Sandstone or Sand - That is the Question-PPT Presentation

PPT Presentation

Pdf Summary

The presentation examines drilled-shaft foundations for the IA-9 Black Hawk Mississippi River Bridge in Lansing, Iowa, where engineers questioned whether the underlying weak, poorly recoverable sandstone should be treated as sand or bedrock.<br /><br />A demonstration shaft was built with an 8.5-foot-diameter rock socket. Despite repeated attempts by multiple drillers, a confirmation boring recovered no core. The shaft was load-tested to evaluate the sandstone’s side and base resistance. Results showed mobilized unit side resistance averaging about 16 ksf and unit base resistance of 444 ksf at just 0.22 inches of displacement. These values were substantially higher than estimates based on sand, although the test results represent mobilized—not ultimate nominal—resistance.<br /><br />The authors emphasize that construction practices strongly affect drilled-shaft performance in weak sandstone. Key practices include using polymer slurry, limiting socket exposure, thorough airlift cleanout and fluid exchange, verifying socket geometry and cleanliness, designing an appropriate reinforcing cage, using neat cement grout and workable concrete, and maintaining informed visual and physical observation.<br /><br />The presentation draws five main conclusions: different drillers can produce widely varying results; zero core recovery and zero rock-quality designation do not necessarily indicate inadequate compressive strength; side and base resistance can mobilize at similarly small displacements; construction quality governs foundation design and performance; and properly constructed and inspected shafts can behave as foundations socketed into bedrock rather than sand.

Keywords

drilled-shaft foundations

Black Hawk Mississippi River Bridge

weak sandstone

load test

side resistance

base resistance

rock socket

core recovery

construction practices

foundation performance