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Foundation settlement

Foundation settlement

(OP)
Engineers,

I have a question about a forensic project i am working on.

This 1 story home is showing some cracks on the walls and the floor was surveyed and settlements of 2 inches in one of the corners were recorded.

The borings were drilled to a depth of 10 ft. Soils (sp) show n values between 4 and 12, ranging from very loose to medium dense sands.

Due to this problem, i got the loads from the structural engineer and he told me they are around 1500 psf.

I decided to run few settlement analyses calcs. Using 1500 psf, The settlement results yielded very low settlement values, nothing compared to 2 inches. Computed values were around 0.1 to 0.2 inches at the most (i played with E values provided by Das and Bowles, using worst case escenarios). For n <10, i used e = 10600 kpa, for n between 11 and 15, i used e = 15000 kpa.

After this i proceeded to compute the bearing capacity using the traditional equations (terzgui, meyerhoff, vesic). The results yielded ultimate bearing capacity values of around 2700 psf, on average.

From this, the computed FS (2700 / 1500)= 1.8.

Per the geotechnical references, A FS > 3 is recommended for bearing capacity purposes.

Do you think the reason of these settlements could be because of such low FS computed)?. I did not see soils around the footing heaving or bulging.

Df = 1 ft, B = 1 ft

Fyi, this project is in florida where lots of sinkholes occur. Somehow client only approved 10 ft of drilling.

Does my calcs make sense?. Please advise.

Thanks.

RE: Foundation settlement

Some added info needed. Age of building, outside drainage situation, basement?, any activity outside such as earth grading?, trees nearby, type of tree, geology of the area, when did settlement occur with respect to life of building?. type of foundation (concrete, block,?), original site before building, such as filled low area., nearby recent construction, sewer and storm water drainage., any service water piping problems? Any vibrations in area (trains, traffic) Some photos will help. I'd be more inclined to suspect something happening, other than footing under designed, etc.

RE: Foundation settlement

Is this boring information from soil borings conducted before or after construction? If this info is from post construction borings and considering you are dealing with SP soils, perhaps some densification occurred due to construction activities. Original soils when the building was constructed could be looser than what you have now. Also, all above additional info required by OG above should be considered as well.

RE: Foundation settlement

(OP)
Yes,

These borings were drilled after construction. That house was built in the 1970s and we drilled these borings about 2 weeks ago.

I understand the requirements by OG are key here.

when you mention "perhaps some densification occurred due to construction activities", do you mean immediate settlement?. If so, that is what I computed, and I am getting very low settlement values, compared to the actual floor survey.

RE: Foundation settlement

The settlement you calculated is based on post construction soil properties. You would need modulus of elasticity values or some other type of insitu testing data available that was performed prior to construction to calculate the settlement and compare it to the survey data.

RE: Foundation settlement

Settlement due to shear failure of the soil is unlikely. You are still 1.8 times the design load.

I would agree with OG some more info needed. I have done some forensic jobs and typical "reasons" are: 1 - planting of a tree that reduced moisture content and caused the clay to shrink over 5-6 yearss, 2 - improper compaction of structural fill material (that was placed as part of the building construction, it was my own house!!!!!) 3 - a house being built on filled ground where they didn't know it was filled (no geotech done before).

If you want to get better answers i would post a typical cross section of the foundation at the worst corner. is it a strip footing with rising walls or is it slab on grade, timber piles?

Also, reviewing google images is very helpfull as it can show you whats gone on at the site over the last few years. i would post them here too.

RE: Foundation settlement

Out of interest too. Are you N values from an SPT too or correlated? Could you be over estimating them with a correlation? Just a thought?

Jmcc - that might be a big ask since it's 1970s house. Alot of the time forensics jobs are old sites with no geotech when the house was built or geotech that far away from the house that would be more or less than soil properties altered by design load (ie becoming denser)

RE: Foundation settlement

Your footings are 1 ft wide? If so, there are small footings. However, you can also check with 12" diameter plate bearing tests. The plate is the same size of your footings so you can have a good idea of the load-settlement behavior.

RE: Foundation settlement

Study the settlement pattern as indicated by floor elevations (easily done with a water level) and cracks in the walls. Diagonal cracks lean towards the location of greater settlement.

Get as much history as you can. Has the condition existed for a long time, or did it recently appear?

Were your borings drilled close to the corner that settled? One close and one farther away where no settlement is evident would be helpful.

Highly localized settlement probably indicates a local soil anomaly or a local event. Examples of an anomaly include a hole dug by the contractor and loosely backfilled, a hole where a big tree was dug out, an old basement, cistern, root cellar, or well. All of these tend to get backfilled with loose soil. A local event usually involves water; a damaged downspout, leaking water line, overwatered vegetation, etc.

If you eliminate all these, perhaps a sinkhole is indeed developing. Investigating that will require several borings or probes to rock or some type of geophysics. You can tell the owner that he needs such an investigation and let him decide, but do not ignore the possibility. Otherwise, the media may name that big sinkhole after you!

RE: Foundation settlement

Take a look at he foundation and ste3m wall in the area of the settlement too.

Are there door, window, or crawl space openings in the area of the cracking that would weaken the foundation if not properly reinforced? The foundation is almost 50 years old. Rebar was available, but may not have been used...

Mike McCann, PE, SE (WA)


RE: Foundation settlement

Have a look at how the surrounding areas drain when it rains and where the gutter down spouts are located if any. Any trees close by that were removed or blown over from the hurricane? If the house has been there for that long most of the long term settlement (if any) would have dissipated by now. I would look at all these possibilities before you jump to investigating for a sink hole. What part of Florida are you in?

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