共查询到20条相似文献，搜索用时 46 毫秒
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非均质是软黏土地基中比较普遍的现象,而目前隧道开挖面稳定研究中比较成熟的理论主要是针对均质土体。因此,从塑性极限分析上限法的基本原理出发,采用平面应变隧道刚体平动破坏模式(多块体上限法),考虑软黏土地基的非均质性,推导了平面应变隧道极限支护压力关于隧道埋深、土体重度及土体强度的上限公式。通过与其他方法的比较分析,证明了极限分析方法在隧道开挖面稳定性方面的可行性;利用该方法的计算结果详细探讨了隧道开挖面稳定的影响因素;而且由计算结果可知,地基土的非均质性在影响隧道开挖面极限支护压力的同时,也影响着隧道开挖破坏面的位置和形状,为工程实践提供重要的理论依据。 相似文献
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基于将塑性上限分析等效为弹性迭代计算的总量虚拟加载上限分析理论,在商业化有限元软件ABAQUS中实现了弹性有限元虚拟加载上限方法（弹性有限元T-EMSD）。应用弹性有限元T-EMSD法分析了不排水黏土中的二维水平受荷桩,其获得的荷载-位移曲线与弹塑性有限元分析结果一致,其极限承载力与塑性解相近。在极限位移加载量下弹性有限元T-EMSD法对应的上限机构从弹性始速度场开始随迭代逐渐演化,迭代收敛后的速度场和解析塑性破坏机构相似。与其他基于可变强度概念（MSD）的方法相比,弹性有限元T-EMSD法对水平受荷桩桩身的分析具有更高的精度。弹性有限元T-EMSD法最大的优势在于可在计算中自然地获得塑性机构,因而可被用于研究一些塑性机构难以构造的复杂问题,并对弹塑性数值方法进行验证。 相似文献
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极限分析方法是土边坡稳定性分析的重要方法之一。刚体有限元上限法是其中的一类,此类方法仍旧存在一些关键问题需要完善。由于单元的刚性假设,系统的塑性变形内能耗散仅发生在单元间的界面上,故此类方法的性能主要取决于界面的布局,即采用非结构化三角形单元计算往往精度较差。为此,提出了基于滑动面摄动的刚体有限元上限法及临界滑动面的搜索方法。首先,在考虑刚体转动的基础上构造刚体有限元上限法的二阶锥规划模型,用于确定在给定试滑动面条件下的运动许可速度场。其次,将试滑动面的控制参数视为决策变量,建立搜索临界滑动面的非线性非凸优化问题模型,并采用非线性单纯形方法和粒子群方法求解此优化问题找出临界滑动面。通过经典边坡稳定问题的分析求解,验证了所提出的新方法,进一步证实了网格类型（即界面的布局）是影响刚体有限元上限法计算精度的主要因素。经过计算结果的对比发现,在刚体有限元上限法中考虑刚体转动是非常必要的,不仅可以提高刚体有限元上限法的计算精度,还可以克服此方法对界面布局的依赖性。 相似文献
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A mesh-free based limit analysis approach is proposed, to determine the upper bound solutions for the collapse loads associated with cohesive soils, under plane strain conditions. In the presented technique, the geometry of problem is just simulated by nodes and there is no need of mesh in the traditional sense. The process of finding an upper bound solution consists of combining limit analysis theory and a mesh-free numerical technique as a discretisation tool. To satisfy the required conditions for the admissibility of the discretised velocity field at the entire problem domain, a strain rate smoothing technique has been adopted. The outcome of proposed combination is a nonlinear optimisation problem which is solved by a direct iterative technique. The solution found by an iterative algorithm is an upper bound for limit load of the stability problem. The efficiency of the proposed method is demonstrated by solving different example problems in the soil mechanics engineering field, at the end of the paper. 相似文献
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Upper bound rigid block methods and finite element limit analysis are applied to investigate the undrained stability of rectangular tunnels. Solutions for a range of soil properties and tunnel geometries are obtained for situations in which the tunnel is wider than it is high. The upper and lower solutions for tunnel stability, which accurately bracket the true collapse load, are summarised in a series of stability charts for use in tunnel design. 相似文献
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In this paper, a fourth-order displacement (cubic strain) Serendipity quadrilateral element is presented. The shape functions and integration scheme are introduced, followed by a series of convergence tests, indicating that the element does not have any spurious zero-energy modes, and passes the patch test and single-element tests. Modifications are then made to enable the element's use in effective stress analysis problems. Examples are finally solved using the element and a comparison is made between some computed and closed-form solutions. It is also shown that the cubic strain quadrilateral may be used in conjunction with or as a substitute for the cubic strain triangle when predicting collapse loads for undrained plane or axisymmetric problems in the fully plastic range. 相似文献
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Undrained stability of a square tunnel in a soil whose strength increases linearly with depth 总被引：1，自引：0，他引：1
This paper examines the undrained stability of a shallow square tunnel in a soil whose strength increases linearly with depth. Rigorous bounds on the loads needed to resist collapse are derived using two numerical techniques which are based on finite element formulations of the classical limit theorems. Both of the numerical procedures assume a linearized perfectly plastic soil model and require the solution of large sparse linear programming problems. For the range of tunnel geometries considered, the numerical results bracket the exact collapse loads closely. The solutions are presented in the form of stability charts which can be used by practising engineers for the purposes of design. 相似文献
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The scaled boundary finite‐element method is derived for elastostatic problems involving an axisymmetric domain subjected to a general load, using a Fourier series to model the variation of displacement in the circumferential direction of the cylindrical co‐ordinate system. The method is particularly well suited to modelling unbounded problems, and the formulation allows a power‐law variation of Young's modulus with depth. The efficiency and accuracy of the method is demonstrated through a study showing the convergence of the computed solutions to analytical solutions for the vertical, horizontal, moment and torsion loading of a rigid circular footing on the surface of a homogeneous elastic half‐space. Computed solutions for the vertical and moment loading of a smooth rigid circular footing on a non‐homogeneous half‐space are compared to analytical ones, demonstrating the method's ability to accurately model a variation of Young's modulus with depth. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
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This paper deals with the problem of T‐bar penetration. New kinematically admissible velocity fields are derived from elastic solutions of incompressible material using Airy stress function. These velocity fields are used to obtain upper bounds to collapse loads. Two particular solutions are presented, one for a rough contact surface between the T‐bar and soil and the other for a smooth contact surface. The merit of the solutions is that within the boundaries of the velocity field, the soil is required to shear compatibly and continuously. Therefore, these solutions can easily be combined with the strain path method to estimate rate and softening effects. Analysis including consideration of strain rate effect showed that the new mechanisms predict, under certain conditions, lower values than previously published upper bound solutions. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
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将极限分析的上限定理、有限元离散思想、随机规划理论和蒙特卡洛方法这四者结合起来,提出了一种土质边坡可靠度分析的上限数值方法。首先采用三节点有限单元离散土质边坡,然后将土体的抗剪参数设为随机变量,根据上限定理构建同时满足三角形单元的塑性流动约束条件、单元公共边的塑性流动约束条件和单元速度边界条件的机动许可速度场,并根据内功功率等于外功功率条件建立目标函数,构建土质边坡可靠度分析的上限法随机规划模型。采用蒙特卡洛方法求解上限法随机规划模型,同时提出了一种基于上限法速度场的边坡失效风险系数估算方法,该方法特别适用于具有多种失效模式的边坡风险分析。对2个经典算例进行了深入分析,验证了方法的正确性。 相似文献
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刚性块体极限分析上限法常应用于岩土工程稳定性研究,然而应用时需假定刚性块体破坏模式并递推繁琐的几何关系。为此,提出一种适应性更广的基于非线性规划模型的刚体平动运动单元上限有限元法,并解决了其优化模型初始值的确定问题。通过引入有限单元思想,将计算区域离散成刚体单元,同时以单元速度和节点坐标作为决策变量,由上限定理建立非线性规划模型获得上限解。利用编制的上限有限元程序进行边坡和浅埋隧道稳定性算例验证,表明运动单元上限有限元法能调整速度间断线至较优方位,所得破坏模式特征鲜明,上限解精度高,可广泛应用于边坡、隧道等稳定性分析研究。 相似文献
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A general decomposition approach for the static method of limit analysis is proposed. It is based on piecewise linear stress fields, on a partition into finite element sub‐problems and on a specific coordination of the subproblem stress fields through auxiliary interface problems. The final convex optimization problems are solved using nonlinear interior point programming methods. As validated for the compressed bar with Tresca/von Mises materials in plane strain, this method appears rapidly convergent, so that very large problems with millions of constraints and variables can be solved. Then the method is applied to the classical problem of the stability of a Tresca vertical cut: the static bound to the stability factor is improved to 3.7752, a value to be compared with the recent best upper bound 3.7776. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
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本文提出一种适用于块状岩体结构的计算模型——刚体-弹塑性夹层模型,它根据岩体的实际结构将岩体离散成刚性元(岩块)和弹塑性夹层(软弱结构面)。本文还在此基础上提出了用有限元定量评价岩体稳定性的另外一种方法——极限分析法,给出了衡量岩体稳定性的相应指标——极限乘子。 相似文献
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This study presents a thermo-hydro-mechanical (THM) model of unsaturated soils using isogeometric analysis (IGA). The framework employs Bézier extraction to connect IGA to the conventional finite element analysis (FEA), featuring the current study as one of the first attempts to develop an IGA-FEA framework for solving THM problems in unsaturated soils. IGA offers higher levels of interelement continuity making it an attractive method for solving highly nonlinear problems. The governing equations of linear momentum, mass, and energy balance are coupled based on the averaging procedure within the hybrid mixture theory. The Drucker-Prager yield surface is used to limit the modified effective stress where the model follows small strain, quasi-static loading conditions. Temperature dependency of the surface tension is implemented in the soil-water retention curve. Nonuniform rational B-splines (NURBS) basis functions are used in the standard Galerkin method and weak formulations of the balance equations. Displacement, capillary pressure, gas pressure, and temperature are four independent quantities that are approximated by NURBS in spatial discretization. The framework is used to simulate strain localization in an undrained dense sand subjected to plane strain biaxial compression under different temperatures and displacement velocities. Results show that an increase in the displacement rate leads to reduction in the equivalent plastic strain while an increase in the temperature leads to an increase in the equivalent plastic strain. The findings suggest that the proposed IGA-based framework offers a viable alternative for solving THM problems in unsaturated soils. 相似文献
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The influence of the plastic potential on plane strain failure 总被引：1，自引：0，他引：1
The influence of the shape of the plastic potential in the deviatoric plane on plane strain collapse is investigated. The most commonly employed elastic‐perfect plastic models are considered, which adopt well‐known failure criteria for defining the yield and plastic potential surfaces, namely the von Mises, the Drucker–Prager, the Tresca, the Mohr–Coulomb and the Matsuoka–Nakai criteria. Finally, the conclusions are also extended to strain hardening/softening models. For simple constitutive models based on perfect plasticity, it is shown that the value of the Lode's angle at plastic collapse in plane strain conditions strongly depends on the specific failure surface adopted for reproducing the plastic potential surface. If the value of the Lode's angle at yield coincides with the failure value prescribed by the plastic potential, the stress–strain curves exhibit the typical perfect plastic behaviour with yield coinciding with failure, otherwise the stress changes after yield and the stress‐strain curves resemble those of strain hardening/softening models. The infinite strength which is in some situations exhibited by the Drucker–Prager model in plane strain condition is investigated and explained, and it is shown that this can also affect strain hardening/softening models. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献