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

Location of Ursa Basin, GOM Gulf of Mexico

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

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

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 m 2 /sec Consolidation coefficient: ~2.2 x m 2 /sec Sedimentation rate: >10mm/y. Sedimentation rate: >10mm/y. Slope: ~2 º Slope: ~2 º

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

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

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

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

K 0 values during consolidation

Determination of preconsolidation pressure P c ’

Stress Condition

1-D flow analysis

Test Record: Undrained triaxial compression

Shear induced pore pressure

Mohr-Coulomb failure criterion

slope expansion Active failure

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

Stress condition for active failure & Landslides

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

Research Summary Experimental simulations of sedimentation: Experimental simulations of sedimentation: –Ratio of  h ’/  v ’: ~0.6 –Overpressure: * = ~ 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