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Analysis of cone tip resistance in sand Ahmadi, Mohammad Mehdi

Abstract

The cone penetration test (CPT) has been used for decades in in-situ geotechnical engineering practice. The reliability and repeatability of the CPT measurements has increased its acceptance as a predominant tool in this field. The cone is pushed into the soil at a standard rate of 20 mm/s, and different measurements such as pore water pressure, sleeve friction, and most importantly cone tip resistance can be made. These measurements are then used to obtain information regarding stratigraphy of the site. Over the years there has been a high demand for validated correlations between cone resistance and engineering properties of soil. The correlations for sands are mostly obtained from experiments in calibration chamber tests with specified boundary conditions. The correlations for clays are mostly obtained from laboratory tests on undisturbed samples. During the course of this study, several approaches to analyze the cone penetration process were investigated; and different codes using the computer program FLAC were written. Only two of these approaches are worth mentioning in this thesis. In the first approach, the cone is placed in a predetermined location in the grid, and is given a downward vertical displacement. Analysis is carried out to seek stresses that remain constant with continued increase in displacement. In this approach, the analytical results show that with continued penetration, the soil stresses around the cone tip do not reach a constant value. This is especially true for sand; and it is unacceptable. This approach was not pursued further in this study. In another approach, the complete process of cone penetration is modeled as the cone starts to penetrate the soil from the ground surface to deeper layers. The results obtained in this approach are reasonable. Based on this approach, numerical results are compared with experimental values from calibration chamber tests on clean, non-cemented, unaged sands. The proposed model is verified by comparing the numerical values with the published experimental results obtained on Ticino sand at the ENEL-CRIS calibration chamber. The results from all four different boundary condition types (BC1 to BC4) used in the experiment are compared numerically. It is shown that the second approach gives numerical values of tip resistance that are in agreement with calibration chamber test results. The agreement is, in general, in the range of ±25%. The Mohr-Coulomb elasto-plastic soil model with stress dependent parameters is used for both approaches. Several applications of the proposed simulation are then presented. The importance of horizontal effective in-situ stresses on the cone tip resistance is addressed in some length in this thesis. It is shown that with the proposed simulation, horizontal stresses play a major role in affecting the magnitude of tip resistance in sand. This is supported by measured experimental results. Numerical simulation is also carried out to investigate the calibration chamber size effect. This study is important to correlate the calibration chamber test results with field measurements for the sand of the same relative density and horizontal and vertical stresses. These simulations are performed for chambers of different sizes under all four different boundary conditions. The simulation is carried out for loose as well as dense sands. The numerical simulation clearly shows that for loose sand, calibration chamber size effect is not significant. For dense sand, however, the effect can be substantial. This is in agreement with the experimental observations. Analysis of cone penetration in layered soil is also addressed. In the analysis, the soil layers consist of sands with two different relative densities, i.e. loose and dense, or layers of two different soil types; i.e. sand and clay. The interface distance that the cone "senses" the approaching new layer is predicted in the numerical analysis. The predictions agree with those measured during experimental tests. Another application of the proposed model is to investigate what property of soil affects the tip resistance to a larger extent. To this end, a sensitivity analysis is carried out. It is seen that the deformational properties of soil, i.e., modulus and dilatancy properties are the most influential in affecting the cone tip resistance values. Finally, a preliminary analysis of pore pressure response during cone penetration in sand is carried out to investigate whether the proposed model can predict, in a reasonable way, the generation of pore pressure around the cone. The preliminary analysis gives results that seem to be promising. However, more work is needed to fully clarify all the intricacies of the pore pressure analysis during cone penetration in sand.

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