Bearing Capacity and Overburden Stress

[Structural.1997]

I have read a forum before saying that overburden loads can be neglected in bearing calculations if that same overburden load was there pre-construction. I asked a geotechnical engineer about this and he said that he has never heard of this. The consensus is my structural office is that this is not true and to design foundations according to Option 1 below.

Example:
Geotechnical report states that the factored Geotechnical Resistance is 130kPa.
Excavated depth of 4m and native soil density of 19kN/m3 ==> pre-construction load on the soil at 4m below grade will be 76kPa
Engineered fill density = 20kN/m3 at 4m depth ==> post-construction overburden load on shallow foundation = 80kPa

Option 1
I was taught to include the overburden as a load in the bearing calculations, this means that even without factoring the overburden weight I'm only left with 130-80=50kPa to carry the factored loads from the building. This sometimes makes spread footings impossible even for relatively small buildings that should be fine with spread footings.

Option 2
Neglect the overburden for bearing calculations (up to the pre-construction overburden levels) and only considering it for stability effects (ex. uplift and overturning).
It was suggested that I accomplish this by treating the factored soil capacity as 130kPa + 76kPa = 206kPa.
This would cause the overburden load to basically cancel out to zero (80kPa-76kPa=4kPa of additional overburden) in bearing checks but still be applied in stability checks. After cancelling out, the capacity would be equivalent to the 130kPa capacity listed in the geotech report.
The theory behind this is that the soil at that level has already experienced that 76kPa that was removed during excavation so the underlying soil wouldnt "feel" a stress increase until after another 76kPa is added.

I'm not sure that I'm describing this correctly, but feedback from Geotechnical Engineers would be greatly appreciated.
Thank you.

 

[HTURKAK]

Geotechnical report states that the factored Geotechnical Resistance is 130kPa.

Is this the bearing capacity with a F.S. =3.0 ? At which height ?
In my one , the allowable bearing capacity is typically provided at 1.0 m depth .
I will suggest you ;
Ask the bearing capacity at 4m depth and follow OPTION 1.

 

[DaveAtkins]

I totally disagree with you and your colleagues. Normally, allowable bearing pressure is NET allowable bearing pressure, excluding the overburden.

 

[HTURKAK]

Is this the bearing capacity with a F.S. =3.0 ? At which height ?
In my one , the allowable bearing capacity is typically provided at 1.0 m depth .

EDIT:
My keyboard has a problem with letter Z. Sometimes it is not printing.
The sentence would be ; In my Zone , the allowable bearing capacity is typically provided at 1.0 m depth .
And typically gross allowable bearing capacity used unless otherwise mentioned. EG; If you design box foundation, you can increase the allowable bearing stress considering the overburden.
The following excerpt from ; Principles of Geotechnical Engineering , BRAJA M. DAS.

 

[Structural.1997]

EDIT:
My keyboard has a problem with letter Z. Sometimes it is not printing.
The sentence would be ; In my Zone , the allowable bearing capacity is typically provided at 1.0 m depth .
And typically gross allowable bearing capacity used unless otherwise mentioned. EG; If you design box foundation, you can increase the allowable bearing stress considering the overburden.
The following excerpt from ; Principles of Geotechnical Engineering , BRAJA M. DAS.


View attachment 3678

I've never seen a Geotech report specify either net or gross bearing capacity. Only terms such as "Factored Geotechnical Resistance (ULS)", "Geotechnical Reaction (SLS) for
25 mm Settlement", and are typically listed at a certain depth or minimum depth.

Is it safe to assume that the ULS resistance should consider the overburden load and the SLS be compared to the net load?
Or is this just something that I'm going to have to just send request for clarification every time I read a geotech report?

Thanks