ASCE 7-22 and the Minnesota State Building Code (Chapter 1305) require a soil mechanics study before any foundation design in Minneapolis. The Mississippi River valley and the glacial history of the Twin Cities create a subsurface profile that changes radically from block to block. One lot hits dense glacial till at 8 ft; the next one, three houses down, finds 25 ft of compressible floodplain silts overlying the Platteville limestone. The test pits program we run in neighborhoods like Prospect Park or near Minnehaha Creek is designed to map those transitions quickly. For deeper bearing layers in the Downtown and North Loop districts, we combine the soil mechanics study with SPT drilling following ASTM D1586-18, because the IBC requires undisturbed strength data below the seasonal frost line at 42 inches.
A 2-ft change in the top of the Platteville limestone can shift the seismic site class from C to B under ASCE 7-22, altering the design base shear by 15%.
