Time-dependent squeezing deformation mechanism of tunnels in layered soft-rock stratum under high geo-stress

Large squeezing deformation of layered soft rock tunnel under high geo-stress has a significant time-dependent deformation behavior.In this paper,we studied the deformation mechanism during the construction period of deep-buried softrock tunnel by means of a combination of field observations and a numerical method.First,a new classification criterion for large deformations based on the power exponent variation law between the deformation and the strength-stress ratio is proposed.Then,the initial damage tensor reflecting the bedding plane(joint)distribution and an equivalent damage evolution equation derived from the viscoplastic strain are introduced based on the geometric research method,i.e.,a new rheological damage model(RDL model)of layered soft rock is established consisting of elastic,viscous,viscoelastic,viscoplastic and plastic elements.A field test was conducted on the Maoxian tunnel in Sichuan province,southwestern China,which is in broken phyllite(layered soft rock)under high geo-stress.The tunnel has experienced large deformation due to serious squeezing pressure,thus we adopted double primary support method to overcome the supporting structure failure problems.The rheological parameters of phyllite in the Maoxian tunnel were recognized by using SA-PSO optimization,and the RDL model does a good job in describing the time-dependent deformation behavior of a layered soft-rock tunnel under high geo-stress.Thus,the RDL model was used to investigate the supporting effect and bearing mechanism of the double primary support method.Compared with the single primary support method,the surrounding rock pressure,secondary lining force,surrounding rock deformation,and the depth of the damage to the rock mass was reduced by 40%-60%after the double primary support method was used.

Effect of stress paths on failure mechanism and progressive damage of hard-brittle rock

During deep buried hard-brittle rock tunnel excavation,the surrounding rock experiences a complicated stress path and stress adjustment process.Once the adjusted stress exceeds the ultimate bearing capacity of rockmass,a rock failure mode defined as stress-cracking type will occur.In order to investigate the effect of stress paths on failure mechanism and progressive damage of deep-buried rockmass,the cyclic loading-unloading,loading-unloading,uniaxial,conventional and unloading triaxial compression tests on samples of hard-brittle sandstone were conducted.According to the experimental results,increase in the confining pressure was beneficial to improve the mechanical parameters of rock,but it will reduce the brittle failure features.Compared with conventional triaxial compression,the sandstone under unloading state had more remarkable stress drop and unstable failure characteristics.Meanwhile,it was found that the energy dissipation and energy release in the whole process of rock deformation were the internal power of driven rock progressive damage.With the increase of confining pressure,the energy hardening and energy accumulation features of rock were weakened,while the progressive damage evolution characteristics could be enhanced.In unloading state,more energy could be converted into elastic energy in the energy softening phase(σeb-σP),so that the prepeak damage rate of rock was lower than that of conventional triaxial compression state.Thus,the energy dissipation rate of rock after peak strength decreased linearly with the increase of confining pressure under conventional triaxial compression state,while in unloading state it showed the opposite law.

四洞交叠盾构隧道近距离下穿既有桥梁桩基施工控制技术研究

在密集的城市建筑环境中,在城市地下交通的大规模兴建过程中不可避免地会出现各类近接施工工程。依托一实际工程,针对四洞交叠盾构隧道近距离下穿既有桥梁桩基施工控制技术开展实践研究,总结提出了“两阶段法理论预测+主动控制与被动保护”的近接施工微扰动控制技术体系。该技术体系的核心内容包括:1)通过引入附加地层损失率,建立多线隧道近接施工时桩基变形的理论计算方法,实现近接施工对邻近建(构)筑物影响的工前预测。2)建立隧道-地层-桩基多元耦合模型,分析四线交叠盾构隧道施工全过程力学行为。3)结合工程实践经验,分别从主动控制和被动保护两方面提出依托工程的控制措施。在依托工程开挖完成过程中,邻近桥梁桩基变形得到有效控制,表明所提出的近接施工微扰动控制技术是合理可行的。

Vertical temperature gradients of concrete box girder caused by solar radiation in Sichuan-Tibet railway

Spatial and temporal temperature variations are critical for concrete box girders,and non-uniform temperature distributions induced by solar radiation depend on the structural shapes and shadows cast on them.There have been many studies of temperature distributions and temperature gradients of concrete box girders,but few have considered a high altitude plateau climatic environment.In this study,the nonlinear temperature distributions of concrete box girders in the Sichuan-Tibet railway caused by solar radiation were investigated based on experimental analysis,real-time shadow-selection algorithm,and finite element method.Furthermore,a vertical temperature gradient model of the concrete box girders was obtained.The vertical temperature gradient values first rise,then decrease,and finally rise again from Chengdu to Lhasa,with samples forming a normal distribution.The recommended vertical temperature gradient value was 25℃with a confidence interval of 95%.This provides a reference for the design and maintenance of concrete box girders on the Sichuan-Tibet railway.

基于蒙特卡洛模拟的图像二值化增强算法（英文）

本文基于蒙特卡洛模拟与局部阈值思想,提出了一种能够适应图像复杂背景、亮度不均条件的灰阶图像二值化分割方法。该方法将灰阶图像划分为多个正方形子图像,每个子图像均反映了灰阶图像的局部信息。先利用大津法对每个子图像进行二值化分割,再将所有二值化后的子图像重新合并后形成灰阶图像的二值化结果。针对局部阈值分割过程中子图像的尺寸选取问题及二值化后子图像合并时的边界效应问题,本文结合蒙特卡洛模拟思想提出了改进算法。将子图像尺寸作为蒙特卡洛计算步中的随机变量,通过大量迭代计算获取灰阶图像中每个像素被分割为目标和背景的概率,并按照指定概率阈值对其进行划分。为验证所述方法的可行性与准确性,本文依托多个灰阶图像案例对方法的二值化结果进行了测试,结果表明,本文提出的方法能够较好地处理复杂背景及亮度不均条件下的灰阶图像。本方法可为区域性遥感影像的解译和地物识别提供支撑。

基于结构参数不确定的列车-轨道-路基系统耦合振动概率分析

列车荷载作用下有砟轨道-路基耦合系统结构参数的不确定性引起的随机振动对行车安全的影响不可忽略。本文提出了一种基于概率密度演化理论的三维列车-有砟轨道-路基耦合系统随机振动概率仿真分析模型,该模型可同时考虑轨道随机不平顺、道砟层和路基层随机刚度、随机阻尼等耦合效应,为列车-轨道-路基系统随机振动分析提供了一种全新的计算分析方法。采用数论方法对多维随机参数进行代表性离散数组点设计。结果表明,与蒙特卡罗模拟方法相比,概率密度演化模型开展随机振动分析更为准确、高效。基于算例开展了多维随机参数的敏感性分析,系统地评估了不确定结构随机参数的相对重要性,并在此基础上探讨了列车-轨道-路基系统的随机概率演化机制。

锚杆框架结构支护三级边坡的地震响应特性研究

结合某工程实践中的典型边坡原型,基于FLAC3D代码建立了锚杆框架结构支护三级土坡的数值模型。通过施加一系列双向汶川地震动激励,研究三级土坡和锚杆框架结构的动力响应,获得了三级结构的破坏状态、土坡的加速度响应、框架结构的位移响应以及锚杆轴向应力。在不同的激励工况下,每级边坡顶面的水平和垂直加速度响应均最大。各级土坡间设置的平台减少了加速度响应的放大效应。随着地震动强度的增加,框架结构的地震残余位移显著增加。每一级框架结构呈现出平移及绕顶部旋转的组合位移模式。框架结构的锚杆轴向应力在汶川地震动激励的第一个振动强烈时段显著增加,且对地震动强度敏感。地震荷载作用下第一级框架结构的锚杆轴向应力增幅最为显著。

振动台试验中刚性模型箱人工反射边界的合理设计试验研究（英文）

采用刚性箱进行隧道等地下工程结构地震动力试验研究时,合理的边界条件是关系到试验结果准确的关键因素之一,特别是与地震激励方向垂直的人工反射边界。本文在大量文献调研的基础上,设置四种典型边界,并考虑地震烈度和地震波类型,分别开展了振动台试验。进而,根据试验结果,分别从加速度峰值偏差、加速度时程谱和傅立叶频谱的相似性以及刚性箱内土体表面不均匀沉降四个方面对四种边界的模拟效果进行了评价分析。结果表明:振动台试验中,边界效应的大小,不仅受边界材料性质的影响,还与地震烈度、地震波类型以及试验箱内填充介质的滤波特型有关;相对于其他三种边界,聚乙烯泡沫板边界模拟效果最为理想,且随着地震烈度的增大,其边界效应有逐步减小的趋势。最后,结合既有研究资料,对振动台试验中的边界效应评价标准,边界材料类型及厚度的确定进行了讨论与总结,给出了相应的设计方法和建议。

高含石率胶结型土石混合体力学性能试验研究（英文）

本文基于单轴压缩以及共振频率试验,对多组不同特征的高含石率胶结型土石混合体试件进行测试,以探究其物理力学特性。试验结果显示,试件的动弹性模量与其单轴抗压强度、弹性模量以及含石率均存在显著的相关性。此外,试件的密度以及基质强度也与其宏观力学性能密切相关。因此,选取以上三项典型指标,对高含石率胶结型土石混合体的力学性能进行预测。除传统的回归分析手段以外,本文通过遗传算法对广义回归神经网络算法进行优化,并建立了相应的预测模型。预测结果表明,尽管多元回归分析相对于一元回归分析而言预测性能有所提高,但基于优化回归神经网络的预测结果更为理想。分别采用试验数据以及文献中的数据,证实了所建立的预测模型具有良好的适应性和理想的预测性能。

考虑桩间泥皮对砂土-混凝土桩接触面剪切特性影响的试验研究

在对地下连续墙、钻孔灌注桩等地下结构进行施工时,需要浇筑混凝土,在土体与混凝土间随之会产生泥皮层。本研究旨在探究泥皮厚度对砂土-混凝土桩界面剪切特性的影响规律。基于桩侧不同粗糙度(I=0,10,20和30 mm)和泥皮厚度(Δh=0,5和10 mm),开展一系列的砂土-混凝土接触面大型界面直剪试验。试验结论表明:对于无泥皮的光滑界面,剪切应力-切向位移曲线呈现软化型;随着泥皮厚度的增加,该曲线由软化型转为双折线型。峰值强度和摩擦角随泥皮厚度的增加呈指数下降。当泥皮厚度由0增加至10 mm的粗糙界面,剪切应力-切向位移曲线则由软化型转变为硬化型,峰值强度和摩擦角随泥皮厚度的增加呈线性下降。发现当存在临界粗糙度Icr=10 mm,接触面粗糙度小于临界粗糙度时,剪切强度随粗糙度的增加而增大;当接触面粗糙度超过临界粗糙度时,剪切强度逐渐降低。此外,泥皮存在会降低粗糙度对接触面强度影响的敏感性。

Stability analysis on tunnels with karst caves using the distinct lattice spring model

The effects of karst caves on tunnel stability were numerically investigated using the distinct lattice spring model(DLSM).The DLSM was validated by investigating the mechanical behavior of Brazilian discs with various sizes of central circular holes.Then,the effects of karst cave on U-shaped tunnel were investigated under various karst caves positions(top,bottom,and right side of the tunnel),tunnelcave distances(0.5-4 times the radius of the tunnel arc),and cave shapes(circular,rectangular flat,and rectangular vertical caves).The failure processes of the tunnel under those various conditions were analyzed and both the failure process and the final failure patterns of the tunnel were discussed.Numerical simulation demonstrated that karst caves around the tunnel could weaken the stability of the tunnel,indicating tunnel-cave distance effects.The closer the cave to the tunnel,the weaker the tunnel under loading.This effect was not significant when the tunnel-cave distance(d)was larger than three times the tunnel arc radius(R).In addition,the final failure pattern of the tunnel and its surrounding rock mass were dependent on both the position and the size of the cave.The larger the cave,the weaker the tunnel and its surrounding rock mass.Furthermore,compared with those cases with top and bottom caves,the tunnel with a right side cave had more impacts on tunnel stability.The main research finding could help engineers carry out stability analysis on tunnels in karst areas and take effective measures to enhance tunnel stability.