Geotechnical characterization of sediments from the Rockall Bank Slide Complex

Abstract

The main reason for this research is the possibility of landslides that could trigger a tsunami together with general interest from the oil and gas industry. The goal of this thesis is to find out what the geotechnical properties are of the 7 different locations at the eastern flank of the Rockall bank which is located to the west of Ireland and to the south of Iceland in the Atlantic Ocean. The possible effects of these properties on slope stability are then discussed. This will be done by geotechnically testing the sediments in a laboratory. All standards used during testing can be seen in chapter 3.1.1. The gravity cores used for this research were found to contain parts of sediment that were quite intact together with other more disturbed parts of sediment. The sediments that were found could be classified as silty SAND, clayey SILTS and silty CLAYS which are calcareous to very higly calcareous,from medium to very high plasticity, low to medium-organic and have extremely low to very low undrained shear strengths. Grain size distribtions were found to be gap-graded and well graded and the the sediments were found to be inactive to normal soil based on the Atterberg limits.Geotechnical properties such as gravimetric water contents are found to range from 0.20 to 1.18, the volumetric water contents range from 0.50 to 0.80, a liquidity index from 0 to 3, specific gravities from 2.72 to 2.79, void ratios from 1 to 3, bulk volumetric weights from 14 kN/m3 to 18 kN/m3, dry volumetric weights from 7 kN/m3 to 13 kN/m3, clay contents from 10% to 60%, a silt content of 15% to 60%, a sand content from 8% to 70%, calcite contents from 17.3% to 57.4%, organic matter contents from 2.6% to 9.0%, liquid limits from 0.40 to 0.82 and plastic limits from 0.23 to 0.49. The undrained shear strength for the original and remoulded sediments from the UU DS tests is found to range from 2 kPa to 8 kPa and from 0 kPa to 4 kPa. The undrained shear strengths from original and remoulded sediments from the fall cone test range from 4 kPa to 33 kPa and from 0 kPa to 4 kPa. The undrained shear strengths from the original pocket vane tests range from 5 kPa to 22 kPa. The sensitivities of the sediments measured by the fall cone tests are found to range from 2 to 28 and the sensitivities obtained by direct shear testing are found to range from 0.5 to 4.Based on these undrained shear strengths a failure mechanism similar to that of a direct shear test is found to be more likely on the undisturbed sediments and a failure mechanism like that of a fall cone test is found to be more likely for the remoulded sediments. The non-phyllosilicate minerals that are present are Smectite, Illite, Muscovite, Chlorite and Kaolinite. The phyllosilicate minerals that are present are Quartz, Alkali feldspar, Plagioclase, Calcite, Ankerite, Siderite, Anatase, Rutile, Hematite, Pyrite, Halite and Apatite. This mineral composition can be logically explained by their possible weathering paths and indicates that the possible parent material is Monzogranite type 2 also called Muscovite-metagranite. Magnetic particles are found to be present in all sediments in small amounts that are not quantified. Similar sediments are found to be present from the research of ( Georgiopoulou, Krastel et al., 2019). This confirms the presence of turbidty deposits. These turbidity deposits are indicated to be present at all sites based on the fininng-upward sequences found. Also the presence of lighter interglacial and darker glacial sediments is found. It was also found that the likley reasons for slope instability on the eastern side of the Rockall Bank are the much higher water contents and clay contents of the sediments present compared to the sediments found on eastern side of the Rockall through.The risk of liquefaction upon distrubance is found at sites 1736, 1672, 1988, 1959 and 1604. No risk of liquefaction upon disturbance is found at site 688. The organic matter content is considered to not have an effect on strenght parameters of the sediments present whereas an increase in the amount of foraminifera shells could increase stability of the sediments. Only differential compaction is not found to be a probable factor in lanslide initiation. A long term instability could arise from weathering of Smectite minerals. Erosion could cause slope instability due to sediments with widely varying grain size distributions. The high water content sediments that are present are prone to liquefaction due a disturbance possibly from seismic activity. It is recommended that in future studies a quantification of the marine shell fraction is made. Also a microscope spectrometry is recommended to be done on the sand fraction together with an X-ray diffraction on the <63µm fraction. The most important recommendation is to do a slope stability analysis on the RBSC using the geotechnical properties presented in this thesis.