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1.
Abstract

As a part of the environmental impact assessment studies, geotechnical properties of sediments were determined in the Central Indian Basin. The undrained shear strength and index properties of the siliceous sediments were determined on 20 box cores of uniform dimension collected from various locations in five preselected sites. The maximum core length encountered was 41 cm and most of the sediments were siliceous oozes consisting of radiolarian or diatomaceous tests. The shear strength measurements revealed that surface sediments deposited in recent times (0–10 cm) have a shear strength of 0–1 kPa; this value increases with depth, reaching 10 kPa at 40 cm deep. Older sediments have greater strength because of compaction. Water content varies in the wide range of 312–577% and decreases with depth. The clay minerals such as smectite and illite are dominant and show some control over water content. Wet density, specific gravity, and porosity do not indicate any notable variation with depth, thereby indicating a uniform, slow rate of sedimentation. The average porosity of sediments is 90.2%, specific gravity 2.18, and wet bulk density 1.12 g/cm3. Sediments exhibit medium to high plasticity characteristics, with the average plasticity index varying between 105% and 136%. Preliminary studies on postdisturbance samples showed an increase in natural water content and a decrease in undrained shear strength of sediments in the top 10- to 15-cm layer.  相似文献   

2.
Abstract

The composition and properties of glacigenic sediments in the southwestern Barents Sea are described based on data from 33 shallow boreholes (< 143 m below seabed) and 11 seabed cores (<4.2m below seabed). The cores are tied into a regional seismostratigraphic framework, illustrating the relationships between different boreholes.

A massive, muddy diamicton (silty, sandy clay with scattered gravel) is found in nearly all cores. Average clay content (<2 pm) of this lithology is about 38%, but varies between about 25% and 50%. Short intervals of finely laminated, waterlain sediments or gravelly sand are cored in a few occasions. A high content of sand and gravel in the cores from near the Norwegian coast shows an influence of sediment input from the mainland, while material eroded from sedimentary rocks dominates farther offshore.

The data presented on physical properties include undisturbed and remolded undrained shear strength, natural water content, bulk density, compressional sound velocity (P waves), Atterberg consistency limits, effective preconsolidation pressure, and consolidation coefficient.

Prediction of overconsolidation from seismic mapping of erosional surfaces is confirmed by the borehole cores. High compaction is found both in Weichselian and older deposits, with a general increase in compaction toward the east as well as toward shallower water. Cores that are “underconsolidated” at their present burial depth are also reported.

The average compressional sound velocity is about 1780 m/s for the borehole cores, 1550 m/s for the seabed cores, and increases with increasing shear strength and consolidation. Both horizontal and vertical sound velocities are measured in several cores, and although the data have a considerable scatter, a slightly aniso‐tropic sound velocity is indicated.  相似文献   

3.
During Leg 75 of the Deep Sea Drilling Project (DSDP) from the D/V Glomar Challenger, a 200‐m deep hole was drilled at Hole 532A on the eastern side of Walvis Ridge at a water depth of 1331 m. Sediment cores were obtained by means of a hydraulic piston corer. All of the cores from this boring were designated for geotechnical studies and were distributed among eight institutions. The results of laboratory studies on these sediment cores were compiled and analyzed. Sediment properties, including physical characteristics, strength, consolidation, and permeability were studied to evaluate changes as a function of depth of burial. It was concluded that the sediment profile to the explored depth of 200 m at Walvis Ridge consists of approximately 50 m of foram‐nannofossil marl (Subunit la) over 64 m of diatom‐nannofossil marl (Subunit 1b) over nannofossil marl (Subunit lc) to the depth explored. All three sediment units appear to be normally consolidated, although some anomalies seem to exist to a depth of 120 m. No distinct differences were found among the sediment properties of the three subunits (la, 1b, and lc) identified at this site.  相似文献   

4.
Radionuclide activities of 210Pb and 226Ra were measured to determine bioturbation coefficients (Db) in seven sediment cores from the Korean licensed block for polymetallic nodules in the Clarion–Clipperton Fracture Zone. Variability in Db is considered in the context of the sedimentological, geochemical, and geotechnical properties of the sediments. Db values in the studied cores were estimated using a steady-state diffusion model and varied over a wide range from 1.1 to 293 cm2/yr with corresponding mixing depths (L) of 26 to 144 cm. When excepting for spurious results obtained from cores where diffusive mixing does not apply, Db values range from 1.1 to 9.0 cm2/yr with corresponding mixing depths (L) of 26 to 63 cm. Such wide variability in Db and L values is exceptional in sites with water depths of ～5000 m and is attributed in this study to an uneven distribution of sediment layers with different shear strengths and total organic carbon (TOC) contents, caused by erosion events. The studied cores can be grouped into two categories based on lithologic associations: layers with high maximum shear strength (MSS) and low TOC content, showing a narrow range of Db values (1.1–9.0 cm2/yr); and layers with low MSS and high TOC content, yielding much higher Db values of over 30 cm2/yr. The distribution of different lithologies, and the resultant spatial variability in MSS and labile organic matter content, controls the presence and maximum burrowing depth of infauna by affecting their mobility and the availability of food. This study provides a unique case showing that shear strength, which relates to the degree of sediment consolidation, might be an important factor in controlling rates of bioturbation and sediment mixing depths.  相似文献   

5.
This study was undertaken to investigate the implication of geoacoustic behaviors in the shallow marine sediments associated with the changes in geotechnical index properties. Two piston cores (270 cm and 400 cm in core length) used in this study were recovered from stations 1 and 2, the western continental margin, the East Sea. Scanning electron microscopy (SEM) was employed to illustrate the effects of microstructure on shear properties. The direct SEM observation of sediment fabrics is inevitable to understand the correlation of the changes in geoacoustic properties to the sediment structure. The consolidation of sediments by overburden stress resulting in the clay fabric alteration appears to play an important role in changing shear properties. Water contents and porosity of sediments gradually decreases with increasing depth, whereas wet bulk density shows a reverse trend. It is interesting to note that shear wave velocities increase rapidly from 8 to 20 m/s while compressional wave velocities significantly fluctuate, ranging from 1450 to 1550 m/s with depth. The fabric changes in sediment with increasing depth for example, uniform grain size and well oriented clay fabrics may cause the shear strength increase from 1 to 12 kPa. Shear wave velocity is, therefore, shown to be very sensitive to the changes in undrained strength for unconsolidated marine sediments. This correlation allows an in-situ estimation of shear stress in the subsurface from shear wave velocity data.  相似文献   

6.
As a part of the environmental impact assessment studies, geotechnical properties of sediments were determined in the Central Indian Basin. The undrained shear strength and index properties of the siliceous sediments were determined on 20 box cores of uniform dimension collected from various locations in five preselected sites. The maximum core length encountered was 41 cm and most of the sediments were siliceous oozes consisting of radiolarian or diatomaceous tests. The shear strength measurements revealed that surface sediments deposited in recent times (0-10 cm) have a shear strength of 0-1 kPa; this value increases with depth, reaching 10 kPa at 40 cm deep. Older sediments have greater strength because of compaction. Water content varies in the wide range of 312-577% and decreases with depth. The clay minerals such as smectite and illite are dominant and show some control over water content. Wet density, specific gravity, and porosity do not indicate any notable variation with depth, thereby indicating a uniform, slow rate of sedimentation. The average porosity of sediments is 90.2%, specific gravity 2.18, and wet bulk density 1.12 g/cm 3 . Sediments exhibit medium to high plasticity characteristics, with the average plasticity index varying between 105% and 136%. Preliminary studies on postdisturbance samples showed an increase in natural water content and a decrease in undrained shear strength of sediments in the top 10- to 15-cm layer.  相似文献   

7.
In Memoriam     
Abstract

The engineering properties of deep continental margin sediments were determined on a worldwide basis. Deep Sea Drilling Project (DSDP) core data and material were utilized from over 900 cores obtained from 89 sites, primarily on the continental margins. Cores were recovered from penetrations to 200 m in water depths averaging 3000 m. Supplementary laboratory testing on selected cores was directed toward determining index properties and shear strength properties of the sediments. The study included a literature review of deep‐sea soil properties, the results of which are to be used by DSDP to evaluate foundation conditions for reentry cones with long casing strings. The results will also be used for a feasibility study of an ultradeepwater marine riser and well‐control system. The marine sediments examined can be divided into three main types: clay, calcareous ooze, and siliceous ooze. Sediment distribution consisted of 48% calcareous ooze, 43% clay, 6% siliceous ooze, and 3% volcanic ash. Because of the sample disturbance inherent in the deepwater coring operation, emphasis was placed on analyzing sediment properties not significantly affected by changes in in situ stresses and structure caused by sampling. Averages and ranges in values of water content and unit weight are presented for the three main sediment types. Plasticity and strength characteristics are discussed in detail and the elastic and compression properties are outlined. The geotechnical properties of deep continental margin soils are summarized.  相似文献   

8.
Abstract

The continental slope off the coast of Israel is riddled with numerous large slump scars at depths greater than 400 m. Recent scar slumps are situated in the steepest central portions of the continental slope (400–450 m depth, α = 6°), frequently disfiguring older slump scars in its lower portions. The slumping materials were probably largely transported downslope in the form of density currents, and occasionally by sliding of large sediment chunks. Upslope retrogressive slumping phases progressively disfigure the shape of the slump scars until they totally disappear, causing net reduction of the thickness of the sedimentary column. To provide a basis for the quantitative analysis of slumping, laboratory vane tests, triaxial consolidated, undrained compression tests with pore‐pressure measurements, drained direct shear tests, and consolidation tests were performed oh undisturbed samples. Because the sediments consist of normally consolidated silty clays, the geotechnical properties measured on the core samples can be readily extrapolated for greater depths, assuming the sediments are homogeneous. Angles of internal friction measured by direct shearing under drained conditions are ?d =24°‐25°, designating the maximum possible angle of a stable infinite slope. These angles are appreciably higher than the steepest slopes in the investigated area, and a drained slumping mechanism is therefore considered unlikely. The slopes of the slump scar walls are about 20°; therefore, in the absence of active erosional, sedimentological, or tectonic agents, these walls have long‐term stability (drained shear). Undrained shear failure resulting in slope instability may be attributable to rapid changes in slope geometry (undercutting or oversteepening of the slope), fluctuations in pore pressure, or accelerations associated with earthquakes. Undrained shear‐strength parameters were determined by both laboratory consolidated‐un‐ drained triaxial tests and by miniature vane shear tests. The angles of internal friction that were measured are ?cu =15°‐17°, and the cu/p o values range between 0.22 and 0.75. An analysis of the force equilibrium within the sediments leads to the conclusion that horizontal earthquake‐induced accelerations, as little as 5–6% of gravity, are sufficient to cause slope failure in the steepest slope zone (400–450 m depth, α=6°, cu/p o=0.25). Collapse resulting from liquefaction is unlikely, as the sediments are normally consolidated silty clays with intermediate sensitivity, St =2–4.

The existence of slump scars in the lower portion of the continental slope, characterized by gentle slopes (α=1°‐3°) and sediments with high shear strength (cu/p o=0.30–0.50) is attributed to large horizontal accelerations (k= 12–16% of gravity). Owing to the wide range of geotechnical properties of the sediments (cu/p o= 0.20–0.75) and the inclination of the continental slope (α=1°‐6°), the same earthquake may generate a wide range of horizontal accelerations in different portions of the continental slope, and slumping may occur wherever the stability equilibrium is disrupted.  相似文献   

9.
An investigation of the geotechnical properties of sediments from 57 boreholes drilled in West Delta and South Pass OSC Lease Areas, offshore Louisiana, indicated that sediment texture, shear strength, water content, and liquid limit vary significantly laterally and with depth. Samples are predominantly soft, fine‐grained sediments with high water content and medium to high plasticity. Firm, coarser‐grained sediments were obtained near the shoreline in South Pass OCS Lease Area and in the northern and eastern parts of West Delta OCS Lease Area. Trends in the distribution of geotechnical properties appear to be influenced by sediment texture and rate of sedimentation.  相似文献   

10.
As part of the STRATAFORM project, a series of cores were obtained from the Eel River Margin area of Eureka, California. The geotechnical analysis of intact specimens and of reconstituted samples provides some insight on the development of shear strength with burial. The results show the effect of bioturbation in the early part of the lifetime of a sediment. SEDCON tests were used to proposed various relationships which help predict the changes in density, liquidity index, and strength as a function of depth. These relationships are found useful from near the water sediment-interface down to a depth of at least 400 m in the sediment column.  相似文献   

11.
To address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries, a series of disturbance and monitoring experiments were conducted at a bay bottom mud site (2.5 m depth) in Galveston Bay, Texas in July 1998 and May 1999. Based on pre- and post-trawl sediment profiles of 7Be; pore water dissolved oxygen and sulfide concentration; and bulk sediment properties, it was estimated that the trawl rig, including the net, trawl doors, and “tickler chain,” excavate the seabed to a maximum depth of approximately 1.5 cm, with most areas displaying considerably less disturbance. Water column profile data in the turbid plume left by the trawl in these underconsolidated muds (85–90% porosity; <0.25 kPa undrained shear strength) demonstrate that suspended sediment inventories of up to 85–90 mg/cm2 are produced immediately behind the trawl net; an order of magnitude higher than pre-trawl inventories and comparable to those observed during a 9–10 m/s wind event at the study site. Plume settling and dispersion caused suspended sediment inventories to return to pre-trawl values about 14 min after trawl passage in two separate experiments, indicating particles re-settle primarily as flocs before they can be widely dispersed by local currents. As a result of the passage of the trawl rig across the seabed, shear strength of the sediment surface showed no significant increase, suggesting that bed armoring is not taking place and the trawled areas will not show an increase in critical shear stress.  相似文献   

12.
Geotechnical characteristics of carbonate sediments from two test sites (Dry Tortugas Keys and Marquesas Keys) in the Lower Florida Keys were investigated as part of the Coastal Benthic Boundary Layer Special Research Program, through an extensive field coring and laboratory testing program conducted by the Marine Geomechanics Laboratory of the University of Rhode Island. Based on results from physical measurements, water content and wet bulk density values for both sites generally showed large variations in the upper 25 cm and little variation below this depth. Sediment samples exhibited low plasticity or nonplastic characteristics. Constant-rate-of-deformation consolidation test results showed strong apparent overconsolidation (stress state ratio >7.5) in the surface sediments (upper 50 cm) at the Dry Tortugas Keys, and light overconsolidation (stress state ratio <1.5) below 50-cm depth at the Marquesas Keys site. In-situ permeability values were between 10-4 and 10-7 cm/s at both sites and showed no strong depth dependence in the upper 2 m. Undrained shear strength profiles for Dry Tortugas Keys sediments indicated a marked stiffening with depth, whereas the Marquesas Keys sediments showed a gradual increase with depth. Consolidated isotropically undrained triaxial shear strength test results indicate that the undisturbed sediments had an average effective angle of internal friction of 38°, which is not fully realized until large axial strains on the order of 11% have accumulated. Evidence of cementation was not found in triaxial compression or consolidation test results. The general behavior and characteristics of these sediments are similar to those of granular materials, which is primarily due to their high calcium carbonate contents and lack of cementation.  相似文献   

13.
Abstract

It is clear from morphology alone that distinctly different dynamic and sedimentary processes can be expected to be associated with the Greater Antilles Outer Ridge relative to those of the adjacent Nares Abyssal Plain. This difference is further substantiated by seismic reflection data which show the ridge to be a very large prism of acoustically transparent sediment in contrast to the stratified deposits of the abyssal plain. An examination of the geotechnical properties of the near‐surface (0 to 2.4m) deposits of the two areas also reveals distinct differences in their sedimentological characteristics. The outer ridge sediments, of more or less homogenous clay‐size material, display much higher water contents, porosities, sensitivities, plasticity, and organic carbon contents in contrast to the abyssal plain deposits which are much less homogenous owing to the presence of turbidite sequences. The turbidites themselves are uniquely contrasted to the other abyssal plain sediments by their higher silt content, wet bulk density, shear strength, and sensitivity.  相似文献   

14.
High‐resolution vertical and lateral gradients and variations in sediment mass physical properties were derived from measurements in box cores, on the scale of millimeters, tens of centimeters, and kilometers from typical, relatively broad areas of the northern California continental slope in the Cape Mendocino area at water depths from 380 to 940 m. Such data are important as a control on comparisons of different sediment suites, as well as providing limits for realistic flux calculations of dissolved inorganic and biochemical species and pollutants. The sediments studied have relatively constant organic carbon contents (OC ? 1.75 wt%) and bulk mineralogy. They range from silty sands (～45% sand, 40% silt) to clayey silts (～63% silt, ～35% clay) and are extensively bioturbated. Physical property variations between subcores (～25 to 35 cm in length), taken from the same box core, increase with increasing clay content. For coarse‐grained sediments, mean down‐core differences in physical property values between related subcores are small, averaging 3.6% for water content, 4% for porosity, 0.026 Mg/m3 for wet bulk density, and 0.1 for void ratio. Subcore variations for fine‐grained sediments are generally significantly larger, averaging 9.8% for water content, 1.52% for porosity, 0.027 Mg/m3 for wet bulk density, and 0.3 for void ratio (box core 125). Millimeter variations of physical properties from horizontal 12‐cm‐long subcores indicate a maximum range of lateral variation of 18.2% for water content, 8% for porosity, 0.14 Mg/m3 for wet bulk density, and ～ 0.6 for void ratio.  相似文献   

15.
Abstract

Vertical variations of geotechnical properties in the uppermost sediment layers characterize the main sedimentary processes acting on the construction and destruction of progressive‐type continental slopes. In the Gulf of Lions, the original thicknesses and distribution of the uppermost sedimentary layers of the continental slope and rise, which consist of Holocene muds overlying Pleistocene muds, have been greatly modified by erosion and several kinds of slope failure processes. Each process is typified through sets of geotechnical properties measured in the eroded or slumped sections and in the associated sediment accumulations.

In slump scars, the water‐rich Holocene muds lie on fine, overconsolidated, Pleistocene muds with high plasticity and low shear strength. In bottom current‐eroded slopes, where modern sedimentation is extremely reduced, the Pleistocene muds frequently outcrop and may sometimes be overlain by a very thin layer of Holocene muds. The Pleistocene muds of eroded slopes are overconsolidated and more silty and less plastic than the Pleistocene muds from slopes affected by slope failure, their shear strength being 10 times greater.

Deposits at the toe of slumps are very often formed by several superposed three‐layer units (triplets of interstratified Holocene, transitional, and Pleistocene layers) issued from retrogressive slumping occurring in the slump scars above their head area. The main body of each layer is then relatively undisturbed, showing the usual burial geotechnical gradients due to overburden pressure (i.e., decrease of water content and increase of unit weight and shear strength). At the toe of bottom current‐eroded slopes, a thick and homogeneous layer of Holocene muds overlies the Pleistocene muds; this Holocene layer has unappreciable burial depth gradient of its geotechnical parameters because of a high rate of modem and continuous deposition.  相似文献   

16.
The first-generation University of Illinois gamma-ray transmission densitometer, designed for the in situ measurement of sediment bulk density, was modified by incorporating in the detector probe (1) an Americium-241 alpha particle pulser and an anti-walk gain stabilization control to maintain better temperature stability and (2) a small power supply and a IC preamplifier to eliminate the need for a high-voltage coaxial cable between the detector and external signal conditioning electronics package. This second-generation Lehigh University system has been successfully deployed since 1971 in routine use from ships and submersibles in the Atlantic and Pacific Oceans and the Gulf of Mexico. Results are presented of system operations to (1) measure bulk density over the range of 1.2–1.8 Mg/m3 in the Hudson Canyon, (2) penetrate 1.9 m into the seafloor in the San Diego trough and, (3) be lowered to a water depth of 3.6 km in the Gulf of Mexico.  相似文献   

17.
Abstract

A differential piezometer was used to monitor excess pore pressure in the soft clayey seafloor sediments of Block 28, South Pass, Mississippi delta, from September 1975 to March 1976. An ambient excess pore pressure of about 32 kPa was measured at a depth of 6.4 m below the mudline in a water depth of 19 m. Storm‐wave‐generated cyclic fluctuations of ± 4 kPa about the ambient were measured during Hurricane Eloise. Irregular, long‐period, small‐amplitude fluctuations in excess pore pressures persisted for 4 days following the storm. An effective stress analysis was made by using excess pore pressures; in situ field vane‐shear strength, t fv, measurements; and laboratory wet unit weights measured by Lehigh and NOAA. The effective stress of the SEA‐SWAB site soil was calculated to be zero to a depth of about 6 m, below which it increased to 3.5 kPa at a depth of 15 m. Values of c´ = 4.6 kPa, ?´ = 56°, and T FV/σvo(c/p) =0.1–0.2 were calculated, and it was concluded that these data do not represent the in situ condition of the soil because of the probability that the measured soil properties were affected by the presence of gas. However, it is clear that the soil is significantly underconsolidated.  相似文献   

18.
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19.
Abstract

The continental slope off the coast of Israel is riddled with numerous large slump scars at depths greater than 400 m. Recent scar slumps are situated in the steepest central portions of the continental slope (400–450 m depth, α=6°), frequently disfiguring older slump scars in its lower portions. The slumping materials were probably largely transported downslope in the form of density currents, and occasionally by sliding of large sediment chunks. Upslope retrogressive slumping phases progressively disfigure the shape of the slump scars until they totally disappear, causing net reduction of the thickness of the sedimentary column. To provide a basis for the quantitative analysis of slumping, laboratory vane tests, triaxial consolidated, undrained compression tests with pore‐pressure measurements, drained direct shear tests, and consolidation tests were performed oh undisturbed samples. Because the sediments consist of normally consolidated silty clays, the geotechnical properties measured on the core samples can be readily extrapolated for greater depths, assuming the sediments are homogeneous. Angles of internal friction measured by direct shearing under drained conditions are ?d =24°‐25°, designating the maximum possible angle of a stable infinite slope. These angles are appreciably higher than the steepest slopes in the investigated area, and a drained slumping mechanism is therefore considered unlikely. The slopes of the slump scar walls are about 20°; therefore, in the absence of active erosional, sedimentological, or tectonic agents, these walls have long‐term stability (drained shear). Undrained shear failure resulting in slope instability may be attributable to rapid changes in slope geometry (undercutting or oversteepening of the slope), fluctuations in pore pressure, or accelerations associated with earthquakes. Undrained shear‐strength parameters were determined by both laboratory consolidated‐un‐drained triaxial tests and by miniature vane shear tests. The angles of internal friction that were measured are ?cu =15°‐17°, and the cu/po values range between 0.22 and 0.75. An analysis of the force equilibrium within the sediments leads to the conclusion that horizontal earthquake‐induced accelerations, as little as 5–6% of gravity, are sufficient to cause slope failure in the steepest slope zone (400–450 m depth, α = 6°, cu /po =0.25). Collapse resulting from liquefaction is unlikely, as the sediments are normally consolidated silty clays with intermediate sensitivity, St =2–4.

The existence of slump scars in the lower portion of the continental slope, characterized by gentle slopes (α=1°‐3°) and sediments with high shear strength (c u /p o=0.30–0.50) is attributed to large horizontal accelerations(k=12–16% of gravity). Owing to the wide range of geotechnical properties of the sediments (cu /po = 0.20–0.75) and the inclination of the continental slope (α=1°‐6°), the same earthquake may generate a wide range of horizontal accelerations in different portions of the continental slope, and slumping may occur wherever the stability equilibrium is disrupted.  相似文献   

20.
Accurate measurements of bulk density are important in the construction of sedimentation-compression curves of water-saturated, fine-grained cohesive sediments for consolidation studies. The bulk density of five cores was measured nondestructively on a laboratory nuclear transmission densitometer and the water content, expressed as void ratio, was derived from an empirical relationship between bulk density and water content. Bulk density and water content measured destructively was determined every 10 cm for a comparison with the nuclear data. Agreement between nuclear and gravimetric methods was within ± 0.02 g/cm3 for bulk density ranging from 1.2 to 1.4 g/cm3. Sedimentation-compression curves made using both methods were in generally good agreement.In the Wilkinson Basin, void ratio gradually decreases with increasing effective overburden pressure. This indicates that consolidation by dewatering the surficial sediments is normal and that the environmental conditions during deposition remained relatively constant.  相似文献   

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