【作者】 林军；

【导师】 蔡国军；

【作者基本信息】 东南大学 ， 岩土工程， 2018， 博士

【摘要】 随着国民经济高速发展,我国的城市建设发展异常迅速,相关的地下工程越来越多,特别是沿海地区城市,大部分深基坑工程位于软土中,对基坑工程的安全性提出了更高的要求。天然沉积土经历漫长的地质年代中而形成,其岩土工程参数具有显著的空间变异性,表现为不同空间位置处,土工参数测试值具有离散性和不确定性。由于软土组成复杂,具有明显的结构性,存在钻孔取样扰动大,室内实验结果可靠性低等缺点。准确评估软土的岩土工程参数的空间变异性,需要采用高精度孔压静力触探(CPTU)原位测试技术。土工参数的变异性导致岩土工程设计方案的不确定性(例如系统功能响应的不确定性)。为了降低设计方案的系统功能响应变化对土工参数变异性的敏感程度,可以采用岩土工程稳健性设计方法。岩土工程稳健性设计方法(Robust geotechnical design),又称岩土工程鲁棒性设计方法,对岩土工程设计参数进行不确定性分析,通过调整易控的设计参数(如支护结构几何结构参数或其它易控参数)来降低系统功能响应对不可控的设计参数(土体参数)变异性的敏感程度;同时,考虑设计方案的成本,达到设计方案功能响应变化最小,设计方案成本最低的要求。论文以国家科技支撑计划项目子课题、国家重点研发计划项目子课题、国家自然科学基金项目、江苏省杰出青年基金项目和江苏省普通高校研究生科研创新计划项目为依托,基于孔压静力触探(CPTU)原位测试数据,识别海相软土的均质土体单元,研究海相软土的均质土体单元的土体参数的空间变异性特征。提出非平稳的对数正态分布随机场的条件模拟方法,研究具有深度趋势和各向异性的土体参数的空间分布规律。采用蒙特卡洛模拟,分析不排水抗剪强度的空间变异性对软土基坑(抗隆起和抗倾覆)可靠性影响。考虑土体参数的空间变异性,优化设计方案的系统功能响应变化和成本造价,获得内撑式地下连续的软土基坑稳健性设计结果。主要研究内容和结论如下:(1)基于CPTU的均质土体单元识别。基于CPTU测试的淤泥质软土不排水抗剪强度预测结果与十字板剪切试验的不排水抗剪强度结果吻合的非常好。在土性构造相同的土层内,相同CPTU钻孔内,基于不同土体参数的均质土体单元识别结果并不相同,其相互间并没有相关性。均质土体单元在Robertson土分类图内分布比较集中,其土体参数的变异性很小。(2)基于随机场理论的均质土体单元参数的空间变异性特征。采用随机场模型,对均质土体单元的土体参数的空间变异性进行研究。竖直方向上采用严格的自相关函数拟合方法研究土体参数的空间变异性,并对波动分量进行修正的Bartlett检验。水平方向上,采用平均零跨法来估计土体参数的空间变异性。结果表明,场地内没有特定的自相关函数模型可以统一描述海相软土的随机场特征。锥尖阻力和不排水抗剪强度的空间变异性具有明显的深度趋势和各向异性。通常,水平方向上的波动范围比竖直方向上的波动范围高一个数量级。(3)提出土体参数空间分布的非平稳随机场的条件模拟方法。结合CPTU测试数据,采用非平稳对数正态随机场的条件模拟方法,推导了土体参数空间分布的条件概率密度,模拟土体参数的深度趋势和各向异性的空间分布规律。该方法采用LDL~T算法分解条件协方差矩阵,对土体参数的趋势函数和协方差结构没有要求,适用于具有任意网格形式的非平稳对数正态随机场的条件模拟。(4)阐明不排水抗剪强度的空间变异性对软土基坑可靠性的影响。采用非平稳对数正态随机场的条件模拟方法,研究了不排水抗剪强度的变异系数,波动范围和空间折减特性对软土基坑抗隆起和抗倾覆可靠性的影响。结果表明,软土基坑抗隆起的安全系数的概率密度函数服从对数正态分布。软土基坑抗倾覆的安全系数的概率密度函数难以估计。若忽略波动范围的影响,会明显高估基坑的失效概率。竖直波动范围对失效概率的影响程度远大于水平波动范围对失效概率的影响程度。较小的波动范围会产生更多的空间平均效应,降低土体参数的变异系数,设计方案的失效概率也随之降低。(5)提出考虑土体参数的空间变异性的软土基坑稳健性设计方法。针对内撑式地下连续墙的软土基坑设计,将不排水抗剪强度的空间变异性引入设计方案的功能函数响应的稳健性分析中,综合考虑设计方案的功能响应变化和成本造价,采用多目标优化方法,取得稳健性设计结果。对比结果表明,稳健性设计方案的系统功能响应的变化要显著小于原始设计方案的系统功能响应的变化,稳健性设计方案的地下连续墙的内力最大值仅为实际设计方案的60%左右。稳健性设计的成本造价要显著小于原始设计方案的成本造价,稳健性设计方案的钢筋面积仅为实际设计方案的70%左右。更多还原

【Abstract】 With the national economy growing,urban construction in China is so fast,and there are more and more underground projects,especially in coastal cities.Most deep excavation projects are located in soft soil,which puts forward more strict safety requirements.Soils has been formed in a long geological history,thus remarkable Spatial variability occurs when geotechnical parameters of soils that are measured at different spatial locations exhibits values that differ across the locations.The complex composition and structure of soft soils result in a high borehole sampling disturbance and low reliability of labs experiment results.To evaluate the spatial variability of geotechnical parameters of soft soils,multifunctional piezocone penetration test(CPTU)is in urgent need.The spatial variability of geotechnical parameters leads to uncertainty in geotechnical design(eg.uncertainty in system response).In order to reduce the sensitivity of the system response of geotechnical design to the spatial variability of the geotechnical parameters,the geotechnical robust design method puts to use.Robust geotechnical design seeks the reduction in the variation of system responses by adjusting only the“easy-to-control”design parameters,and not the“hard-to-control”noise factors,on the basis of uncertainty analysis of geotechnical parameters,simultaneously satisfying safety and cost requirements.The paper is supported by the National Key R&D Program of China,the National Key Research and Development Program Subproject,the National Natural Science Foundation Project,the Foundation of Jiangsu Province Outstanding Youth and the Jiangsu Provincial University Graduate Research Innovation Program.The discerning results of homogeneous soil units(HSU)of soft soils are presnented,and the spatial variability of of soft soils are studied detailed based on the identification results of homogeneous soil uinsts with CPTU data.A conditional simulation method of non-stationary log-normal distribution random field is proposed to study the spatial distribution of soil parameters with a depth trend and anisotropy in scale of fluctuation.The Monte Carlo simulation was used to analyze the effects of spatial variability of undrained shear strength on the reliability of excavation in soft soil.The Robust geotechnical design of braced excavation is achieved by optimizing the system response and design cost considering the spatial variability of geotechnical parameters.Here are some research conclusions:(1)Identification of homogeneous soil units based on CPTU.The undrained shear strength is predicted based on the CPTU data,match the results of Field Vane test very well.In the soil layer with a same soil structure and CPTU borehole,the identification results of homogeneous soil units of different soil parameters differs from each other,and there is no correlations between them.The homogeneous soil uints distributed closely in the Robertson soil classification chart,and the variability of soil parameters is relatively low.(2)Spatial variability of homogeneous soil unit parameters based on CPTU data The spatial variability of soil parameters of homogeneous soil units is studied detailed with the random field model.In the vertical direction,a rigorous method is used to study the spatial variability of soil parameters,the modified Bartlett test is used to inspect the stationary of fluctuation components.In the horizontal direction,the expeditive method is used to estimate the spatial variability of soil parameters.The results show that there is no specific autocorrelation function model in the site describing the random field of marine soft soil well.The spatial variability of cone tip resistance and undrained shear strength has remarkable depth trend and anisotropy in scale of fuctuation.Generally,the fluctuation range in the horizontal direction is an order of magnitude higher than the fluctuation range in the vertical direction.(3)A conditional simulation method for the non-stationary log-normal random field of soil parameters is proposed.Combined with the CPTU data,the conditional probability density of the spatial distribution of soil parameters is derived.By using the conditional simulation method,the spatial distribution of soil parameters with a depth trend and aniostropy in scale of fluctuaiton is simulated.The method uses the LDL~T method to decompose the conditional covariance matrix.There are no requirements for the trend function and covariance structure of the soil parameters.It is also suitable for the conditional simulation of non-stationary lognormal random fields with arbitrary grid.(4)Revealing the influence of spatial variability of undrained shear strength on the reliability of braced excavation in soft soil.The impact of the coefficient of variation,scale of fluctuation and spatial reduction of undrained shear strength on the reliability of braced excavation in soft soil are studed with conditional simulation method.The results show that the probability density function of the safety factor of the basal heave failure obeys the lognormal distribution.The probability density function of the safety factor for push in failure is difficult to estimate.If the influence of scale of fluctuation is neglected,the failure probability will be significantly overestimated.The vertical scale of fluctuation affects the failure probability much greater than the horizontal scale of fluctuation.Smaller scale of fluctuation will cause more spatial averaging effects,lowering the coefficient of variation of soil parameters,and decreasing the failure probability.(5)A robust design method for braced excavation is proposed considering spatial variability of soil parameters.For the braced excavation design,the spatial variability of the undrained shear strength is introduced into the robustness analysis of system response of the geotechnical design.Taking into account of system response and construction cost,multi-objective optimization method is used to achieve robust design results.The comparison results show that the system function response of the robust geotechnical design is significantly smaller than the change of the system function response of the original design scheme.The maximum internal force of the diaphragm wall of the robust geotechnical design scheme is only about 60%of the original geotechnical design.The cost of robust geotechnical design is significantly less than the cost of original geotechnical design.The design area of rebar of robust geotechnical design is only about 70%of the actual design.更多还原