PUBLICATION NR.70 Direct Simple-Shear Tests on a Norwegian Quick Clay -Geotekniske problemer ved utførelsen av Tunnelbanen fra Jernbanetorvet til Tøyen -Anvendelse av kompensert fundamentering i Norge - Falling Drop used for Grain-Size Analysis of Fine-Gr

摘要

The Paper gives a short description of a recently developed direct simple-shear apparatus. The cylindrical sample (diameter 8 cm, height 1 cm) is confined in a reinforced rubber membrane allowing vertical deformations and horizontal displacements with no change in diameter. Consolidated con¬stant-volume shear tests showed shear strengths which were lower than those found in comparable triaxial tests. The ratio of the constant-volume shear strength to vertical consolidation pressure with the new equipment was found to be 0.18 com¬pared with 0.29 measured in the triaxial tests and 0.13 found by vane tests in situ. The drained tests on samples consolidated at a pressure equal to the overburden in the field indicate that the structure of the clay might be stable for small shear stresses but starts to collapse at a certain critical value. This phenomenon might contribute to explain some landslides which have occurred in slopes in quick clay at a shear stress much below the ultimate drained shear strength.nConstruction of the subway through the central part of Oslo where the tunnel crossed through heavy deposits of soft clay, provided a number of geotechnical problems, the most difficult of which was that of ensuring the stability of the deep excavations.nSeveral different methods have been used to stabilise the excavations, the choice being dependent on the conditions at the various locations. The main method consisted of excavating in compressed air beneath a loaded tunnel roof and between sheet pile walls. This method, which had not been used before, seemed to be an adequate construction procedure.nIn connection with the tunnel work, a number of measurements have been taken, of earth pressures on strutted sheet piles, strut loads, pore pressures in the clay, deformations and settlements, etc. The results are published in the Norwegian Geotechnical Institute's publication series, and the data will be of great importance for the design of future deep excavations in clay, among others the remaining tunnel work in the centre of Oslo.nTo facilitate simultaneous performances of numerous grain-size analyses covering the silt and clay fractions, a method has been worked out and the necessary equipment constructed for utilizing a falling drop for determining densities of particle suspen¬sions. A drop is taken out by means of a micro-syringe at the desired depth in a sedi¬mentation vessel, and ejected into a column of an organic liquid with a slightly lower density. The drop then falls with a constant speed and the time measured with a stopwatch over a certain distance is a measure of the density of the drop, and hence also of the particle concentration in the sedimentation vessel where the drop is taken. In this way it is easily possible to work through twenty samples in a normal working day, including removal of organic matter, dispersion, washing (to remove effects of soluble salts) and evaluating the final results.