Excavation near foundations - rule of thumb? 1

[Rockjoint]

I wish to know of any rules of thumb for staying away from (or getting close to) residential and commercial foundations during shallow (less than 20 feet) excavating.

I frequently need to excavate contaminated soil in the vicinity of buildings, and do not want to undermine or otherwise compromise the integrity of the foundations or walls themselves. Soils typically range from silts to silty sands, and often encounter glacial till at depth. Can I somehow use Boussinesq theory? Thanks for any input. I am not an engineer, but wish I was.
Rockjoint
rockjoint@yahoo.com

 

[dik]

For soils in our locale, we typically stay outside a line from the edge of the footing that is 10 horizontal to 6 vertical. This should be OK for most soils that have some cohesion. Sands having little or not 'binding' fines may require a more shallow slope. If the excavation is going to be of a very short duration something in the order of 1H:1V may be in order.

You may want to check with some local geotekkies who may be more familiar with the soils.

 

[VAD]

Hello Rockjoint:

It appears to me that you have some procedure in doing this work based on your statement made."frequently need to excavate..." Hence, I am sure that you do not just go in and excavate. There is no particular rule of thumb for this type of activity. Dik has given you information on his experience in his locale. Others can give the same depending on their experiences.

I can also suggest that if the building is residential and has a basement then you can exacvate as close as possible to the basement wall but you should not go below the footing level. For commercial buildings if they are founded on piles the same could apply. However, the above is not always correct unless one understands the foundation elements of the prevailing structure and soil conditions.

I am curious on what information you obtain on the structure and site before deciding on your methodology and equipment in undertaking the exercise.

Remember, that sandy soils could be problematic and that lateral soil movement could result in settlement of the existing structure. For a depth of 20 ft below the foundation level, I am not sure that this is shallow, or maybe the depth is based on some other reference point -existing ground level. It all depends of course on how close you would like to get to the existing structure in taking out the contaminated material.

Unless you have money to offset liability issues then this type of decision should be, as Dik said, left to the local geotekkies to provide advice. You may be lucky most of the time but it only takes one mishap to cause you grief and your pocketbook.

The answer to this type of question lies in the understanding of fundamental soil behaviour, type of soil, ground water conditions, the concept of the zone of influence used in determining the bearing capacity of the foundation, the equipment to be used for excavation and others.

I am sure that you do not just dig on any site without having the utility companies locate their facilities. Well, the same thinking should result in the geotechs being solicited for input unless of course that you are confident in digging, examining the soil yourself and making the decisions.

Am I misreading the intent of the query?.

 

[Rockjoint]

Thanks to both of you for the advice. You are right. I do indeed inspect the soils and the conditions prior to excavating. I said 20 feet, but usually the depth is much less than that, typically only 5 feet. Often we use mini-excavators, and they often have a reach not much beyond that. And it is often at that depth where we encounter rock.

I look at the general construction of the building and whether it is slab-on-grade, etc. to identify the type of foundation. Also, I inspect for old stone walls (staying at a distance from them) and for obvious cracks in the walls/foundations prior to doing anything near them.

I never get closer than 2 feet to the wall, regardless of condition. That may be highly conservative, but that is the way I go about it. If I *should* get closer, I would remove the uppermost soil and then excavate away in a step-wise fashion, forming a bench if necessary.

Thanks for the valuable information.

Rockjoint
rockjoint@yahoo.com