Underground footing for underground facilities bearing check

[Civilling]

We want to design an underground support similar to a combined footing for a heavy underground asset. It will all be underground and filled and compacted at the end. We would not be using geotech reports and will be using the presumptive values given by IBC 1806.2 (1500psf). When performing a bearing check under the face of the support, would the load used for dividing by the area include the weight of the soil above the support, or just the weight of the asset? Would I also add the weight of the concrete support minus the weight of the soil displaced?

 

[oldestguy]

Answer: Maybe yes and maybe no. Generally with some yielding of the support you may develop arching in the soil above, transferring some of that upper load around the zone of the facility. A geotech at least looking at the job might be worth your while.

 

[Civilling]

Is it typically needed for something like a shallow foundation? That is the closest thing I can compare it to.

 

[MotorCity]

Based on the questions you are asking, I think you need to seek assistance from another senior engineer. But in the meantime, I will try to point you in the right direction.

So, in order to use the presumptive values in IBC, you need to know generally what type of soil you have. Are you able to do this without the input of a geotechnical engineer?

Generally, allowable bearing capacity is given as the NET bearing capacity. This means any additional weight the soil will see other than the weight of itself in the existing, undisturbed condition. Calculate the weight of the structure and any loads on the structure and the weight of the displaced soil. Compare these values to determine if you have any net increase in soil bearing pressure and if so, then compare the increase to the allowable soil bearing pressure.

 

[Civilling]

Thanks for your input about it being a net bearing capacity. That helps a lot. I haven't had a geotechnical engineer's input, but will use the worst soil of 1,500psf. If it passes the bearing check at this pressure, should it be ok to use 1,500 without input from a geotechnical engineer?

 

[MotorCity]

Anytime you assume a bearing capacity without qualification you take on a risk, you decide if its worth it. You can cover yourself by stating the bearing capacity you assumed for design on the drawings and add a note that it shall be field verified by the contractor's geotechnical engineer. Otherwise people reading the drawings will assume that you have verified the bearing capacity yourself or by some other means. You are probably safe using 1500 psf, however....there could be a layer of organic material just below your foundation that's only good for 750 psf.....or what if the soil is actually good for 3000 psf...then you are doing your client a disservice by overdesigning and costing him or her $$$.

 

[Civilling]

Very helpful information. Thanks.