What you find in excavating is either competent rock, which is somewhat solid (it all breaks with the right equipment) or you have concomitant rock where the rock (limestone, granite, whatever) is intermingled with other rock or shale and comes apart easily without serving as a bearing layer for your planned construction.
What they found under the spillway when it was removed was some areas of competent rock, but also other areas of weathered shale intermixed with limestone or overburden. The non-competent areas had to be dug out until a good solid base was obtained and then a leveling “mud-slab” of roller compacted concrete was placed so that the final slab would have a uniform sub-base composed of either competent rock or mud-slab place to the planned subgrade of the new spillway slab placement.
I understand your point, that uplifting mountain building tore apart earlier layers of limestone in many cases and plain shelf rock is rarely what you get. But all we are looking for is consistency uniform enough to bear upon.
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Precious little granite on the west side until you get above 6500 feet.
At our place we have a one acre soft limestone bluff above 10+ acres of crumbled crystalline limestone intermingled with crystaline blue SiO2 that is hard enough to burn out a moil point in about two minutes.
It all comes apart with a ripper on a D5 effortlessly.
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