P.B. Flemings (1), I. Song (2,3) and D.M. Saffer (3)P.B. Flemings (1), I. Song (2,3) and D.M. Saffer (3)

(1) Jackson School of Geosciences, University of Texas, (1) Jackson School of Geosciences, University of Texas, Austin, USAAustin, USA

(2) Korea Institute of Geoscience and Mineral Resources, (2) Korea Institute of Geoscience and Mineral Resources, KoreaKorea

(3) Department of Geosciences, Pennsylvania State (3) Department of Geosciences, Pennsylvania State University, USAUniversity, USA

Laboratory investigation of coupled Laboratory investigation of coupled deformation and fluid flow in deformation and fluid flow in

mudrock: implications for slope mudrock: implications for slope stability stability

in the Ursa Basin, Gulf of Mexicoin the Ursa Basin, Gulf of Mexico

Location of Ursa Basin, Location of Ursa Basin, GOMGOM

Gulf of MexicoGulf of Mexico

Seismic Profile: IODP Exp. Seismic Profile: IODP Exp. 308308

Mud/clay

MTDs

(Flemings et al., 2005)

ObjectivesObjectives

Characterization of consolidation and Characterization of consolidation and shear behaviors of core samples from shear behaviors of core samples from IODP Sites U1324 in the Ursa Basin, GOMIODP Sites U1324 in the Ursa Basin, GOM

Estimation of the in situ state of stress and Estimation of the in situ state of stress and pressure during sedimentationpressure during sedimentation

Analysis of slope stability in the continental Analysis of slope stability in the continental slope at passive margin in the Ursa Basin, slope at passive margin in the Ursa Basin, GOMGOM

Description of the Description of the sedimentary basin at Site sedimentary basin at Site U1324U1324 Sample depth: 30 ~ 160mbsfSample depth: 30 ~ 160mbsf Clay content: 40 ~ 60%Clay content: 40 ~ 60% Consolidation coefficient: Consolidation coefficient:

~2.2 x 10~2.2 x 10-8-8 mm22/sec/sec

Sedimentation rate: >10mm/y.Sedimentation rate: >10mm/y. Slope: ~2Slope: ~2ºº

Stress pathStress path

2

)''(' hvp

2

)( hvq

p’q

active failure line

passive failure line

K0 line

sedimentationunloadin

g

reloading

shear

Triaxial Pressure Triaxial Pressure SystemSystem Load signal

Oil Brine

GD

S p

ump

GD

S p

ump

vacu

um

vacu

um

vacuum

Axial force

Base pressure

Pore pressureConfining pressure

loadcell

sample

Electric DevicesElectric Devices

load cell

load signal

axial LVDTs

radial LVDT

radial caliper

LVDT signals

temperature signal

base pedestal

pressure vessel base

Test Record: Test Record: Uniaxial ConsolidationUniaxial Consolidation

h’ v’

'

'0

v

hK

KK00 values during values during consolidationconsolidation

Determination of Determination of preconsolidation pressure preconsolidation pressure PPcc’’

StressStressConditionCondition

'' 0 vh K

'vvu

dzzgv )(

'' cv P

1-D flow analysis1-D flow analysis

Test Record: Test Record: Undrained triaxial Undrained triaxial compressioncompression

Shear induced pore Shear induced pore pressurepressure

Mohr-Coulomb failure Mohr-Coulomb failure criterioncriterion

slope expansion

Active Active failurefailure

Slope stability analysisSlope stability analysis

'*

vh

u

ii

is cossin

cosFS

*2

FS: Safety Factors: Sliding friction coefficienti: Slope angle (~2º)*: normalized over pressure

• Overpressure given by tests;

• Sliding friction coefficient s; 0.03~0.12 (s = 1.7 ~ 6.8º)

• Assuming that s = 0.424;

• Normalized overpressure *: 0.92

i'

Stress Stress condition condition for active for active failure &failure &LandslideLandslidess

ii cossin*

*2* cos i

*

*

*

** s

*1

*2/45

slope expansion

slip

line

slip line

*

(a)

(b)

(c)

slope surface

failureinitiation

*1

i

i

2/45

MTD

Research SummaryResearch Summary

Experimental simulations of sedimentation:Experimental simulations of sedimentation:– Ratio of Ratio of hh’/’/vv’: ~0.6’: ~0.6– Overpressure: Overpressure: **= -0.27 ~ 0.707 increasing = -0.27 ~ 0.707 increasing

with depthwith depth– Shear failure criterion: Shear failure criterion: = = tan(23º)tan(23º)– Shear strain may cause additional pore Shear strain may cause additional pore

pressure to increasepressure to increase The slope area is stable during The slope area is stable during

sedimentationsedimentation Slope stability analysis reveals that the Slope stability analysis reveals that the

instability of the continental slope is more instability of the continental slope is more sensitive to active failure than landslidessensitive to active failure than landslides