【作者】 封晓伟；

【导师】 闫澍旺；

【作者基本信息】 天津大学 ， 岩土工程， 2010， 博士

【摘要】 饱和软粘土地基在波浪荷载的循环往复作用下,土体产生软化效应。表现为土体的强度降低,变形模量减小,从而使软粘土地基的承载能力大幅度降低,影响结构的安全稳定性。对长江口深水航道治理工程导堤失稳原因进行了分析和总结,提出对于修建在软土地基上的防波堤、导堤等港口结构物和建筑物,必须考虑波浪循环荷载引起的地基强度弱化现象。以天津港原状软粘土为研究对象,通过室内试验手段研究半圆体沉箱、筒型基础等港口结构物地基土在波浪作用下的强度软化特性及其稳定性分析方法。本文主要的研究内容包括:1.粘土的循环软化程度与土的灵敏度有关。灵敏度对粘性土循环软化作用的重要性与松散程度对饱和砂土液化的影响类似。采用室内十字板测试仪进行了灵敏度试验,重塑土样(充分扰动)的强度降低到原状土样的强度的22%～42%,可用来判断土样充分扰动情况下的软化程度;2.在总结前期进行的长江口航道治理工程中的导堤软粘土地基软化问题的研究的基础上,增加了新的验证性和对比性试验,试验结果取得了与前期研究较为一致的规律性成果。提出了考虑不同的应力组合和实际的应力路径加载的室内动三轴试验方法,动、静三轴试验结果经过归一化分析,呈现比较明显的规律性,建立了软粘土强度软化程度与环境条件、荷载条件的关系,确定了软粘土动力软化的判别标准,为工程应用提供了便捷手段;3.正常固结的饱和软粘土,经不排水循环后,土体抗剪强度发生降低,类似于金属的“疲劳”现象。主要原因是循环荷载使软粘土内部产生累积孔隙水压力和不可恢复的结构性破坏。基于循环荷载下累积孔隙水压力和累积变形规律的分析,从能量转化角度,探讨了软粘土在波浪动荷载作用下的强度弱化机理;4.根据半圆体防波堤上的波浪力以及地基应力分布特征,利用软粘土强度弱化程度的室内试验成果,对半圆体防波堤软土地基加固前、后的稳定性进行了有限元分析,计算结果与实测数据吻合较好,提出在强度发生软化的情况下半圆体防波堤的稳定性分析方法。5.通过对天津港原位十字板剪切强度试验结果的统计回归,提出根据原位十字板剪切试验结果确定天津港软粘土抗剪强度指标的方法。由此方法得到的抗剪强度统计回归指标可以直接用来进行边坡稳定分析,土压力计算以及地基承载力评估。根据推算的软粘土抗剪强度指标,提出箱筒型基础防波堤稳定性的简化分析方法。更多还原

【Abstract】 Strength softening may take place for the saturated soft clay under the action of wave cyclic loading. The strength of soft clay may further decrease and the modulus of deformation becomes smaller. As a result, the bear capability of the soft clay foundation will become significant lower and the stabilization of the structures will be worsened. Based on the analysis of the failure of the guide dikes for deepening of navigation channel along the Yangtze Estuary, it was proposed that the strength softening be considered when the harbor architectural or construct buildings such as guid dikes and breakwaters were buit on the soft soil foundations. The strength sofening property of the foundations soil on which the harbor buildings such as semi-circular caisson or suction caisson breakwaters were built was studied by means of performing indoor tests on the undisturbed samples taken from Tianjin harbor.1. Cyclic softening of the clay is related to the soil sensitivity. The sensitivity infulnece on clay is as important as that of the density on sand. The soil sensitivity were tested using indoor vane test device and the results revealed that the remodeled (fully disturbed) samples strength reduced to 22%～42% that of the undistrubed ones on anvarage, which can be used to determine the softening extent of the soil under fully disturbed condition.2. Depending on the research on the soft clay strength softening for the Yangtze Estuary guide dike foundations, more verification and comparison tests were performed. The tests gave similar results to those of the pervious research. The interior dynamic triaxial test method was introduced taking into account different load combinations and the actual loading path. The test results revealed obvious regularity when normalization was carried out on the dynamic and static triaxial test results. The relationship between discounting ratio and environmental and load conditions was established and then the strength discounting criterion of the soft clay could be determined which brought convinence to the engineering paractice.3. The shear strength of normal consolidated saturated clay will lowered after undrained cyclic shear which is similar with the metal fitague. The main reason is that cumulative pore pressure and unrecoverable structural failure has been caused in the clay. Based on the analysis of the accumulated pore pressure and accumulated deformation behaviour of the clay under cyclic loading, the strength discounting mechanics of the soft clay was also analyzed in the view of the energy transformation.4. According to wave loading distribution on the semi-circular breakewater and stress state of the foundation soil, the stability of the semi-circle breakwater foundation was analyzed before and after soil improvement combining the clay shear strength disconting ratio obtained from the test results with FE methods. The calculation results agree well with the observed data. The breakwater stability analysis method involving the soil softening effects was then put forward.5. By means of statistical regression analysis of the field vane test results of Tianjin Harbor soft clay, the thesis presented a method for how to determine the shear strength parameters of the clay using the in situ vane shear test results. The regression strength parameters can be used for slope stability analysis, earth pressure calculation and bearing capacity assessment. According to the correlated soil parameters, a simple analysis method for the suction caisson breakwaters was established also.更多还原