1. P.B. Flemings (1), I. Song (2,3) and D.M. Saffer (3) (1) Jackson School of Geosciences, University of Texas, Austin, USA (2) Korea Institute of Geoscience and Mineral Resources, Korea (3) Department of Geosciences, Pennsylvania State University, USA Laboratory investigation of coupled deformation and fluid flow in mudrock: implications for slope stability in the Ursa Basin, Gulf of Mexico

2. Location of Ursa Basin, GOM Gulf of Mexico

3. Seismic Profile: IODP Exp. 308 Mud/clay MTDs (Flemings et al., 2005)

4. Objectives Characterization of consolidation and shear behaviors of core samples from IODP Sites U1324 in the Ursa Basin, GOM Characterization of consolidation and shear behaviors of core samples from IODP Sites U1324 in the Ursa Basin, GOM Estimation of the in situ state of stress and pressure during sedimentation Estimation of the in situ state of stress and pressure during sedimentation Analysis of slope stability in the continental slope at passive margin in the Ursa Basin, GOM Analysis of slope stability in the continental slope at passive margin in the Ursa Basin, GOM

5. Description of the sedimentary basin at Site U1324 Sample depth: 30 ~ 160mbsf Sample depth: 30 ~ 160mbsf Clay content: 40 ~ 60% Clay content: 40 ~ 60% Consolidation coefficient: ~2.2 x 10 -8 m 2 /sec Consolidation coefficient: ~2.2 x 10 -8 m 2 /sec Sedimentation rate: >10mm/y. Sedimentation rate: >10mm/y. Slope: ~2 º Slope: ~2 º

6. Stress path p’p’ q active failure line passive failure line K 0 line sedimentation unloading reloading shear

7. Triaxial Pressure System Load signal OilBrine GDS pump vacuum Axial force Base pressure Pore pressure Confining pressure load cell sample

8. Electric Devices load cell load signal axial LVDTs radial LVDT radial caliper LVDT signals temperature signal base pedestal pressure vessel base

9. Test Record: Uniaxial Consolidation h’h’ v’v’

10. K 0 values during consolidation

11. Determination of preconsolidation pressure P c ’

12. Stress Condition

13. 1-D flow analysis

14. Test Record: Undrained triaxial compression

15. Shear induced pore pressure

16. Mohr-Coulomb failure criterion

17. slope expansion Active failure

18. Slope stability analysis FS: Safety Factor  s : Sliding friction coefficient i: Slope angle (~2º) *: normalized over pressure Overpressure given by tests; Sliding friction coefficient  s ; 0.03~0.12 (  s = 1.7 ~ 6.8 º ) Assuming that  s   = 0.424; Normalized overpressure *: 0.92 i

19. Stress condition for active failure & Landslides

20. slope expansion slip line (a) (b) (c) slope surface failure initiation i i MTD

21. Research Summary Experimental simulations of sedimentation: Experimental simulations of sedimentation: –Ratio of  h ’/  v ’: ~0.6 –Overpressure: * = -0.27 ~ 0.707 increasing with depth –Shear failure criterion:  =  tan(23º) –Shear strain may cause additional pore pressure to increase The slope area is stable during sedimentation The slope area is stable during sedimentation Slope stability analysis reveals that the instability of the continental slope is more sensitive to active failure than landslides Slope stability analysis reveals that the instability of the continental slope is more sensitive to active failure than landslides