geotechnical properties of two deep-sea marine soils from the labrador sea

Geotechnical properties of two deep-sea marine soils from the Labrador Sea

PIERRE MORIN Faculty of Engineering and Applied Sciences, Memorial University of Newfoundland, St. John's, Nfld., Canada AIB 3x5

A N D

C. ROY DAWE' C-CORE, Memorial University of Newfoundland, St. John's, Nfld., Canada AIB 3x5

Received September 22, 1986

Accepted June 22, 1987

Cores from two deep-sea sites from the Labrador Sea have been extensively studied using various analyses. X-ray techniques provided a continuous profile of bulk densities and an accurate description of the bedding. The soil structure was analyzed by scanning electron microscopy. Detailed identification, laboratory shear strength, and compressibility tests have been used to determine the geotechnical properties of the soils, both in their intact and remolded states. Geological data and fabric observations are compared with geotechnical properties in order to explain the observed overconsolidation of the surfi- cia1 sediments. Characterization of the seabed materials is further improved by using existing empirical correlations with soils of onshore and offshore origin.

Key words: marine sediments, deep-sea investigation, overconsolidation, laboratory testing, offshore.

Des carottes provenant de deux sites de forage en eaux profondes dans la mer du Labrador ont CtC CtudiCes en detail B l'aide de diverses techniques. Les radiographies par rayons-X ont fourni un profil continu de densite et une description precise de la stratification. La structure des sols a CtC analysCe B I'aide du microscope Clectronique B balayage. Des essais dttaillCs d'identification, de determination de la rCsistance au cisaillement au laboratoire et de compressibilit6 ont servi ?i dCterminer les pro- priCtCs gCotechniques des sols, intacts et remaniCs. Les donnCes gCologiques et les observations faites sur la structure ont Cti: comparkes aux propriCtCs gkotechniques pour expliquer la surconsolidation observCe dans les sediments de surface. Les conk- lations empiriques avec des sols d'origine terrestre ou marine ont permis de mieux cerner les caractCristiques de ces matCriaux.

Mots elks : sediments marins, reconnaissance en eaux profondes, surconsolidation, essais de laboratoire, off-shore.

Can. Geotech. J. 24, 536-548 (1987)

Introduction

Geotechnical investigations in the area of the Labrador Sea have been very scarce and generally limited to the continental shelves, where there is oil and gas exploration activity. In 1984 the Geological Survey of Canada conducted a sea-bottom investigation at planned Ocean Drilling Program sites (ODP leg 105). The survey included collecting sediment cores from deep-sea sites in the Labrador Sea. Laboratory tests were performed on these cores with the intention to widen the available data base for deep-sea sediments.

The tests performed on the sediment cores comprised quali- tative or semiqualitative observations (X-ray radiographs, microscopy, X-ray diffractometry, visual description) as well as quantitative measurements of geotechnical parameters like water content, consistency limits, specific surface, carbonate content, undrained shear strength, and compressibility parameters. The results are discussed in light of the available regional geological data, and the geotechnical properties of both sites are compared with existing correlations for offshore and onshore deposits.

Presentation of the sites

The site referred to as site 02 in this paper (Fig. 1) is located on the northwest Labrador slope, east of the Saglek Bank (58"301N, 57'56'W). The water depth is 2666 m at the location of the borehole with an average sea-bottom slope of 0.7% on the west side of the North Atlantic Middle Ocean Channel. The topography of the seabed is ondulated (water depth is

'Present address: Department of Health, Government of New- foundland, St. John's, Nfld., Canada A1C 5T7.

ranging from 2650 to 3030 m) and the surface of the site may be affected by deep currents (Chough et al. 1985).

Site 05 is located southeast of the Greenland Rise and northwest of the Eirik Ridge (58"04'N, 48'24'W). The water depth is 3326 m at borehole location and the average slope is 0.5 %.

Data collected in the Labrador Sea are limited and consist mainly of geological description and analysis (Fillon and Duplessy 1980). The surficial sediments were deposited during and since the Quaternary period (Pleistocene). The Labrador Sea has known successive glacial and interglacial cycles during the Wisconsinan and Holocene periods. Aksu and Mudie (1985) indicate different rates of sediment deposition during interglacial and glacial periods (22 mm/1000 years and 41 mm/1000 years respectively). These authors also analyzed cores from the northwest Labrador Sea and correlated the paleoflora and paleofauna with the oxygen isotope time scale proposed by Shackleton and Opdyke (1973). According to this model, sea-ice formation occurred rapidly during the early stages of the glacial epochs and the ice sheet was maintained by the northward transportation of moisture from the northern Atlantic Ocean. The glacial cycles are an important factor affecting the deposition process. The bottom and surface current patterns are characterized by their counterclockwise direction and can also greatly influence the sediment transportation, especially on the West Greenland coast.

More recently, a detailed investigation (De Vernal 1986) was canied out on samples collected during the same cruise as for the present study and from previous investigation. De Vernal worked on establishing a reliable stratigraphy based on sedimentology, isotope analysis (vertical distribution of isotopes 180 and 13C in foraminifera), and palynology (vertical distribution of pollen, spores, and dinoflagelate cysts in the

Primed in Canada I LrnprimC au Canada

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MORIN AND DAWE

FIG. 1. Location of sites

sediments). The results of this research will be presented later in this paper.

Method of investigation

Offshore sampling During the summer of 1984 the Atlantic Geoscience Centre

(AGC), Geological Survey of Canada, conducted a geophysi- cal survey and sampling cruise on the CSS-HUDSON (Srivas- tava 1984). A 5.47 m core (referenced as 84-030-022) was collected at site 02 using a free fall gravity piston corer (inside diameter 64 mm), activated by a trigger-weight corer (see, for example, Shepard 1973). A 11.20 m core (referenced as 84-030-05) was similarly obtained at site 05. The use of standard sampling equipment did not allow for keeping the samples pressurized at the in situ hydrostatic pressure. The cores were cut into 1.5 m sections and the ends sealed with wax on board. The samples were brought to the laboratory at Memorial University of Newfoundland, St. John's, New- foundland, and stored vertically at a temperature of 8°C and relative humidity of 85 % . Laboratory testing

Laboratory tests were carried out between January 1985 and September 1985. The cores were first fluoroscoped and then X-rayed at appropriate orientations to identify any sedimentary features and nonuniformities found during fluoroscopy. The

X-ray radiographs were used to select representative sub- samples for classification and mechanical testing. For site 02, radiographs provided a downcore profile of bulk density (Fig. 2) using a technique similar to that described by Wahlgren and Lewis (1977).

A Hitachi S 570 scanning electron microscope (SEM) was used to study the fabric of the sediments at specific depths under magnifications up to 6000. The method of preparation of the samples followed the recommendations of Smart and Tovey (1982). Parallelipedic specimens (3 x 8 x 50 mm) were cut from the sediment cores with a wire saw, deep frozen in liquid nitrogen, desiccated, and fractured by bending along horizontal or vertical planes. The specimens were then mounted on a pedestal and coated with a carbon conductive layer.

Semiquantitative X-ray diffractometry was used to determine the mineralogy of the clay fraction of the sediments.

The relative density of the solids and the grain size distribution have been measured according to ASTM standards D 854 and D 422. The plastic limit has been determined following ASTM standard D 4318. The liquid limit was evaluated using the Casagrande percussion device (ASTM D 4318) and the Swedish fall cone (Bureau de Normalisation du Quebec 1986). Recent investigation by Wasti (1987) shows that the liquid limit obtained with the cone is on average 5% higher than the limit using the Casagrande method. That trend was confirmed

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538 CAN. GEOTECH. J I . VOL. 24, 1987

Bulk D e n s ~ t y F rom D ~ r e c t Labora to ry Measurements

I I

BULK DENSITY (kg/m3)

FIG. 2. Bulk density profile from X-ray radiogmphs.

in the present study as was the validity of the single-point method proposed by the BNQ standard. For clarity, the results presented hereafter refer to the fall-cone determination only.

The total specific surface of the solids was evaluated using the methylene blue test (Lan 1977) and the procedures described by Boust and Prive (1984) for deep-sea sediments. According to these authors the presence of carbonates can be taken into account by dividing the measured specific surface by the percentage of noncarbonate minerals present in the sediment, and organic carbon does not significantly affect the specific surface if its content is less than 4-5 % in weight, which was the case in the present study. The corrected specific surface has been expressed in terms of metres squared per gram of dry soil.

The carbonate content was determined using a combustion method: the sample was oven-dried at 1 10°C and the corresponding mass taken as a reference. The temperature was raised to 500°C and then to 800°C and the carbonate content was determined from the amount of C02 burned between these two temperatures.

The undrained shear strength of intact samples was evaluated using the Swedish fall cone. The sensitivity was determined from the shear strength measured by dropping the appropriate cone on the remolded material (see, for example, Leroueil et al. 1985). A laboratory shear vane was also used; it had a height of 12.5 mm and a diameter of 12.5 mm and rotated at a rate of 10°/min.

Direct shear box tests were performed on 50 mm diameter circular samples. The shear surface corresponded to a horizontal direction in situ and the rate of displacement allowed for full dissipation of the excess pore pressure (0.8 x mm/s). Large displacements were obtained by reversing the direction of shear after initial failure.

Consolidated undrained triaxial tests were carried out in compression and extension. The samples were trimmed to 38

TABLE 1. Mineralogy of the clay fraction (site 02)

Sample Content (%) depth

(m) (1) (2) (3) (4) (5) (6) (7) (8) (9)

0.72 2.8 6.1 9.6 8.4 13.4 44.8 0.5 14.2 0.2 2.60 1.8 1.5 9.5 8.7 20.8 44.4 4.3 8.2 0.7 4.47 2.2 1.2 17.6 16.3 9.3 30.8 8.3 13.7 0.6

NOTE: (1) dolomite, (2) hematite-calcite, (3) feldspar, (4) quartz, (5) mix- ed layer clays, (6) illite, (7) kaolinite, (8) chlorite, (9) montmorillonite.

mm diameter by 76 mm height. After back-pressure saturation and isotropic consolidation, the specimen was sheared at a vertical rate of 4.3 x 10-4%/s under undrained conditions, and the pore pressure was recorded.

Standard consolidation tests were carried out on 50 mm diameter by 20 mm high specimens. Drainage was permitted at both ends and the load was increased daily, with a load ratio of 1.5. A small-increment load schedule was used in the early stages of the tests to provide a better definition of the oedometer curve. The preconsolidation pressure was evaluated using the Casagrande classical method. The interpretation, from the plot, of the tangent modulus as a function of the vertical effective pressure (Janbu 1969) and the interpretation using the cumulative stored work (Becker et al. 1987) were also developed in the present study (Dawe and Morin 1986).

The engineering classification of the soil, shown in paren- theses in the following description, refers to the updated ASTM standard D 2487. The visual detailed description fol- lows the recommendations of the Canadian Foundation Engi- neering Manual (1985).

Results from site 02 The position of the borehole 84-030-22 on site 02 is given by

58"30.16'N, 57'56.25'W. The results of the tests performed on the samples from that borehole are summarized in Fig. 3. Organic carbon, carbonate content, and specific surface measurement are given in Table 1, whereas the mineralogical analysis is indicated in Table 2.

The visual description of the core and inspection of the geotechnical profiles from Fig. 3 indicate that the upper part of the seabed can be divided in three main units.

Unit I : from the seabed to 1.6 m depth (CH fat clay) This layer is composed of grey silt and clay of high plasticity

with traces of sand and gravel at the bottom of the layer. The clay fraction is predominantly illite. The natural water content increases with depth and the liquidity index ranges from 1.1 to 1.4. The organic content is less than 5% and the carbonate content is about 14%. The corrected specific surface is typically 66 m2/g. The variability of the uncorrected specific surface is attributed to the grain size and mineralogical composition of the sediments. The vertical effective stress profile of Fig. 3 shows that the preconsolidation pressure indicated by Casagrande plots constitutes a lower envelope of the probable range of values from Janbu or cumulative work plots. Typical oedometer curves are shown in Fig. 4. The overburden pressure is also indicated in Fig. 3, assuming a hydrostatic pore pressure distribution. Unit 1 shows apparent overconsolidation with an overconsolidation ratio (OCR) of about 4.5 at its bottom. Direct shear box tests lead to a value of 5 kPa for the cohesion and an angle of internal friction of 30°C. The

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MORIN AND DAWE 539

plasticity with sand

. Natural water X Laboratory vane test (Intact) I Casagande's plat

FIG. 3. Geotechnical properties at site 02.

TABLE 2. Organic matter, carbonate content, and specific surface (site 02)

Specific surface Hygroscopic water Carbonate Specific surface corrected for

Depth and organic matter content noncorrected carbonate content (m) (%) (%) (m2/g) (m2/g)

undrained shear strength ranges between 5 and 10 kPa within the sediment unit, with a low to medium sensitivity.

SEM analysis (Fig. 5a) shows that the soil at 1.5 m is composed of shells and mineral particles that are generally larger than 1 pm in diameter and are aggregated in 5 - 10 pm clusters. Figure 56 is a lower magnification view of the same specimen showing a linear pattern in the soil fabric, which can possibly be caused by the freezing process.

The profile of bulk densities computed from the optical densities of the X-ray radiographs is shown in Fig. 2. Despite its limitations (Wahlgreen and Lewis 1984), that method shows clearly the great advantage that continuous profiling has over discrete sampling and direct laboratory measurements.

Unit 2: from 1.60 to 3.20 m depth (CL lean clay with sand) This layer is characterized by a drop in the average water

content and consistency limits. The visual inspection shows a variety of sublayers of clayey silt and sandy silt. The stratifica-

tion is quite complex and does not follow a well-defined pattern. The average liquidity index is 0.7 and the presence of coarse grains is reflected by a corresponding low value of the uncorrected specific surface (34 m2/g), whereas the finer materials have values close to those measured in unit 1 (66 - 70 m2/g).

The determination of the preconsolidation pressure is made difficult by the low compressibility of the material and possible sample disturbance. Typical curves are shown in Fig. 4. The undrained shear strength markedly increases in this layer, from 10 to -25 kPa at 3.2 m depth, with low sensitivity. Direct shear box tests indicate an angle of friction of 32" for a cohesion of 5 kPa.

SEM photographs Figs. 5c and 5d show the fabric at 2.1 and 2.65 m respectively. The particle size ranges from 1 to - 100 pm. At 2.1 m depth the soil particles appear to be tightly interlocked with small interparticle voids. The individual particles seem to be fused together and it is difficult to

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540 CAN. GEOTECH. J. VOL. 24, 1987

differentiate between them. This dense appearance correlates VERTICAL EFFECTIVE STRESS (kPo)

well with the low values of the water content and compressibility at this depth.

Unit 3:from 3.2 m to the end of the core (CH fat clay with sand becoming elastic silt with sand)

From 3.2 m depth the water content and consistency limits increase regularly up to 4.5 m. Despite a drop in these values below 4.5 m depth, the liquidity index remains about constant at 0.8. The undrained shear strength is constant with depth and has an average value of 20-25 kPa. The clay exhibits a small amount of preconsolidation (the OCR is ranging between 1.5 and 2.0) and tends toward a normally consolidated material with increasing depth. Local values of the apparent preconsolidation pressure can be high.

Typical horizontal and vertical SEM views of soil sampled in that unit are shown in Figs. 5e and Sf respectively. The pore spaces are relatively small except for the 3 pm wide voids that divide the fabric into 10 pm diameter clusters.

A detailed geological analysis has been carried out (De Vernal 1986) on samples collected at a horizontal distance of 30 km southeast of the borehole presently discussed, under a water depth of 2853 m. The analysis of sediment types, isotopes concentration, paleoflora (pollen, spores), and paleofauna (cysts of dinoflagelates) provides a reliable stratigraphy related to geological epochs (Fig. 6). According to De Vernal, three zones can be defined: -From the sea bottom to 1.0 m depth: extending to an age of 3800 years before present (BP), this zone covers the late Holo- cene epoch, characterized by a sedimentation rate higher than 260 mm/1000 years, with the possibility of a high biogenic productivity. Low concentration of coarse sand and reworked biogenic material indicates a low level of erosion and glacial activity. -From 1.0 to 2.3 m depth: corresponding to 3 800 - 11 200 years BP, this unit is associated with mid- and early Holocene epochs, with a sedimentation rate of 170 mm/1000 years. The sedimentology and palynology show a low level of local ice drafting, biogenic debris having been remolded and transported from remote regions (Arctic and possibly Hudson Bay) by meltwater. -From 2.3 to deeper than 8.0 m depth: the depth 4.35 m has been dated 16 700 BP (late Wisconsinan). The rate of sedimentation is 370 mm/1000 years. The isotope 180 analysis indicates a high contribution of meltwater during that period with a corresponding decrease in the salinity. The high amount of coarse material witnesses an intensive glacial activity and the low concentration of biogenic material is explained by the existence of a continuous ice cover that prevented phytoplanc- tonic activity.

The comparison between the stratigraphy as proposed by De Vernal (1986) for site 02 and the units previously defined on the basis of physical and geotechnical properties is shown in Fig. 6. The water content profile has been chosen as an indi- cator of the basic physical parameters. The correlation between the time scale and the geotechnical units is found satisfactory within the limits of accuracy of both analyses. The overconsolidated material corresponds to the postglacial epoch (Holocene), dominated by the influence of meltwater and sub- sequent increase in biogenic activity. The measured overconsolidation is not likely to be explained by a mechanical event (e.g., removal of overburden or wave loading) because of the deep water. Therefore, aging of the sediment, chemical

FIG. 4. Typical oedometer curves (site 02).

alteration, and changes leading to a partial cementation of grain contacts are possible explanations.

Results from site 05 The location of the borehole 84-030-005 on site 05 is given

by 58'04.28'N and 48"23.601W under 3326 m of water. A summary of the tests performed in the laboratory has been pre- pared in Fig. 7 and Tables 3 and 4. The upper part of the seabed can be divided in three main units.

Unit 1: from the seabed to depth 5.0 m (MH, the soils plot below but close to the A-line)

This yellowish brown to olive grey mottled silty clay has a high average natural water content (100- 110%). The liquid limit increases with depth, from 75 at the surface to 100 at 4.5 m and the liquidity index ranges between 1.2 and 1.5 in this unit.

Table 3 shows that the carbonate and organic carbon contents are low. The corrected specific surface is typically 120 m2/g and the clay content is high (70%).

Typical oedometer curves are shown in Fig. 8. The Casa- grande plot for evaluating the preconsolidation pressure leads to lower values than Janbu or cumulative work estimates. The vertical effective stress profile indicates that the soil is overconsolidated at the top of the core and becomes normally consolidated at 5.0 m.

The undrained shear strength as measured with the Swedish fall cone increases with depth from 3 kPa at 1.6 m to 11 kPa at 5.0 m. The direct shear box tests indicate a cohesion of 5 kPa and an angle of internal friction of 36" for both depths 1.15 and 2.15 m. Stress paths for triaxial tests performed on samples at depth 4.55 m are shown in Fig. 9. They indicate angles of internal friction of 41" in compression and 26" in extension.

A typical microstructure representing the soil at 0.95 m

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MORIN AND DAWE

( a ) S i t e 0 2 - 1 . 5 m ( b ) S i t e 0 2 - 1 . 5 m

( C ) Site 0 2 - 2 . l m ( d ) Site 0 2 - 2 . 6 5 m

( e ) Site 0 2 - 4 . 4 3 m ( f ) Site 0 2 - 4 . 4 3 ~ 1

FIG. 5. SEM photographs (site 02).

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CAN. GEOTECH. J. VOL. 24, I987

"

FIG. 6. Comparison between geological and geotechnical units (site 02).

VERTICAL EFFECTIVE STRESS (kPa)

end of core

0 Fall-cone tests (Intact)

I Casagandei plot

FIG. 7. Geotechnical properties at site 05.

below the seabed is shown in Fig. 10a. The predominantly platy soil particles range in size from about 1 pm upwards and appear to be clustered into 10 pm randomly packed aggregates.

The soil at 1.5 m depth (Fig. lob) is composed of angular agglomerates made of shells and mineral particles. Narrow contact bridges interconnect these aggregates and large voids are seen, indicating potential high compressibility.

A stereo pair (Figs. 10c and 10d) at 2.88 m depth shows a particular chain structure that appears to be unaffected by the inclusion of denser soil nodules (Fig. 10e). That linear matrix pattern is not likely to be caused by the method of preparation:

stereo views as well as observations on vertical fractured samples (Dawe and Morin 1986) indicate that individual shells follow that pattern.

Unit 2: from 5.0 to 9.5 m depth (CH, all samples plot above A-line)

The soil is a homogeneous soft grey silt and clay with traces of sand. The water content is higher than in the overlying material and ranges from 110 to 130%. The liquidity index is about constant and equal to 1.4. The carbonate content is slightly higher than in unit 1 but the specific surface is very

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MOMN AND DAWE 543

TABLE 3. Mineralogy of the clay fraction (site 05)

Sample Content (%) depth

(m) (1) (2) (3) (4) (5) (6) (7) (8) (9)

NOTE: (1) dolomite, (2) hematite-calcite, (3) feldspar, (4) quartz, (5) mix- ed layer clays, (6) illite, (7) kaolinite, (8) chlorite, (9) montmorillonite.

comparable (Table 3). The soil is normally consolidated, with a low undrained

shear strength (less than 5 Wa). These low values can be associated with high water contents and difficulty in obtaining undisturbed specimen.

Direct shear box tests at depth 8.0 m indicate an angle of internal friction of 36" and a cohesion of 5 Wa. Triaxial stress paths in compression and in extension (9.3 m depth) are shown in Fig. 11. Compression tests indicate an internal angle of friction of 36", whereas extension tests indicate 28". The axial strains are also shown in Fig. 11.

Figure 12a shows a typical fabric of the silty clay at 8.5 m depth and is a view looking into the open pore tubes similar to those observed at 2.88 m (Figs. 10c and 10d). At 8.9 m depth Figs. 126 and 12c are typical examples of a structure that has been observed at several depths (8.6 and 8.9 m) where nodules of 200 pm diameter are embedded in a more loosely oriented matrix. Evidence for the orientation is shown in Fig. 12d where shells and debris are disposed along linear clusters. A possible explanation of that structure is that the nodules are rip-up clast worn round during transportation and further deposition.

Unit 3: from 9.5 m to the end of the core (CH, all samples plot on or above A-line)

The natural water content drops to 80% at 9.5 m depth and increases with depth to 90% at 11 m. The liquidity index is close to 1.0. The soil is normally consolidated and the undrained shear strength increases with depth. This 2 m core length appears to be the top of a uniform layer of normally consolidated material at its liquid limit.

Fillon and Duplessy (1980) analyzed samples from borehole HU-75-37, located 120 krn north of the present sampling location under 3208 m of sea water. Additional work has been carried out by De Vernal (1986), using palynology techniques. Despite the relatively great distance between both sampling sites, it is interesting to compare the proposed geological stratigraphy with the geotechnical units previously defined (Fig. 12). De Vernal defines three main vertical zones: -From the surface to 2.85 m: the bottom of that unit is dated approximately 10 000 years BP and this zone covers the Holo- cene epoch, with a high phyloplanctonic activity and a sedimentation rate of about 400 mm1100 years. The bottom of the unit has been associated with an increase of glacial activity (melting) due to the interrelated influence of the north Atlantic water mass and the Arctic Greenland Current. -From 2.85 to 4.30 m: the low concentration in biogenic materials (dinocysts or microforaminifera, for example) and the presence of coarse sand at the top of the unit indicate important ice drafting and a continuous ice cover along the

VERTICAL EFFECTIVE STRESS ( k P a )

I 10 100 5 0 0

FIG. 8. Typical oedometer curves (site 05).

4 0 1 1 1 1 1 1 1 1 1 1 1 1 1

FIG. 9. Stress paths for CIU triaxial tests (site 05, depth 4.55 m).

Greenland coast, associated with the last glaciation (rate of sedimentation of about 80 mml1000 years). -From 4.30 to 8.20 m: this zone corresponds to the late and mid-Wisconsinan epoch and witnesses variable ice conditions. Ice erosion, ice drafting, and transportation by meltwater characterize the deposition mode.

Figure 13 indicates that unit 1 previously described in the geotechnical analysis can be associated with the Holocene epoch. The base of the unit corresponds to the last maximum glaciation and to the transition between over- and normally consolidated sediments. However, geological analysis does

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( a ) Site 0 5 - 0 . 9 5 ~ 1 ( b ) Site 05 -1.5m

( C ) S i t e 0 5 - 2 . 8 8 m ( d ) Site 0 5 -2 .88rn

. VOIDS NODULE

GRAINS OR DEBRIS

(e 1

FIG. 10. SEM p h o t o g r a p h s (site 05).

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TABLE 4. Organic matter, carbonate content, and specific surface (site 05)

Specific surface Hygroscopic water Carbonate Specific surface corrected for

Depth and organic matter content noncorrected carbonate content (m) (%I (%I (m2/g) (m2/gI

0.07 83 1.40 5.48 6.56 96 103 2.10 5.56 6.84 116 124 2.98 107 3.67 5.95 7.14 4.10 4.35 6.74 124 133 5.06 5.16 6.39 73 78 6.07 100 6.27 7.50 9.07 132 145 7.95 5.13 8.27 102 109 8.70 4.89 8.60 101 109 9.61 4.57 6.82

l l I l I 1 I I ~ l l

Site 05 : depth 9.50m ,,,,+'= 36O

1 0 0 b l I 1 I I ' l l 10 20 30 40 50

(ui +u;)/2 ( k P o )

FIG. 11. Stress paths for CIU triaxial tests (site 05, depth 9.50 m).

not provide sufficient information to adequately discuss the transition between units 2 and 3. De Vernal (1986) reports that the temperate climatic conditions from the north Atlantic water mass interact with the arctic influences and there is therefore no indication of past presence of seabed permafrost in these regions, as the high water contents could suggest. The apparent overconsolidation of the upper sediments (unit 1) is related to the mode of deposition (salinity, temperature, transportation process) that characterized the late Wisconsinan and the Holo- cene epochs. However, the microfabric analysis does not provide definite evidence of strong cementation for site 05.

General correlations General correlations are of interest to improve the data base

and empirically relate physical and engineering properties of sediments.

The plasticity chart (Fig. 14) indicates that most of the materials from both sites fall on or above the A-line. As indicated before, the clay minerals are predominantly illite and a useful comparison can be made between the specific surface of the solid particles and the liquid limit (Fig. 15). The uncorrected specific surface has been plotted to allow consistent compari-

son with previous investigators (Locat et al. 1984). Agreement is generally good but low plasticity sediments fall below the line proposed by these authors. The salinity of the pore water can possibly affect these results. Boust and Prive (1984) show that the specific surface measurements are little affected by the salinity but there is no general agreement on the influence of the salinity on the liquid limit: Torrance (1975) and Chasse- fiere and Monaco (1982) indicate opposite trends for sensitive clays of low salinity and marine sediments respectively. More data are therefore required for low plasticity soils.

The compressibility properties of these deep-sea sediments are shown in Figs. 16 and 17. Figure 16 compares the results of the present investigation with data given by Lambe and Whitman (1979) as a plot of CJ(1 + eo) versus e,. The sediments from site 05 are less compressible than the average shown by them. The difference is-possibly due to the disturbance of the specimen during the laboratory preparation. Figure 17 shows the compression index as a function of the natural void ratio and the sensitivity as proposed by Leroueil et al. (1983). A very good agreement is found in this case.

Finally, the relationship between the liquidity index and the undrained shear strength of the sediments after remolding is shown in Fig. 18. The results of the present study fall between the relations proposed by Leroueil et al. (1983) and Wroth and Wood (1978).

Discussion and conclusions The data presented in this paper show that the sediments

constituting the late Quaternary deposits in the Labrador Sea have geotechnical properties (plasticity, compressibility, shear strength) very comparable to those of other onshore and offshore sediments. Microstructure features observed in the core sample sediments are helpful in understanding possible modes of deposition. The influence of these features is likely to be more important in situ, where possible anisotropy is not hidden by the sampling process.

There is evidence on both sites of an overconsolidation of the upper sediments. The depth of the transition to normally consolidated soils depends on the local conditions and rate of deposition. Such a phenomenon has been clearly evidenced by various authors for deep-sea deposits (Richards 1976; Silva and Jordan 1984; Christian and Morgenstem 1986). According to Richards (1984), most deep-sea deposits also exhibit under- consolidation below a certain depth, due to an excess in pore-

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CAN. GEOTECH. .I. VOL. 24, 1987

( a ) S i t e 0 5 - 8 . 5 m ( b ) Site 0 5 - 8 . 9 m

( C ) Si te 0 5 - 8 . 9 m ( d ) Site 0 5 - 8 . 6 m FIG. 12. SEM photographs (site 05).

FIG. 13. Comparison between geological and geotechnical units (site 05).

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MORIN AND DAWE 547

0 SlTE 0 2

SlTE 05

3 0

90

80-

70-

60-

a - X 50-

n 2

40- > t F 30-

4 a 20-

lo-

NATURAL VOID RATIO eo

FIG. 17. Compressibility properties.

FIG. 14. Plasticity chart.

LIQUID LIMIT IWLl

CL

5 0 100 150 180

SPECIFIC SURFACE (rn2/g)

FIG. 15. Comparison between specific surface and liquid limit.

0

C? ,& A< M L

0 SITE 0 2 SITE 05

A-Lane Ip -0 731WL-201

0 SITE 02

SITE 05

0.3 0.5 0.7 1 ; i i ; UNDRAINED SHEAR STRENGTH OF REMOLDED SOIL (FALL-CONE TEST1 ( k P a l

o , , , , o 10 eo 30 40 50 60 70 80 90 IW ~ i o izo I:

FIG. 18. Undrained shear strength of remolded soil against liquidity index.

activity during the Holocene epoch. That period of time corresponds to cyclic changes in salinity and temperature of the ocean water. Biogenic activity, erosion, and sediment transportation are also important. Richards (1984) proposed various - explanations for the overconsolidation of the top soils: the presence of invertebrate mucous is an attractive explanation for

- this phenomenon and also supports high water contents, as o SITE 02 observed on site 05 for example. However, this interpretation

SITE05 - is difficult to demonstrate and other factors like interparticle bonding. cementation, or vreferred cluster orientation are to be . A

called upon to account for the apparent overconsolidation. I 4 0 60 80 100 10 200 300 400 These phenomena require further attention and research, as NATURAL WATER CONTENT(%)

FIG. 16. Compressibility properties. they are important for-understanding basic properties of deep- sea sediments.

water pressure. As pore pressures have not been recorded at sites 02 and 05, it is impossible to validate this statement in the Acknowledgments present case. The shape of the oedometer curves also makes The cores from both sites were provided by the Atlantic the determination of the preconsolidation pressure difficult in Geoscience Centre, Geological Survey of Canada, Bedford many cases. Institute of Oceanography, Dartmouth, Nova Scotia, Canada.

Geological observations indicate that, for both sites, the C-CORE (Centre for Cold Ocean Resources Engineering) and overconsolidated sediment units are associated with glaciation the Faculty of Engineering and Applied Sciences, Memorial

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548 CAN. GEOTECH. J. VOL. 24, 1987

University of Newfoundland, are also acknowledged for their financial and technical support.

AKSU, A. E., and MUDIE, P. J. 1985. Late Quaternary stratigraphy and paleoecology of northwest Labrador Sea. Marine micropaleon- tology, 9: 537-557.

BECKER, D. E., CROOKS, J. H. A., BEEN, K., and JEFFERIES, M. G. 1987. Work as a criterion for determining in situ and yield stresses in clays. Canadian Geotechnical Journal, 24, this issue.

BOUST, D., and PRIVE, P. 1984. Mesure des surfaces spCcifiques de sediments marins par la mCthode au bleu de mCthylbne. Bulletin de Liaison des Laboratoires des Ponts et ChaussCes, no. 134: 59-66.

BUREAU DE NORMALISATION DU Q U ~ B E C . 1986. Sols-Dktemination de la limite de liquidit6 ?i l'aide du pCnCtromktre 5 c6ne suCdois et de la limite de plasticitC. BNQ 2501-092. QuCbec, Que.

CANADIAN FOUNDATION ENGINEERING MANUAL. 1985. 2nd ed. Canadian Geotechnical Society, Rexdale, Ont.

CHASSEFIERE, B., and MONACO, A. 1982. On the use of Atterberg limits on marine soils. Marine Geotechnology, S(2): 153- 178.

CHOUGH, S. K., MOSHER, D. C., and SRIVASTAVA, S. P. 1985. Ocean Drilling Program (ODP) site survey (Hudson 84-030) in the Labra- dor Sea. Geological Survey of Canada, Paper 85-10, pp. 33 -41.

CHRISTIAN, H. A., and MORGENSTERN, N. R. 1986. Compressibility and stress history of Holocene sediments in the Canadian Beaufort Sea. Preprints of the 3rd Canadian Conference on Marine Geo- technical Engineering, St. John's, Nfld.

DAWE, R., and MORIN, P. 1986. Laboratory testing performed on soil samples from the 1984 Advanced Ocean Drilling Programme in the Labrador Sea. Ocean Engineering Group, Memorial University of Newfoundland, St. John's, Nfld., Report 108P.

DE VERNAL, A. 1986. Analyses palynologiques et isotopiques de sediments de la mer du Labrador et de la Baie de Baffin : ClCments d'une climatostratigraphie du Quaternaire supCrieur dans 1'Est du Canada. Ph.D. thesis, Department of Geography, UniversitC de MontrCal, Montreal, Que.

FILLON, R. H., and DUPLESSY, J. C. 1980. Labrador Sea bio-, tephro-, oxygen isotopic stratigraphy and Late Quaternary pale- oceanographic trends. Canadian Journal of Earth Sciences, 17: 83 1 - 854.

JANBU, N. 1969. The resistance concept applied to deformations of soils. Proceedings, 7th International Conference on Soil Mechanics and Foundation Engineering, Mexico, Vol. 1, pp. 191 - 196.

LAMBE, T. W., and WHITMAN, R. V. 1979. Soil mechanics. Wiley, New York, NY.

LAN, T. N. 1977. Un nouvel essai d'identification des sols : l'essai au bleu de mCthylbne. Bulletin de Liaison des Laboratoires des Ponts

et ChaussCes, no. 88: 136- 137. LEROUEIL, S., TAVENAS, F., and LE BIHAN, J. P. 1983. PropriCtCs

caracttristiques des argiles de l'est du Canada. Canadian Geo- technical Journal, 20: 68 1 - 705.

LEROUEIL, S., MAGNAN, J. P., and TAVENAS, F. 1985. Remblais sur argiles molles. Laboratoire Central des Ponts et ChaussCes et Tech- nique et Documentation Lavoisier, Paris.

LOCAT, J. , LEFEBVRE, G., and BALLIVY, G. 1984. Mineralogy, chemistry, and physical properties interrelationships of some sensitive clays from Eastern Canada. Canadian Geotechnical Journal, 21: 530-540.

RICHARDS, A. F. 1976. Marine geotechnics of the Oslo-fjorden region. In Contributions to soil mechanics Laurits Bjermm memorial volume. Norwegian Geotechnical Institute, Oslo, Norway, pp. 41 -64.

1984. Modelling and the consolidation of marine soils. In Seabed mechanics. International Union of Theoretical and Applied Mechanics, pp. 3-8.

SHACKLETON, N. J., and OPDYKE, N. D. 1973. Oxygen isotope and palaeomagnetic stratigraphy of Equatorial Pacific core V28-238: oxygen isotope temperatures and ice volumes on a lo5 year and lo6 year scale. Quaternary Research (New York), 3: 39-55.

SHEPARD, F. D. 1973. Submarine geology. Harper and Row, New York, NY.

SILVA, A. J. , and JORDAN, S. A. 1984. Consolidation properties and stress history of some deep sea sediments. In Seabed mechanics. International Union of Theoretical and Applied Mechanics, pp. 25-39.

SMART, P., and TOVEY, N. K. 1982. Electron microscopy of soils and sediments: techniques. Clarendon Press, Oxford, United Kingdom.

SRIVASTAVA, S. P. 1984. Report of cruise No 84030 CSS-HUDSON. Atlantic Geoscience Centre, Bedford Institute of Oceanography, Dartmouth, N.S. Internal report.

TORRANCE, J. K. 1975. On the role of chemistry in the development and behavior of the sensitive marine clays of Canada and Scan- dinavia. Canadian Geotechnical Journal, 12: 326-335.

WAHLGREN, R. V., and LEWIS, C. F. 1977. Estimation of bulk density and water content of Beaufort Sea sediment cores using radiographs. In Report of activities, part A. Geological Survey of Canada, Paper 77-1A.

WASTI, Y. 1987. Liquid and plastic limits as determined from the fall cone and the Casagrande methods. Geotechnical Testing Journal, lO(1): 26-30.

WROTH, C. P., and WOOD, D. M. 1978. The correlation of index properties with some basic engineering properties of soils. Cana- dian Geotechnical Journal, 15: 137 - 145.

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geotechnical properties of two deep-sea marine soils from the labrador sea

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