Upper limit for soils' bearing capacity (ULS)

[MramJ]

Hi everyone,

I have a geotechnical report that provided an Ultimate Limit State (ULS) bearing capacity of 2000 kPa for silty sand soils with N-values greater than 50 (reaching about 70 blows per 0.3 m). My question is: Is there an upper limit to the ULS value where a geotech would not consider for soils? I ask this because I’ve noticed that similar or even lower values are typically given for bedrock. For example, according to CEFM guidelines, a preliminary design bearing capacity of around 1000 kPa to 4000 kPa can be used for metamorphic rock (medium to high strength). Does this indicate that there is a practical or theoretical threshold beyond which soil ULS values are not realistic? Could someone please clarify this?

Thank you

 

[Greenalleycat]

Different soil and rock types will have typical ranges - your metamorphic example means that they will have tested a bunch of different rocks consistent with a "metamorphic rock" description and that's where most samples fell
There is probably a material more firm than 'metamorphic rock' that will pick up at the higher end and give better capacities
There will be a high end where the materials encountered at earth's surface just don't give higher bearing capacities - we're not expecting to find high-grade steel alloys littering the earth's crust neatly under our foundations, are we?

As someone that also does a lot of shallow geotechnical work...I commonly cap the ultimate capacities I give to what I consider are realistic 'good enough values'
I do this to lead the engineers to using more distributed foundations, rather than trying to chuck down big point loads
This is a safer approach that leads to better overall building performance
I'm primarily a structural engineer so I think I have a good feel for what is realistic in these cases