A common mistake we see from contractors in Austin is assuming the stiff Taylor Marl that underlies most of the city will behave uniformly when tunneling. That assumption can be costly. In our experience, the soft soil zones near the Colorado River floodplain and along creeks like Shoal Creek and Waller Creek contain interbedded clay, silt, and sand with highly variable properties. Without a site-specific geotechnical analysis for soft soil tunnels, you risk unexpected ground loss, face instability, or excessive surface settlement. We have seen projects stall because the geotechnical baseline report did not capture the thin sand lenses that caused running ground during excavation. A proper investigation starts with boreholes advanced to at least two tunnel diameters below invert, combined with In-Situ like SPT and a dilatometer test to measure lateral stress and modulus in soft clays.
In Austin's soft soils, pore pressure response to rainfall can double tunnel face support requirements within 24 hours.