Geotechnical characteristics and slope stability on the Ebro margin, western Mediterranean

Marine Geology, 95 (1990) 379-393 Elsevier Science Publishers B.V., Amsterdam

379

Geotechnical characteristics and slope stability on the Ebro margin, western Mediterranean Jesus Baraza, a H o m a J. Lee, b R o b e r t E. Kayen, b and M o n t y A. H a m p t o n b alnstituto de Ciencias del Mar, C.S.I.C., Paveo Nacional s/n, 08039 Barcelona, Spain bU.S. Geological Survey, Mail Stop 999, 345 Middlefield Road, Menio Park, CA 94025, U.S.A. (Revision accepted N o v e m b e r 9, 1989)

ABSTRACT Baraza, J., Lee, H.J., Kayen, R.E. and Hampton, M.A., 1990. Geotechnical characteristics and slope stability on the Ebro margin, northwestern Mediterranean. In: C.H. Nelson and A. Maldonado (Editors), The Ebro Continental Margin, Northwestern Mediterranean Sea. Mar. Geol., 95: 379-393. Sedimentological and geotechnical analyses of core samples from the Ebro continental slope define two distinct areas on the basis of sediment type, physical properties and geotechnical behavior, The first area is the upper slope area (water depths of 200-500 m), which consists of upper Pleistocene prodeltaic silty clay with a low water content (34% dry weight average), low plasticity, and high overconsolidation near the seafloor. The second area, the middle and lower slope (water depths greater than 500 m), contains clay- and silt-size hemipelagic deposits with a high water content (90% average), high plasticity, and a low to moderate degree of overconsolidation near the sediment surface. Results from geotechnical tests show that the upper slope has a relatively high degree of stability under relatively rapid (undrained) static loading conditions, compared with the middle and lower slopes, which have a higher degree of stability under long-term (drained) static loading conditions. Under cyclic loading, which occurs during earthquakes, the upper slope has a higher degree of stability than the middle and lower slopes. For the surface of the seafloor, calculated critical earthquake accelerations that can trigger slope failures range from 0.73 g on the upper slope to 0.23 g on the lower slope. Sediment buried well below the seafloor may have a critical acceleration as low as 0.09 g on the upper slope and 0.17 g on the lower slope. Seismically induced instability of most of the Ebro slope seems unlikely given that an earthquake shaking of at least intensity VI would be needed, and such strong intensities have never been recorded in the last 70 years. Other cyclic loading events, such as storms or internal waves, do not appear to be direct causes of instability at present. Infrequent, particularly strong earthquakes could cause landslides on the Ebro margin slope. The Columbretes slide on the southwestern Ebro margin may have been caused by intense earthquake shaking associated with volcanic emplacement of the Columbretes Islands. Localized sediment slides on steep canyon and levee slopes could have been caused by less intense shaking. In general, the slope is stable under present environmental loading conditions and is fundamentally constructional. Nevertheless, rapid progradation caused by high sedimentation rates and other processes acting during low sea-level periods, such as more intense wave loading near the shelfbreak, may have caused major instability in the past.

Introduction

Instability of sedimentary deposits on the slopes of continental margins can be an important mechanism for sediment transport and redeposition, as well as a hazard to offshore development. The occurrence of submarine-slope instability has been reported frequently in the literature. Previous investigations (e.g., Embley and Jacobi, 1977; McGregor et al., 1979; Moore, 1977; Nardin et al., 1979; Kenyon, 1987) have shown landslide de0025-3227/90/$03.50

posits on all types of continental margin slopes throughout the world's ocean. Most of these investigations were qualitative and based on interpretation of high-resolution subbottom acoustic profiles. A few investigators (e.g., Almagor and Wiseman, 1977; Booth and Garrison, 1978; Hampton et al., 1978; Suhayda and Prior, 1978; Edwards et al., 1980; Olsen et al., 1982; Lee and Edwards, 1986; Kayen, 1988) have attempted to apply mechanical models to understand the causes of observed failure features, in addition to

© 1990 - - Elsevier Science Publishers B.V.

380 evaluating the relative stability of specific undersea slopes. The U.S. Geological Survey and the "Instituto de Ciencias del Mar", C.S.I.C., have been working together on quantitative analyses of the various factors that influence slope stability in the area of the Ebro continental slope (Fig.l). Sediment mechanical response to both static and cyclically varying environmental forces that are active on the slope have been evaluated by laboratory measurements of shear strength, consolidation, water content, grain size, and Atterberg limits of sediment core samples. With these results, we have evaluated the relative stability of this section of the Spanish continental margin.

J. BARAZA ET AL.

40°30'N

[]

Setting The Ebro River, which has a drainage basin of 86,000 km 2, is one of the largest rivers draining into the Mediterranean Sea (Maldonado, 1972). The river empties onto a continental margin that has a smooth, convex-upward physiography and a continental shelf as much as 70 km wide. The dominant winds in the area are from the northwest to northeast; wind speeds are often between 20 and 50 km/h. Winds greater than 50 km/h that can produce wave heights of greater than 4 m make up approximately 14% of the annual total (Flos, 1985). Wind-driven surface currents are from the northwest and extend to water depths of 150-200 m. They represent a permanent flow along the continental slope, modified by interactions with slope topography (Lacombe and Tchernia, 1971; Font et al., this issue). The Levantine Intermediate Water (LIW), a mass of homogeneous water identified by maxima of temperature and salinity, is present between the depths of 200 and 600 m (Font, 1987). Because of high insolation and few storms, evaporation exceeds precipitation during each month of the year. Accordingly, the sea is very saline, and during the summer there is a well-defined thermocline with a temperature gradient of 0.17°C/m occurring by the end of July (Flos, 1985). During periods of water-mass stratification, internal waves may form on the interface between water masses of different density (Southard and Stanley, 1976).

40 °

N

I°E

l°30,E

Fig.l. Locations of coring stations on the continental slope of the Ebro margin. Open symbols indicate cores measured for water content and vane shear strength only. Closed symbols indicate cores subjected to triaxial and consolidationtests. Note that the contour interval is not constant. Limited high-resolution subbottom profiles (Field and Gardner, 1990) suggest that the Ebro margin is fundamentally a constructional, prograding slope and that erosional and mass-wasting processes are of limited significance. One exception in terms of mass wasting is the large, thick Columbretes slide in the southwestern part of the Ebro margin, which may be associated with seismic activity related to volcanic emplacement of the Columbretes Islands. Other exceptions are the base-of-slope debris aprons located downslope of large gullied canyons (Nelson et al., 1984; Alonso et al., 1985) and thin, localized sediment slides on the steep slopes of levees and on canyon walls. According to Field and Gardner (1990), these features represent a "minor but ongoing process of mass-movement on the steeper slopes within the continuing prograding margin system."

EBRO MARGIN SLOPE STABILITY

We investigated an area of the Ebro continental slope on which two sectors can be differentiated (Fig. 1): (1) A northern sector bordered by Vinaroz (north) and Oropesa (south) canyons (Alonso, 1986) which is deeply incised by gullies and submarine canyons with V-shaped cross sections. This sector reaches 25 km in width and extends to a water depth of 1400 m. The slope gradients average 4°-6 °, but in canyon heads and walls the gradient may exceed 18° (Alonso, 1986). The morphology suggests a region of potentially unstable terrain. (2) A southern sector which is characterized by either a lack of gullying and a thin sediment cover or by slope valleys which have been shallowly eroded, into the underlying deposits. This sector appears to be relatively stable (Alonso et al., 1985). However, slopes in this sector range between 4 ° and 5°, which may indicate potential instability given that sliding has been recognized on more gentle seafloor slopes (Lewis, 1971). The southern sector is 10 km wide and extends to the 1000 m water depth. To the south it is bordered by the canyon-incised slope of the Columbretes Islands area, where large failures are evident in the subsurface and in a major slide area (Field and Gardner, 1990; Alonso et al., this issue).

Sediment types Four different sediment types have been identified on the Ebro slope (Baraza, 1988). (1) On the upper slope (200-500 m), gray, massive prodeltaic mud (average 44% silt, 54% clay size) is dominant in cores, and shows a characteristic black mottling of monosulfides. In the southern area, this mud shows a thin, parallel and oblique bedding, and is interbedded with thin, terrigenous sandy silty intervals with erosional basal contacts. This mud was deposited as a distal prodelta deposit derived from the Ebro River during lowstands of sea level. (2) Above this prodeltaic sediment, there is a thin (10 60 cm), irregularly distributed, offshelf spillover sequence of Holocene, heterogeneous, bioturbated sediment that covers almost the entire uppermost slope. It is light brown and poorly sorted (19-37% sand, 23 40% silt, 37-53% clay),

381

and the sand fraction is mainly terrigenous but may have important amounts of shelly debris and benthic foraminifera, and lesser amounts of planktonic foraminifera. This sediment is interpreted as having been deposited over the upper slope as a result of storms during the early stages of the Holocene transgression and then biologically mixed with Holocene hemipelagic deposits. (3) On the middle and lower slope (> 500 m), greenish gray, massive, monosulfide-rich, bioturbated, mixed terrigenous-biogenic, hemipelagic slope mud (average 27% silt, 68% clay) is dominant. This type of sediment is the most abundant over the entire slope and is a result of hemipelagic settling and downslope gravitational processes. As a result of such a combination of processes, there is a gradation in the sedimentological characteristics of middle and lower slope muds. The difference can be mainly observed by a downslope decrease of silt content and an increase in the percentage of planktonic foraminifera in the sand fraction. (4) Beneath the slope mud, on the lower slope (>1000 m), gray terrigenous, thinly laminated turbiditic mud and silt (average 44% silt) is common in canyon areas. In some cores there is well-developed parallel bedding, and thin, discrete silt and sand layers with basal erosive contacts occur. These muds are possibly the result of sediment failures on the upper slope during periods of high sedimentation rates, generating low-density, canyon-funnelled turbidity currents that deposited thin-bedded turbidites on lower slope intercanyon areas and channel levees, Most of the sediment recovered in cores from the Ebro continental slope was deposited during Pleistocene lowstands of sea level and the early stages of the Holocene transgression. Limited radiocarbon dating (Nelson, this issue) gives uncorrected ages ranging from 19,000 to 25,000 yrs B.P. for prodeltaic silty mud at the bottom (100 cm) of one short upper slope core and similar ages for turbidite sediment at the bottom (200 cm) of a lower slope core. These ages indicate that turbidite deposition on the lower slope of the Ebro margin was contemporaneous with deposition of the late Pleistocene shelf margin delta on the upper slope (Farrfin and Maldonado, this issue). Middle slope

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hemipelagic deposits have ages of 9000-11,000 yrs B.P. at depths of between 200 and 250 cm in the cores (Nelson, this issue). Methods

Field methods Twenty-three gravity core samples (locations in Table 1 and Fig.l) were collected on the continental slope of the Ebro margin in water depths ranging between 200 and 1100 m. Navigation during sampling was by satellite and LORAN-C (hyperbolic). The cores have a diameter of 7.5 cm and a maximum length of 2.5 m. After recovery, the cores were cut into 1 m long sections. Vane shear measurements were made on the ends of the core sections by inserting a 1.27 cm diameter, 1.27 cm long steel vane into the sediment and rotating it at 90°/min. An electronic torque cell was used to TABLE

determine the peak torque from which shear strength of the sediment was calculated. Water content samples were taken from the locations of the vane shear tests. Sections from eleven of the cores (designated geotechnical cores and identified in Table 1 and Fig.l) were capped, wrapped in cheesecloth, coated with microcrystalline wax, and stored vertically in a refrigerator at 4°C before consolidation and triaxial testing. The remaining cores were split longitudinally and described.

Laboratory methods Onshore, X-radiographs were made of the geotechnical cores so that sections free of cracks or complex stratigraphy could be selected for advanced testing. Intact core sections remaining after triaxial and consolidation testing were split lengthwise for vane shear strength and water content determinations at roughly 10 cm intervals, and a

1

Core locations Core number

Latitude

Longitude

Water d e p t h (m)

Geotechnical core

1° 0 0 . 6 3 ' E 1° 0 0 . 9 4 ' E 1°01.38'E 1° 0 3 . 5 0 ' E 1° 0 4 . 2 2 ' E 1°05.10'E 1° 0 7 . 4 Y E 1°07.66'E 1°12.15'E 1°20.09'E 1° 18.23'E I°17.76'E l ° 17.76'E 1°20.96'E

248 248 447 679 703 799 1000 1000 1183 1186 1044 952 959 724

no yes no yes no no yes no no no no no yes yes

1°24.5 I ' E l °22.93 'E 1° 18.86'E 1° 19.26'E 1° 14.73'E 1° 16.95'E 1° 15.03'E 1° 12.07'E 1° 0 8 . 9 8 ' E

980 950 457 705 419 1110 1087 639 212

yes no yes yes no no yes yes yes

Cruise GC3-84-MS E10-TG10 E10-TG11 E11-TGI2 E 1 2 - T G 13 E 1 2 - T G 14 E 13-TG l 5 E 1 4 - T G 16 EI4-TG17 EI5-TG18 EI6-TG19 E17-TG20 E22-TG25 E22-TG26 E23-TG27

40°01.32'N 40°01.03'N 40°01.19'N 40°00.87'N 40°00.67'N 40°00.03'N 39 ° 5 9 . 2 3 ' N 39°59.2YN 39°56.57'N 40°05.54'N 40°05.40'N 40°09.00'N 40°09.00'N 40°19.85'N

Cruise GC4-85-MS E3-TG05 E4-TG06 E5-TG07 E6-TG08 E7-TG09 E8-TGI0 E 9 - T G 12 E 1 0 - T G 13 E 11 - T G 14

40 ° 18.48'N 40°21.12'N 40°22.52'N 40 ° 1 9 . 3 Y N 40 ° 1 2 . 0 5 ' N 40°11.12'N 40°04.39'N 40°04.63'N 40°06.03'N

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visual description of the sediment was made. Atterberg limits were determined according to standard ASTM procedures on the sections selected for advanced geotechnical testing. Specific gravity of oven-dried sediment was measured using an air comparison pycnometer. On selected levels of stratigraphic cores, 2 cm thick sediment slabs were cut and X-radiographed in order to recognize detailed sedimentary structures. Textural analysis of selected subsamples was conducted by hand sieving the >50 pm fraction and by the SEDIGRAPH procedure (Micromeritics Model 5000D*) for the < 50 pm fraction. Composition of the sand fraction was determined by visual identification and by counting 400-500 grains per sample with the aid of a binocular microscope. Consolidation characteristics of the geotechnical cores were evaluated using the constant-rate-ofstrain (CRS) consolidation test procedure (Wissa et al., 1971) conducted within a back-pressured, fixed-ring consolidometer. The test results were used to obtain an estimate of the maximum past vertical effective stress that had been exerted on the sediment (Casagrande, 1936). Triaxial shear strength was measured by monotonic axial loading under consolidated, undrained conditions. These tests were run in pressurized cells on small cylindrical samples that had been previously consolidated to various effective stress conditions. Readings of axial load, pore pressure and deformation were obtained at frequent intervals throughout the tests (Bishop and Henkel, 1964). Cyclic triaxial loading tests were run to determine the susceptibility of the sediment to strength loss during repeated loading, such as from earthquakes or storm waves. Sinusoidally varying axial loads were applied with a frequency of 0.1 Hz under alternating and equal levels of tension and compression at a predetermined percentage of the static shear strength. Cyclic loading continued until either an extensional or compressional strain of 15% was reached. These procedures are similar to those described in greater detail by Lee and Edwards (1986).

Results Grain size

Mean grain-size determinations at various locations downcore show relatively coarse and varied sediment on the upper slope (water depths less than 500 m), finer and more homogeneous sediment on the middle slope (water depth between 500 and 1000 m), and more heterogeneous sediment on the lower slope (/> 1000 m), due to the presence of thin silt and sand layers within the Pleistocene turbiditic sediment (Fig.2). Water content

Water content (% dry weight) at a depth of 1 m in the cores increases from average values of 33% on the upper slope to values approaching 90% at the base-of-slope (Fig.3). There is little difference between the northern and southern sectors in the general trend of the water content-water depth data. However, there is a pronounced increase in

sea level shelf

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*Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey.

Fig.2. Mean grain size compared with water depth. Each point indicates one determination. A b o u t ten determinations are shown for each of six cores.

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northern sector

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100

Fig.3. Water content at 1 m deep in core compared with water depth. Notice incre&se in water content between 500 and 600 m isobaths in both sectors.

water content between the 500 m and 600 m isobaths in both sectors. Within each core from the middle and lower slope, water content decreases by as much as 50% with burial depth from 0 to 2 m in the continuous hemipelagic sediment. Such a decrease is typical of normal consolidation (gravitational compaction, McGregor et al., 1979). In contrast, the water content through the first few meters of prodeltaic sediment on the upper slope is nearly constant (Fig.4).

I 20

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n o d h e m sector

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Fig.4. Water content compared with depth in core. Water content is nearly constant in first few meters of sediment on upper slope. O-

200

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Liquid limit generally increases downslope from 34% on the upper slope to nearly 70% on the lower slope. Plasticity index (liquid limit minus plastic limit) also increases, from 15% on the upper slope to 38% on the lower slope (Fig.5). These values compare favorably with those from a similar depositional environment on the Rhone continental slope (Chassefiere et al., 1985). The plasticity index and liquid limit decrease slightly with depth in each core. The Atterberg limit data

[] 1000

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1200 10

•

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I 30 index

a0

(%)

Fig.5. Average plasticity index compared with water depth. Liquid limit and plasticity index increase from upper slope to lower slope.

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SLOPE STABILITY

parallel the A-line on the plasticity chart (Fig.6), suggesting sediment of consistent composition. Sediment from the upper slope falls within the zone corresponding to low-plasticity clay (CL), whereas sediment from the middle and lower slope falls within the zone of high-plasticity clay (CH, liquid limit > 50%). There is no apparent difference in the variation of the Atterberg limits with water depth between the northern and southern sectors (Figs.5 and 6).

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£ Vane shear strength 200 -

All the sediment in the cores shows vane shear strength increasing with burial depth from average values of 2 kPa at the top of the cores to about 10 kPa at 150 cm (Fig.7). The remolded vane shear strength also increases with depth. Such increases with burial depth are typical of marine sediment. Decreased vane shear strength below 150 cm in some cores is probably a result of sampling disturbance. Ebro slope sediment shows a decreasing vane shear strength (at a depth of 100 cm in the cores) from values of about 10 kPa on the upper slope to about 5 kPa on the middle and lower slope. Cores from the northern sector have slightly lower vane shear strengths compared with those from the southern sector (Fig.7).

70

60-

upper middleand slope lowerslope northern sector

A

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A50CH

CL 40-

t

>, •~ 30Q" 20

10

0

MH A lineS i 10

r 20

i I 30 40 liquid limit (%)

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50

60

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Fig.6. Plasticity chart showing sediment engineering classification according to Atterberg limits. A-line and vertical line at liquid limit of 50% separate chart into zones corresponding to different engineering classifications ( C L - clay of low plasticity; C H - - clay of high plasticity; M L - - silt of low plasticity; M H - - silt of high plasticity).

a

•

i •

r

300

upper middleand slope lowerslope northern sector

[]

southern sector

•

10 20 shear strength (kPa)

a r • 30

vane

Fig.7. Vane shear strength compared with depth in core.

Consolidation

tests

The results of consolidation tests were used to determine a quantity known as the m a x i m u m past stress, arm', which is purportedly the greatest effective overburden stress to which the sediment has ever been exposed. However, other processes, such as diagenetic bonding, can affect ~vm" The average m a x i m u m past stress for each core was highest in cores from the upper slope; the amount decreased steadily toward the base-of-slope (Fig.8). The trend does not vary between the northern and southern sectors. The overconsolidation ratio ( O C R ) , the ratio of the m a x i m u m past stress to the effective stress at the time of sampling, typically has values ranging between 2 and 3 at a depth of 1 m in the sediment, implying slightly overconsolidated sediment. Within one core from the upper slope (E10-TGI 1), the O C R at 1 m was 8, indicating a strongly overconsolidated condition. High values of O C R can suggest the loss, for example through erosion, of part of the sediment load that previously existed. Perhaps some overconsolidation developed during sea-level low-

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200

400

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J~ 600

800

1000

1200

iD• I 20

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no~hern sector

I

southern sector

I 40

average maximum past stress,

I 60

80

~vm' (kPa)

Fig.8. Maximum past stress (average of all values for a core) compared with water depth.

stands, when cyclic-wave loading of sediment near the upper slope could have been important. Finally, high values of OCR in short core samples can also indicate apparent overconsolidation resulting from bioturbation, interparticle bonding, or secondary compression, all of which often dominate the stress behavior of sediment near the seafloor (Lee et al., 1981). The compression index, C¢, is the amount of compression (change in void ratio) that occurs during a tenfold increase in vertical effective stress. The compression index ranges from 0.17 to 0.55 for the continental slope on the Ebro margin; the highest values are from the lower slope.

Slope stability analysis Stability under gravitational loading conditions The stability of a sedimentary deposit on a given slope depends on the shear strength of the sediment, how that strength varies with depth below the seafloor, and how that strength compares with the environmental stresses that are

imposed on the sediment. A slope will become unstable whenever the average shearing stress along the potential surface of sliding becomes equal to the average shearing resistance along this same surface (Scott and Zuckerman, 1970). In the absence of downslope forces other than gravity, the stability of a sedimentary body on a slope is inversely related to the thickness of the potentially unstable mass (H), the slope angle (@, and the unit weight of the sediment @) (Hampton et al., 1978). The stability is directly proportional to the shear strength of the sediment at the depth, H. The shear strength of the sediment on the Ebro margin was determined using the normalized soil parameter (NSP) method (Ladd and Foott, 1974). Static undrained triaxial tests were conducted on samples consolidated to four times their estimated maximum past stress to determine the normalized shear strength of normally consolidated sediment. The ratio, S, of the measured undrained shear strength of these samples to the consolidation stress decreases for both the northern and southern sectors from average values of about 0.45 on the upper slope to about 0.40 on the lower slope (Fig.9). If sediment becomes normally consolidated well below the level of sampling (i.e., measured overconsolidation in the cores was apparent and thereby lost with increasing burial), the sine of the maximum stable slope under static undrained loading conditions is roughly equal to S (Lee and Edwards, 1986). By this reasoning, the maximum stable slope would range from 27 ° on the upper slope to 24 ° on the lower slope. Such declivities are probably lower bounds because there may be some overconsolidation at depth in the sediment and most long-term gravitational failures of seafloor slopes would be partially or fully drained, rather than undrained. Both overconsolidation and partial drainage increase the stability of slopes. Under fully drained conditions the maximum stable slope under gravitational loading approaches the effective friction angle of the sediment, qY (Edwards et al., 1980). Within the study area the friction angle ranges from 31° on the upper slope to 43 ° on the lower slope. According to these results, the upper slope may be slightly more stable under undrained or relatively rapid static loading conditions compared

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EBRO MARGIN SLOPE STABILITY

200

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Stability under cyclic loading conditions

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magnitude greater during the Pleistocene than during the Holocene (Nelson, this issue). A state of underconsolidation, with corresponding low values of shear strength, could have been present and could have led to increased instability in the past.

05

average value of S

Fig.9. Value of S (average of all values for a core), the ratio of shear strength to consolidation stress for the normal consolidated state, compared with water depth. with the lower slope, which is more stable under drained or very long-term static conditions. No matter what the rate of loading, however, the maximum stable slopes (24o-43 °) are much greater than the actual slopes observed in the area, with the possible exception of some canyon walls, where local failure has been observed (Alonso et al., this issue; Field and Gardner, 1990). Some factors (e.g., high sedimentation rate, presence of bubble-phase gas, or the occurrence of artesian pressures in deep formations) could result in greatly reduced maximum stable slopes under static loading conditions. However, the Holocene sedimentation rate on the Ebro continental slope is low, and no evidence of high, excess pore-water pressure has been found. Hence, the slope appears to be stable under static loading conditions at present, except perhaps for highly localized environments, such as steep canyon walls, in which the slope geometry has been changed rapidly by oversteepening or excavation due to erosive currents. Sedimentation rates of prodeltaic mud on the upper slope, however, were an order of

Repeated shear-stress fluctuations caused by storm waves or earthquakes are superimposed on the static downslope gravitational loading produced by the weight of sediment (Lee and Edwards, 1986). These repeated shear stresses can also increase the pore-water pressure within the sediment, thereby reducing the shear strength. In order to evaluate the strength loss from cyclic loading, samples were placed in a triaxial cell and isotropically consolidated to stress levels identical to those imposed on the static triaxial samples. In this way the shear strength of sediment under cyclic loading conditions can be evaluated relative to the predetermined static undrained shear strength. During shear testing, repeated cycles of shear stress were applied in both tension and compression until failure (defined as 15% axial strain) was achieved. For a given sediment, more loading cycles are required to reach failure as the cyclic shear stress level is reduced. A strength degradation factor, Ar, equal to the cyclic shear stress normalized by the static shear strength, was determined at the ten cycles-to-failure level. A repeated loading of ten cycles represents the major stress cycles of a typical moderate to strong earthquake (Seed and Idriss, 1971). Values of A r greater than 1.0 indicate that the rate, or the viscous effects, associated with rapid cyclic loading more than compensates for the pore pressures generated by the cyclic loading. However, values of A r less than 1.0 indicate that cyclic loading diminishes the resistive strength of the sediment. Ar equals 0.73-0.80 on the upper slope and increases to 1.53 on the lower slope (Fig. 10). Values ofA r are slightly lower for the northern sector than the southern sector. As has been observed in other environments (Lee and Edwards, 1986), the degradation factor increases in proportion to the water content of the sediment. To produce instability under cyclic loading

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Fig. J0. The cyclic strength degradation factor, At, compared

with water depth. conditions (Scott and Zuckerman, 1970; Hampton et al., 1978; Lee and Edwards, 1986), the total shear stress acting on the slope (static plus cyclic) must exceed the available mobilized shear strength (static shear strength modified by a cyclic degradation factor). Lee and Edwards (1986), using equations modified from Morgenstern (1967) and Seed and Rahman (1978), proposed a method for analyzing the stability of slopes subjected to cyclic loading. According to their method, the relative stability of a slope can be expressed in terms of either the seismically induced critical acceleration or the storm wave height needed to produce failure. Seismically induced instability

As shown by Lee and Edwards (1986), the relative stability of slopes undergoing seismic loading can be assessed by considering: kc = (7'/7)[A¢ArUS( O C R ) m - sin c~]

where k~ is the critical earthquake acceleration, 7'/Y is the ratio of submerged to total density,

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k c (g's) f o r level s e a f l o o r

Fig.ll. The estimated critical earthquake acceleration factor, kc, compared with water depth for a levelseafloorcomposed of sediment having the overconsolidation state measured for sediment at a subbottom depth of I m.

A c is a factor that corrects for consolidation anisotropy (it was found to be 1.19 in our tests). U is the degree of consolidation ( U = I for normal and overconsolidation and is <1 for underconsolidation), m is a sediment parameter typically equal to about 0.8 (the one applicable test performed in our program showed m = 0.76), ~ is the slope angle, and the other terms have been defined previously. In a region in which the expected intensity of seismic shaking is essentially uniform, slopes that have a lower critical acceleration have a higher probability of failure than those with higher values of critical acceleration. We calculated several sets of values of k o on the basis of different assumptions regarding consolidation state and bottom slope. First, we assumed that the estimates of consolidation state obtained from CRS consolidation testing were real and are present at depth beneath the seafloor. For the calculated values of O C R at a depth of 1 m and for a level seafloor, values o f k c ranged from about 0.3

EBRO MARGIN SLOPESTABILITY

20o]

389

•

200 "1 /

IF-]

400

slope angte

~=10

400 •

~'600 -

II D 800 800 t

ii

1000 northernsector [~ southernsector • 1200 I

0.0

0.1

1000 ~ 1200

OCR = 1.0

o12

0.3

0,4

critical earthquake acceleration, kc (g's) for level seafloor

Fig.12. The estimated critical earthquake acceleration factor, k~, versus water depth for a level seafloorcomposedof normally consolidated sediment. Such a condition may exist at great depth in the sediment column (> 20 m). Linear regression fit of data points is shown. g for the lower slope to nearly 0.8 g for the upper slope (Fig.l l). A slope of 10° would reduce the value of k~ by about 0.07 g, yielding a k¢ that ranges from 0.23 to 0.73 g for the lower and upper slope respectively. Lee and Edwards (1986) found that k~ values greater than 0.14 correspond to stable slopes in a few seismically active areas of the California and Alaska margins. The Ebro margin is less seismically active, and its slopes have undergone lower seismic accelerations. Accordingly, the value ofkc separating stable and unstable slopes must be lower in the Ebro margin that in the active areas of Alaska and California. The exact value of k¢ separating stable and unstable slopes cannot be determined, however, because we did not sample any known unstable areas. In any case, we can conclude that a condition of overconsolidation throughout the Ebro margin sediment column (k¢ > 0.2 g) would result in a high level of stability on known slopes with respect to seismic loading. In separate calculations, we determined what k~

0O

OCR = 1.0 C a h f o r n l a ~

01

02

critical earthquake acceleration, kc (g's)

03

Fig.13. The estimated critical earthquake acceleration factor, k¢, compared with water depth for normally consolidated sediment and a variety of seaftoor slopes. A normally consolidated condition may be present at great depth in the sediment column (>20 m). Only linear regression fits of calculated data are shown. Notice that the value of kc in the Ebro margin can be somewhat lower than the value in the active seismic areas of the Alaska and California margins. would be if the overconsolidtion observed in the surficial sediment cores is apparent, that is, present only near the seafloor surface as a result of interparticle bonding or reworking by bottom currents. If the overconsolidation is apparent, the sediment would actually be normally consolidated (OCR = 1) below the near-surface zone of apparent overconsolidation. For this condition and a level seafloor, k c ranges from 0.18 g for the upper slope to nearly 0.30 g for the lower slope (Fig.12). As the bottom slope gradient increases the value of kc decreases (Fig.13). For a 10° bottom slope, k~ is 0.09 g on the upper slope and increases to 0.17 g for the lower slope. There is no apparent difference between northern and southern sectors. If a sediment like that of the upper slope of the Ebro margin were normally consolidated and was present on a 10° slope in the areas of high seismicity off California or Alaska, it would

390

probably fail during one of the large earthquakes that those areas frequently undergo. However, the Ebro margin is passive and, as such, is not highly seismically active. Within the last 70 years, only two moderate earthquakes with epicenters near the study area have occurred and both had maximum intensities below IV (M.S.K. scale) (GaibarPuertas, 1979). The earthquake intensity I o can be correlated with peak acceleration by the following (Smith, 1973): a = 10 exp [Io - q)/P] where a is the acceleration in cm/s 2, and p and q are empirical constants ranging from 2 to 3 and 1.5 to 3.5 respectively. Therefore, if the intensity of the earthquake is known, the peak acceleration can be estimated. For an earthquake of intensity V and minimum values ofp and q, the upper bound of the peak acceleration is 0.06 g. Peak accelerations and critical accelerations for slope stability are not directly comparable, but if the peak acceleration is less than the critical acceleration, slope failure is not expected. Because the estimated peak acceleration during intensity V shaking is below the 0.09 g minimum-critical acceleration for a 10° seafloor slope, the possibility that extensive seismically induced slope failures occurred during the last 70 years seems slight. However, oversteep slopes in canyons might have failed. Also, the acceleration relationship with intensity is exponential, and a peak acceleration of of 0.18 g might be reached with an intensity VI earthquake. Infrequent strong earthquakes with intensities of VI or greater and with return periods of much greater than 70 years are possible on the Ebro margin. These events could cause extensive failures on the continental slope. The large Columbretes slide to the southwest of our study area was probably caused by earthquake shaking associated with the volcanic emplacement of the Columbretes Islands (Field and Gardner, 1990).

Storm wave-induced instability Whether storm waves can induce submarine slope failures depends on the wave height, wave length, and water depth, as well as on the

J. BARAZA ET AL.

properties of the seafloor. By analyzing these factors in the Gulf of Alaska, Lee and Edwards (1986) showed that 37 m high storm waves are capable of causing slope failures to a maximum water depth of 81 m. Wave impact decreases rapidly with increasing water depth and is practically negligible at depths greater than one-half the wave length (Edwards et al., 1980). For these reasons and given that the Ebro slope has a minimum depth of 150 m and that storm waves are generally less than 4 m high, there is little chance that storm waves can create bottom cyclic shear stresses of sufficient magnitude to produce instability. Therefore, storm waves can be eliminated as a factor causing the landslides on the Ebro slope at present. Nevertheless, they were conceivably a factor at lower sea levels, when high sedimentation rates may have produced underconsolidated sediment, and shallower depths at the shelfbreak may have induced a stronger direct storm wavegenerated loading. Then, significant failures were occurring in canyons and causing deposition on the Ebro aprons (Nelson and Maldonado, this issue).

Instability induced by internal waves Slope stability could be affected by internal waves developed during periods of water-mass density stratification by either: (1) contributing to sediment transport and influencing the nature and geotechnical properties of sediment remaining, or (2) directly loading the slope and degrading the sediment strength by repeated loading (Mayer et al., 1981). Cacchione and Southard (1974) suggested that internal waves reach their highest velocities and maximum shear stress near the top of steep continental slopes. Such a process could therefore be an important factor in basins such as the western Mediterranean, which is bordered by high-gradient slopes. Under certain conditions, breaking internal waves may generate very high bottom current velocities (Southard and Stanley, 1976), which could resuspend fine sediment (Cacchione and Southard, 1974). By winnowing out fine sediment, such a resuspension could cause a change in the consolidation state and the sorting of grain sizes. The resulting shear strength and slope

391

EBRO MARGIN SLOPE STABILITY

stability relative to loading by some other mechanism could be altered. In the case (2) of direct loading, the energy associated with internal waves on the Ebro slope area does not appear to have sufficient power to induce major seafloor shear stresses (D. A. Cacchione, pers. commun., 1987). Therefore, these waves are probably not capable of failing the sediment by direct loading.

Regional variations in physical properties From the results of the geotechnical tests, two different areas can be distinguished on the basis of sediment index properties: the upper slope (water depth less than 500 m) and the middle and lower slope (water depth greater than 500 m). On the upper slope, prodeltaic mud with a high silt content and a low to moderate sand content is dominant. The average water content is 33% dry weight, slightly below the liquid limit, which is about 34%. The plasticity index is about 15%, and the sediment is highly to moderately overconsolidated (OCR as high as 8). In contrast, the hemipelagic sediment from the middle and lower slope has a lower sand and silt content than that of the upper slope. In addition, the water content is higher (approaching 90%) and is above the liquid limit (ranging from 55 to 75%), the plasticity index is higher, and the degree of overconsolidation is lower (OCR of 2 3). Vane shear strength and the normalized strength parameter S (ratio of strength to consolidation stress for normal consolidation) are both lower, whereas the cyclic strength degradation factor, Ar, is higher than that of the upper slope. When inserted into a simplified model for seismic stability, these results show that the middle and lower slope would be more susceptible than the upper slope to earthquake-induced failure at shallow depth but less susceptible to failure at much greater depth in the sediment column (where the sediment is presumed to be normally consolidated). These results indicate a correlation between physical properties, as well as slope stability, with the decrease in sand and silt content that occurs in increasing water depth. The downslope fining of the sediment is probably a result of greater sand and silt deposition on Pleistocene prodeltas and normal sorting of sediment by downslope trans-

port processes. Winnowing of fines by breaking internal waves within the stratified water column above 600 m may also have contributed to this downslope fining of the sediment on the Ebro slope. Along a transect across the continental slope in the southern sector, Holocene winnowing is suggested by slow sedimentation rates, which are about 5 cm/103 yrs for the superficial posttransgressive sediment covering the upper slope, increasing to about 150 cm/103 yrs for the Pleistocene prodeltaic deposits of the upper slope, and decreasing to about 20 cm/103 yrs for two cores of hemipelagic sediment from the middle and lower slope (Nelson, this issue). Such results suggest that processes forming sediment on the upper slope have been different from those of the middle and lower slope. We found only very slight differences in measured physical properties between the northern and southern sectors. The differences are certainly insufficient to account for the significant differences in seafloor morphology - - i.e. canyons and generally irregular topography to the north, less gullying to the south. The differing morphology must have been caused by variations in the physical properties of deeper, older, and unsampled sediment, regional variations in the processes that affect the stability of the seafloor and its underlying sediment, different structural features such as faulting, or the differing relative position of sediment discharge points due to the meandering character and bifurcations of the Ebro River mouth during the Quaternary (Nelson and Maldonado, 1988).

Conclusions During the lowered sea levels of the late Neogene, shelf margin deltas were deposited over the outer continental shelf (Farrfin and Maldonado, this issue). The resulting relatively coarsegrained Pleistocene prodeltaic deposits are exposed at the seafloor or, on the upper slope, thinly covered by bioturbated, Holocene mixed hemipelagic and spillover deposits. The combined effect of potential underconsolidation due to fast prodeltaic sedimentation rates and the action of external factors such as intense storms active in relatively

392

shallow water may have produced Pleistocene instability events on the upper slope. Holocene sediment accumulation rates on the Ebro continental slope are not high because terrigenous material derived from the Ebro River either does not reach or bypasses the outer continental shelf. Also, Holocene sediment above the 600 m lower boundary of the homogeneous intermediate water mass (Levantine Intermediate Water) has apparently been winnowed, perhaps by breaking internal waves. Accordingly, sediment on the Ebro continental slope is presently normally consolidated to overconsolidated. Holocene water depths are sufficiently great that storm waves are presently incapable of causing slope failures. According to the geotechnical test results, the prodeltaic and winnowed deposits of the upper slope are slightly more stable under undrained static loading conditions relative to the hemipelagic deposits of the lower slope. The lower slope is more stable under drained or very long term static conditions. Maximum slopes in our study area appear to be stable under static (gravitational) loading. Seismically induced instability on most of the Ebro margin seems unlikely given the low seismicity of the region. Earthquakes of approximately intensity VI or greater would be needed to cause failures, and these have not occurred in the last 70 years. Likewise, the action of other cyclic loading events, such as storm waves or internal waves, can be ignored as possible causes of instability on the Ebro slope because they are incapable of producing sufficiently large shear stresses in the sediment. Nevertheless, localized instability might be produced by a combination of oversteepening and infrequent, more intense seismic loading. These results are supported by the findings of Field and Gardner (1990) and Alonso et al. (this issue) that the margin has been essentially stable and constructional with significant mass wasting occurring only in limited environments.

Acknowledgements We are grateful for the cooperation of the members of the Marine Geology Group of "Consejo Superior Investigaciones Ciencias" (C.S.I.C). and the officers and crew of the B.O. Cornide de

J. B A R A Z A ET AL.

Saavedra and B.O. Garcia del Cid during the sampling cruises. We also thank H.W. Olsen and J.V. Gardner for helpful reviews of this manuscript. Funding for the study was provided by the U.S.-Spain Joint Committee for Scientific and Technological Cooperation (award CCA 8309/047).

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