Slope stability along active and passive continental margins ... - E-LIB

2 Geotechnical characteristics and slope stability along the Uruguayan and northern Argentine margin
2.4 Results
2.4.1 Physical and geotechnical properties of sediments
One gravity core GeoB13854-1 comes from the upper slope of the scar S2 of NS at a water depth of
~2120 m (Fig. 2.2B). Two MeBo cores were also used: Core GeoB13860-1 is located upslope of a
scour ~60 km southwest of the scar S2 of NS at water depths of ~1220 m (Fig. 2.2C) and core
GeoB13868-1 is located at the nearly flat portion of the middle continental slope (the Ewing terrace)
~20 km north of the Querendi Canyon of SC at water depths of ~1140 m (Fig. 2.2C). Sediment
lithofacies were defined on the basis of visual core description (see Fig. 2.3) and quantitative
parameters such as magnetic susceptibility (see Fig. 2.3), grain size distribution (see Fig. 2.4A) and
Atterberg limits (see Fig. 2.4B). Visual core description shows that sediments of NS (cores
GeoB13854-1 and GeoB13860-1) mainly consist of fine-grained materials (silt and clay) with
interbedded sand layers, while the sediments of SC (GeoB13868-1) consist of very fine sand to
medium sand (Fig. 2.3). Magnetic susceptibility provides a first order estimate of ferromagnetic
mineral abundance in sediments and is sensitive to grain size variations, with higher grain size resulting
in higher magnetic susceptibility of our cores (Fig. 2.3) (Bozzano et al. 2011). Grain size distribution
and Atterberg limits from whole round samples give a more refined classification for sediments (Fig.
2.4). The representative grain size of sediments from NS is clayey silt (~20% clay, 10-20% sand) to
sandy silt (core GeoB13854-1 at 4.71 m) with intermediate to high plasticity. The representative grain
size of sediments from SC is silty sand (2-8% clay, 18-38% silt).
The key physical properties of sediments from NS and SC are compared in Figure 2.3. Physical
properties such as bulk density, water content, and void ratio have close association with grain size
distribution. The bulk density values obtained from MAD are slightly lower than the values obtained
from MSCL. Silty sediments from NS show lower bulk density (1.5-2.1 g/cm 3 ) comparing to sandy
sediments from SC (1.7-2.4 g/cm 3 ). Silty sediments from NS show high water content (65-95%) at
surficial depth (GeoB13854-1 at 5 m) and lower water content (~20%) at deep depth (GeoB13860-1 at
35 m). Sandy sediments (GeoB13868-1) from SC show low water content between 20 and 50%. Void
ratio has similar trends as water content showing highest void ratios (1.6-3.0) in core GeoB13854-1 and
lowest void ratios (0.6-1.2) in core GeoB13868-1.
The values of the undisturbed and remoulded shear strength are presented in Figure 2.3. Undrained
shear strengths from vane shear tests are lower than the values from cone penetrometer measurements
at the same depth. Sediments of core GeoB13854-1 have high undrained strength (~20 kPa) near the
top of the core and gradually increase to ~30 kPa at 5 m core depth. Coarse-grained sediments in the
top 1.5 m of core GeoB13860-1 have low shear strength (0-20 kPa) and increase rapidly up to ~200
kPa at 16 m bsf (below sea floor). Undrained shear tests were not carried out below 16 mbsf because
the sediments were found stiff to hard. The strength sensitivity of the sediments from NS obtained from
vane shear test is low (1.0-2.7). Vane shear test were not conducted on the sandy sediments from SC.
The values of shear strength obtained from cone penetrometer are in range of 0-30 kPa. The ratio of the
undrained shear strength to vertical effective stress (S u /σ v ) gives an indication of the consolidation state
of sediment. Typical values of S u /σ v for normally consolidated sediments range between 0.2-0.4 (Locat
and Lee 2002). It is indicated from the stiffness of the sediments at a given depth that materials from
NS are highly overconsolidated, while the sediments from SC are underconsolidated to normally
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