Excavation in existing crawl space

[jtstruct]

Thanks in advance for your time.

I'm involved with a project where the designers plan to excavate in a crawl space to create a basement for a mechanical room. The designers have drawn a 45 degree line from the edge of the existing footings into the crawl space area. Their argument is that everything is a-okay (meaning no need to do anything special to the existing footings and no need to hire a geotech) as long as they don't excavate past this 45 degree line. I'm not sure where they came up with this 45 degree line (i.e. why not 30 degrees?). It seems that this imaginary work line would be completely dependent on the properties of the soil. I say we need to hire a geotech. Do you all know if this is common practice for excavations adjacent to existing footings?

FYI, this is a very old, 3-story building with stone footings.

 

[eureka]

By all means get a geotech involved. The excavation line is very much dependent on the soil properties. Special care is required when excavating next to existing foundations.

 

[ERV]

Not following the proper steps can lead to disaster. Particularly on old structures with stone foundations. Hire a geotech, get an existing foundation evaluation as well as soil stability, bearing capacity, etc.

You may also want to calc the bearing on the existing foundation prior to the foundation investigations.

 

[cap4000]

Definitely go to a Geo-Tech/Foundation Engineer!! The 45 degree line is just a "rule of thumb" as far as the load spreading out from your stone footings. The 3 story building may have column spread foootings and of cause continuous wall footings. Boussinesq and Spangler have special formulas you should consider for the actual load
spread soil pressures and distribution. Don't rule out "Underpinning" as also a way to save space should the existing conditions allow for it. Good Luck.

 

[PEinc]

It is common - a common mistake! I'd guess that no one has sampled the soils or made any borings. Hire a geotech. Any ground water seeping into the deepened basement could undermine the existing foundations. Any flooding of the basement due to broken mechanical (water) lines could also cause the same problem. I agree with cap4000. Seriously consider underpinning.

 

[ashjun]

I beg to differ.
45 degree is a general practice; that load under a footing is dispersed within that angle. The theory comes out from standard Boussinesq equation and other allied derivations. So, your designers are not totally wrong. You may find this reference in many standard text book (I remember looking at one code that two adjacent footings at different elevations can be installed as long as the line joining the bottom edge of the higher footing and top edge of the lower footing (essentially the line joining the two footings) should not incline less than 45 degrees (or was it less, I need to check that if I am able to remember the code). Anyway, it is within the 45 degrees.

Although concern raised by PEinc (rise in GWT) should definitely be checked by a competent engineer if GWT is high.

 

[DRC1]

Bousineeque (pardon my spelling) can be approximated by a 2:1 slope To be conservative, a 1:1 slope is sometimes used. If a lower footing, wall,excavation is located within this influence line it will pick up load from the higher footing. Objects outside the should not see a significant increase. It is au seful tool for doing preliminary layout, but I have seen the rule misused more than used properly. Particularly in your situation. For example, the approximation does not consider the capacity of the footing or the excavation. Once you actually excavate a slope below a footing, you have substatally reduced the capacity of that footing. Even with out groundwater it could fail suddenly. Also excavating in a crawl space can be difficult to maintain a desired slope. It may be tight and cramped and easy to over excavate. On the otherhand, the setback may be okay and the footing loads low enough that you might not have a problem. But you should know. The guys doing the work are trusting that someone has decided it is safe. So let me be clear about this:
GET A GEOTECHNICAL ENGINEER BEFORE YOU FINALIZE THE PLANS OR LET A CONTRACT FOR THE WORK. GET ONE FAMILAR WITH LOCAL CONDITIONS AND CONSTRUCTION EXCAVATION.

Good luck

 

[Focht3]

I'm a bit tired - been dealing with lawyers all day. So I'm a bit grumpy. Please excuse my bluntness - but relying on a 45&[ignore]deg[/ignore]; projection in order to save the geotech fees is just plain stupid. A very old stone building that's three stories high - with stone footings? Can you say, "Collapse"?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[PEinc]

Well said, DRC1. The 1:1 or 2:1 influence of a higher footing on an adjacent lower footing is a different problem than removing confining or overburden soils from a footing.

 

[clayR]

Hi all,



The issue here is no soil study or design! I wont even comment on the 1:1 layback slope. What slope is safe with the given surcharge? It could only be a guess.

As stated by others you need help. Do not proceed without engineering.