Soil Subgrade Modulus on Rock 1

[LPPE]

My Geotech is on vacation...
So, I've got a mat foundation on a hard rock (gneiss) of very good quality. The allowable bearing pressure on the rock is at 80 ksf. Does anyone have a suggestion as to a reasonable modulus of subgrade reaction? I'm figuring on somewhere between 12,000 pci and 8,000 pci.

 

[MotorCity]

A subgrade modulus for rock is not of much use. The modulus is defined as pressure/measured deflection. Rock, unlike soil, does not compress when pressure is applied (i.e. it will crack before its compressibility can be mearured). Thus, the rock will experience a brittle fracture failure instead of a slow compression as can be seen in most soils. If your allowable bearing pressure is exceeded, there most likely will be an immediate failure of the rock.

 

[WillisV]

If you are just needing that value to put something in a matt design program I would use something in the order of 36*bearing pressure.