COLLAPSE BEHAVIOR OF SAND

Loose cohesionless materials can collapse during either static or dyna mic loading, resulting in a rapid buildup of pore pressure and associa ted reduction in shear resistance. As the cohesionless material collap ses, it rapidly looses resistance until the acting shear stress decrea ses to the available residual or steady-state strength. Specially desi gned stress-path testing has been performed on sand to investigate thi s collapse process. Results from this test program and previously publ ished data show that a state boundary can be defined when a cohesionle ss material moves from peak to steady state along a constant void rati o stress path regardless of whether it is loaded drained or undrained. Further. it is demonstrated that the state boundary represents a surf ace in the effective mean normal stress-deviator stress-void ratio spa ce. Hence, flow slides and liquefaction can be initiated when the stre ss path followed during either drained or undrained loading attempts t o cross this state boundary surface.