SMW工法+环形支撑体系在基坑支护中应用研究

SMW工法桩与环形支撑的联合支护体系逐渐在基坑工程中实际应用,分析其支护效果和支护桩的受力性能对工程实际具有重要意义。基于某基坑工程实际,运用有限差分程序建立数值模型,分析其降水及多次开挖工况下土体的位移和支护桩的变形特征。研究结果表明：降水工况下,基坑的土体沉降主要集中在开挖范围内及基坑的边缘处,随着开挖的进行,基坑边缘土体沉降线呈圆弧状,坑底土体因卸荷会产生回弹,最大回弹值约为4.5 mm;基坑开挖对距边缘25 m范围内的土体影响较大,从地表至地下6.5 m影响逐渐减小;基坑侧壁的变形随开挖加深而增大,呈现出先增大后减小的趋势,支护桩在坑边中心位置变形最大,承受较多的荷载,且对于环形内支撑体系,整体受力均匀,研究结果可为基坑支护工程提供参考。     

基坑工程开挖对桩锚支护变形特性模拟分析

当前,地下工程的建设十分常见。地下工程对土体稳定性的要求高,为了预防施工风险,必须加强土体支护的设计、支护施工质量的控制。以长春某工程基坑为例,采用有限元分析方法对基坑进行数值模拟,探讨基坑开挖对基坑变形的影响,并对基坑隆起、桩身水平位移、锚具轴力等数据进行分析。将模拟数据和监测数据进行比较,验证了支护结构的合理性。     

基于极差分析法的基坑参数优化研究

结合淄博市舒香苑项目的基坑工程实践,采用正交试验建立了土钉支护结构内部整体稳定性安全的计算模型,利用极差分析方法分析了基坑开挖深度、基坑周围建(构)筑物距离基坑开挖边缘的水平距离、开挖土层的弹性模量、土层的粘聚力、土层的内摩擦角等因素之间的关系,得到了一些具有价值的分析结果,为土钉支护优化设计提供了重要依据,对淄博地区土钉支护设计和施工具有一定的指导意义。     

硬土场地基坑变形监测与分析

南京阳光雅居4期基坑工程处于硬土场地中,基坑开挖深度5.7 m,局部7.0 m,围护体系采用了人工挖孔灌注桩和土钉墙2种支护结构形式.施工过程中分别对桩顶圈梁水平位移、土钉墙墙顶水平位移、围护桩桩侧土体深层水平位移、邻近建筑物沉降、邻近道路沉降进行了长达8个月的监测.依据硬土的物理力学特性和本次基坑变形监测结果,分析表明:硬土场地中快速挖土卸载,可致使基坑支护结构产生明显水平位移,而周围土体水平位移相对较小,由于两者变形不协调,通常导致支护结构和土体间出现裂缝;硬土场地中基坑开挖引起的邻近建筑物和道路沉降较小,对周围环境影响不明显.     

深基坑施工中的测量控制

当前,基坑支护设计中对基坑周围土体变形的计算尚无成熟的方法,而在施工中通过准确的监测,可以指导基坑开挖和支护,有利于及时采取应急措施,避免或减轻破坏性后果.     

槽壁与坑底加固对地铁基坑支护结构变形影响研究

三轴搅拌桩加固地连墙槽壁及基坑坑底有效减小支护结构的变形,保障周边管线、既有建筑物、基坑工程安全。以杭州汽车北站基坑工程为例,建立有限元分析模型模拟地铁基坑开挖过程,并结合现场数据,研究了三轴搅拌桩加固地连墙槽壁与坑底土体对基坑支护结构变形特性的影响。     

深厚软土深基坑支护结构变形的实测与分析

为探讨深厚软土地区深基坑开挖基坑支护结构变形及周边土体的位移特性。以龙海某深厚软土深基坑支护工程为例,通过有效的监测手段,确保了施工过程中深基坑及周边环境安全可控,并对监测数据进行分析,为类似的深基坑开挖及支护结构设计提供了参考。     

青岛车站深基坑开挖支护稳定性数值分析

本文首先对如何模拟基坑开挖进行了阐述。基坑开挖全过程数值模拟研究主要包括:开挖土体与周围护墙体相互作用的模拟,二维基坑接触面单元的模拟,基坑预应力施加的模拟和基坑开挖的模拟等等。本文结合青岛地铁西流庄站深基坑工程,利用迈达斯模拟了开挖过程。     

地铁车站深基坑开挖稳定性与抗震分析

以合肥市轨道交通4号线金寨路站深基坑为背景,利用FLAC 3D软件建立了地铁车站深基坑的数值计算模型,分析了基坑开挖过程中基坑周边土体和桩体的竖向位移、水平位移和支撑轴力,并进一步研究了地震荷载作用下基坑主体结构的竖向位移、水平位移和内力。研究结果表明:基坑开挖过程中,最大地表沉降、桩体水平位移和支撑轴力分别为33.6 m、27.8 m和1528.5 kN;主体结构顶板以及侧墙的竖向位移和水位位移的最大差异值分别出现在地震后4 s和8.5 s,分别为18 mm和10 mm;地震荷载作用下车站立柱比侧墙更容易发生压裂破坏。     

基坑开挖和支护对周边建筑的安全影响分析

近年来软土地区基坑的开挖与支护受到普遍关注,基坑开挖模拟过程根据基坑设计的开挖方法与支护措施采用ANSYS进行安全影响分析;模拟开挖过程分为八个工况,并逐一分析每个工况的沉降及侧向位移。从位移及力学角度上分析,该工程K0+050~K0+080段基坑开挖与支护对基坑、市政建筑物及设施的安全性影响,基坑自身安全性:侧向变形满足规范要求,但是沉降变形不满足规范要求,应在基坑支护中加大支护参数;周边市政建筑物及设施由于基坑的开挖而产生的变形满足规范要求。     

Influence of random pore defects on failure mode and mechanical properties of SiC ceramics under uniaxial compression using discrete element method

To investigate the relationship between micro-defects in ceramic materials and macro mechanical properties and behaviours, a computational model of SiC ceramics with randomly oriented elliptical pores was established using the discrete element method (DEM). The effects of pore defect content and its aspect ratio on the failure mode, stress-strain curve and mechanical properties of specimen were investigated under uniaxial compression. The effective Young's modulus which was obtained from DEM simulations was compared with the predictions of Mori-Tanaka scheme (MTS) and Self-Consistent scheme (SCS) at various pore defect densities. The results showed that the compressive strength and crack initiation stress decrease nonlinearly as the pore defect content increases. Furthermore, the smaller the aspect ratio of the elliptical pore defects was, the more obvious the weakening trend was. As the pore defect content increases, the failure mode of the specimen changed from brittle fracture to tensile-shear mixing and then to axial splitting. The stress-strain curves showed a certain “softening” period during the loading process. The effective Young's modulus obtained from the DEM simulations coincides with the approximations of MTS and SCS at low pore densities. However, when the pore defect density became larger, the DEM simulation results were slightly lower than the theoretical results of the Mori-Tanaka scheme, which only considers the weak interaction between defects.     

Study of cold powder compaction by using the discrete element method

The discrete element method (DEM), based on a soft-sphere approach, is commonly used to simulate powder compaction. With these simulations a new macroscopic constitutive relation can be formulated. It is able to de-scribe accurately the constitutive material of powders during the cold compaction process. However, the force-law used in the classical DEM formulation does not reproduce correctly the stress evolution during the high density compaction of powder. To overcome this limitation at a relative density of about 0.85, the high density model is used. This contact model can reproduce incompressibility effects in granular media by implementing the local solid fraction into the DEM software, using Voronoi cells. The first DEM simulations using the open-source YADE software show a fairly good agreement with the multi-particle finite element simulations and experimental results.     

Monte Carlo simulation of copolymerization and compositional inhomogeneity of copolymers: comparison to experimental data

A review is given of the methods used to calculate copolymer composition as a function of chain length. A Monte Carlo simulation of copolymerization, which considers the initiation and propagation steps and yields cumulative copolymer composition as a function of chain length, is described in detail. The results of the simulations for three copolymer systems show that the initiation step significantly influences the short chains but its effect rapidly vanishes with increasing chain length. The results of the simulations for the three copolymer systems are compared to experimentally determined composition-chain length distributions. The agreement of the simulations and the experimental data is good for long chains but poor for short chains.     

Compaction of aggregated ceramic powders: From contact laws to fracture and yield surfaces

This work describes a methodology based on Discrete Element Method (DEM) simulations to generate yield and fracture surfaces for aggregated ceramic powders. The DEM simulations, which consider the length scale of porous aggregates, are used as numerical triaxial experiments to obtain the behavior of a small volume element of powder under a given load. The experimental identification procedure, which relies on the Design Of Experiment method, is designed to limit the number of experiments and simulations needed to obtain the model material parameters. These material parameters, which model the interactions between aggregates in the DEM simulations are identified using two simple experiments on a Uranium diOxide powder: closed-die compaction and diametrical compression test. The yield and fracture surfaces obtained from the DEM simulations provide valuable information on the behavior of the powder for stress states that are difficult or impossible to attain in complex triaxial tests.     

Monte Carlo simulation of linear polymer melts in shear flow. Effect of shear stress and confined space on chain dynamics

Linear homopolymer melts under shear stress were studied using a Monte Carlo method in a dense system, using the cooperative motion algorithm. The simulations were performed in a plane-parallel geometry, between reflective walls, for chains up to 640 beads (entangled). Behavior of the melts under shear stress was simulated by appropriate biasing of the bead move probability. Chain dynamics was monitored during reaching the equilibrium flow in the step-shear experiment (creep) simulation, in the equilibrium flow and during melt relaxation. The results were compared with those obtained for non-flowing melt. Significant differences between step-shear and steady-state simulations were observed, while chain dynamics during recovery is almost identical with that in non-flowing melt. Dynamics of the melt depended strongly on the chain length and shear stress, showing nonlinear effects for long chains. Chain orientation, coil deformation and relaxation were analyzed. Relaxation time of long chain decreased by orders of magnitude under shear following the power law. Diffusion of beads and chains was analyzed and an anomalous diffusion was observed, also in the directions perpendicular to the flow direction. In the flowing melt the effect of confined space was important, even if the wall spacing was much bigger than R_g.     

Assessment of hydrophobic and anticorrosion properties of composite silane–zeolite coatings on aluminum substrate

The effect of matrix compounds on the behavior of silane–zeolite composite coatings on a 6061 aluminum alloy was evaluated by electrochemical test performed at long immersion time in 3.5% NaCl solution. It was observed that silane chains influence the hydrophobic behavior and the electrochemical performances of the zeolite-based composite coating. All composite coatings showed a long-term protection action in 3.5% NaCl solution. The use of silane compounds with long alkyl chain or with bi-hydroxyl groups made it possible to obtain electrochemically stable coating during long immersion time. Best results have been obtained with a multicomponent silane mixture (short and long molecular chains).     

表面接枝二分散聚合物的自洽场理论

建立了表面接枝多分散聚合物的自洽场理论.通过对二分散聚合物进行自洽场模拟,研究了排除体积参数对分子链密度分布的影响,考察了分子链的摩尔分数与自由端分布的关系并将模拟结果与强伸展理论进行了比较.通过计算分子链的链段密度分布和伸展轨迹,讨论了二分散聚合物分子链的伸展行为.     

Cracking and shape deformation of cylindrical cavities during constrained sintering

During constrained sintering of thin films, in which a cylindrical cavity with axis perpendicular to the substrate has been introduced before sintering, cracks emerge that initiate at the cavity surface. By combining experiments with continuum mechanical and particle based simulations, the fundamental causes and effects of this kind of crack formation are identified. A stress analysis performed by finite element (FEM) simulations matches with the cracking behavior observed in experiments. A comparison of discrete element (DEM) results with experiments shows the applicability of this simulation method to describe the effect of cross-sectional stripe dimensions and cavity diameters on the cracking behavior. Moreover, DEM simulations reveal that hair-line cracks in narrow stripe samples formed during pre-sintering manufacturing steps might be a dominant cause for the observed crack damage in such systems.     

Simulation study of the effect of wall roughness on the dynamics of granular flows in rotating semicylindrical chutes

A discrete element model (DEM) is used to investigate the behavior of spherical particles flowing down a semicylindrical rotating chute. The DEM simulations are validated by comparing with particle tracking velocimetry results of spherical glass particles flowing through a smooth semicylindrical chute at different rotation rates of the chute. The DEM model predictions agree well with experimental results of surface velocity and particle bed height evolution. The validated DEM model is used to investigate the influence of chute roughness on the flow behavior of monodisperse granular particles in rotating chutes. To emulate different base roughnesses, a rough base is constructed out of a square close packing of fixed spherical particles with a diameter equal to, smaller, or larger than the flowing particles. Finally, the DEM model is used to study segregation in a binary density mixture for different degrees of roughness of the chute. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2117–2135, 2015     

The thickness profile of ultra‐high molecular weight polyethylene films during sequential biaxial drawing

In this study, the evolution of the thickness profile of UHMW‐polyethylene films during sequential biaxial drawing is studied. The basic assumption is that a nonuniform thickness distribution of the drawn film is the result of the amplification of initial imperfections. Finite element simulations of the drawing process have been performed in which the drawing of a piece of film with a geometric imperfection is considered. For these numerical simulations, a viscoelastic model is adopted. Although the model does not describe the behavior of UHMW‐PE films in full detail, it is able to capture typical features of inhomogeneous deformation. The most striking observation is that severe localization occurs at the beginning of the second drawing step (i.e. in the direction transverse to the first step). In this stage of the drawing process, a localization band develops. These phenomena are also observed in validation experiments. A discrepancy between the simulations and the validation experiments is observed at the end of the drawing process. In the simulations, the deformation is stabilized again, and the resulting thickness profile is practically uniform. In the experiments however, the bad thickness profile at the beginning of the second drawing step is conserved up to the end of the drawing process. The simulations have been performed for different process parameters. Hence, it is possible to access the influence of these parameters on the evolution of the thickness profile. Furthermore, a suggestion is done to explain the deformation phenomena. The stability of the deformation can be determined from stress‐strain curves during the biaxial drawing process. It is concluded that, although no absolute values are acquired, trends are predicted correctly and they can serve as quidelines for process control.