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Vol. 11 10 No. 12 2004 2003 19 61
Ingeokring Newsletter THESIS ABSTRACTS Abstract thesis Jeroen van Nes TU Delft Application of computerized tomography to investigate strain fields caused by cone penetration in sands. Everywhere in the Netherlands, where a structure is built, a study of the shallow subsurface is done. It is of major importance to know the geotechnical properties of the soils on which the structure is to be founded. One of the most important methods to explore the subsurface is the cone penetration test (CPT). A large number of correlations between cone penetration test parameters and several geotechnical soil properties exist but have a low degree of repeatability. Empirical relationships require local tuning. Analytical relationships make strong assumptions on failure structures, which are believed to be created by the CPT test but have not been really investigated. Numerical codes have problems in simulating cone penetration correctly and their results are only partially validated with cone resistance measurements. The objective of this thesis is to visualize and characterize the deformation patterns taking place during cone penetration using X-ray Computerized Tomography (CT). Its outcome can be used to improve hypotheses made in analytical models or validate new numerical models, either finite element models or discrete element models. For this purpose, homogenous dry sand samples of various densities are prepared with dry sand pluviation. Baskarp sand is selected due to the finesses, well sorting and small amount of contamination by deviate minerals and materials. Sand samples are created with a density varying between 1,51 g/cm3 and 1,68g/cm3. Small-scale penetration tests are conducted with a 6 mm diameter aluminium cone in calibration chambers with varying density, confining pressures and boundary properties. Measured cone penetration resistances range from 5 MPa in loose sand under low confining pressure (40 kPa) up to18 MPa in dense sand under high confining pressures (80 kPa). The measured lateral strains ranges from almost zero in loose sand, up to 0,0035 in dense sand. Taking in consideration the measurements of the cone penetration resistance and lateral strain measurements it can be concluded that the repeatability of the small scale CPT test is adequate. To investigate the deformation patterns and failure Spatial distribution of the absolute density of the initial sand sample (left) and penetrated sand sample (right). The horizontal lines in the sand sample are placed on purpose in order to obtain a better visualization of the deformation occurring during the CPT test. structures, computerized tomography, a nonedestructive investigation method, is used to analyse the sand sample before and at several stages during the CPT tests. By obtaining the linear relationship between the Hounsfield Values observed in the CT-analyses and the sand density it is possible to define the density changes and volumetric strain in the sand samples. Several scanning parameters are to be set before any CT-scanning can begin: the X-ray intensity X-ray beam strength and slice width. Also the diameter of the sample influences the image quality obtained by the CT-scanner. Several tests show that the optimum scanning parameters are a high X-ray intensity (to decrease noise), a moderate high X-ray beam strength (to decrease beamhardening effects and to keep the loss of beam sensitivity to a minimum), and a slice thickness of 1 mm (to obtain an adequate resolution in the direction perpendicular to the slices). The diameter of the sand samples used during this thesis study is 10 cm but it is also possible to use larger diameters up to 15 cm. In general the test results showed that a dilating area is formed ahead of the cone tip during the penetration, creating a dilating zone around the probe. A very dilating zone was found at 1-2 mm from the probe and shear bands are believed to be present in these zones. Outside this dilating area an area is observed which is not influenced or slightly compacted by the penetration test. In some cases some compaction takes place in front of 62
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