A contractor called us last fall about a deep excavation on the Rimrock Road corridor. The cut was nearly 30 feet into the Eagle Sandstone formation, and the shoring plan called for a tieback system that wouldn't hold. The original design missed a critical joint set in the rock mass. We re-engineered the active and passive anchor design using a bonded length that extended past the fractured zone, verifying capacity with a sacrificial test anchor on site. Billings geology isn't uniform across town. Beneath the topsoil, you find the Eagle Sandstone on the Rims, fractured shale in the South Hills, and deep alluvial gravels along the Yellowstone River floodplain. Each formation demands a different bond stress assumption. A design that works on the West End will fail downtown if you ignore the water table or the presence of cobbles in the gravel. We tie every anchor calculation to site-specific SPT drilling data and shear strength parameters from lab testing.
A 100 psi bond stress assumption that works in granite will cause a creep failure in the Cody Shale south of Billings.
