Load transfer analysis of rock-socketed drilled shafts by coupled soil resistance

The load distribution and deformation of rock-socketed drilled shafts subjected to axial loads are evaluated by a load transfer method. The emphasis is on quantifying the effect of coupled soil resistance in rock-socketed drilled shafts using 2D elasto-plastic finite element analysis. Slippage and shear-load transfer behavior at the pile–soil interface are investigated by using a user-subroutine interface model (FRIC). It is shown that the coupled soil resistance acts as pile-toe settlement as the shaft resistance is increased to its ultimate limit state. Based on the results obtained, the coupling effect is closely related to the ratio of the pile diameter to soil modulus (D/E_s) and the ratio of total shaft resistance against total applied load (R_s/Q). Through comparison with field case studies, the 2D numerical analysis reasonably estimated load transfer of pile and coupling effect, and thus represents a significant improvement in the prediction of load deflections of drilled shafts.     

Finite element analyses of two-tier geosynthetic-reinforced soil walls: Comparison involving centrifuge tests and limit equilibrium results

This study presents the procedure and results of the finite element (FE) analyses of a series of centrifuge tests on geosynthetic-reinforced soil (GRS) two-tier wall models with various offset distances. The objectives of this study were to evaluate the applicability of FE for analyzing GRS two-tier walls with various offset distances and to investigate the performance and behavior of GRS two-tier walls in various stress states. The FE simulations were first verified according to the centrifuge test results by comparing the locations of failure surfaces. The FE results were then used to investigate the effective overburden pressure, mobilization and distribution of reinforcement tensile loads, and horizontal deformation at the wall faces. The interaction between two tiers was investigated based on the FE results, which were also used to examine the modeling assumption of reinforcement tensile loads in limit equilibrium (LE) analysis and to evaluate the design methods in current design guidelines. This study demonstrated favorable agreement between FE and the centrifuge model in locating the failure surface. The FE results indicated that as the offset distance increased, the reinforcement tensile load and wall deformation decreased in both the upper and lower tiers, suggesting the attenuation of interaction between the two tiers. The maximum tensile loads of all reinforcement layers at the wall failure predicted using FE analysis and LE method assuming uniform distribution of reinforced tensile loads were comparable. Compared with the FE results, the Federal Highway Administration (FHWA) design guidelines are conservative in determining the effect of overburden pressure, required tensile strength, location of maximum tension line (for designing the reinforcement length), and the critical offset distance. Furthermore, the FHWA design guidelines do not account for the influence of the lower tier on the upper tier that was observed in this study.     

Slope stability analysis by means of finite element limit analysis and finite element strength reduction techniques. Part II: Back analyses of a case history

This paper deals with a back analysis of a slope failure. The case history investigated is located in an alpine environment in central Europe and is characterized by a very steep original terrain, indicating in situ soil with high strength. To study the factor of safety, two different approaches applying the so-called φ′/c′ reduction are used, namely finite element limit analysis and strength reduction finite element analysis. Comparison of a strength reduction technique with rigorous finite element limit analysis confirms that the factors of safety (FoS) obtained are very similar for associated plasticity, an intrinsic assumption of limit analysis. For non-associated plasticity, a modified version of the so-called Davis approach has been applied because it has been shown that the original formulation proposed by Davis works well when the FoS is defined in terms of loads but is not appropriate when the FoS is defined in terms of soil strength. The results show that, with the modified Davis parameter, both strength reduction finite element analyses and finite element limit analyses provide very similar factors of safety. The key advantage of limit analysis, however, is that the value of the FoS can be bracketed from above and below with upper and lower bound calculations.     

Ovalization of steel energy pipelines buried in saturated sands during ground deformations

Cross-sectional ovalization of buried steel pipes subjected to bending moment induced by end displacements is discussed. A three dimensional finite element analysis was conducted employing Abaqus/CAE. The pipe was simulated using 3D shell elements while the saturated sand soil medium was simulated by employing discrete nonlinear springs along the pipeline. The effects of normalized burial depth (H/D), diameter to wall thickness ratio (D/t), sand density and level of the internal pressure on the ovalization are investigated, and resulting ovalization distribution with respect to bending moment at critical sections is presented. The results of this study enable simple one dimensional finite element models to consider geometrical cross-sectional nonlinearities in the analysis of buried pipelines.     

Disturbed state model for sand–geosynthetic interfaces and application to pull-out tests

Successful numerical simulation of geosynthetic-reinforced earth structures depends on selecting proper constitutive models for soils, geosynthetics and soil–geosynthetic interfaces. Many constitutive models are available for modelling soils and geosynthetics. However, constitutive models for soil–geosynthetic interfaces which can capture most of the important characteristics of interface response are not readily available. In this paper, an elasto-plastic constitutive model based on the disturbed state concept (DSC) for geosynthetic–soil interfaces has been presented. The proposed model is capable of capturing most of the important characteristics of interface response, such as dilation, hardening and softening. The behaviour of interfaces under the direct shear test has been predicted by the model. The present model has been implemented in the finite element procedure in association with the thin-layer element. Five pull-out tests with two different geogrids have been simulated numerically using FEM. For the calibration of the constitutive models used in FEM, the standard laboratory tests used are: (1) triaxial tests for the sand, (2) direct shear tests for the interfaces and (3) axial tension tests for the geogrids. The results of the finite element simulations of pull-out tests agree well with the test data. The proposed model can be used for the stress-deformation study of geosynthetic-reinforced embankments through numerical simulation. Copyright © 1999 John Wiley & Sons, Ltd.     

上覆土嵌岩扩底桩抗拔承载特性离心模型试验及计算方法研究

随着我国西部输电线路工程的快速发展,杆塔基础抗拔问题日益突出,岩体的破坏模式、极限抗拔承载力的计算方法都是目前亟待解决的关键问题。根据离心模型试验结果,分析了岩体破坏模式、荷载-位移曲线、桩身轴力分布和桩侧摩阻力曲线。基于试验结果对岩体破坏形状进行无量纲化处理并拟合,得到了统一的函数描述形式。在此基础上,利用极限平衡法推导出上覆土嵌岩扩底桩极限抗拔承载力的计算方法,并对该计算方法作了简化分析,将积分计算值和简化计算值分别与离心模型试验结果进行对比。研究结果表明:上覆土嵌岩扩底桩的软质岩破坏模式为喇叭型曲面,可用统一的幂函数形式进行描述。简化计算方法的结果与离心模型试验结果吻合良好,提出的新方法对西部山区杆塔基础的抗拔设计具有参考和应用价值。     

关于地铁地震响应的模型振动试验及数值分析

为明确大地震时地铁的破坏过程及原因,以1995 年日本阪神－淡路大地震中遭受严重破坏的神户大开地铁车站为对象,进行了一系列的模型振动试验和动力有限元分析。对于作用于地下结构的地震动土压的发生原理、地震波输入方向、结构的埋设深度、地基与结构间的刚性比对地震动土压的影响及在地基-结构系统的非线性响应下结构周围地基终局状态时的地震动土压进行了研究。明确了地震动土压的极限值及在大地震时周围地基的残余应变引起的静止土圧力的存在。为今后改进地震土压力计算方法、提高地下结构抗震设计水平提供了依据。     

基于Web的金刚石钻头模具钢体设计

介绍了应用网页技术进行金刚石钻头模具钢体设计计算的方法。该方法丰富了模具钢体设计的远程教育,免去了传统人工求解的繁杂过程,界面友好,精度高,结果可靠,具有实际使用价值。      

土-混凝土接触面特性环剪单剪试验比较研究

对可用于接触面应力变形及强度特性试验的直剪仪、单剪仪及环剪仪的优缺点进行了比较分析。对某土料与混凝土接触面的应力-变形及强度特性进行了单剪试验研究,试验包括土-混凝土接触面及土-泥皮-混凝土接触面两种。同时,将已有的环剪试验成果与单剪试验结果进行了比较分析。研究了环剪试验与单剪试验成果的差异。结果表明,单剪试验测得的抗剪强度比环剪试验测得的低30%～40％,环剪试验用于接触面强度特性研究较合适,而单剪试验用于较低应力水平下的应力-变形试验更好,泥皮的存在使得接触面强度降低30%左右。     

筋土之间直剪试验与拉拔试验的对比分析

直剪试验和拉拔试验是研究土工合成材料与土界面剪切特性的两种手段,但二者的试验机理存在明显的差异。本文根据筋土界面直剪试验和拉拔试验结果,对这两种试验方法进行了对比分析,探讨了直剪试验和拉拔试验得到的界面剪切强度之间的差别及其原因。试验结果表明:对于机织土工布来说,直剪试验的结果与拉拔试验得到的界面剪切强度相近,但峰值位移差别较大;对于玻纤格栅来说,直剪试验得到的界面剪切强度大于拉拔试验的结果。     

Discrete element modeling of direct shear tests for a granular material

A succinct 3D discrete element model, with clumps to resemble the real shapes of granular materials, is developed. The quaternion method is introduced to transform the motion and force of a clump between local and global coordinates. The Hertz–Mindlin elastic contact force model, incorporated with the nonlinear normal viscous force and the Mohr–Coulomb friction law, is used to describe the interactions between particles. The proposed discrete element model is used to simulate direct shear tests of the irregular limestone rubbles. The simulation results of vertical displacements and shear stresses with a mixture of clumps are compared well with that of laboratory tests. The bulk friction coefficients are calculated and discussed under different contact friction coefficients and normal stresses. Copyright © 2009 John Wiley & Sons, Ltd.     

Mechanical aspects of thrust faulting driven by far-field compression and their implications for fold geometry

In this paper we present a mechanical model that intends to captures the kinematical aspects of thrust fault related folds induced by regional-scale far-field contraction. Fold shapes may be the only surface evidence of the geometry of underlying faults, so complex fault interactions are assessed in terms of how they influence fold geometry. We use the finite element method to model the fold and finite deformation frictional contact to model the activation and evolution of slip throughout preexisting faults. From several simulated 2D fault patterns we infer how one may form an anticline similar to that observed at Sheep Mountain Anticline, Wyoming.     

土与结构界面位移特性静动力单剪试验研究

土与结构接触面是工程中经常遇到的,剪切试验是研究界面特性的主要方法之一。单剪试验既可以研究界面的特性,又可以考虑土体受力变形的实际特征,土体变形和界面错动位移的关系是尚未被注意的问题。通过淤泥质粉质黏土的静、动力单剪试验和进行不同法向应力、不同剪切应力幅值的对比试验,得到界面位移与循环周数、土体应变的试验曲线。结果表明,静力单剪试验的初期剪应力迅速增大,其后增长速率逐渐减小,没有出现软化现象;在剪切初期,主要是土体发生剪切位移,达到一定位移量后,土体和钢板之间开始出现较为明显的滑移;在一定的土体应变范围内静力单剪界面错动位移和土体应变有近似线性关系;动力单剪的界面错动位移和土体应变有很好的线性关系;静动力试验界面位移与土体应变关系线的斜率总体上静力试验结果高于动力试验值,线性关系的斜率变化范围不大     

青藏粉土-玻璃钢接触面力学特性直剪试验研究

青藏直流±400 kV输变电工程中采用表面光滑的玻璃钢覆盖基础表面,以减少切向冻胀力对基础的冻拔作用。过去的研究鲜有涉及土体特别是冻土与玻璃钢基础接触面的力学特性,为指导冻土区基础设计和安全评价,采用应变直剪仪开展了多种含水率和温度条件下青藏粉土-玻璃钢接触面直剪试验研究。结果表明,青藏粉土-玻璃钢接触面屈服时相应剪切位移很小,应变硬化阶段短暂或不显著;冻结状态下接触面应力-位移性状呈脆性破坏型,存在明显峰值;融化状态时接触面的剪应力-位移性状呈塑性破坏型,其应力-位移关系曲线为弱软化型和屈服型,没有明显峰值;融化状态时接触面抗剪强度值随含水率的增加而缓慢减小,冻结状态时其强度随负温绝对值和含水率的增加而增大,且随着土体含水率的增大,温度降低导致接触面抗剪强度增强效果更加显著,土体含水率大于19%后抗剪强度趋于稳定;温度对抗剪强度的影响主要体现于黏聚力的改变,且随着含水率的增加,温度影响增强。接触面内摩擦角随负温绝对值增加而减小,随含水率的增加而减小。     

复合单元法的物理和数值试验验证

简述了复合单元法的基本原理和算法,该方法的主要优点在于可以明显地减少前处理的工作量和难度,且可以与有限单元很好地耦合在一个系统中。介绍了宝珠寺重力坝的物理模型超载试验。分别采用三维弹黏塑性复合单元法和三维弹黏塑性有限单元法对宝珠寺重力坝的超载工况进行了数值试验,并与物理试验进行比较。结果表明,三种方法得到的特征点位移吻合得较好,两种数值试验方法的整体位移和应力也基本一致。研究验证了复合单元法的正确性,也说明了该算法的主要优点：前处理简单,计算精度与有限单元法相当,同时还证实了复合单元单元法可以较好地解决复杂岩基上重力坝的变形与破坏问题。     

Ground reinforced embankments for rockfall protection: design and evaluation of full scale tests

Infrastructures such as roads and railways as well as urbanised areas, in mountainside regions, can frequently be endangered by rockfalls and, therefore, need to be protected against the impact of rolling blocks. Among the various protection works that can be used, ground reinforced embankments can be considered a feasible technique. A set of full-scale tests on embankments made of ground reinforced by geogrids are presented and discussed. The experiments were performed in a specifically designed and constructed test facility, where concrete blocks up to 9,000 kg in weight were thrown onto a geogrid reinforced embankment at a speed of about 30 m/s. Several embankments made of different geogrid types, different soils and construction layouts were tested at different impact energy levels, permitting a quantitative assessment of the resistance impact of these structures. The experimental results were compared with those obtained from a dynamic finite element method numerical model, and a good agreement was obtained.     

三轴试验中砂土刚度的统计和回归分析及应用

砂土的密度和应力状态对其刚度有很大的影响。计算岩土工程中许多硬化土体模型都是基于邓肯−张模型得出的,没有考虑到密度对砂土刚度的影响。而在极致密或松散的砂土的三轴压缩过程中,剪切应变的上升会引起密度的显著变化。为了评估粒径分布、密度及应力状态对砂土刚度的影响,使用统计和回归方法对来自莫斯科和明斯克的15个建筑工地的962个土壤样本的各向同性三轴试验数据进行分析。基于密度和应力状态参数的影响,提出了评估不同粒径砂土刚度的经验方程。对来自欧洲、印度和美国的冲积土和陆地土试验的比较分析表明,其砂土的刚度与莫斯科和明斯克的砂土在同一范围。所提出的方程可用于初步估计有限元法计算里的刚度参数,也可应用于岩土工程模型（允许考虑刚度的变化、水平和垂直分布）。此外,还提出了基于邓肯−张模型的半经验关系。当密度的变化影响土的刚度时,该半经验关系可为受到大变形和（或）复杂加载路径影响的松散和非常致密的砂土提供更为真实的结果。一般来说,岩土工程师可将获得的结果应用于更为复杂的土体模型设计中。     

钢衬钢筋混凝土压力管道外包混凝土的裂缝控制研究

基于钢衬钢筋混凝土压力管道在设计荷载下外包混凝土将带裂缝工作的基本承载特性,以缓解开裂与限裂间的矛盾为指导思想,将减小坝后背管外包混凝土裂缝宽度和提高开裂管道的耐久性作为研究目的,结合模型试验和有限元法,研究了一些裂缝控制措施的应用效果,以及有效的计算内水压力下钢衬外包混凝土裂缝宽度的公式。结果表明：总用钢量相同时,减薄钢衬,加大钢筋用量,减小裂缝宽度效果明显;减薄钢衬外包混凝土厚度,也可减小裂缝宽度;性能可靠的防水涂料具备黏接性强和变形能力高的基本性质,可进一步研究其对提高管道耐久性的长期作用;采用П-780-83规范中的裂缝宽度公式,计算结果与模型试验结果较为符合。     

Numerical Procedure for Dynamic Simulation of Discrete Fractures Due to Blasting

Summary  This paper describes development of a generic nonlinear, dynamic modelling technique to simulate discrete rock fractures due to blasting using the finite element method. The element elimination technique together with a brittle, Rankine failure-type material model are used as a means to simulate the initiation and growth of fractures in the rock under the effect of blast-induced dynamic pressure pulse. Dynamic loads representing ideal and non-ideal detonations are simulated and a new method, termed as optimised pressure profile, is proposed to approximate the pressure-time profile of the blast load to model the dynamic load. Comparison of numerical model results with previously reported observations from the literature reveals the ability of the model as a predictive tool and supports the validity of the developed modelling procedure. Author’s address: Hani S. Mitri, Department of Mining, Metals and Materials Engineering, McGill University, 3450 University Street, Montreal, Canada H3A 2A7     

Load and resistance factor design (LRFD) calibration for steel grid reinforced soil walls

This paper reports the results of load and resistance factor design (LRFD) calibration for pullout and yield limit states for steel grid reinforced soil walls owing to soil self-weight loading plus permanent uniform surcharge. The calibration method uses bias statistics to account for prediction accuracy of the underlying deterministic models for reinforcement load, pullout capacity and yield strength of the steel grids, and random variability in input parameters. A new revised pullout design model is proposed to improve pullout resistance prediction accuracy and to remove hidden dependency with calculated pullout resistance values. Load and resistance factors are proposed that give a uniform probability of failure of 1% for both pullout and yield limit states. The approach adopted in this paper has application to a wide variety of other reinforced soil wall technologies.