SIMULATING SUBMARINE SLOPE INSTABILITY INITIATION USING CENTRIFUGE MODEL TESTING

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1 SIMULATING SUBMARINE SLOPE INSTABILITY INITIATION USING CENTRIFUGE MODEL TESTING S.E. COULTER and R. PHILLIPS C-CORE, Memorial University of Newfoundland, St. John s, Newfoundland & Labrador, Canada, A1B 3X5 Abstract COSTA is addressing the questions of why seafloor slope failures occur where they do, and with what frequency they occur. The original program has been complemented by COSTA- Canada. One of the tasks involves the study of the initiation of slope instability through numerical and centrifuge modelling. This paper reviews previous centrifuge studies related to submarine slope failure and presents the preparations for this task. The initiation of submarine slope instability has been attributed to triggers such as earthquakes, erosion, oversteepening, wave loading, and sedimentation. Centrifuge modelling has been used to simulate most of these loading conditions in similar boundary value problems. Keywords: Submarine slope instability, triggering mechanisms, centrifuge modelling, earthquake simulation 1. Introduction Submarine slope stability has been identified as a major concern in offshore resource development. Research programs have attempted to characterise and understand submarine failures worldwide. These programs have included the Arctic Delta Failure Experiment (ADFEX) , Geological Long Range Inclined Asdic (GLORIA) , Sediment Transport on Atlantic Margins (STEAM) , ENAM II , STRATAFORM and Continental Slope Stability (COSTA) 2000 to present. The original COSTA program on seafloor stability has been complemented by COSTA- Canada, Locat and Héroux (2001) and One of the 6 tasks in this complementary study involves the initiation of slope instability through numerical and centrifuge modelling. The initiation of submarine slope instability has been attributed to triggers such as earthquakes, waves, tides, sedimentation, gas, erosion and diapirism, Hampton et al.. (1996). Centrifuge modelling has been used to simulate many of these loading conditions and relevant soil conditions in similar seafloor stability studies. Figure 1 shows a submarine slope failure model of a submerged 8 o slope that flowed to an angle of 2 o in normally consolidated silty clay due to an increase of excess pore water pressure in the slope. Following an overview of centrifuge modelling, other examples reviewed in this paper 29

2 30 Coulter and Phillips include submarine failures from: (1) material softening, (2) sedimentation, (3) wave loading, and (4) earthquakes. Figure 1. Submarine Slope Failure in Silty Clay. 2. Centrifuge Modelling Centrifuge model testing is a physical modelling tool for geotechnical engineers. Analogues to this technique exist in other branches of civil engineering, such as wind tunnel testing in aeronautical engineering and flume testing in hydraulic engineering. To achieve mechanical similitude in geotechnical models it is necessary to reproduce the material behaviour both in terms of strength and stiffness. This behaviour is primarily a function of the effective stress resulting from self-weight and other external forces. Centrifuge modelling then is a technique for investigating gravity dependant phenomena, such as soil slope behaviour, using reduced-scale physical models. As a full-scale soil structure is in equilibrium under earth s gravitational field g, a reduced-scale model on a centrifuge, at radius R, is in equilibrium under an acceleration field of Rω 2, where ω is the rotational speed of the centrifuge. The model will then have its weight increased N times, where N = Rω 2 /g. For a typical value of N = 100, if a model is made at 1/100th scale and is accelerated to 100g, the stresses due to self-weight will be similar to the stresses in the prototype at homologous points. The model can then reproduce the phenomena of cracking, rupture or, flow that would be observed in the prototype because the stress dependency of soil behaviour has been correctly simulated. The principles, scaling laws and some applications of centrifuge modelling are more fully described by Murff (1996) and Taylor (1995). 3. Material Softening The Canadian Liquefaction Experiment (CANLEX) by Robertson et al. (2000) investigated the liquefaction potential of loose sand deposits under monotonic shear stress increments. The program included high quality soils testing, numerical analyses and a full-scale field event designed to cause a flowslide. Centrifuge model tests were added to the CANLEX program to provide additional physical data and to simulate the physical response of the

3 Centrifuge Modelling of Submarine Slope Instabilities 31 field event. The oil sand tailings from the field site used in the model tests were strain softening under triaxial extension. One centrifuge model simulated the failure of a 16 o, 8.8m high loose sand submerged slope, Phillips and Byrne (1994). Surcharging the slope crest caused the model slope to liquefy and flow with deep-seated lateral movements to an angle of 7 o, Figure 2. This submarine slope failure occurred primarily due to the strain softening response of the sand at the toe of the slope that caused pore pressure increases and destabilisation of the slope. Oil Level Surcharge Final Initial PPT 2 PPT 1 Figure 2. CANLEX Submerged Slope Flowside Failure. 4. Sedimentation There are other external loading conditions that can cause pore pressure increases in the vicinity of slopes. Continuous sedimentation has been identified as one such loading condition. Hurley (1999) has modelled the sedimentation and primary consolidation of a highly permeable cohesive kaolin sediment from 580% water content. The equivalent of 10m depth of aqueous slurry sedimented and consolidated into a 2m thick silty clay layer, Figure 3. Continuous measurements of bulk density (from gamma ray attenuation), pore pressure, and shear and compression wave velocities were made at 100g. The excess pore pressures generated by this sedimentation process may be sufficient to destabilise a submarine slope, Figure 4. The excess pore pressure at PPT6, 5m down in the consolidating clay layer, was 30kPa at the prototype time of 2 years after commencement of sedimentation. This is equivalent to an excess pore pressure ratio of about unity. The associated effective shear strength is low. If sufficient material is subject to such pore pressures then seafloor instability can ensue. 80 PPT6 Initial Level PPT1 PPT2 PPT3 PPT4 Excess Pore Pressure (kpa) 60 PPT5 PPT4 40 PPT3 z(mm) PPT1 37 PPT2 188 PPT3 340 PPT4 493 PPT5 645 PPT6 840 Final Level PPT5 PPT6 20 PPT2 PPT Time (mins) Figure 3. Sediment Column Test Setup. Figure 4. Excess Pore Pressures in Slurry Sediment.

4 32 Coulter and Phillips 5. Wave Loading Wave loads can also cause excess pore pressures leading to seabed mobility and liquefaction. The techniques for modelling wave seabed interaction were developed by Phillips and Sekiguchi (1992). Sekiguchi et al.. (2000) simulated at 50g the response of a 4.5m thick sand layer under 4m water depth subjected to 0.3m high waves with a 6s period, Figure 5. Significant seabed mobility and liquefaction was observed to propagate down from the seafloor to the base of the sand layer. Figure 6 shows the excess pore pressure response u e at the sand base to pressure cycles at the seafloor. Three additional configurations were modelled with a gravel cap of various lengths on the seabed to develop a mitigation strategy to minimise damage to the seafloor. Figure 5. Wave Loading Model Test Setup. Figure 6. Excess Pore Pressure Response in Seabed. 6. Earthquake Effects 6.1 VELACS The VELACS (Verification of Earthquake Liquefaction Analysis by Centrifuge Studies) program considered the effects of earthquake-like loading on a variety of soil models, Arulanandan and Scott (1993 &1994) and VELACS was aimed at better understanding the mechanisms of soil liquefaction and at acquiring data for the verification of various analysis procedures. The response of each of nine boundary value configurations was predicted by between 4 to 16 numerical analysts. The Class-A predictions were then compared to the measured response from nominally the same physical model test conducted at between 1 to 3 centrifuge centres. Model 2 involved the simulation of lateral spreading of a submerged slope due to earthquake effects, Figure 7. The submerged 2 o slope comprised a saturated 10m deep layer of Nevada sand at 40% relative density. The base of the layer was subjected to about 22 cycles to 0.1g lateral acceleration at 2Hz. The toe of the slope at the seafloor, LVDT3 was observed to move downslope a distance of 0.5 m due to this earthquake, Figure 8.

5 Centrifuge Modelling of Submarine Slope Instabilities Lateral Spread, cm Time, seconds Figure 7. VELACS Model 2 Test Setup. Figure 8. VELACS Model 2 Lateral Spread. 6.2 COSTA-Canada Earthquake Simulation C-CORE will commission a C$0.5m earthquake simulator (EQS) on its 5.5m radius centrifuge in early This single axis EQS shown in Figure 9 will excite a 400kg moving payload of 1m length by 0.5m width by up to 0.6m high with a maximum force of 200kN. The peak lateral acceleration of the full payload will be 40g under a constant (centrifugal) acceleration of 80g. The lateral acceleration envelope is shown in Figure 10 over the frequency range of Hz. The COSTA-Canada tests will involve test configurations similar to Figure 7, using the selected sand discussed in Section Lower permeability silt layers will be introduced into the sand layer to simulate a stratified profile. The effect of these layers on pore pressure generation and subsequent dissipation will be examined. The silt layers will impede drainage of the sand as shown by Konrad and Dubeau (2002). The migration of pore pressure towards potential drainage boundaries is also expected to cause continued movement of the slope after cessation of the earthquake. Figure 9. C-CORE Earthquake Simulator Layout. Figure 10. EQS Performance Envelope Model Container Any model container used in dynamic geotechnical tests should cause stress and strain in the model similar to the soil layer being tested in infinite lateral area and finite depth. This

6 34 Coulter and Phillips requires a model box with flexible boundaries. Among the several options available the one chosen for these tests is the equivalent shear beam (ESB) container introduced by Zeng and Schofield (1996). The ESB container has been designed with the following principles: (1) the end walls function as shear beams with equivalent dynamic shear stiffness to the adjacent soil, (2) each end wall has similar friction to the adjacent soil so that it can sustain complimentary shear stresses induced by the shaking, (3) the side walls are frictionless to avoid induced shear stress between soil and side walls, (4) the walls maintain a K o condition, and (5) the frictional end walls have similar vertical settlement of the soil layer to avoid initial boundary shear stresses. The ESB container is constructed of stacked aluminium rings alternating with rubber layers to achieve its complimentary flexibility, Figure 11. Figure 11. Equivalent Shear Beam Model Container Substitute Pore Fluid Centrifuge modelling of dynamic events in saturated soil environments arouses one major difficulty in recognized scaling laws. There exists a difference in scale factors (N) with respect to time between static events and dynamic events. Stewart (1998) shows that by increasing the pore fluid viscosity by N times a reduction in permeability of the soil of N times can be achieved. It is shown that this reduction in permeability leads to an agreement of the two scaling relationships. The substitute pore fluid selected for these tests is chemically known as hydroxypropyl methylcellulose (HPMC), which has been used previously by Stewart (1998), Dewoolkar et al. (1999a), and Dewoolkar et al. (1999b). HPMC is considered a superior product for this purpose because it is biodegradable, can be easily mixed with deionised water to create fluids with a wide range of viscosities, has many properties similar to water, is inexpensive, and readily available. A kinematic viscosity of 50cSt has been selected for this research. Mixing trials have been undertaken and fluids with required kinematic viscosities can be reliably reproduced.

7 Centrifuge Modelling of Submarine Slope Instabilities Selected Sand The sand that has been selected for use in the COSTA-Canada centrifuge tests is Fraser River sand from the west coast of Canada. A considerable amount of this sand has been processed and delivered to C-CORE. Fraser River sand has been used extensively by project collaborators at the University of British Columbia in numerous laboratory tests relating to liquefaction studies, including those noted in Uthayakumar and Vaid (1998) and Vaid et al. (2001). This sand is uniform, grey coloured, and medium grained with subangular to subrounded particles. Fraser River sand features an average mineral composition of 40% quartz, 11% feldspar, 45% unaltered rock fragments, and 4% other minerals. The engineering properties presented in the literature have been confirmed within reasonable tolerances for the batch of Fraser River sand that is to be used at C-CORE for the centrifuge tests Model Preparation The sand models will be placed into the ESB by means of air pluviation, similar to methods discussed by Ueno (1998). A hopper containing the test sand will be moved over the model container at a specified drop height that will ensure a relative density of approximately 30-32%. Some densification in the range of 8-10% is expected during movement of the test slope in the laboratory and saturation with the substitute pore fluid. The saturation of the model ground will be done under vacuum in a method similar to that explained by Ueno (1998). A high degree of saturation is required to avoid any capillary action amongst the sand particles that may produce cohesion. Vacuum will be applied to the model container to remove the air and then carbon dioxide will be introduced to displace less soluble air particles that may be present in the voids. Following this step, de-aired pore fluid will be introduced slowly under vacuum conditions by means of a small differential driving head so as not to cause any disturbance in the sample Instrumentation Sufficient saturation will be verified in flight using a miniature air hammer to generate P- waves, similar to the device used by Malvick et al. (2002). The arrival of these P-waves will be measured by two in-line accelerometers that will be placed a known distance apart in the sand model during pluviation. The arrival times of the generated P-wave signals will be compared. As shown by Ishihara et al. (2001) a P-wave velocity of greater than 750m/s indicates a degree of saturation in excess of 99%, which can be considered adequate to ensure that liquefaction would be insensitive to the degree of saturation. Additional instrumentation will be installed during pluviation and used to examine the reaction of the model slope before, during, and after earthquake actuation. Pore pressure transducers equipped with sintered bronze stones will be used to monitor the changes in pore pressure. Miniature accelerometers will be placed at key locations to measure the shaking response of the slope. Linear variable differential transformers and laser transducers will be employed to measure surface deformations. All instrumentation will be connected to the new high-speed 24 channel data acquisition system that will be integrated with the new EQS. With this new system data acquisition can be performed automatically while the shaker control is in operation.

8 36 Coulter and Phillips 7. Summary Centrifuge modelling has been shown to be a valuable tool for investigations into various triggering mechanisms of submarine slope instabilities. It is upon this foundation that COSTA-Canada hopes to build. The preparations for the COSTA-Canada centrifuge tests are advancing and model preparation methods are currently being perfected. Testing will commence shortly after commissioning of the new earthquake simulator in early Initial results can be expected by mid 2003 and the results of this work will be offered in future publications. 8. Acknowledgements This work is supported by C-CORE and COSTA-Canada, a NSERC Collaborative Research Opportunities Grant. 9. References Arulanandan, K., and Scott, R.F. (eds.), 1993 & Verification of Numerical Procedures for the Analysis of Soil Liquefaction Problems. International Conference on the Verification of Numerical Procedures for the Analysis of Soil Liquefaction Problems, Vols. 1 and 2. Balkema, Rotterdam, Netherlands. Dewoolkar, M.M., Ko, H.Y., and Pak, R.Y.S., 1999a. Centrifuge Modelling of Models of Seismic Effects on Saturated Earth Structures. Geotechnique, 49: Dewoolkar, M.M., Ko, K.Y., Stadler, A.T., and Astaneh, S.M.F., 1999b. A Substitute Pore Fluid for Seismic Centrifuge Modeling. ASTM Geotechnical Testing Journal, 22: Hurley, S.J Development of a Settling Column and Associated Primary Consolidation Monitoring Systems for use in a Geotechnical Centrifuge. Master of Engineering Thesis, Memorial University of Newfoundland. Hampton, M.A., Lee, H.J., and Locat, J., Submarine Landslides. Reviews of Geophysics, 34: Ishihara, K., Tsuchiya, H., Huang, Y., and Kamada, K., Recent Studies on Liquefaction Resistance of Sand Effects on Saturation. 4 th International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. San Diego, USA. Konrad, J.M. and Dubeau, S., Cyclic Strength of Stratified Soil Samples. 55th Canadian Geotechnical Conference, Niagara Falls, Canada. Locat, J., and Héroux, M.C., COSTA-Canada: A Canadian Contribution To The Study Of Continental Slope Stability An Overview. 54th Canadian Geotechnical Conference, Sept. 2001, Calgary, Canada. Malvick, E.J., Kulasingam, R., Kutter, B.L., and Boulanger, R.W., (2002). Void Redistribution and Localized Shear Strains in Slopes During Liquefaction. Physical Modelling in Geotechnics: ICPMG 02. Balkema. Rotterdam, Netherlands. Murff, J.D., The Geotechnical Centrifuge in Offshore Engineering. OTRC Honors Lecture, Offshore Technology Conference, Houston, USA. Phillips, R., and Sekiguchi, H., Generation of Water Waves in a Drum Centrifuge. Techno-Ocean '92, Yokohama, Japan. Phillips, R., and Byrne, P.M., Modelling Slope Liquefaction Due to Static Loading. 47th Canadian Geotechnical Conference, Halifax, Canada. Robertson, PK, Phillips, R...et al. (34 authors), The Canadian Liquefaction Experiment: An Overview. Canadian Geotechnical Journal, 37: Sekiguchi, H., Sassa, S., Sugioka, K., and Miyamoto, J., Wave Induced Liquefaction Flow Deformation and Particle Transport in Sand Beds. GeoEng 2000, Melbourne, Australia. Stewart, D.P., Chen, Y.R., and Kutter, B.L., Experience with the Use of Methylcellulose as a Viscous Pore Fluid in Centrifuge Models. ASTM Geotechnical Testing Journal, 21: Taylor, R.N., Geotechnical Centrifuge Technology, Blackie Academic & Professional. London, UK. Ueno, K., Methods for Preparation of Sand Samples. Centrifuge 98. Balkema. Rotterdam, Netherlands. Uthayakumar, M., and Vaid, Y.P., Static Liquefaction of Sands Under Multiaxial Loading. Canadian Geotechnical Journal, 35: Vaid, Y.P., Stedman, J.D., and Sivathayalan, S., Confining Stress and Static Shear Effects in Cyclic Liquefaction. Canadian Geotechnical Journal, 38: Zeng, X., and Schofield, A.N., Design and Performance of an Equivalent-shear-beam Container for Earthquake Centrifuge Modelling. Geotechnique, 46: