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This paper describes the design of an automated slurry consolidometer and calibration chamber system used to prepare largesize cohesive soil specimens for testing in-situ devices, model foundations, and ground anchors. A two-stage technique for the preparattion of homogeneous cohesive specimens subjected to a known stress history is described. Briefly summarized are the data acquisition/control system and the instrumentation details for monitoring the spatial pore pressure distributions in the specimen, the vertical and lateral stresses on the specimen, and specimen settlement during slurry consolidation and subsequent reconsolidation in the chamber. The specimens prepared were reproducible and of uniform quality as indicated by the settlement and pore pressure dissipation histories and by the water content results obtained from samples taken from chamber specimens. The homogeneity of the specimens is confirmed by the cone penetration test results.
Cone penetration test (CPT) is a fast and reliable site investigation tool for exploring soils and soft ground. While the interpretation of the test results in clay has advanced considerably from a theoretical and numerical viewpoint that of tests in sands still relies largely on empirical correlations. A major source of such correlations comes from tests done in calibration chambers (CC), where soil state and properties might be tightly controlled. Calibration chambers are relatively large pieces of equipment, and calibration chamber testing is expensive and time consuming. Moreover, CC tests are performed on freshly reconstituted sands whose fabric may vary from that of natural sand deposits. Hence, correlations developed for one type of sand might not be suitable for another sand deposit. Numerical DEM-based calibration chambers might offer an interesting alternative to the more cumbersome physical tests. This study is the first attempt to perform a three-dimensional DEM-based simulation of cone penetration test. The three-dimensional commercial DEM code (PFC3D) is used to develop Virtual Calibration Chamber CPT (VCC CPT) model. To achieve that objective, several steps were necessary. First, calibration of an analogue discrete material to represent Ticino sand was performed using single-element tests. Afterwards, the mechanical response of the discrete material was further validated by performing additional triaxial tests with different initial conditions. The VCC CPT model was then constructed. Comprehensive dimensional analysis showed that the best option to balance computational efficiency and realism was to fill the chamber with a scaled-up calibrated discrete material. An original filtering technique was proposed to extract steady state cone resistances. A basic series of simulations was performed to explore the effect of initial stress and relative density in cone resistance. The results obtained from the simulations did fit closely the trends that had been previously established using physical chambers. That result was taken as a general validation of the proposed simulation approach. From the micromechanical point of view, the granular material is highly discontinuous and inhomogeneous. Obtaining a homogeneous initial state (especially in the zone of the penetrating cone) is crucial to obtain easily interpretable results. Specific procedures to assess initial state inhomogeneities were developed. DEM-based models can provide results at various level of resolution i.e. the microscale, the meso-scale and the macro-scale. A large series of VCC CPT has been performed. Simulations were performed for models with different horizontal servo-control walls, various sizes of chamber, cone and particles and two boundary conditions. The results were analyzed, focusing on aspects such as chamber size, particle size and boundary condition effects on steady state cone resistance values. A smaller number of tests have also been examined from the point of view of shaft resistance. Most trends and results obtained are shown to be in agreement with previous physical tests. When disagreements appear, the causes are identified: the most severe disagreements result from initial inhomogeneities in the discrete model. The work described in this thesis showed ease the burden of future CPT calibrations in granular materials.
This paper describes the design of a calibration chamber suitable for the preparation of uniform clay beds in which the performance of full-size field test devices may be studied. Details are given of the clay bed preparation procedure by which clay slurry is initially consolidated under K0 conditions and then further conso using equal or unequal horizontal and vertical stresses. Field equipment may be tested in the clay bed, which is maintained under known horizontal and vertical boundary stresses during the test. Self-boring pressuremeter tests have been successfully carried out in the clay bed.
Cone penetration testing (CPT) has become the industry standard for in situ testing of cohesionless soils, and in particular, field liquefaction evaluation. The empirical methods for the interpretation of CPT data are either based on field data or the observation of CPT measurements in laboratory samples. In this study, a miniature cone penetrometer (with a diameter of 6 mm) is developed for understanding the response of loose to medium-dense sands. A modified triaxial cell is used for sample preparation and containment of the sample during cone penetration. The miniature cone can measure cone tip resistance, sleeve friction, and excess pore water pressure developed at the cone tip. While cone tip resistance is measured by a separate load cell, sleeve friction is obtained by subtracting cone tip resistance from a combined measurement of tip resistance and sleeve frictional force. Due to the free-draining nature of the sand tested in this study, no excess pore water pressure is developed during cone penetration. The measured data from the miniature cone are verified by comparison with CPT resistances measured in several other calibration chamber experiments on similar sands. Compared to a large calibration chamber with a standard size cone, the miniature cone allows quicker and less expensive CPT experiments in a more uniform sample.