huangyhg

soil subgrade modulus on rock
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.
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.
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.

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