Alignment of stone‐pavement clasts by unconcentrated overland flow – implications of numerical and physical modelling

The crucial role of stone pavements in arid environments for aeolian or alluvial processes and as numerical dating tools is increasingly acknowledged. This role is based on the assumption that stone pavements are stable landforms, formed gradually over time and predominantly by vertical processes. However, this is challenged by evidence of stone‐pavement clast reworking or burial. Bimodal, mostly slope aspect‐symmetrical clast orientation is a frequent phenomenon in various study areas. It implies that stone pavements may be influenced by unidirectional lateral processes besides vertical ones. Here, the finding of lateral processes contributing to stone‐pavement evolution is supported by numerical modelling and physical experiments. These unequivocally show that unconcentrated overland flow can transport clasts to form a closely packed stone mosaic with characteristics similar to those of natural stone pavements. The commonly observed length‐axes orientation angle of 40 ± 14° for natural stone‐pavement clasts is consistently reproduced by angle‐dependent force equilibrium. Monte Carlo runs confirm the natural scatter and allow characterization of the control parameters of clast orientation. The model explains up to 70% of the natural variance. It is further validated by flume experiments, which confirm model predictions of single object orientation angles. Experiments with multiple objects yield artificial stone pavements with properties similar to those found in the field. The unidirectional lateral process acting on natural stone pavements requires the presence of a vesicular horizon. This underlines the tight genetic coupling of this common epipedon feature and the clast cover. The presented findings highlight the role of stone pavements as process and environment proxies. However, stone pavements represent information since the last surface disturbance only. This has to be considered when using them as age indicators. Copyright © 2013 John Wiley & Sons, Ltd.     

Three‐dimensional finite element modeling of tire/ground interaction

Tire/ground interaction has been an important issue in terramechanics, transportation and pavement engineering. Characterization of tire/ground interaction has been majorly investigated based on empirical approaches and field tests. So far very few dynamic tire/ground interaction models have been presented. This paper presents a three‐dimensional finite element model for tire/ground interaction. The rubber material is modeled as nearly incompressible finite strain hyperelasticity instead of being modeled as a rigid wheel. The tire is also modeled as bias type and steel rebars are embedded within the tire structure. The tire model is combined with the ground model to form a tire/ground interaction model. Both single tire/ground interaction and four‐tire vehicle/pavement interaction models are presented, which allow us to investigate a lot of issues easily and accurately, such as compaction, pavement response and pavement damage. Numerous simulations are carried out to demonstrate that the dynamic tire/terrain interaction model can be used to predict the dynamic ground response due to moving vehicle. Different tire rolling conditions can be easily incorporated into the tire/ground interaction model, which further substantiates the broad application of the model in transportation and pavement engineering. Copyright © 2011 John Wiley & Sons, Ltd.     

山区公路典型沥青路面结构容许横向沉降差研究

大量的研究表明,路基不均匀沉降导致上部路面结构在车辆重复荷载作用下出现早期的破坏,严重影响了路面服务性能,而现有的路面结构设计未考虑路基不均匀沉降对路面结构的影响。在以往对路基不均匀沉降研究的基础上,采用有限元程序ABAQUS分析了在路基发生不均匀沉降条件下,路面承受车辆重复荷载作用的动力学响应以及路面开裂的变化过程,引入损伤变量定量描述了路面的破坏过程,分析了路基横向沉降差对路面开裂的影响,并参照路基边坡稳定安全系数的定义及规范取值。在此基础上,提出了山区公路典型沥青路面结构的容许横向沉降差值为2.2‰,并建议将其作为路基路面结构整体设计的一个控制指标加以考虑,以避免路基不均匀沉降导致的路面早期破坏,从而提高山区高等级公路的设计及建造水平。     

多年冻土区不同路面材料路基热稳定性分析

针对多年冻土区路基路面病害多发的现状,从路面材料入手,对水泥混凝土和沥青两种路面下路基温度场的变化规律进行研究。研究结果表明,在计算高度范围内,对于8.5 m路面宽幅的路基来说,沥青路面下的最大融深大于混凝土路面下的最大融深,尤其在路基高度较低时两种路面下的融深差异更大。最大融化深度和最大融化深度发展速率均与路基高度和路面宽幅有紧密关系,通过研究可以提出有效的措施来保持路基热稳定性,从而减少路基路面病害的发生和公路的运营成本,为今后类似工程提供参考     

冲击碾压改建路面施工对路基动力作用的试验研究

依托冲击碾压破碎旧水泥路面现场施工,沿路面横断面和不同深度位置埋设土动压力传感器,对不同冲击碾压遍数、冲压路线、行驶速度下路基的竖向和侧向动土压力进行了试验研究。研究表明：冲击碾压施工引起的地基振动具有周期瞬态冲击特性,冲击荷载附近动土压力存在叠加增长现象。路基动土压力与冲击荷载位置、行驶速度和碾压遍数密切相关。动土压力随冲击压路机由近及远为先增加再减小,纵向距离30 m以内的路基均受到振动影响,纵向水平距离15 m范围内影响开始显著。不同的冲击遍数下动土压力最大值总体有一个先增大再减小的趋势,由于路面板和基层的荷载扩散作用,冲击碾压改建产生的土动压力相比直接冲击路基下降了1个数量级。在试验工况下,冲击碾压改建对旧路基的实际影响深度比有限元计算的深度范围更大,沿路纵向水平影响距离可取为10 m。     

含碎石层沥青加铺结构外部耦合计算方法

碎石化沥青加铺路面结构中不同层位材料性质差异显著,难以由同一种力学模型加以描述。针对这一问题,将沥青加铺层及碎石化混凝土视为连续及非连续颗粒介质,分别采用有限差分法(DEM)与颗粒离散单元法(PCC)模拟沥青加铺层及碎石结构层的力学行为,并采用耦合的方式,通过两域对应边界的相容条件与平衡条件的控制,实现了连续域与离散域的平滑过渡。作为应用,结合现场情况,将破碎混凝土简化成碎粒层、碎石表层和碎石底层,通过模拟计算,分析了破碎粒径不同组合条件下沥青加铺层对荷载的响应。结果表明,碎粒层粒径越大,沥青层的拉应力、应变越大;碎石表层粒径越小,即多锤头破碎机(MHB)破碎的越碎,原水泥混凝土面板的强度下降的越多,整体性就越差,沥青层底的拉应力、应变也就越大,因此要将破碎状态控制在一定程度为宜。     

地下厂房岩锚梁锚杆涂沥青段有限元模拟研究与应用

基于AutoCAD和OpenGL有限元可视化建模,利用Fortran语言对模型文件中的单元信息进行改写,提出了一种在地下厂房有限元模型中生成沥青单元网格的方法,显著减少了模型单元数与节点数,提高了建模与计算效率。采用隐式杆单元法模拟涂沥青锚杆,隐式柱单元法模拟普通锚杆,推导了其刚度矩阵及有限元迭代计算公式,并应用于地下厂房岩锚梁锚杆涂沥青段的有限元分析中。结果表明,吊车梁附近围岩破坏区减少,围岩深部锚杆应力增大,说明围岩受力特性得以改善,围岩和吊车梁整体安全度增大,符合工程实践经验。所提出的计算和单元再分方法可操作性和实用性大,可为类似工程有限元分析提供参考。     

全无缝桥梁接线路面拉伸力学特性理论和实验研究

为了掌握和模拟全无缝桥梁接线路面弹塑性拉伸特性,本文基于钢筋和混凝土的单轴拉伸平均应力-应变本构模型,推导出接线路面的宏观弹塑性拉伸P-Δ理论模型;并对室内总长28 m的接线路面足尺模型,进行了拉伸试验。结果显示:(1)接线路面的理论与实验拉伸P-Δ骨架曲线在钢筋屈服前后都吻合得很好,可以直接用于全无缝桥梁抗震设计和损伤评估;(2)接线路面实验模型的拉伸变形量Δ高达14 cm时,仍未拉断,其延性甚好、有利于抗震。     

山东蓬莱新生代岩石圈地幔的异常锂同位素组成：橄榄岩包体的离子探针分析

应用离子探针技术对山东蓬莱新生代玄武岩中橄榄岩包体的主要组成矿物进行了原位测定,结果显示了强烈偏离正 常地幔矿物组成和平衡的Li 含量和同位素比值。橄榄石、斜方辉石和单斜辉石的Li 含量和δ7Li 分别为（2.0~10.4）×10-6, （2.8~23.8）×10-6 和（3.3~59.7）×10-6;-28.6‰ ~+0.5‰,-12.7‰ ~+12.8‰和-41.5‰ ~+22.6‰。这种强烈的非平衡体系反 映了最近强烈和复杂的地幔交代作用。辉石的高Li（~ 60×10-6）和δ7Li（~ +20‰）反映了最近富Li地幔熔体/ 流体的特征, 而橄榄石的中等Li 富集（<5×10-6）和强烈的δ7Li 亏损（~ -20‰）则可能记录了更早一期低δ7Li 的地幔交代。同位素剖面 的扩散反演以及Li 强烈富集与LREE 相对亏损之间的解耦显示了两期地幔交代作用的短暂性和近期性。因此,我们认为华 北东部克拉通上地幔中存在与再循环有关的锂同位素异常组分,它们的形成应该与中生代以来太平洋板块向东亚大陆下的 俯冲有关. 这些组分能够在地幔中较长的保存直至最近被热扰动所释放,并被橄榄岩包体所记录。     

Shaking table experimental study of recycled concrete frame-shear wall structures

In this study, four 1/5 scaled shaking table tests were conducted to investigate the seismic performance of recycled concrete frame-shear wall structures with different recycled aggregates replacement rates and concealed bracing detail. The four tested structures included one normal concrete model, one recycled coarse aggregate concrete model, and two recycled coarse and fi ne aggregate concrete models with or without concealed bracings inside the shear walls. The dynamic characteristics, dynamic response and failure mode of each model were compared and analyzed. Finite element models were also developed and nonlinear time-history response analysis was conducted. The test and analysis results show that the seismic performance of the recycled coarse aggregate concrete frame-shear wall structure is slightly worse than the normal concrete structure. The seismic resistance capacity of the recycled concrete frame-shear wall structure can be greatly improved by setting up concealed bracings inside the walls. With appropriate design, the recycled coarse aggregate concrete frame-shear wall structure and recycled concrete structure with concealed bracings inside the walls can be applied in buildings.