Excavation in existing crawl space
Excavation in existing crawl space
(OP)
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.
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.





RE: Excavation in existing crawl space
RE: Excavation in existing crawl space
You may also want to calc the bearing on the existing foundation prior to the foundation investigations.
RE: Excavation in existing crawl space
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.
RE: Excavation in existing crawl space
RE: Excavation in existing crawl space
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.
RE: Excavation in existing crawl space
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
RE: Excavation in existing crawl space
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Excavation in existing crawl space
RE: Excavation in existing crawl space
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.