【作者】 王伟；

【导师】 石名磊；

【作者基本信息】 东南大学 ， 岩土工程， 2020， 硕士

【摘要】 随着我国进入高速时代,高速公路及拼宽路基等基础设施的大规模建设,因路基被动区软土剪切塑流造成的失稳问题日益受到行业内专家学者的关注和重视。疏桩补偿软土路基以其深层加固效果好、控沉稳定、经济合理等优点在软土地区修建的高速公路、高速铁路中得到广泛应用。但是软土剪切塑流失稳机制和疏桩路基的稳定控制理论研究滞后于工程实践,仍需对其工作机制进行深入系统的研究,结合工程实践完善相关分析理论和设计方法。本文基于国内外对软土在荷载作用下的侧移塑流稳定问题的相关研究,重点关注软土剪切失稳机制及其稳定控制两方面。采用理论分析和数值模拟相结合的综合研究手法,构建了疏桩补偿软土地基协力模型、疏桩荷载转移模型,提出了软土塑性区开展程度判定依据,揭示了路基被动区软土失稳机制及疏桩稳定控制方法,并结合工程实例验证了相关理论的实用性。本文主要的研究内容及成果有:（1）基于Mesri（1989）的软土地基平均不排水强度经验算式,并考虑软土不排水强度S_u随深度的演化规律,提出了简化的不排水抗剪强度计算方法。（2）基于路基疏桩的补偿协力设计原理,运用Boussinesq弹性理论应力解答叠加Mathematica算法分析得到了疏桩间软弱基体附加应力场,探讨了软基临塑荷载判定方法;基于饱和软黏土不排水强度S_u理论分析,完善了路基下软土地基剪切塑流的弹塑性理论分析方法,阐明了桩间软弱基体附加应力显著减小的疏桩补偿软土的“减沉”机制。进一步通过算法分析了软土路基加载情况下被动区坡度、反压台设置方式及硬壳层对路基稳定的影响,同时关联被动区坡趾不利地貌影响机制,结合工程案例提出塑性区开展面积稳定控制指标（105 m²～141m²）,完善了软土地基附加应力场分析方法。（3）根据极限状态设计原理中的使用极限状态设计原则,完善了基于路基基底荷载疏桩分担（应力集中）补偿稳定概念模型,同时提出路基疏桩协力承载力检算方法,可用于软土地基路基疏桩间距与桩长的协配设计或检算。（4）基于Unit Cell单元分析模型,结合Marston路基内柱面剪切位移“土拱”效应理论原理,推导出柱面微单元微分控制方程组的剪切位移解析解答。同时引入柱面剪切位移理想弹塑性模型,建立了碎石垫层应力扩散分析模型,推导出疏桩桩帽顶碎石垫层扩散后的内土柱底面（h=0）荷载分担比λ₀及其与基底λ_b的演化规律。通过上述疏桩地基桩土剪切位移的荷载传递理论分析方法和路基内柱面剪切位移的基底荷载转移理论分析方法,进一步完善了路基下部灰土层、中部填土和上部灰土路床典型三层体系的柱面弹塑性状态分析理论,并结合工程实例验证了路基疏桩基底构造检算方法的适用性。（5）对路基加载稳定控制标准（[VD]=5mm/d、[VS]=10mm/d）和路基预压沉降收敛控制标准给出了清晰的论述,用于控制填筑速率的加载期稳定监测和预压期收敛监测,除了确保路基安全稳定,还起到把握卸载时机与面层施工时机的作用。（6）基于路基欠载预压、等载预压、路基填土联合预压三种工况,推导简化出工后沉降关联沉降率表达通式;根据路基填筑分级加载,对被动区软土水平位移增量与路基沉降增量作归一化处理,提出了相对位移稳定特征指标,结合上述路基相关控制标准,可指导工程实践;同时,提出施工图设计和预压期沉降率收敛检算方法。更多还原

【Abstract】 As China enters the era of high speed,the large-scale construction of infrastructure such as highways and wide roadbed,the instability caused by the erosion flow of soft soil in the passive area of roadbed has been paid more and more attention by experts and scholars in the industry.Scattered piles as compensation of soft soil subgrade is widely used in highway and high-speed railway in soft soil area,because of its advantages such as better deep reinforcement effect,settling control stability and reasonable economy.However,the stability control theory of scattered pile subgrade lags behind the engineering practice,so it is urgent to carry out systematic research on its working mechanism and improve the relevant analysis theory and design method.Based on the research of stability problem caused by lateral plastic flow in soft soil area under load at home and abroad,this paper focuses on the shear instability mechanism of soft soil and its stability control.The laboratory test,theoretical analysis combined with numerical simulation technique,constructs the synergistic model of scattered pile as compensation to soft soil,the load transfer model of scattered pile,put forward the judgement on the basis of the plastic zone of soft soil,reveals the instability mechanism of soft soil roadbed in passive area and stability control method of scattered pile,and the practicability of relevant theories are proved combined with engineering example.The main research contents and achievements of this paper are:（1）Based on the ratio of the average undrained strength of soft soil foundation to the previous consolidation pressure was constant 0.22（Mesri,1989）,and the calculation method of undrained shear strength was simplified by considering the evolution law of the undrained strength of soft soil with depth.（2）Based on the cooperative design principle of roadbed scattered piles,this paper uses Boussinesq elastic stress theory and mathematical algorithm to gain the additional stress field of the weak part between scattered piles,proposes a method for judging the critical load of soft soil foundation.Based on the theoretical analysis of undrained strength（S_u）,the paper also complete the elastic-plastic theoretical analysis method of shear flow of soft soil under subgrade,reveals the instability problem of passive zone caused by plastic flow,and indicates the mechanism of"reducing settlement"of soft soil compensated by scattered piles because of the significant reduction of additional stress of weak part between piles.Furthermore,the paper analyzes the influence of the passive slope ratio,the setting mode of the anti-pressure platform and the hard-shell layer on the stability of the subgrade under loading of the soft soil foundation by the algorithm.Meanwhile,the mechanism of the adverse landform of the slope toe and the area index（105 m²～141m²）in the passive area,is correlated to improve the analysis method of the additional stress field of the soft soil subgrade.（3）According to the design principles of limit state,the stability concept model of scattered pile as compensation to soft soil was proposed that scattered piles sharing load（stress concentration）can inhibit shear plastic deformation of subgrade,and puts forward the bearing capacity computation method of subgrade scattered piles,which can be used in the soft soil roadbed to design and check the spacing and length of scattered piles.（4）Based on unit cell analysis model and combining with the theory of"soil arch"of subgrade cylindrical shear displacement proposed by Marston,this article derives the analytical solution of cylindrical micro-element differential control equations of shear displacement.And this paper introduces the ideal elastic-plastic model of cylindrical shear displacement,establishes the stress diffusion analysis model of the gravel cushion,and derives the evolution law of the load with single ratio ofλ₀ on the bottom surface of the scattered pile（h=0）and the baseλ_b,after the diffusion of the gravel cushion on the top of the cap of the scattered pile.Based on the theoretical analysis of load transformation of scattered pile shear displacement and base load transformation of cylindrical shear displacement in the subgrade.Based on the theory of cylindrical elastic-plastic state analysis,the typical three-layer system of the lime-soil layer under roadbed and the filling in the middle and upper lime-soil roadbed are further established,the applicability of the method to check the foundation structure of roadbed scattered piles is verified with an engineering example.（5）This paper systematically indicates that stability-control standard of roadbed loading（[VD]=5mm/d、[VS]=10mm/d）and the convergence-control standard of roadbed preloading settlement.The stability monitoring during the filling period is mainly used to control the filling rate and ensure the stability of roadbed.The main unloading time and the construction time of surface layer are monitored during the preloading period.（6）Based on the consolidation expression of equal load preloading proposed by Zen Guoxi（1981）,the paper puts forward the stability index of the increment of horizontal displacement of soft soils in the passive zone during graded loading and the increment of embankment settlement,and derives the general formula of subgrade under load preloading,preloading,and overload preloading associated with settlement and sedimentation rate.Combining with the relevant control standards,which can guide the engineering practice.In this paper,the design of construction drawing and the convergence checking method of settlement rate in preloading period are put forward.更多还原