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Geotechnical and sedimentological investigations of deep-sea sediments from a manganese nodule field of the Peru Basin
Deep-Sea Research II 48 (2001) 3593–3608
Geotechnical and sedimentological investigations of deep-sea sediments from a manganese nodule field of the Peru Basin Bernd Grupe*, Hermann J. Becker, Horst U. Oebius Technische Universitat D-10623 Berlin, Germany . Berlin, Sekr. VWS-FB6, Muller-Breslau-Strae/Schleuseninsel, .
Abstract Deep-sea mining of manganese nodules will significantly modify the surface layer of deep sea. Surface sediments are mechanically disturbed, and a cloud of sediment particles suspended and redeposited. Assessment of sedimentological and soil mechanical characteristics of undisturbed top layer sediments from a manganese nodule field of the Peru Basin are necessary to provide baseline data for soil mechanical estimations and for the physical and numerical modelling of environmental impacts of future ocean mining activities. This will assist the International Seabed Authority to make decisions on how to formulate the mining code, and will increase our understanding of sedimentation processes in the Peru Basin. The deployment of a new sampling device (maxicorer) guaranteed undisturbed sediment samples in which soil mechanics-related parameters were measured including sediment density, water content, shear strength, and penetration force. Data evaluation show a close relationship between the soil mechanical parameters and various sedimentological properties such as grain-size distribution, mineral composition and micro-structure. From the results it can be concluded that in the worst case a deep-sea nodule mining vehicle would sink about 15–20 cm into the sea bed. r 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction In the marine environment human activities such as the exploitation of deep-sea resources as well as natural events like earthquakes, volcanic eruptions or downslope failure are inevitably connected with large-area disturbances of the sea floor. They can change the geological and geochemical boundary conditions and also cause biological alterations.
*Corresponding author. Tel.: +49-0-30-31184-260; fax: +49-0-30-31184-200. E-mail address: [email protected] (B. Grupe). 0967-0645/01/$ -see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 1 ) 0 0 0 5 8 - 3
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The most severe anthropogenic impact will result from marine mining, which involves two main processes: the upper layer of the deep-sea sediment will be partly or completely compressed, squeezed, sheared or eroded by the collector and the miner, and the biogeochemical milieu will be altered by erosion, suspension of sediment (Oebius et al., 2001), the release of chemically active substances (Koschinsky et al., 2001), and resettlement of the sediment particles (Becker et al., 2001). To assess and evaluate these impacts reliable sedimentological and soil mechanical baseline data of the undisturbed benthic milieu are essential. Such data also are required by the engineer to support the design and construction of stable and environmentally acceptable sub-sea structures and bottom vehicles (Oebius, 1998). The sub-project ‘‘Mechanical Parameters of Sediments in Deep-Sea Areas with great Manganese Nodule Abundance (MEPARSED)’’ was undertaken to provide these data, supporting some of the other studies compiled in this volume (Becker et al., 2001; Jankowski and Zielke, 2001; Oebius et al., 2001). As there is a lack of suitable equipment, methodology and standards for the assessment, evaluation and interpretation of geotechnical information concerning deep-sea sediments, special methods and instruments had to be developed (Grupe et al., 1994). This refers particularly to the recovery of large-volume, undisturbed sediment samples from the deep sea floor. Therefore, in advance of the Sonne 106 cruise a new sediment sampling device, the maxicorer, was developed by the Technical University of Berlin (Gerber et al., 1996).
2. Experimental areas Sediment samples from selected areas of the Peru Basin showing changing morphological conditions were used for the experiments. (See ‘‘Use and Protection of the Deep-Sea’’, introduction to this volume.) The sedimentological and soil mechanical baseline studies were accomplished in samples from Area 2 (DISCOL Experimental Area/DEA) (Schriever et al., 1996), Area 5 (SEDIPERU area) (Wiedicke and Weber, 1996; Stummeyer and Marchig, 2001), and for special comparison of the water content of sediments additionally in samples from Area 6 in the SEDIPERU area. A detailed description of the geological and biogeochemical milieu is given in the above-mentioned papers and in Koschinsky et al. (2001). The sediment characteristics of the Peru Basin are strongly influenced by the carbonate compensation depth (CCD), which lies between 4200 and 4350 m. Calcareous ooze occurs down to about 4000 m, calcareous–siliceous mud between 4000 and 4200 m, and siliceous mud deeper than about 4200 m. The top layer (5–14 cm) is composed of homogeneous dark brown sediments, which are underlain by greyish- to yellowish-brown and strongly bioturbated sediments. Depending on the water depth, the ratio of siliceous and calcareous tests changes, as does the amount of clay minerals. To investigate the influence of the different compositions of the sediment on the water content, sediment samples from different water depths and different locations (SEDIPERU area, DISCOL area/DEA) were compared with respect to horizontal and lateral variations.
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3. Background and methods 3.1. Sediment samples Geotechnical investigations of shear strength measurements, compression tests, penetration tests, structural analyses and trafficability require mechanically undisturbed sediments. Therefore, the maxicorer, combining the advantages of both the box corer and the multicorer, needed to be developed beforehand (Gerber et al., 1996) and its suitability for the assessment of soil mechanical parameters proven during the SO 106 cruise. It was deployed successfully 15 times and provided large-volume samples that allowed, for the first time, soil mechanical investigations directly in undisturbed deep-sea sediments. The surface and the semi-liquid top layer remained unaffected by the sampling procedure (some samples were recovered with nodules still embedded in undisturbed state, see CD ROM, Annex 2.7, Photo 1). Furthermore, the samples were covered with in situ deep-sea water at a temperature of about 3–41C, allowing biological and geochemical studies. Consequently, the maxicorer samples served as a reference and calibration standard for geotechnical measurements from box corer and multicorer samples which also were collected and used in the investigations.
3.2. Grain-size analyses by laser technique The grain-size distribution of undisturbed deep-sea sediment is necessary to characterise the sedimentological composition and possible genesis, and also to serve as a baseline indicator for the evaluation of the effects of anthropogenic impacts (Oebius et al., 2001; Becker et al., 2001). The grain-size distribution was determined on-board the research vessel and in the on-shore laboratory by use of a Fritsch laser particle analyser, type Analysette 22. Sediment samples were desalted by washing with purified water and treating them for 2 min in an ultrasonic bath. Sodium pyrophosphate decahydrate in a concentration of 0.5 g/1000 cm3 was used as a disperser to avoid coagulation of the fine particles in the treated suspension.
3.3. Semi-quantitative analysis of the clay fraction Clay minerals dominate the soil mechanical characteristics and behaviour of cohesive deep-sea sediments. Their small size, flaky shape and electrical charge are responsible for different particle structures of the sediments, which in turn influence its water content, plasticity, and shear strength. One has to distinguish between clay minerals that do not hydrate, such as kaolinite and illite, and those that do, especially montmorillonite. The determination of the clay mineral content was accomplished by a PHILIPS X-ray diffractometer, type PW 1710/1729, with a Cu anode, 40 kV/30 mA, and monochromatic Ka radiation. The pretreatment of the samples followed standard procedures. The diffractograms were semi-quantitatively analysed using the MacDiff software programme developed by R. Petschick, University of Frankfurt/Main, Germany.
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3.4. Water content In saturated sediments the water content strongly influences the soil mechanical behaviour as well as the biogeochemical processes. Its assessment is necessary for the determination of soil stability (void ratio, pore water pressure, consolidation), trafficability, particle concentration of suspended sediment clouds (Oebius et al., 2001), and also for preparation of artificial deep-sea sediments for subsequent laboratory tests as a part of the PARTRANS project (Becker et al., 2001). To ensure detection of minor variations, the water content was determined in 1-cm steps downcore. The overlying water of each core or subcore was carefully siphoned off, the core was slowly pressed out of the liner, and sections were sliced in 1-cm intervals down to a core depth of about 30 cm. These samples were preserved (from drying) and packed for transportation to the laboratories of the Technical University Berlin for further analyses. In total, 210 determinations of water content were made.
3.5. Soil mechanical investigations These investigations, in particular shear strength, bearing characteristics, nodule adhesion forces, and sediment structure, were essential to provide baseline data for environmental impact assessments (Becker et al., 2001; Oebius et al., 2001). Since many geotechnical deep-sea sediment characteristics differ from their terrestrial or coastal counterparts, a direct transfer of terrestrial soil mechanical data to deep-sea conditions is not possible (Inderbitzen, 1974). In some cases there are not even comparable standards, methods or appropriate instrumentation. Therefore, for some of the investigations new methodologies had to be developed and adopted.
3.6. Shear strength The shear strength is assumed to be the most significant soil mechanical parameter for the study of water-saturated clayey sediments. Many methods have been applied to measure the shear strength of cohesive sediments in the semi-liquid top layer in the deep-sea (cones, box shear meters, ring shear meters, triaxial tests, vane shear meters). Since these methods cannot be directly compared with each other (Bode, 1991), a method had to be chosen that allows intercalibration with other standard methods, is easy to handle on-board a research vessel, and allows an assessment of shear strengths in small steps (10 mm steps) in liquid pastes. The only instruments that meet all the requirements, are vane shear meters (Lettau, 1978). On-board the RV Sonne all tests were carried out with the HAAKE viscosimeter RV 20 equipped with the M5 measuring system and a vane shear-strength device as sensor. Two vanes of different sizes (d ¼ 10 mm; h ¼ 8:8 mm and d ¼ 20 mm; h ¼ 10 mm) were applied and usually rotated at 301/min, but in some cases the speed was varied to investigate the influence of different shear velocities on the measurement. The measurements generally were conducted in vertical steps of 10 mm in order to obtain a vertical profile, which can be correlated with other sedimentological parameters such as water content and grain size.
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About 1650 measurements were made on sediment from 40 different locations. Together with results from the SO 79 cruise (Grupe et al., 1994), the shear strength measurements allow general statements about the soil mechanical conditions in larger areas of the Peru Basin. 3.7. Plate-penetration tests Plate-penetration tests are used to determine the position of the layer with the necessary bearing capacity to cope with a static (deep-sea structure) or dynamic (miner) load. Such information is needed to evaluate the effective disturbance depth of miners, the trafficability of deep-sea sediments, or the pore water pressure and possible slope instabilities. Circular plates of different sizes (20, 50, 100 cm2) were either loaded by a fixed weight and the time-dependent sinking depth into the smooth clayey and silty sediments was measured, or they were pressed slowly and with constant penetration velocity into the sediment in which case the penetration-dependent force was constantly measured. For these tests a specially constructed multipurpose soil mechanical test instrument was used (see CD ROM, Annex 2.7, Photo 2). Penetration tests were carried out on eight maxicorer and box corer samples. Tests on the maxicorer samples were the first ones to be carried out in undisturbed deep-sea sediments with overlying deep-sea water.
Table 1 Results from the lifting tests of manganese nodules Sample
Water Lifting Total Nodule Sediment Sediment/ Nodule-type according to von depth (m) force (N) weight (N) weight (N) weight (N) nodule (%) Stackelberg and Marchig (1987)
1518KG 4155 185KG
4111
201MX2 4260 215KG
4004
223MX3 4115
20.47 3.06 5.81 4.16 7.46 0.74 3.00 0.88 3.80 3.55 2.54 0.95 0.89 0.79 6.63 2.58 3.41 2.84
9.08 15.40 0.87 1.35 0.93 5.14 0.50 1.07 0.39 2.71 1.38 0.53 0.40 0.82 0.19 1.96 1.28 1.86 1.71
7.95 14.40 0.64 1.15 0.80 3.95 0.39 0.74 0.17 2.20 1.36 0.44 0.20 0.65 0.16 1.52 0.44 0.82 0.71
1.12 1.00 0.23 0.19 0.13 1.19 0.11 0.33 0.21 0.51 0.03 0.09 0.20 0.16 0.03 0.45 0.84 1.04 1.00
14.1 6.9 35.7 16.9 16.7 30.2 27.3 45.3 125.1 23.1 2.0 20.5 98.0 24.7 20.4 29.3 190.7 127.8 139.7
Shape Surface
Size (l, w, h)
S SI SI EI E2 S S E E E2 EI E E E E DI E2 D SI
10.0, 9.5, 9.5 12.0, 10.0, 11.5 4.7 6.6, 5.4, 4.9 6.0, 5.1, 3.3 8.8, 7.5, 8.4 3.5, 3.4, 3.2 7.7, 6.0, 2.5 5.4, 3.6, 2.1 8.9, 7.2, 5.7 6.0, 5.0, 4.9 5.0, 4.5, 2.6 4.8, 3.8, 2.3 6.0, 5.0, 2.5 5.1, 2.6, 1.6 7.2, 6.4, 6.8 5.7, 5.0, 5.1 6.4, 6.6, 5.9 6.2, 5.6, 6.2
cf; s, r, s sf; s, r, s cf; r, r, r cf; r.s, r, r.s cf; r.s, r, r.s cf; r, s, r cf; r, r, r cf; r, r, r cf; r.s, r, r.s cf; r.s, r.s, r.s cf; r.s, r, r.s cf; r.s, r.s, r.s cf; r.s, r, r.s cf; r.s, r.s, r.s cf; r.s, r, r.s cf; r.s, r, r.s cf; r, r, r cf; r, r, r cf; r.s, r, r.s
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3.8. Nodule traction tests Traction tests with a variety of differently shaped manganese nodules were performed to determine adhesion between the nodules and the sediment. These tests quantify the amount of sediment adhering to the different types of nodules after their removal and potential discharge into the water column after recovery of manganese nodules (Oebius et al., 2001). Because the adhesion of sediment to nodules strongly depends on the shape and size of the nodules (discoidal, ellipsoidal, spheroidal), nodules were classified according to von Stackelberg and Marchig (1987) (Table 1). For these tests the special multipurpose soil mechanical test equipment (see above) was again used, in this case utilising a specially designed claw to grab the nodules without disturbing the sediment interface. The traction was provided by an electric drive, and forces were measured by means of strain gauges.
4. Results and discussion 4.1. Grain-size distribution Fig. 1 shows a typical laser particle analyser output from the grain-size distribution in the Peru Basin. In general, the sediments from the SEDIPERU area and the DEA are silty clays or clayey
Fig. 1. Example of particle-size distribution in sediments of the Peru Basin.
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Fig. 2. Scanning electron microscope (SEM) photograph of undisturbed sediment of the semi-liquid layer.
silts with little regional variation. The sand fraction is less than 5% in most samples and is mainly composed of foraminiferal residues and debris of various shells (Fig. 2). The percentage depends very much on the amount of biogenic carbonate present in the sediment. Fig. 3 shows a comparison of the measured clay, silt, and sand fractions as a function of the sediment depth. It can be seen that the share of the sand fraction (larger particles) is greater in the very young semiliquid layer. Chemical dissolution and crushing of tests presumably by grazing macrofauna, especially of the plankton shells in the top layer, is mainly responsible for the breakdown of particles and reduction in grain size. In the deeper parts of the sediment this process is terminated. 4.2. Semi-quantitative analysis of the clay fraction X-ray investigations of 70 surface-sediment samples showed that smectite was by far the most abundant clay mineral with contents between 35% and 65%, followed by illite with 20–30%, chlorite with 9–26% and kaolinite with 0–13%. Local variations exist between the SEDIPERU
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Fig. 3. Grain-size distribution as a function of the sediment depth.
area where montmorillonite of pelagic origin is dominant and the DEA where more continental kaolinite and illite were found. Due to the relatively great distance from the continent, transport of fluvial clay minerals to the locations investigated probably plays a minor role. Most of the smectite in this part of the Peru Basin is formed authigenically and as the product of hydrothermal alteration of volcanic rocks. The analyses of the semi-liquid top layer show that the clay mineral content has a relationship to the water depth. For instance the amount of smectite decreases from 65% to 35% with increasing water depth (Fig. 4). Table 2 shows some examples of X-ray analyses of the clay mineral contents in the semi-liquid top layer, the transition zone, and the consolidated sub-layer. The different clay minerals appear in all three layers with no significant relationship to the sediment depth. Instead they seem to be well mixed, which could be attributed to bioturbation caused by burrowing organisms. 4.3. Water content Fig. 5 shows a compilation of the average water content of sub-samples taken at selected 1-cm downcore steps. In the ca. 15-cm-thick semi-liquid top layer, the mean water content decreases linearly from nearly 85% at the sediment surface to about 73% at the interface of the transition
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Fig. 4. Clay mineral contents as a function of the water depth.
Table 2 Clay mineral content as function of the sediment depth
230 KG top layer 230 KG transition layer 230 KG sub-layer (50 cm depth)
Illite (%)
Smectite (%)
Chlorite (%)
Kaolinite (%)
27.96 30.09 26.99
35.11 45.27 45.19
31.78 18.77 22.53
5.15 5.87 5.29
zone. Below this zone the water content reduces more slowly to 70% within the next 30 cm. This decreasing water content indicates an increasing consolidation of the sediment also reflected in the shear strength and plate-penetration tests. The compilation of water content data from the SO 79 and SO 106 cruises (Fig. 6) exhibits a relationship between water depth and water content of the pelagic surface sediments. The water content of the semi-liquid layer increases with increasing water depth. This is caused by the decrease of calcareous skeletons and debris with increasing water depth as a consequence of the CCD so that the relative amount of swelling clay minerals (smectites) increases.
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Fig. 5. Mean water content as a function of the sediment depth.
4.4. Shear strength The results of the vane shear measurements in sediment samples from the DEA and SEDIPERU areas show two clear distinct layers. The shear strength increases from values below 0.1 kPa for the first few cm of a non-consolidated semi-liquid layer to about 3–5 kPa at a depth of about 10–15 cm (transition layer), followed by a sharp increase of the shear strength below this depth, characterising the more consolidated sub-layer. At greater depths the shear strength increases rapidly reaching about 10 kPa at 30 cm depth. Below 15 cm depth the shear strengths vary strongly due to the influence of burrowing organisms (bioturbation). All measurements show a significant change at the interface between the semi-liquid layer and the transition zone, which is not due to normal geostatic consolidation. This might result from intense bioturbation within the semi-liquid layer. There are two typical types of shear strength curves. Type 1 (Fig. 7a) shows almost no increase in the shear strength within the semi-liquid top layer (0–13 cm), but a sharp increase of the gradient at the top of the transition zone, which has a reduced thickness of o3 cm. The consolidated sub-layer is characterised by a lower shear strength gradient. Type 2 (Fig. 7b) shows a subsequently increasing shear strength curve of the semi-liquid top layer up to values of 2 kPa at 9 cm sediment depth. Within the transition zone from 9 to 14 cm the gradient of the shear curve increases similarily as in type 1, but is obviously thicker. The consolidated sub-layer is likewise
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Fig. 6. Water content of the semi-liquid layer as a function of the water depth.
characterised by a lower shear strength gradient as in Fig. 7a. It has yet to be proven whether these differences in the sediment layers, especially the sharp contrast between the semi-liquid top layer and the transition zone in type 1, are either caused by erosional events or depend on the high rate of bioturbation. The shear strength of the sediment, which closely depends on the above-mentioned parameters such as water content and grain size, has great importance for environmental impact assessments. The semi-liquid top layer houses the manganese nodules that penetrate this layer and mostly rest on the consolidated sub-layer. Mining of these nodules simultanously means the removal of the semi-liquid layer by the collector with a suspension of particles as well as soluble matter into the benthic boundary layer. A vehicle (e.g., a manganese nodule miner) moving on the sea floor will need bearable sediment conditions, which will be in deep-sea clay at a shear strength of about 2.5–4 kPa. The measurements therefore allow a first estimation of how deep vehicles will sink into the sediment. In case that more exact plate bearing tests confirm the above data, a mining system designed in Germany would sink into the sediment for about 15–20 cm as a worst case scenario. Additionally, the deeper layers would be compressed within the tracks of the miner.
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Fig. 7. (a) Transition from unconsolidated to consolidated soil conditions; with a reduced transition zone. (b) Transition from unconsolidated to consolidated soil conditions; with a distinct transition zone.
4.5. Plate-penetration tests Fig. 8 shows results from dynamic plate-penetration tests. In Fig. 8a the measured penetration depth is plotted as a function of the load and of different plate sizes. The curves reflect the different plate (loading) areas but seem to follow the same tendency. In Fig. 8b the measured penetration depth is plotted as a function of the plate pressure (load/plate area). These curves exhibit a significant influence of the plate area on the test result and, in part, reflect shear strength effects of the plates. Since plate-bearing tests should not reflect the shear strength but rather the bearing capacity, only plates with a size of 100 cm2 were used for the following tests. Fig. 9 shows two results from
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Fig. 8. (a) Penetration force as a function of the penetration depth using different plate sizes. (b) Ground pressure per unit area as a function of the penetration depth.
tests in a maxicorer liner (with water coverage) and in a box corer (without water), which were both accomplished directly after recovery to avoid effects from de-watering. The plate-penetration curve elaborated with the maxicorer samples shows a similar pattern as the shear strength distribution with a distinct increase of the penetration force between the semi-liquid layer, the transition zone and the consolidated sub-layer below 10 cm core depth. The curve gained with the
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Fig. 9. Plate-penetration tests with and without water coverage.
box corer samples does not exhibit these differentiations, demonstrating that the overlying water column has a strong influence on the penetration forces. 4.6. Nodule traction tests Most of the investigated nodules had a ‘‘cauliflower’’ structure and an ellipsoidal shape (Table 1), i.e. their surface texture was mainly rough representing the ‘‘basin-type’’ of nodules in the Peru Basin (Marchig et al., 2001). The evaluation of the nodule traction tests indicated that generally about 35–50 g of sediment adhere to nodules of 4–6 cm in diameter. Assuming 5–80% nodule coverage of the sediment surface, and an average of 40% being equivalent to 200 nodules of 5 cm in diameter per m2, in the Peru Basin and in the Clarion-Clipperton Fracture Zone (von Stackelberg and Marchig, 1987), about 7000–10,000 t (wet weight) of sediments/km2 mining area would be recovered together with the nodules (Oebius, 1998). This amount of mud needs to be discharged constantly into the surface water by a mining ship if the nodules are not rinsed off during or prior to recovery.
5. Summary A necessary precursor for the accomplishment of the sedimentological and soil mechanical tests was the recovery of undisturbed soil samples with large sediment surfaces. The maxicorer has fully met these requirements and has proven to be a valuable tool in addition to the conventional soil samplers. Manganese nodules in their original and undisturbed position also were recovered using this tool.
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The water content of the undisturbed samples indicated different void ratios in the semi-liquid and sub-layers. They can be correlated to the state of consolidation. Contrary to terrestrial soil mechanics the correlation between shear strength and consolidation seems to be a functional relationship between water depth and water content in the top layer. The same seems to apply to the thickness of the semi-liquid layer. A correlation between the locations of the samples could not be established. The most valuable tool for the description of deep-sea sediments is shear strength and its distribution. It has to be realised, however, that the results of shear strength measurements are restricted by the method applied and that no international standards exist. The same is valid when using other standard equipment of different systems, and thus intercalibration is recommended (Oebius, 1998). Although local differences in value (up to 40%) and extension of measured vertical shear strength profiles have been detected (i.e., due to bioturbation), all samples exhibited a typical shear-strength distribution. This is characterised by a zone of low shear strength (semi-liquid top layer), a transition zone of increasing shear strength of different vertical extension, and a sub-layer of rapidly increasing shear strength. The existence and extension of the transition zone seems to be a function of the degree of bioturbation. The availability of the maxicorer for the first time permitted carrying-out tests in undisturbed sediment samples. The advantage of such measurements is clearly demonstrated by the results of the soil mechanical tests conducted with and without an overlying water column. The results of our measurements of geotechnical properties in undisturbed and water-covered samples also allowed some initial and realistic estimates of the miner’s penetration depth into the sediment surface. The impact is calculated to be not more than 15–20 cm, but further tests are needed to verify these results. Acknowledgements The authors thank the crew of the RV Sonne for their help and co-operation during cruises SO 106a and 106b. Thanks are also due to all the members of the TUSCH group for the helpful discussions and assistance received. Project MEPARSED II was funded by the German Bundesministerium fur . Forschung und Technologie through Contract 03 G 0106F. The reviews of A. Skinner and T. Scheytt helped to improve the final version of the manuscript. The authors are responsible for the content of this paper. Appendix Photos 1 and 2 are not included in the printed volume but can be found on CD-ROM, Annex 2.7. References Becker, H.J., Grupe, B., Oebius, H.U., Liu, F., 2001. The behaviour of deep-sea sediments under the impact of nodule mining processes. Deep-Sea Research II 48, 3609–3627. . Bode, O., 1991. Simulation der Fahrt von Raupenfahrwerken auf Tiefseeboden. Dissertation, University Hannover, Germany.
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Gerber, H.W., Oebius, H.U., Grupe, B., 1996. MaxicorerFa device for taking undisturbed samples of sediment including the benthic boundary layer. Sea Technology 37, 66–69. Grupe, B., Halbach, P., Gronenberg, M., Oebius, H.U., 1994. Bodenmechanische Eigenschaften von Tiefseesedimenten in Maganknollen-Feldern als Funktion des sedimentpetrographischen Aufbaus. Technische Universit.at Berlin, VWS, Eigenverlag, Germany. Jankowski, J.A., Zielke, W., 2001. The mesoscale sediment transport due to technical activities in the deep sea. Deep-Sea Research II 48, 3487–3521. Inderbitzen, A.L. (Ed.), 1974. Deep-sea SedimentsFSeminar Workshops I–III. Plenum Press, New York. Koschinsky, A., Winkler, A., Fritsche U., 2001. Role of marine particle types for the adsorption of heavy metals in the deep sea. Applied Geochemistry, submitted for publication. Lettau, O., 1978. Bodenmechanische Bord-und Landlaboruntersuchungen an pazifischen Tiefseesedimenten. Bundesministerium fur . Forschung und Technologie, FE-M78 03, Bonn, Germany. Marchig, V., von Stackelberg, U., Hufnagel, H., Durn, G., 2001. Compositional changes of surface-sediments and variability of manganese nodules in the Peru Basin. Deep-Sea Research II 48, 3523–3547. Oebius, H.U., 1998. Deep-sea mining and its environmental consequences. Technische Universit.at Berlin, VWS, Eigenverlag, Germany. Oebius, H.U., Becker, H.J., Rolinski, S., Jankowski, J.A., 2001. Parametrization and evaluation of marine environmental impacts produced by deep-sea manganese nodule mining. Deep-Sea Research II 48, 3453–3467. Schriever, G., Koschinsky, A., Bluhm, H., 1996. Cruise Report ATESEPP: auswirkungen technischer eingriffe in das . okosystem der tiefsee im sud-ost-pazifik . vor peru. Ber. Zentrum Meeres-u. Klimaforschung Universit.at Hamburg 11, 20–21. Stackelberg, U.V., Marchig, V., 1987. Manganese nodules from the equatorial North Pacific. Geologisches Jahrbuch D 87, Hannover, Germany, pp. 123–227. Stummeyer, J., Marchig, V., 2001. Mobility of metals over the redox boundary in Peru Basin sediments. Deep-Sea Research II 48, 3549–3567. Wiedicke, M., Weber, M., 1996. Small-scale variability of sea floor features in the northern Peru Basin: results from acoustic survey methods. Marine Geophysical Researches 18, 507–526.
Geotechnical and sedimentological investigations of deep-sea sediments from a manganese nodule field of the Peru Basin Bernd Grupe*, Hermann J. Becker, Horst U. Oebius Technische Universitat D-10623 Berlin, Germany . Berlin, Sekr. VWS-FB6, Muller-Breslau-Strae/Schleuseninsel, .
Abstract Deep-sea mining of manganese nodules will significantly modify the surface layer of deep sea. Surface sediments are mechanically disturbed, and a cloud of sediment particles suspended and redeposited. Assessment of sedimentological and soil mechanical characteristics of undisturbed top layer sediments from a manganese nodule field of the Peru Basin are necessary to provide baseline data for soil mechanical estimations and for the physical and numerical modelling of environmental impacts of future ocean mining activities. This will assist the International Seabed Authority to make decisions on how to formulate the mining code, and will increase our understanding of sedimentation processes in the Peru Basin. The deployment of a new sampling device (maxicorer) guaranteed undisturbed sediment samples in which soil mechanics-related parameters were measured including sediment density, water content, shear strength, and penetration force. Data evaluation show a close relationship between the soil mechanical parameters and various sedimentological properties such as grain-size distribution, mineral composition and micro-structure. From the results it can be concluded that in the worst case a deep-sea nodule mining vehicle would sink about 15–20 cm into the sea bed. r 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction In the marine environment human activities such as the exploitation of deep-sea resources as well as natural events like earthquakes, volcanic eruptions or downslope failure are inevitably connected with large-area disturbances of the sea floor. They can change the geological and geochemical boundary conditions and also cause biological alterations.
*Corresponding author. Tel.: +49-0-30-31184-260; fax: +49-0-30-31184-200. E-mail address: [email protected] (B. Grupe). 0967-0645/01/$ -see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 1 ) 0 0 0 5 8 - 3
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The most severe anthropogenic impact will result from marine mining, which involves two main processes: the upper layer of the deep-sea sediment will be partly or completely compressed, squeezed, sheared or eroded by the collector and the miner, and the biogeochemical milieu will be altered by erosion, suspension of sediment (Oebius et al., 2001), the release of chemically active substances (Koschinsky et al., 2001), and resettlement of the sediment particles (Becker et al., 2001). To assess and evaluate these impacts reliable sedimentological and soil mechanical baseline data of the undisturbed benthic milieu are essential. Such data also are required by the engineer to support the design and construction of stable and environmentally acceptable sub-sea structures and bottom vehicles (Oebius, 1998). The sub-project ‘‘Mechanical Parameters of Sediments in Deep-Sea Areas with great Manganese Nodule Abundance (MEPARSED)’’ was undertaken to provide these data, supporting some of the other studies compiled in this volume (Becker et al., 2001; Jankowski and Zielke, 2001; Oebius et al., 2001). As there is a lack of suitable equipment, methodology and standards for the assessment, evaluation and interpretation of geotechnical information concerning deep-sea sediments, special methods and instruments had to be developed (Grupe et al., 1994). This refers particularly to the recovery of large-volume, undisturbed sediment samples from the deep sea floor. Therefore, in advance of the Sonne 106 cruise a new sediment sampling device, the maxicorer, was developed by the Technical University of Berlin (Gerber et al., 1996).
2. Experimental areas Sediment samples from selected areas of the Peru Basin showing changing morphological conditions were used for the experiments. (See ‘‘Use and Protection of the Deep-Sea’’, introduction to this volume.) The sedimentological and soil mechanical baseline studies were accomplished in samples from Area 2 (DISCOL Experimental Area/DEA) (Schriever et al., 1996), Area 5 (SEDIPERU area) (Wiedicke and Weber, 1996; Stummeyer and Marchig, 2001), and for special comparison of the water content of sediments additionally in samples from Area 6 in the SEDIPERU area. A detailed description of the geological and biogeochemical milieu is given in the above-mentioned papers and in Koschinsky et al. (2001). The sediment characteristics of the Peru Basin are strongly influenced by the carbonate compensation depth (CCD), which lies between 4200 and 4350 m. Calcareous ooze occurs down to about 4000 m, calcareous–siliceous mud between 4000 and 4200 m, and siliceous mud deeper than about 4200 m. The top layer (5–14 cm) is composed of homogeneous dark brown sediments, which are underlain by greyish- to yellowish-brown and strongly bioturbated sediments. Depending on the water depth, the ratio of siliceous and calcareous tests changes, as does the amount of clay minerals. To investigate the influence of the different compositions of the sediment on the water content, sediment samples from different water depths and different locations (SEDIPERU area, DISCOL area/DEA) were compared with respect to horizontal and lateral variations.
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3. Background and methods 3.1. Sediment samples Geotechnical investigations of shear strength measurements, compression tests, penetration tests, structural analyses and trafficability require mechanically undisturbed sediments. Therefore, the maxicorer, combining the advantages of both the box corer and the multicorer, needed to be developed beforehand (Gerber et al., 1996) and its suitability for the assessment of soil mechanical parameters proven during the SO 106 cruise. It was deployed successfully 15 times and provided large-volume samples that allowed, for the first time, soil mechanical investigations directly in undisturbed deep-sea sediments. The surface and the semi-liquid top layer remained unaffected by the sampling procedure (some samples were recovered with nodules still embedded in undisturbed state, see CD ROM, Annex 2.7, Photo 1). Furthermore, the samples were covered with in situ deep-sea water at a temperature of about 3–41C, allowing biological and geochemical studies. Consequently, the maxicorer samples served as a reference and calibration standard for geotechnical measurements from box corer and multicorer samples which also were collected and used in the investigations.
3.2. Grain-size analyses by laser technique The grain-size distribution of undisturbed deep-sea sediment is necessary to characterise the sedimentological composition and possible genesis, and also to serve as a baseline indicator for the evaluation of the effects of anthropogenic impacts (Oebius et al., 2001; Becker et al., 2001). The grain-size distribution was determined on-board the research vessel and in the on-shore laboratory by use of a Fritsch laser particle analyser, type Analysette 22. Sediment samples were desalted by washing with purified water and treating them for 2 min in an ultrasonic bath. Sodium pyrophosphate decahydrate in a concentration of 0.5 g/1000 cm3 was used as a disperser to avoid coagulation of the fine particles in the treated suspension.
3.3. Semi-quantitative analysis of the clay fraction Clay minerals dominate the soil mechanical characteristics and behaviour of cohesive deep-sea sediments. Their small size, flaky shape and electrical charge are responsible for different particle structures of the sediments, which in turn influence its water content, plasticity, and shear strength. One has to distinguish between clay minerals that do not hydrate, such as kaolinite and illite, and those that do, especially montmorillonite. The determination of the clay mineral content was accomplished by a PHILIPS X-ray diffractometer, type PW 1710/1729, with a Cu anode, 40 kV/30 mA, and monochromatic Ka radiation. The pretreatment of the samples followed standard procedures. The diffractograms were semi-quantitatively analysed using the MacDiff software programme developed by R. Petschick, University of Frankfurt/Main, Germany.
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3.4. Water content In saturated sediments the water content strongly influences the soil mechanical behaviour as well as the biogeochemical processes. Its assessment is necessary for the determination of soil stability (void ratio, pore water pressure, consolidation), trafficability, particle concentration of suspended sediment clouds (Oebius et al., 2001), and also for preparation of artificial deep-sea sediments for subsequent laboratory tests as a part of the PARTRANS project (Becker et al., 2001). To ensure detection of minor variations, the water content was determined in 1-cm steps downcore. The overlying water of each core or subcore was carefully siphoned off, the core was slowly pressed out of the liner, and sections were sliced in 1-cm intervals down to a core depth of about 30 cm. These samples were preserved (from drying) and packed for transportation to the laboratories of the Technical University Berlin for further analyses. In total, 210 determinations of water content were made.
3.5. Soil mechanical investigations These investigations, in particular shear strength, bearing characteristics, nodule adhesion forces, and sediment structure, were essential to provide baseline data for environmental impact assessments (Becker et al., 2001; Oebius et al., 2001). Since many geotechnical deep-sea sediment characteristics differ from their terrestrial or coastal counterparts, a direct transfer of terrestrial soil mechanical data to deep-sea conditions is not possible (Inderbitzen, 1974). In some cases there are not even comparable standards, methods or appropriate instrumentation. Therefore, for some of the investigations new methodologies had to be developed and adopted.
3.6. Shear strength The shear strength is assumed to be the most significant soil mechanical parameter for the study of water-saturated clayey sediments. Many methods have been applied to measure the shear strength of cohesive sediments in the semi-liquid top layer in the deep-sea (cones, box shear meters, ring shear meters, triaxial tests, vane shear meters). Since these methods cannot be directly compared with each other (Bode, 1991), a method had to be chosen that allows intercalibration with other standard methods, is easy to handle on-board a research vessel, and allows an assessment of shear strengths in small steps (10 mm steps) in liquid pastes. The only instruments that meet all the requirements, are vane shear meters (Lettau, 1978). On-board the RV Sonne all tests were carried out with the HAAKE viscosimeter RV 20 equipped with the M5 measuring system and a vane shear-strength device as sensor. Two vanes of different sizes (d ¼ 10 mm; h ¼ 8:8 mm and d ¼ 20 mm; h ¼ 10 mm) were applied and usually rotated at 301/min, but in some cases the speed was varied to investigate the influence of different shear velocities on the measurement. The measurements generally were conducted in vertical steps of 10 mm in order to obtain a vertical profile, which can be correlated with other sedimentological parameters such as water content and grain size.
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About 1650 measurements were made on sediment from 40 different locations. Together with results from the SO 79 cruise (Grupe et al., 1994), the shear strength measurements allow general statements about the soil mechanical conditions in larger areas of the Peru Basin. 3.7. Plate-penetration tests Plate-penetration tests are used to determine the position of the layer with the necessary bearing capacity to cope with a static (deep-sea structure) or dynamic (miner) load. Such information is needed to evaluate the effective disturbance depth of miners, the trafficability of deep-sea sediments, or the pore water pressure and possible slope instabilities. Circular plates of different sizes (20, 50, 100 cm2) were either loaded by a fixed weight and the time-dependent sinking depth into the smooth clayey and silty sediments was measured, or they were pressed slowly and with constant penetration velocity into the sediment in which case the penetration-dependent force was constantly measured. For these tests a specially constructed multipurpose soil mechanical test instrument was used (see CD ROM, Annex 2.7, Photo 2). Penetration tests were carried out on eight maxicorer and box corer samples. Tests on the maxicorer samples were the first ones to be carried out in undisturbed deep-sea sediments with overlying deep-sea water.
Table 1 Results from the lifting tests of manganese nodules Sample
Water Lifting Total Nodule Sediment Sediment/ Nodule-type according to von depth (m) force (N) weight (N) weight (N) weight (N) nodule (%) Stackelberg and Marchig (1987)
1518KG 4155 185KG
4111
201MX2 4260 215KG
4004
223MX3 4115
20.47 3.06 5.81 4.16 7.46 0.74 3.00 0.88 3.80 3.55 2.54 0.95 0.89 0.79 6.63 2.58 3.41 2.84
9.08 15.40 0.87 1.35 0.93 5.14 0.50 1.07 0.39 2.71 1.38 0.53 0.40 0.82 0.19 1.96 1.28 1.86 1.71
7.95 14.40 0.64 1.15 0.80 3.95 0.39 0.74 0.17 2.20 1.36 0.44 0.20 0.65 0.16 1.52 0.44 0.82 0.71
1.12 1.00 0.23 0.19 0.13 1.19 0.11 0.33 0.21 0.51 0.03 0.09 0.20 0.16 0.03 0.45 0.84 1.04 1.00
14.1 6.9 35.7 16.9 16.7 30.2 27.3 45.3 125.1 23.1 2.0 20.5 98.0 24.7 20.4 29.3 190.7 127.8 139.7
Shape Surface
Size (l, w, h)
S SI SI EI E2 S S E E E2 EI E E E E DI E2 D SI
10.0, 9.5, 9.5 12.0, 10.0, 11.5 4.7 6.6, 5.4, 4.9 6.0, 5.1, 3.3 8.8, 7.5, 8.4 3.5, 3.4, 3.2 7.7, 6.0, 2.5 5.4, 3.6, 2.1 8.9, 7.2, 5.7 6.0, 5.0, 4.9 5.0, 4.5, 2.6 4.8, 3.8, 2.3 6.0, 5.0, 2.5 5.1, 2.6, 1.6 7.2, 6.4, 6.8 5.7, 5.0, 5.1 6.4, 6.6, 5.9 6.2, 5.6, 6.2
cf; s, r, s sf; s, r, s cf; r, r, r cf; r.s, r, r.s cf; r.s, r, r.s cf; r, s, r cf; r, r, r cf; r, r, r cf; r.s, r, r.s cf; r.s, r.s, r.s cf; r.s, r, r.s cf; r.s, r.s, r.s cf; r.s, r, r.s cf; r.s, r.s, r.s cf; r.s, r, r.s cf; r.s, r, r.s cf; r, r, r cf; r, r, r cf; r.s, r, r.s
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3.8. Nodule traction tests Traction tests with a variety of differently shaped manganese nodules were performed to determine adhesion between the nodules and the sediment. These tests quantify the amount of sediment adhering to the different types of nodules after their removal and potential discharge into the water column after recovery of manganese nodules (Oebius et al., 2001). Because the adhesion of sediment to nodules strongly depends on the shape and size of the nodules (discoidal, ellipsoidal, spheroidal), nodules were classified according to von Stackelberg and Marchig (1987) (Table 1). For these tests the special multipurpose soil mechanical test equipment (see above) was again used, in this case utilising a specially designed claw to grab the nodules without disturbing the sediment interface. The traction was provided by an electric drive, and forces were measured by means of strain gauges.
4. Results and discussion 4.1. Grain-size distribution Fig. 1 shows a typical laser particle analyser output from the grain-size distribution in the Peru Basin. In general, the sediments from the SEDIPERU area and the DEA are silty clays or clayey
Fig. 1. Example of particle-size distribution in sediments of the Peru Basin.
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Fig. 2. Scanning electron microscope (SEM) photograph of undisturbed sediment of the semi-liquid layer.
silts with little regional variation. The sand fraction is less than 5% in most samples and is mainly composed of foraminiferal residues and debris of various shells (Fig. 2). The percentage depends very much on the amount of biogenic carbonate present in the sediment. Fig. 3 shows a comparison of the measured clay, silt, and sand fractions as a function of the sediment depth. It can be seen that the share of the sand fraction (larger particles) is greater in the very young semiliquid layer. Chemical dissolution and crushing of tests presumably by grazing macrofauna, especially of the plankton shells in the top layer, is mainly responsible for the breakdown of particles and reduction in grain size. In the deeper parts of the sediment this process is terminated. 4.2. Semi-quantitative analysis of the clay fraction X-ray investigations of 70 surface-sediment samples showed that smectite was by far the most abundant clay mineral with contents between 35% and 65%, followed by illite with 20–30%, chlorite with 9–26% and kaolinite with 0–13%. Local variations exist between the SEDIPERU
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Fig. 3. Grain-size distribution as a function of the sediment depth.
area where montmorillonite of pelagic origin is dominant and the DEA where more continental kaolinite and illite were found. Due to the relatively great distance from the continent, transport of fluvial clay minerals to the locations investigated probably plays a minor role. Most of the smectite in this part of the Peru Basin is formed authigenically and as the product of hydrothermal alteration of volcanic rocks. The analyses of the semi-liquid top layer show that the clay mineral content has a relationship to the water depth. For instance the amount of smectite decreases from 65% to 35% with increasing water depth (Fig. 4). Table 2 shows some examples of X-ray analyses of the clay mineral contents in the semi-liquid top layer, the transition zone, and the consolidated sub-layer. The different clay minerals appear in all three layers with no significant relationship to the sediment depth. Instead they seem to be well mixed, which could be attributed to bioturbation caused by burrowing organisms. 4.3. Water content Fig. 5 shows a compilation of the average water content of sub-samples taken at selected 1-cm downcore steps. In the ca. 15-cm-thick semi-liquid top layer, the mean water content decreases linearly from nearly 85% at the sediment surface to about 73% at the interface of the transition
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Fig. 4. Clay mineral contents as a function of the water depth.
Table 2 Clay mineral content as function of the sediment depth
230 KG top layer 230 KG transition layer 230 KG sub-layer (50 cm depth)
Illite (%)
Smectite (%)
Chlorite (%)
Kaolinite (%)
27.96 30.09 26.99
35.11 45.27 45.19
31.78 18.77 22.53
5.15 5.87 5.29
zone. Below this zone the water content reduces more slowly to 70% within the next 30 cm. This decreasing water content indicates an increasing consolidation of the sediment also reflected in the shear strength and plate-penetration tests. The compilation of water content data from the SO 79 and SO 106 cruises (Fig. 6) exhibits a relationship between water depth and water content of the pelagic surface sediments. The water content of the semi-liquid layer increases with increasing water depth. This is caused by the decrease of calcareous skeletons and debris with increasing water depth as a consequence of the CCD so that the relative amount of swelling clay minerals (smectites) increases.
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Fig. 5. Mean water content as a function of the sediment depth.
4.4. Shear strength The results of the vane shear measurements in sediment samples from the DEA and SEDIPERU areas show two clear distinct layers. The shear strength increases from values below 0.1 kPa for the first few cm of a non-consolidated semi-liquid layer to about 3–5 kPa at a depth of about 10–15 cm (transition layer), followed by a sharp increase of the shear strength below this depth, characterising the more consolidated sub-layer. At greater depths the shear strength increases rapidly reaching about 10 kPa at 30 cm depth. Below 15 cm depth the shear strengths vary strongly due to the influence of burrowing organisms (bioturbation). All measurements show a significant change at the interface between the semi-liquid layer and the transition zone, which is not due to normal geostatic consolidation. This might result from intense bioturbation within the semi-liquid layer. There are two typical types of shear strength curves. Type 1 (Fig. 7a) shows almost no increase in the shear strength within the semi-liquid top layer (0–13 cm), but a sharp increase of the gradient at the top of the transition zone, which has a reduced thickness of o3 cm. The consolidated sub-layer is characterised by a lower shear strength gradient. Type 2 (Fig. 7b) shows a subsequently increasing shear strength curve of the semi-liquid top layer up to values of 2 kPa at 9 cm sediment depth. Within the transition zone from 9 to 14 cm the gradient of the shear curve increases similarily as in type 1, but is obviously thicker. The consolidated sub-layer is likewise
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Fig. 6. Water content of the semi-liquid layer as a function of the water depth.
characterised by a lower shear strength gradient as in Fig. 7a. It has yet to be proven whether these differences in the sediment layers, especially the sharp contrast between the semi-liquid top layer and the transition zone in type 1, are either caused by erosional events or depend on the high rate of bioturbation. The shear strength of the sediment, which closely depends on the above-mentioned parameters such as water content and grain size, has great importance for environmental impact assessments. The semi-liquid top layer houses the manganese nodules that penetrate this layer and mostly rest on the consolidated sub-layer. Mining of these nodules simultanously means the removal of the semi-liquid layer by the collector with a suspension of particles as well as soluble matter into the benthic boundary layer. A vehicle (e.g., a manganese nodule miner) moving on the sea floor will need bearable sediment conditions, which will be in deep-sea clay at a shear strength of about 2.5–4 kPa. The measurements therefore allow a first estimation of how deep vehicles will sink into the sediment. In case that more exact plate bearing tests confirm the above data, a mining system designed in Germany would sink into the sediment for about 15–20 cm as a worst case scenario. Additionally, the deeper layers would be compressed within the tracks of the miner.
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Fig. 7. (a) Transition from unconsolidated to consolidated soil conditions; with a reduced transition zone. (b) Transition from unconsolidated to consolidated soil conditions; with a distinct transition zone.
4.5. Plate-penetration tests Fig. 8 shows results from dynamic plate-penetration tests. In Fig. 8a the measured penetration depth is plotted as a function of the load and of different plate sizes. The curves reflect the different plate (loading) areas but seem to follow the same tendency. In Fig. 8b the measured penetration depth is plotted as a function of the plate pressure (load/plate area). These curves exhibit a significant influence of the plate area on the test result and, in part, reflect shear strength effects of the plates. Since plate-bearing tests should not reflect the shear strength but rather the bearing capacity, only plates with a size of 100 cm2 were used for the following tests. Fig. 9 shows two results from
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Fig. 8. (a) Penetration force as a function of the penetration depth using different plate sizes. (b) Ground pressure per unit area as a function of the penetration depth.
tests in a maxicorer liner (with water coverage) and in a box corer (without water), which were both accomplished directly after recovery to avoid effects from de-watering. The plate-penetration curve elaborated with the maxicorer samples shows a similar pattern as the shear strength distribution with a distinct increase of the penetration force between the semi-liquid layer, the transition zone and the consolidated sub-layer below 10 cm core depth. The curve gained with the
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Fig. 9. Plate-penetration tests with and without water coverage.
box corer samples does not exhibit these differentiations, demonstrating that the overlying water column has a strong influence on the penetration forces. 4.6. Nodule traction tests Most of the investigated nodules had a ‘‘cauliflower’’ structure and an ellipsoidal shape (Table 1), i.e. their surface texture was mainly rough representing the ‘‘basin-type’’ of nodules in the Peru Basin (Marchig et al., 2001). The evaluation of the nodule traction tests indicated that generally about 35–50 g of sediment adhere to nodules of 4–6 cm in diameter. Assuming 5–80% nodule coverage of the sediment surface, and an average of 40% being equivalent to 200 nodules of 5 cm in diameter per m2, in the Peru Basin and in the Clarion-Clipperton Fracture Zone (von Stackelberg and Marchig, 1987), about 7000–10,000 t (wet weight) of sediments/km2 mining area would be recovered together with the nodules (Oebius, 1998). This amount of mud needs to be discharged constantly into the surface water by a mining ship if the nodules are not rinsed off during or prior to recovery.
5. Summary A necessary precursor for the accomplishment of the sedimentological and soil mechanical tests was the recovery of undisturbed soil samples with large sediment surfaces. The maxicorer has fully met these requirements and has proven to be a valuable tool in addition to the conventional soil samplers. Manganese nodules in their original and undisturbed position also were recovered using this tool.
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The water content of the undisturbed samples indicated different void ratios in the semi-liquid and sub-layers. They can be correlated to the state of consolidation. Contrary to terrestrial soil mechanics the correlation between shear strength and consolidation seems to be a functional relationship between water depth and water content in the top layer. The same seems to apply to the thickness of the semi-liquid layer. A correlation between the locations of the samples could not be established. The most valuable tool for the description of deep-sea sediments is shear strength and its distribution. It has to be realised, however, that the results of shear strength measurements are restricted by the method applied and that no international standards exist. The same is valid when using other standard equipment of different systems, and thus intercalibration is recommended (Oebius, 1998). Although local differences in value (up to 40%) and extension of measured vertical shear strength profiles have been detected (i.e., due to bioturbation), all samples exhibited a typical shear-strength distribution. This is characterised by a zone of low shear strength (semi-liquid top layer), a transition zone of increasing shear strength of different vertical extension, and a sub-layer of rapidly increasing shear strength. The existence and extension of the transition zone seems to be a function of the degree of bioturbation. The availability of the maxicorer for the first time permitted carrying-out tests in undisturbed sediment samples. The advantage of such measurements is clearly demonstrated by the results of the soil mechanical tests conducted with and without an overlying water column. The results of our measurements of geotechnical properties in undisturbed and water-covered samples also allowed some initial and realistic estimates of the miner’s penetration depth into the sediment surface. The impact is calculated to be not more than 15–20 cm, but further tests are needed to verify these results. Acknowledgements The authors thank the crew of the RV Sonne for their help and co-operation during cruises SO 106a and 106b. Thanks are also due to all the members of the TUSCH group for the helpful discussions and assistance received. Project MEPARSED II was funded by the German Bundesministerium fur . Forschung und Technologie through Contract 03 G 0106F. The reviews of A. Skinner and T. Scheytt helped to improve the final version of the manuscript. The authors are responsible for the content of this paper. Appendix Photos 1 and 2 are not included in the printed volume but can be found on CD-ROM, Annex 2.7. References Becker, H.J., Grupe, B., Oebius, H.U., Liu, F., 2001. The behaviour of deep-sea sediments under the impact of nodule mining processes. Deep-Sea Research II 48, 3609–3627. . Bode, O., 1991. Simulation der Fahrt von Raupenfahrwerken auf Tiefseeboden. Dissertation, University Hannover, Germany.
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