Proc. IODP | Volume 308 doi:10.2204/iodp.proc.308.210.2009 Flemings, P.B., Behrmann, J.H., John, C.M., and the Expedition 308 Scientists Proceedings of the Integrated Ocean Drilling Program, Volume 308 Abstract Triaxial strength experiments completed on specimens from Inte- grated Ocean Drilling Program Expedition 308 Sites U1322 and U1324 document internal friction angles from 17° to 29° and nor- malized undrained shear strength from 0.21 to 0.36 at maximum shear. Just prior to undrained shearing, lateral stress ratios for these triaxial specimens range from 0.50 to 0.73. In general, inter- nal friction angle and normalized undrained shear strength de- crease with increasing vertical consolidation stress. Lateral stress ratio does not vary systematically with vertical consolidation stress. All 18 specimens in this study were collected with the ad- vanced piston corer (APC). Specimens from Site U1322 were from 27.3–72.9 meters below seafloor (mbsf), and those from Site U1324 were from 89.1–304 mbsf. For each specimen we com- pleted a K o -consolidated undrained triaxial test to provide data on the stress-strain behavior, undrained shear strength, internal fric- tion angle, and secant modulus. Undrained shearing for each specimen was conducted at a vertical consolidation stress exceed- ing the hydrostatic vertical effective stress for that specimen. Introduction Integrated Ocean Drilling Program (IODP) Expedition 308 was dedicated to understanding fluid flow, in situ pressure, and slope stability in rapidly deposited sediments of the northern Gulf of Mexico. The Ursa region was one of the focus areas for Expedition 308 because of high-quality multichannel seismic data illuminat- ing depositional architecture (Sawyer et al., 2007), previous indus- try drilling that documented overpressure in shallow sand and mud (Ostermeier et al., 2001; Pelletier et al., 1999), and a series of stacked mass-transport complexes (MTCs) (see the “Expedition 308 summary” chapter). The Ursa region is ~210 km southeast of New Orleans, Louisiana (USA) in ~1000–1300 m water depth (Fig. F1). Sites U1322 and U1324 are separated by 12 km along a west- southwest–east-northeast transect (Fig. F1). Three-dimensional seismic data over the Ursa region show a basin floor fan (Blue Unit) overlain by mud-dominated levee deposits (see the “Expedition 308 summary” chapter). These levee depos- its are thick near Site U1324 and thin toward Site U1322. The stacked MTCs are confined within these levee deposits. Drilling was proposed in this region to understand how flow within the Data report: strength characteristics of sediments from IODP Expedition 308, Sites U1322 and U1324 1 Brandon Dugan 2 and John T. Germaine 3 Chapter contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Laboratory testing methodology . . . . . . . . . . . 2 Experimental results . . . . . . . . . . . . . . . . . . . . . 3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 4 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1 Dugan, B., and Germaine, J.T., 2009. Data report: strength characteristics of sediments from IODP Expedition 308, Sites U1322 and U1324. In Flemings, P.B., Behrmann, J.H., John, C.M., and the Expedition 308 Scientists, Proc. IODP, 308: College Station, TX (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.308.210.2009 2 Department of Earth Science, Rice University, Houston TX 77005, USA. [email protected]3 Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge MA 02139, USA.
Transcript
Proc. IODP | Volume 308
Flemings, P.B., Behrmann, J.H., John, C.M., and the Expedition 308 ScientistsProceedings of the Integrated Ocean Drilling Program, Volume 308
Data report: strength characteristics of sediments from IODP Expedition 308, Sites U1322 and U13241
1Dugan, B., and Germaine, J.T., 2009. Data report: strength characteristics of sediments from IODP Expedition 308, Sites U1322 and U1324. In Flemings, P.B., Behrmann, J.H., John, C.M., and the Expedition 308 Scientists, Proc. IODP, 308: College Station, TX (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.308.210.20092Department of Earth Science, Rice University, Houston TX 77005, [email protected] of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge MA 02139, USA.
AbstractTriaxial strength experiments completed on specimens from Inte-grated Ocean Drilling Program Expedition 308 Sites U1322 andU1324 document internal friction angles from 17° to 29° and nor-malized undrained shear strength from 0.21 to 0.36 at maximumshear. Just prior to undrained shearing, lateral stress ratios forthese triaxial specimens range from 0.50 to 0.73. In general, inter-nal friction angle and normalized undrained shear strength de-crease with increasing vertical consolidation stress. Lateral stressratio does not vary systematically with vertical consolidationstress. All 18 specimens in this study were collected with the ad-vanced piston corer (APC). Specimens from Site U1322 were from27.3–72.9 meters below seafloor (mbsf), and those from SiteU1324 were from 89.1–304 mbsf. For each specimen we com-pleted a Ko-consolidated undrained triaxial test to provide data onthe stress-strain behavior, undrained shear strength, internal fric-tion angle, and secant modulus. Undrained shearing for eachspecimen was conducted at a vertical consolidation stress exceed-ing the hydrostatic vertical effective stress for that specimen.
IntroductionIntegrated Ocean Drilling Program (IODP) Expedition 308 wasdedicated to understanding fluid flow, in situ pressure, and slopestability in rapidly deposited sediments of the northern Gulf ofMexico. The Ursa region was one of the focus areas for Expedition308 because of high-quality multichannel seismic data illuminat-ing depositional architecture (Sawyer et al., 2007), previous indus-try drilling that documented overpressure in shallow sand andmud (Ostermeier et al., 2001; Pelletier et al., 1999), and a series ofstacked mass-transport complexes (MTCs) (see the “Expedition308 summary” chapter). The Ursa region is ~210 km southeast ofNew Orleans, Louisiana (USA) in ~1000–1300 m water depth (Fig.F1). Sites U1322 and U1324 are separated by 12 km along a west-southwest–east-northeast transect (Fig. F1).
Three-dimensional seismic data over the Ursa region show a basinfloor fan (Blue Unit) overlain by mud-dominated levee deposits(see the “Expedition 308 summary” chapter). These levee depos-its are thick near Site U1324 and thin toward Site U1322. Thestacked MTCs are confined within these levee deposits. Drillingwas proposed in this region to understand how flow within the
B. Dugan and J.T. Germaine Data report: strength characteristics of sediments
Blue Unit affects pressure and stability in the bound-ing mud. The overpressure profiles at Sites U1324and U1322 have been characterized through directmeasurements (Flemings et al., 2008) and laboratoryexperiments (Long et al.). We present constraints onthe strength of sediments from these sites based ontriaxial experiments.
Whole-round samples were used in detailed triaxialexperiments to evaluate the stress-strain behavior,undrained strength, friction angle, and modulus.These results complement grain size analyses (Saw-yer et al.), consolidation and hydraulic conductivityanalyses (Long et al.) of sediment, and permeabilityanalyses (Schneider et al.) of sediments from this re-gion. Together these studies provide a full suite ofgeotechnical parameters which will help us under-stand coupling between flow, deformation, and sta-bility and can be used for inputs into basin models.
Laboratory testing methodologyAll laboratory tests (Table T1) were conducted in ac-cordance with the American Society for Testing andMaterials (ASTM) standards. For tests where ASTMstandards do not exist, the procedures followed theestablished Massachusetts Institute of Technology(MIT) geotechnical laboratory protocols.
RadiographyRadiography allows the selection of the best qualitymaterial for advanced geotechnical testing. Allwhole-round samples were X-rayed following a pro-cedure similar to ASTM standard D4452 (ASTM Inter-national, 2003). X-ray images were used to assesssample quality, presence of inclusions, general soiltype, and variation in soil layering. Radiographs areavailable in Nelson et al.
Specimen index propertiesWater content is measured by taking the differencein the weight of a soil before and after oven dryingand then dividing this difference by the oven-driedweight. For each experiment we provide the watercontent of the test specimen (wn) (Tables T2, T3). Wealso provide the initial void ratio (ei) of each speci-men (Tables T2, T3). Void ratio is defined as the vol-ume of voids divided by the volume of solids. Fromthe void ratio and water content we calculate the ini-tial saturation (Si = wnGs/ei) for each specimen (TablesT2, T3). We assume a constant specific gravity of thesolid grains (Gs = 2.78) for these clay-rich samples.All variables are defined in Table T4.
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Undrained strength testingConsolidation and strength properties were mea-sured from the results of Ko-consolidated undrained(CKoU) triaxial tests on specimens from Sites U1322and U1324. The MIT geotechnical laboratory has de-veloped a standard method for performing CKoUtests. In addition, ASTM standard D4767 (ASTM In-ternational, 2004) was used as a reference for the tri-axial testing. Undrained strength testing can be di-vided into four stages. The first stage of the testinvolves sample preparation by trimming the speci-men in a trimming jig using a wire saw. After thesample is trimmed (~1.75 cm radius; ~8 cm height),it is placed on the triaxial base with a nylon filterfabric and a porous stone placed on each end. Sidedrains were not used. Two thin, impermeable mem-branes are rolled over each specimen and sealed withthree O-rings each at the top cap and bottom base ofthe triaxial chamber. The triaxial cell is then filledwith silicon oil and tightly sealed. Distilled waterwas used as the fluid in the drainage system, whichis connected to the top and base of the specimen.
Backpressure saturation is the second stage of thetest. This phase ensures full saturation of the speci-men. To do this, a modest pressure is applied to dis-solve any air bubbles in the specimen. Next, a small,isotropic effective stress is applied to the specimensuch that there is minimal to no axial strain. This ef-fective stress is applied to seat the specimen in thetriaxial cell. For the specimens from the Ursa region,the applied effective stress ranged from 16 to 76 kPa(Table T2). This isotropic effective stress is main-tained while the axial stress and cell pressure are in-creased incrementally by the same value. To test forspecimen saturation, the drainage lines are closedand the axial stress and cell pressure are increased in-crementally and the B value (ASTM International,2004) is measured. A B value of 0.98–1.00 is desired;however, it was not achieved in all experiments (Ta-ble T2). After the B value is measured, the drainagelines are opened for the consolidation phase of thetest.
The third test stage is Ko consolidation. During Ko
consolidation the specimen is consolidated one-di-mensionally in the axial direction (i.e., no radialstrain) following the SHANSHEP testing technique(Ladd, 1986). Ko consolidation allows vertical strainon the specimen but maintains the radius of thespecimen. This simulates burial of the sediment in aconfined basin where sediments deform verticallybut are confined laterally and do not strain in thelateral direction. Ko is the ratio of the radial effectivestress to the vertical effective stress required to main-
B. Dugan and J.T. Germaine Data report: strength characteristics of sediments
tain no radial strain (Ko = σr′/σa′). We define Ko at themaximum vertical consolidation stress (σvc′) as theconsolidation lateral stress ratio (Kc) (Table T2). Ko
consolidation rates are provided in Table T2. Afterreaching the desired consolidation stress, total verti-cal stress, cell pressure, and pore pressure were heldconstant for a set time (ts) to allow excess pore pres-sure to dissipate and to allow some secondary com-pression (Table T2). For all specimens the maximumvertical consolidation stress exceeds the in situ effec-tive vertical stress to ensure the specimen is on theprimary consolidation path.
The final stage of the test is undrained shearing.Prior to starting the shear, a leak check is performedby closing the drainage valves for 30 min. Duringthis time, the backpressure should remain constant.After the leak check, the specimen is sheared withthe drainage lines closed. Shearing with the drainagelines closed prevents any fluid drainage during shear,maintains a constant volume of the specimen, andallows us to define the undrained strength parame-ters for each specimen. The shear rates are providedin Table T3. Positive shear strain data indicate a com-pression test, whereas negative shear strain data indi-cate an extension test.
Experimental resultsAll specimen locations, depths, and correspondingtriaxial experiments are provided in Table T1. TableT2 summarizes the consolidation phase of each ex-periment, and Table T3 provides information on theshear phase of each experiment. Complete experi-mental data are provided in “Supplementary mate-rial.”
Consolidation resultsWe describe the initial conditions and consolidationparameters for each specimen (Table T2). The speci-men data define the initial conditions of the speci-men including wn, total density (ρt), ei, and Si. Speci-men water contents ranged from 28% to 50%, andinitial void ratios ranged from 0.84 to 1.4. Gs was as-sumed to be 2.78. Initial consolidation conditionsdefine the specimen and cell conditions prior to thestart of Ko consolidation. σi′ is the initial vertical ef-fective stress applied to the specimen and εa is theaxial strain after this initial stress is applied. ub is thebackpressure applied to the specimen. εvol is the totalvolume strain to the specimen during backpressuresaturation. B is the pore pressure parameter (B = Δu/Δσr).
The consolidation results portion of the table de-scribes the parameters of the Ko consolidation. The
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strain rate (εa/h) defines the constant rate of axialstrain maintained during consolidation. Strain ratesfor each experiment were constant but varied from0.05%/h to 0.23%/h between experiments (TableT2). The specimen conditions at maximum stress de-fine the conditions at the end of primary Ko consoli-dation. These include εa, εvol, maximum effective ver-tical stress (σvm′), and lateral stress ratio (Kc) at σvm′.The time allowed for secondary compression afterreaching σvm′ is ts. The final parameters define theconditions at the end of secondary compression in-cluding σvc′ and overconsolidation ratio (OCR). Allexperiments were controlled to have an OCR = 1such that σvc′ was the greatest effective stress towhich the specimen was exposed. Prior to initialshearing, the consolidation stress ratio (Kc) for thespecimens ranged from 0.50 to 0.73 (Fig. F2).
Standard results from the Ko-consolidation phase in-clude the stress-strain behavior, total work done tothe specimen, and the lateral stress ratio (Fig. F3).From the stress-strain data, basin model inputs canbe constrained and compression behavior can be de-fined as is done for constant-rate-of-strain consolida-tion experiments (e.g., Long et al.). The total workdata can be used to infer preconsolidation stress foreach specimen (Long et al.; Becker et al., 1987).Work is calculated as
.
Example Ko-consolidation phase data are provided inTable T5. Complete Ko-consolidation phase data areprovided as plots and tables in the CONSOL folder in“Supplementary material.”
Strength resultsThe shear phase conditions and results are providedin Table T3. The specimen data are identical to thosein the consolidation phase (Table T2). The shearphase conditions provided in Table T3 define thestrain rate for shearing (εa/h) and the stress condi-tions (Kc, σvc′, and OCR) prior to shearing. The nextsection of the table provides the strength results atmaximum shear and obliquity (Table T3). Obliquityrefers to the ratio of the normalized shear stress (q) tothe normalized mean effective stress (p′). The nor-malized undrained strength ranges from 0.21 to0.36, whereas the friction angle ranges from 17° to29° at maximum shear (Fig. F2; Table T3). Both prop-erties show a relation to the vertical consolidationstress prior to shearing.
Example data from the shear phase are shown in Fig-ure F4 and Table T6. The summary plots include nor-malized shear stress (q/σvc′) versus εa, internal friction
B. Dugan and J.T. Germaine Data report: strength characteristics of sediments
angle (φ) versus εa, normalized excess pore pressure(Δue/σvc′) and normalized shear-induced pore pres-sure (Δus/σvc′) versus εa, and normalized stress path(q/σvc′ versus p′/σvc′) for each test. The followingequations define p′, q, φ, secant modulus (E), andpore pressure parameter (A), which are key parame-ters in the shear-phase results. All variables are de-fined in Table T4.
Complete data from the undrained shearing portionsof the triaxial tests are in the SHEAR folder in “Sup-plementary material.”
SummaryTriaxial strength experiments on 18 specimens fromSites U1322 and U1324 show that internal frictionangle and normalized undrained shear strength gen-erally decrease with increasing vertical consolidationstress. Internal friction angle ranges from 17° to 29°and normalized undrained shear strength rangesfrom 0.36 to 0.21 at maximum shear for vertical con-solidation stresses from 230 to 1840 kPa. Lateralstress ratios do not exhibit any relation with verticalconsolidation stress and range from 0.50 to 0.73prior to undrained shearing. In conjunction withother physical properties analyses, these strengthdata help define a complete geotechnical character-ization of sediments from the Ursa region.
AcknowledgmentsWe thank the participants and technical staff of Ex-pedition 308 for their efforts and assistance in col-lecting samples for this study. Samples and/or data
p′σa′ σr′+
2--------------------=
qσa σr–
2----------------=
φ sin 1– qp′----⎝ ⎠
⎛ ⎞=
E 2Δqε----------=
Aue Δσr–
Δσa Δσr–------------------------=
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were provided by the Integrated Ocean Drilling Pro-gram (IODP). Funding for this research was providedby the Consortium for Ocean Leadership.
ReferencesASTM International, 2003. Standard test method for con-
solidated undrained triaxial compression test for cohe-sive soils (Standard D4767-02). In Annual Book of ASTM Standards (Vol. 04.08): Soil and Rock (I): West Consho-hocken, PA (Am. Soc. Testing and Mater.), 911–934.
ASTM International, 2004. Standard test method for con-solidated undrained triaxial compression test for cohe-sive soils (Standard D4767-04). In Annual Book of ASTM Standards (Vol. 04.08): Soil and Rock (I): West Consho-hocken, PA (Am. Soc. Testing and Mater.).
Becker, D.E., Crooks, J.H.A., Been, K., and Jeffries, M.G., 1987. Work as a criterion for determining in situ and yield stresses in clays. Can. Geotech. J., 24(4):549–564. doi:10.1139/t87-070
Flemings, P.B., Long, H., Dugan, B., Germaine, J., John, C.M., Behrmann, J.H., Sawyer, D., and IODP Expedition 308 Scientists, 2008. Pore pressure penetrometers docu-ment high overpressure near the seafloor where multi-ple submarine landslides have occurred on the continental slope, offshore Louisiana, Gulf of Mexico. Earth Planet. Sci. Lett., 269(3–4):309–325. doi:10.1016/j.epsl.2007.12.005
Ladd, C.C., 1986. Stability evaluation during staged con-struction. J. Geotech. Eng., 117(4):540–615.
Ostermeier, R.M., Pelletier, J.H., Winker, C.D., and Nichol-son, J.W., 2001. Trends in shallow sediment pore pres-sure—deepwater Gulf of Mexico. Proc.—SPE/IADC Drill. Conf., 1–11.
Pelletier, J.H., Ostermeier, R.M., Winker, C.D., Nicholson, J.W., and Rambow, F.H., 1999. Shallow water flow sands in the deepwater Gulf of Mexico: some recent Shell experience [International Forum on Shallow Water Flows Conference, League City, TX, October 1999].
Sawyer, D.E., Flemings, P.B., Shipp, R.C., and Winker, C.D., 2007. Seismic geomorphology, lithology, and evolution of the late Pleistocene Mars-Ursa turbidite region, Mis-sissippi Canyon area, northern Gulf of Mexico. AAPG Bull., 91(2):215–234. doi:10.1306/08290605190
Initial receipt: 6 May 2007Acceptance: 23 July 2008Web publication: 27 March 2009MS 308-210
B. Dugan and J.T. Germaine Data report: strength characteristics of sediments
Figure F1. A. Regional basemap showing the location of the Ursa region, northern Gulf of Mexico. B. SitesU1322 and U1324 in Ursa region of the Mississippi Canyon.
95°W 91° 89° 87°
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B. Dugan and J.T. Germaine Data report: strength characteristics of sediments
Figure F2. Summary of internal friction angle, normalized shear stress, and lateral stress ratio as a function ofmaximum vertical consolidation stress. Data are from Tables T2 and T3.
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Figure F3. Example data from the consolidation phase of triaxial experiment TX729. Axial strain and void ratiovs. vertical consolidation are plotted on a logarithmic scale that can be used to define stress-strain behavior.Total work and lateral stress ratio (Ko) vs. vertical consolidation stress are plotted on a linear scale. Work datacan be used for interpreting in situ stress conditions. Ko data define how horizontal stress changes during uni-axial (no radial strain) loading.
101 102 103101 102 103
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B. Dugan and J.T. Germaine Data report: strength characteristics of sediments
Figure F4. Example data from the shear phase of triaxial experiment TX729. Axial strain is during undrainedshearing. PP = pore pressure.
0 5 10 150.20
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B. Dugan and J.T. Germaine Data report: strength characteristics of sediments
Table T1. Summary of samples used in triaxial experiments.
Hole, core, section
Depth (mbsf) Test Comments
308-U1322D-1H-2 42.9 TX725U1322D-2H-2 72.9 TX728U1322D-2H-2 72.7 TX729U1324B-10H-7 89.2 TX730U1324B-10H-7 88.9 TX734U1322B-4H-3 27.3 TX735 No void ratio data; included second shearU1322D-2H-2 72.6 TX736 No void ratio data; included second shearU1324B-10H-7 89.1 TX737 No void ratio dataU1324C-6H-3 304.0 TX763U1324B-18H-6 161.6 TX764U1322D-1H-2 42.6 TX767U1324C-6H-3 303.9 TX770U1322D-1H-2 42.7 TX773U1322D-1H-3 44.2 TX774U1322D-1H-3 44.1 TX775U1322D-1H-3 44.0 TX776 No consolidation dataU1322D-1H-3 43.9 TX778 No void ratio dataU1322D-1H-3 43.8 TX779 No void ratio data
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iments. (See table note.)
Consolidation results
Test conditions General At maximum stress At preshear
B. Dugan and J.T. Germaine Data report: strength characteristics of sediments
Table T6. Example data from shear phase of triaxial experiment TX729. (See table notes.)
Notes: Vertical consolidation stress = 610 kPa. Complete data available in the SHEAR folder in “Supplementary material.” See Table T4 for head-ing definitions.
Time (s) εa (%) q/σvc′ p′/σvc′ Δue/σvc′ Δus/σvc′ Δq/qmax E/σvc′ A φ (o) Area (cm2)
