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Ocean Drilling Program Scientific Results Volume Rea, D.K., Basov, I.A., Scholl, D.W., and Allan, J.F. (Eds.), 1995 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 145 36. SUBMARINE SLOPE STABILITY ANALYSIS ON THE DETROIT SEAMOUNT, SITE 8831 John A. Roberts,2 Anne K. Rutledge,3 and Aarno T. Kotilainen4 ABSTRACT Diatom- and clay-rich sediments sampled from the Detroit Seamount were tested to determine the parameters required for slope stability analysis. Consolidation tests performed show that the sediments are apparently overconsolidated in the upper 24 m of the section and become normally consolidated at greater depths. Triaxial testing shows that the major sediment types (clays and diatom oozes) cannot be distinguished on the basis of normalized strength parameters. The results of a slope stability analysis using the infinite slope theory indicate that the sediments are unstable when subjected to earthquake ground accelerations in excess of 7% to 8% of gravity on the flanks and 10% to 11 % on the top of the seamount. Examination of stress path plots show that most of the samples tested exhibit a nondisintegrative nature, that would be expected to produce slump blocks or slides. INTRODUCTION Consolidation Tests The study of the stability of submarine slopes has increased in One-dimensional consolidation tests were performed using Soil interest to marine geologists over the past 20 yr, as the extent of down- Testing Inc. oedometers at Texas A&M University. The samples (cut to slope mass movement processes has become clear. Very large slope a size of 6.35 cm diameter by 2.54 cm high) were laterally constrained failures have been documented from around the world (e.g., Lewis, while being incrementally loaded up to 800 kPa and then rebounded to 1971;Embley, 1976; Masson et al., 1992). Slope failure can be caused 50 kPa, while changes in sample height were measured using linear by a number of factors in the deep sea; some of the more popular strain conversion transducers (LSCTs). The maximum load used (800 hypotheses include earthquake loading (perhaps the most likely), kPa) is rather low, because of restrictions during the testing program. sea-level rise and fall, and gas hydrate destabilization. Relationships of vertical displacement and time were obtained and The sediments cored at Site 883, located on the Detroit Seamount analyzed using the procedure outlined in BS 1377 (1975), and Head 1 in the northwest Pacific, have been divided into six lithologic units (1982) to yield values for cv (coefficient of consolidation [cmV ]), mv 4 (see Shipboard Scientific Party, 1993). The sediments of Unit I were (coefficient of volume compressibility [m2MN ]) and k (permeability tested in this study using 14 whole-round samples that were taken [ms"1]). The void ratio at the end of each loading stage was calculated from Holes 883A and 883B (to a maximum depth of 45 mbsf) for and plotted on a graph vs. log effective stress. The Casagrande (1936) geotechnical testing. These sediments are clays and diatom oozes analysis was performed on these plots, and the overconsolidation ratio with minor lithologies of spiculite, carbonate, and some ash layers. (OCR) was calculated (OCR = σ'vm/?'v, the ratio of the maximum past The purpose of this study is to assess the composition, in situ overburden pressure and the present overburden). An OCR of one strength, and physical properties of the sediments on top of the Detroit indicates that the sample is normally consolidated, values less than one Seamount and to relate these to (1) the sediment stability, (2) the and greater than one indicate under and overconsolidation, respec- seismicity of the region, and (3) the sediment composition. The effect tively. The disadvantages of the Casagrande method have been re- of biogenic silica content on the geotechnical properties of a sediment ported by both the Marine Geotechnical Consortium (1985) and has been considered by other workers, such as Pittenger et al. (1989), Bryant and Rack (1990), and will not be discussed here. who concluded that siliceous layers on the V0ring Plateau may rep- resent zones of weakness in sedimentary sections. Bryant and Rack Triaxial Tests (1990) concentrated on consolidation characteristics of siliceous sed- iments from the Weddell Sea and found that all the surficial diatoma- The sediment strength was determined using triaxial testing meth- ceous sediments they sampled were overconsolidated. They attrib- ods (isotropic consolidation and undrained shear, or CIU) following uted this to diatom tests forming a strong microfabric (with cal- the normalized strength parameter (NSP) approach devised by Ladd careous and clay particles) and thus attaining a high permeability and Foott (1974) and Ladd et al. (1977). This methodology has also which would permit the rapid drainage of pore waters and an in- been used by researchers in the Gulf of Alaska and off the coast creased sediment strength. of California (e.g., Lee and Edwards, 1986; Hampton, 1989). The method uses elevated consolidation pressures (σ'c: consolidation stress METHODS in the triaxial test) in an attempt to overcome the effects of distur- bance, and to normalize the shear strength with respect to σ'c, to yield In addition to the methods used to determine index properties and the ratio S (the ratio of shear strength [SJ to σ'c [Su = q/2, where q = vane shear strength (Shipboard Scientific Party, 1993), the following the deviatoric stress, defined as σ^ - σ'3]). Also, if the sediments procedures have been employed. sampled are assumed to be lithologically similar to those at greater depths, then the method can be used to predict the in situ shear strength profile, and therefore the sediment stability, deeper in the sediment column. 1 Rea, D.K., Basov, I.A., Scholl, D.W., and Allan, J.F. (Eds.), 1995. Proc. ODP, Sci. Results, 145: College Station, TX (Ocean Drilling Program). The samples were tested at Cardiff University using a Wykeham Marine Geoscience Research Group, Department of Earth Sciences, University of Farrance Ltd. triaxial rig following the procedures of Bishop and Wales, P.O. Box 914, Cardiff CF1 3YE, United Kingdom. 3 Henkel (1962) and Head (1986). The data were recorded using an Department of Oceanography, Texas A&M University, College Station, TX 77843, electronic data logger attached to a microcomputer and printer. Sam- U.S.A. 4 Department of Earth Sciences, University of Cambridge, Downing Street, Cam- ples were first subjected to a "B" value (Skempton, 1954) test to bridge CB2 3EQ, United Kingdom. ensure complete saturation and were then isotropically consolidated 547 J.A. ROBERTS, A.K. RUTLEDGE, A.T. KOTILAINEN to approximately four times σ'rm (to induce an OCR of 1) and finally however, that the effect of such vertical movements is less significant, sheared over a period long enough to allow complete equalization of as they are damped by the water overburden pressures (Kastens, 1984). pore pressures. During undrained shear axial load, axial displacement and pore pressure were measured and recorded. Using these data, RESULTS stress-strain and stress path plots were produced (in the Cambridge Shipboard Measurements stress field: p' = the mean all round effective stress, The results of the shipboard measurements are presented in Figure σ',+ 2σ', 1 on a composite depth scale constructed using the gamma-ray attenu- ation and porosity evaluator (GRAPE) and magnetic susceptibility records (see Roberts, 1994). The index properties show the expected and q = the deviatoric stress). Failure points were chosen as either the interrelationships, with water content and porosity exhibiting fairly peak deviatoric stress, the deviatoric stress at the critical state (see similar profiles, and the wet bulk density plot mirroring these. Water Roscoe et al., 1958; Schofield and Wroth, 1968), or the deviatoric contents are generally high (notably below 34 m composite depth, stress at a limiting strain of 20%, dependent on the shape of the where they are sometimes in excess of 300% dry weight) and corre- stress-strain curve. The shear strength (SJ was then normalized; it spond to lower values of wet bulk density. Variations in the index can be related to the in situ shear strength using the following equation properties profiles are most likely the result of changes in sediment modified from Ladd et al. (1977): composition, in which the diatom-rich sediments are generally less dense and have higher water contents. The shear strength of the — = S (OCR) Λ, (1) sediment steadily increases downhole, but there is a transition zone at σ „ 18 m composite depth and a break of slope at 24 m composite depth. In addition to the smear slide descriptions conducted onboard, where Su = undrained shear strength (kPa), and A = a sediment constant relating OCR to normalized strength. the samples tested in this study have been described, and the results The parameter A is assumed to have a value of 0.84, as reported are presented in Table 1. by Hamilton (1989) from tests on similar sediments in the Gulf of Alaska. Equation 1 was modified to Equation 2 by Lee and Edwards Consolidation Tests (1986) to correct the calculation for anisotropic consolidation (the field situation) and cyclic strength degradation (loss of strength be- Table 2 gives a summary of the results from the consolidation tests cause of cyclic loading). The former correction factor was not mea- performed. The void ratio vs. log effective stress curves (Fig. 2) are sured, but a value of 0.8 has been used previously after a compre- representative of disturbed samples, with flattened curves. The virgin hensive survey of published data by Mayne (1985). The Marine compression line is not fully developed in any of the plots as loading Geotechnical Consortium (1985) conducted several anisotropically was terminated at 800 kPa; this makes the estimation of σ'vm difficult consolidated triaxial tests (CAU) on samples from Deep Sea Drilling and somewhat subjective.
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