Study On The Particle Size Effect And Scale Effect In Model Test

With the development of centrifugal model tests, the centrifuge has become a valid method in many fields. Though centrifugal model test, the environmental engineering, civil engineering, hydraulic projection, offshore engineering, highway bridge construction project and mining can be studied in the same stress level as the prototype. But because of the influence of the particle size effect on the centrifuge test, the centrifuge test can't reflect the characters of the prototype. In this thesis, the influence of the depth on the particle size effect in bearing capacity centrifuge tests is studied at first, and then the particle size effects on the centrifuge bearing capacity tests of the end bearing pile are researched. The influence of the scale effect and the particle crushing on the particle size effect is also studied.Fristly, the influence of the depth on the particle size effects of the circle shallow footings in centrifugal bearing capacity tests are studied by modelling of models. In the tests, the ground is made by the river sand whose grain diameter is adjusted by sieving and the mean grain diameter is 0.06cm. The diameter of the model footings changed from 0.5 to 4cm and the ratio of the footing diameter to the mean grain diameter is 8.3 to 66.7. The ratio of the depth to the footing diameter is 0,0.5 and 1.0 and the equivalent footing diameter is 20 to 150cm. The evaluating index is used to discuss the particle size effects in the tests and the results show that the influence of the particle size effects is decreased as the depth increased. And based on the test results, the figure which indicates the scope of the particle size effects is deduced and it can be used to choose the reasonable model footing size and the ground material.Secondly, the bearing capacity tests are conducted in a centrifuge to examine the influence of the footing embedment on the scale effect of the footings. Test results show that the scale effect becomes more significant with the increasing of footing embedment and there also exists a scale effect for the bearing capacity Nq, and Nq decreases more sharply as the footing embedment increases. The scale effect also becomes more significant with the increasing in the model footing diameter for the same footing embedment and in the centrifuge model test, the scale effect become smore significant in the case of the particle size effect less obvious.Thirdly, the particle size effect on the centrifuge bearing capacity tests of the end bearing pile are studied by modeling of models. For the same prototype, because the different model piles had different length and compressibility, the mobilization of the shaft resistance is different. In this thesis, the influence of the particle size effect on the end resistance, the shaft resistance and the total pile resistance are discussed respectively. The test results show that the rule of particle size effects on the end resistance is the same as the shallow foundation, and the index evaluating the particle size effects on the shallow foundation can be used to assess the influence on the end resistance. The particle size effect on the shaft resistance is more obvious than the end resistance. And because of the shaft resistance, the particle size effect on the total pile resistance is also more marked than the end resistance in the same condition. For both the end resistance and the total pile resistance, the particle size effect decreases as the slender ratio increased.Lastly, the particle size analysis is conducted and the electron microscope is used to study the particle crushing in the bearing capacity centrifuge tests. The results show that the curves of grains size distribution of different footing diameter are almost the same when the equivalent footings and depth are small and the particle crushing is not significant. But as the equivalent footing diameter and the depth increase, the curves of grains size distribution are different from others, especially the part of small grains size. The particle crushing becomes significant with the bearing capacity increasing. Though the electron microscope, the sand under the model footings present the particle crushing in the most tests, and the particle crushing becomes obvious when the ultimate bearing capacity is greater than 2174kN. The influence of the stress level on the particle crushing is more than that of the depth. Because the particle size effect becomes more significant with the equivalent footing diameter increasing, the particle size effect is more obvious when the particle crushing is marked.