Stress overlap of existing building.

[ITsSB]

We have a two basement building to be constructed near a G+7 building (total height = 24m. Due to the slope of the ground, we are to excavate 11m near this building. Initially we were going to provide shoring but we chose to remove 1 basement near the existing building (not as a whole but near the building). My boss (structural)says we need to offset our footing based on 1H:1V for rock and 2H:1v for soil in order to avoid stress overlap without rigorous calculation. But the only thing I found is 1H:2V. So my question is: -

1. Is he right (like for 6m depth I need 6m offset?)
2. Can Bousinesq's theory be used to calculate stress a required depth when the soil is influenced by a retaining wall provided (I think one of its assumption says semi infinite but Mine is discontinuous at the wall retained edge.)

 

[SlideRuleEra]

Initially we were going to provide shoring but we chose to remove 1 basement near the existing building (not as a whole but near the building).

In the USA, decisions on temporary shoring are often part of a Contractor's "means and methods". Since you (Engineer?) have decided not to use shoring, conditions during temporary, open-cut excavation need to be evaluated. Temporary excavation may well be the overriding factor whether the new building can be safely located as shown. Without shoring, the excavation will be "deep" and relatively close to the existing building. Not only may the slope of the excavation be "steep" (and potentially unstable), but weight the existing building is a surcharge load that makes things worse.

Suggest getting a geotechnical engineer involved for both temporary and permanent conditions.

 

[geotechguy1]

Stress bulbs are generally simplified as 1H:2V or bousinesq or westergaard stress distribution in soil (or similar - different countries / professors / standards will be slightly different).

Commonly people use 1H:1V or 2H:1V as rules of thumb to determine these setback distances for footings, which I suppose is an implicit way of incorporating a factor of safety of 2 to 4.

As SlideRule says though, I'd be more worried about the potential for failure / displacement depending on how the excavation for the new building is done. Your stress bulb / overlap for the permanent case is one thing, the soil under the old footing sloughing out during excavation of the new building or unacceptable displacements occurring due to the shoring method or dewatering is a much more immediate concern that needs to be addressed.