How Are Foundations Built on Spokane's Basalt Bedrock?

The foundation design starts with knowing which rock the footing will actually sit on. Borings and test pits map the basalt flows and the sedimentary interbeds between them, and the geotechnical engineer assigns bearing values to each: high for fresh, massive basalt, reduced for weathered or fractured rock, and conditional for the Latah interbeds, which perform well undisturbed but degrade when exposed. The report specifies the required bearing stratum by description and elevation, so the excavator knows when to stop digging and the inspector knows what to verify. Protection of the bearing surface is a construction operation with engineering consequences. The contractor keeps the exposed rock covered and dry, places a mudslab or gravel blanket promptly after the geotechnical engineer approves the surface, and maintains the dewatering or diversion that keeps groundwater and stormwater off the excavation. Where the bearing surface crosses from basalt onto an interbed, the engineer decides whether to overexcavate the weaker material, bridge it, or found deeper, because a footing bearing half on rock and half on softened sediment settles differentially.

Basalt excavation is selected for the rock's actual hardness. Weathered and closely fractured basalt rips with heavy excavators; fresh, massive flows need hydraulic hammers or hoe-rams; and production blasting, where the site and permits allow it, moves the largest volumes fastest. Vibration monitoring protects neighboring structures when blasting or heavy hammering occurs near them, and the excavation plan keeps slopes stable in the fractured rock, with rockfall protection where the cut stands open. Below-grade walls in rock cuts are designed for the lateral pressures the fractured basalt and any soil overburden impose, plus hydrostatic pressure where groundwater is present, with drainage behind every wall. Exterior footings bear below the frost line so seasonal freezing cannot lift them, and grading carries surface water away from the foundation. The structural drawings show the bearing elevations, the required rock description, and the drainage details together, because on basalt the foundation's success is decided as much by water management as by bearing pressure.