipe installion backfill. could be as others suggest but this seemed more likely to me. could explain why pipe isnt in terrible condition (although i've seen sinkholes open up from soil loss and take cars from a 6inch x 2inch gap in a storm structure adjacent to pipe to structure connection--mortar fell out). the backfill/trashpit theory could/would also tie it to pipe construction if it is the case...proving it in court may be different story.

 

[foundationman]

We have seen similar problems in Texas in silty/clay soils. The settlement has generally had one of two sources, 1] loss of material into the drainage pipe, or 2, consolidation of poorly consolidated fill in the trench containing the pipe. For problem 1, a solution is to use polyurethane injection to seal off leaks in the pipe. If practical, this avoids the cost of excavation. Care has to be taken not to obstruct the pipe with polyurethane. For problem 2, we have generally underpinned the foundation with pilings driven under the footing for the exterior wall.

 

[mjr6550]

I'd like to thank everyone for the replies. To clarify some points, there are some drainage issues around the house, especially a downspout at the front corner where the most settlement has occurred. This was piped underground to the storm drain, but apparently has not been connected for many years. I do not think that the fire contributed in any way. There has been some soil infiltration into the pipe, but it did not seem to be excessive and with the pipe being 20+ feet from the foundation wall I did not think it would have that much effect. water table seems to be significantly below foundation and drain pipe. The municipality is planning on replacing the pipe to deal with flooding issues that have occurred several miles away. There was almost no flow in the pipe when I inspected it, but I assume that surcharge likely occurred. Soil is mostly silt. It looks like the most cost effective solution will be underpinning with helical or push piers. I plan to do more investigation when a contractor excavates to do repairs. Also I may be able to investigate further when excavation is done to replace the storm water drain pipe. Considering the amount of settlement and the light load, I'm guessing some type of soil loss is the cause. Based upon the limited excavation I performed near the foundation the soil was well compacted with no signs of fill. I think some settlement occurred very long ago, but the most significant movement at the area shown in the photos occurred within the past 14 years.

 

[cntw1953]

"...a downspout at the front corner where the most settlement has occurred. This was piped underground to the storm drain, but apparently has not been connected for many years."

Is there a drainage basin in between the storm sewer and the house drain pipe (from the down spout). It's not clear at which point the house drain pipe was disconnected. I am thinking is there any chances the silt was washed into the storm pipe/drainage basin due to back-up of the flood dowstream.

 

[palmahouse]

I did not have time to read all the posts, but just wanted to point out that I have investigated a piping problem in the past, and I hired a leak detection company to do a camera survey of the pipe. For my project, the data was quite valueable.