【作者】 柳飞；

【导师】 郑西来； 豊澤康； 杨俊杰；

【作者基本信息】 中国海洋大学 ， 环境工程， 2010， 博士

【摘要】 随着土工离心模拟技术的不断发展,离心模型试验已成为解决工程问题的一种强有力的手段,在环境工程、建筑工程、水利水电、近海、交通、采矿等工程中广泛应用。但由于承载力离心模型试验中粒径效应问题的存在,使得离心模型试验结果存在误差,不能很好的模拟原型的各种特性。本文在研究基础埋深对圆形浅基础粒径效应的影响的基础上,详细研究了端承型桩承载力离心模型试验中的粒径效应问题,并讨论了基础埋深对基础尺寸效应的影响,探讨了承载力离心模型试验中颗粒破碎性与粒径效应之间的关系。首先利用modelling of models(模型的模拟)的方法研究基础埋深对圆形浅基础承载力离心模型试验中的粒径效应的影响规律。使用的地基材料为经过粒度调整的河砂,其平均粒径为0.06cm,模型基础直径为0.5～4cm,模型基础直径与地基材料平均粒径的比值在8.3～66.7之间,模型基础埋深与直径的比值为0,0.5和1.0；模拟原型直径为20～150cm。利用粒径效应评价指标对试验中的粒径效应进行定量讨论,结果表明,随着基础埋深的增大,粒径效应的范围及对试验结果的影响程度减小。同时,给出粒径效应范围图,用于选择合理的模型基础尺寸或地基材料尺寸。通过一系列不同埋深情况下的圆形基础承载力离心模型试验研究基础埋深对基础尺寸效应的影响,并探讨粒径效应和基础尺寸效应之间的关系。试验结果表明,基础尺寸效应的程度和影响范围均随基础埋深的增大而增大,即随着基础埋深的增加,承载力试验中的基础尺寸效应逐渐变得显著,由此推测,承载力系数Nq同样存在基础尺寸效应问题,且随着基础埋深的增加,Nq的基础尺寸效应逐渐明显。同时,对于相同的基础埋深,基础尺寸效应的影响随基础直径的增大而增大,即在离心场中,基础尺寸效应随粒径效应的减小而变得显著。利用不同长径比情况下的端承型桩承载力离心模型试验,研究桩基承载力离心模型试验中的粒径效应问题。在模拟同一原型时,不同桩径的模型桩,桩身压缩性及桩长均不同,导致侧摩阻力发挥机理及程度不同,本文分别探讨了桩端阻力,侧摩阻力及承载力(桩顶荷载)的粒径效应对承载机理和承载特性的影响。结果表明,桩端阻力的粒径效应的作用规律与浅基础一致,可以借用浅基础的粒径效应定量评价方法评价端承桩承载力离心模型试验中的粒径效应。侧摩阻力的粒径效应比桩端阻力的粒径效应显著。由于侧摩阻力的影响,相同条件下承载力的粒径效应比桩端阻力有所增强。对于极限桩端阻力和极限承载力,粒径效应均随长径比的增加而减弱。为了探讨承载力离心模型试验中的颗粒破碎性与粒径效应之间的关系,对承载力离心模型试验中的基底砂进行了颗粒分析试验,并用电子显微镜对基底砂进行了观察。结果表明,当换算基础直径和埋深较小时,不同直径模型基础承载力试验基底土样的颗粒级配曲线基本重合,颗粒破碎现象不明显。而随着换算基础直径和埋深的增大,不同直径模型基础承载力试验基底土样的颗粒级配曲线则并不完全重合,尤其粒径较小部分的曲线差别较大。即随着承载力的增加,颗粒破碎现象变得明显。通过电子显微镜的观察,在大多数试验条件下,基础底部的砂土颗粒都出现了破碎现象,且当极限承载力大于2174kN时,破碎现象比较明显。应力水平的增加对颗粒破碎性的影响大于基础埋深(长径比)对颗粒破碎性的影响。而随着换算基础直径的增大,粒径效应增强,因此,随着颗粒破碎性的增强,粒径效应变得明显。更多还原

【Abstract】 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.更多还原