Dynamic analysis of strain localization in water‐saturated clay using a cyclic elasto‐viscoplastic model

A computational framework is presented for dynamic strain localization and deformation analyses of water‐saturated clay by using a cyclic elasto‐viscoplastic constitutive model. In the model, the nonlinear kinematic hardening rule and softening due to the structural degradation of soil particles are considered. In order to appropriately simulate the large deformation phenomenon in strain localization analysis, the dynamic finite element formulation for a two‐phase mixture is derived in the updated Lagrangian framework. The shear band development is shown through the distributions of viscoplastic shear strain, the axial strain, the mean effective stress, and the pore water pressure in a normally consolidated clay specimen. From the local stress–strain relations, more brittleness is found inside the shear bands than outside of them. The effects of partially drained conditions and mesh‐size dependency on the shear banding are also investigated. The effect of a partially drained boundary is found to be insignificant on the dynamic shear band propagation because of the rapid rate of applied loading and low permeability of the clay. Using the finer mesh results in slightly narrower shear bands; nonetheless, the results manifest convergency through the mesh refinement in terms of the overall shape of shear banding and stress–strain relations. Copyright © 2013 John Wiley & Sons, Ltd.     

Modeling the 2D behavior of dry‐stone retaining walls by a fully discrete element method

The dry‐stone retaining walls (DSRW) have been tipped as a promising solution for sustainable development. However, before recently, their behavior is relatively obscure. In this study, discrete element method (DEM) approach was applied to simulate the plane strain failure of these walls. A commercial DEM package (PFC2D™) was used throughout this study. The authors used a fully discrete approach; thus, both the wall and the backfill were modeled as discrete elements. The methodology for obtaining the micromechanical parameters was discussed in detail; this includes the three mechanical sub‐systems of DSRWs: wall, backfill and interface. The models were loaded progressively until failure, and then the results were compared with the full‐scale experimental results where the walls were loaded, respectively, with hydrostatic load and backfill. Despite its complexity and its intensive calculation time, DEM model can then be used to validate a more simplified approach. Copyright © 2015 John Wiley & Sons, Ltd.     

Reaction softening by dissolution–precipitation creep in a retrograde greenschist facies ductile shear zone,New Hampshire,USA

We describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host schist of the Littleton Formation near Claremont, New Hampshire. Here, Na‐poor muscovite and chlorite progressively replace first staurolite, then garnet, and finally biotite porphyroblasts as the core of the fault zone is approached. Across the transect, higher grade fabric‐forming Na‐rich muscovite is also progressively replaced by fabric‐forming Na‐poor muscovite. The mineralogy of the new phyllonitic fault‐rock produced is dominated by Na‐poor muscovite and chlorite together with late albite porphyroblasts. The replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement. In contrast, element mapping shows that fabric‐forming Na‐rich muscovite is selectively replaced at high‐strain microstructural sites, indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite. The replacement of strong, high‐grade porphyroblasts by weaker Na‐poor muscovite and chlorite constitutes reaction softening. The crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated Na‐rich muscovite‐defined foliation destroys not only the metastable high‐grade mineralogy, but also its stronger geometry. This process constitutes both reaction and microstructural softening. The deformation mechanism here was thus one of dissolution–precipitation creep, activated at considerably lower stresses than might be predicted in quartzofeldspathic rocks at the same lower greenschist facies conditions.     

Contrast in stress–strain history during exhumation between high- and ultrahigh-pressure metamorphic units in the Western Alps: Microboudinage analysis of piemontite in metacherts

Our analyses of microboudinage structures of piemontite grains embedded within six samples of metachert, one collected from an ultrahigh-pressure (UHP) metamorphic unit at Lago di Cignana in Italy of the Western Alps, and the other five from surrounding high-pressure (HP) metamorphic units in Italy and France, have revealed that the structures are all symmetrical in type, and were presumably produced in coaxial strain fields. Stress–strain analyses of the microboudinaged grains revealed significant contrasts in the stress and strain histories of the UHP and HP metamorphic units, with the differential stress recorded by the UHP sample being unequivocally lower than that recorded by the five HP samples. In addition, our analyses showed that the UHP sample underwent stress-relaxation during microboudinage, whereas the five HP samples did not. On the basis of these observations and analyses we discuss the mechanical decoupling of the UHP and HP units that led to different histories in differential stress between the units during exhumation of the Western Alps.     

Logistic判别模型在强降水预报中的应用

利用Logistiv判别模型进行强降水预报,并设计3种方案进行对比分析。方案1直接使用14个影响因子进行判别预报,受因子共线性作用及噪音信号影响,虽然拟合效果较好,但预报效果明显下降。方案2对14个影响因子进行主成分分析,利用前6个主成分建模,虽然拟合效果较方案1降低,但由于消除了因子共线性作用以及噪音信号影响,预报效果较方案1提高。方案3运用Bootstrap抽样技术得到符干样本并建模计算模型参数,打乱了原有时间序列中的波动,仪保留平稳信息,拟合自由度进一步降低,导致拟合效果较方案案2下降,但预报效果却是3种方案中最好且最稳定的。在上述研究基础上,利用欧洲中心数值预报模式的预报场资料,建立基于Logistic判别模型的强降水客观预报系统,并在中央气象台业务运行。2013和2014年连续两年汛期预报检验结果表明,概模型对强降水预报的TS评分高于数值模式本身,具有一定的业务参考价值。     

Landsat 8陆地成像仪影像的缨帽变换推导

针对Landsat 8陆地成像仪影像,该文提出了一种缨帽变换系数的推算方法。利用准同步的Landsat ETM+影像缨帽变换结果为目标值,采用普鲁克算法对不同地理分布下的陆地成像仪影像样本点的主成分轴分别进行旋转,得到相应的初始变换系数;通过广义普鲁克分析对所得初始变换系数进行平均化,并以格拉姆-施密特算法保持其正交性,最终得到基于陆地成像仪大气表观反射率数据的缨帽变换系数。验证结果表明,与现有陆地成像仪变换系数相比,该文提出的缨帽变换精度相对较高,误差较小,变换后的不同地物覆盖类型具有更好的可分性。     

Modification of strain wedge method for lateral soil-pile interaction in sandEI北大核心CSCD

A modified strain wedge (SW) method for analyzing the behavior of laterally loaded single piles in sand is proposed. The modified model assumes that the lateral displacements of a pile behind the three-dimensional passive soil wedge are nonlinear, which makes the horizontal soil strain variable with depths instead of a constant value in the original strain wedge model, and also employs two different hyperbolic models, one for describing horizontal stress increment-strain behavior of soil in the wedge, and the other for describing the shear stress-displacement property at the interface between soil and pile shafts. An example is analyzed to demonstrate the effectiveness of the modified method, and a good agreement is obtained. Finally, the effects of modifications on the lateral bearing capacity of pile shafts are discussed. The results show that the problem of overestimating the lateral bearing capacity of piles with strain wedge method can be ameliorated by introducing the assumption of nonlinear lateral displacements of piles. It makes the SW method more convenient and effective in analyzing the behavior of laterally loaded piles by introducing the new relationships of horizontal stress increment-strain and shear stress-displacement.     

兴蒙造山带—华北克拉通地球化学走廊带沉积物主成分的含量与空间分布研究

选取兴蒙造山带—华北克拉通地球化学走廊带上沉积物的SiO_2、Al_2O_3、CaO、MgO、Na_2O、K_2O、TFe_2O_3、FeO、TiO_2、MnO、P_2O_5、CO_2、H_2O~+、pH十四个指标,研究其在不同二级构造单元、地理景观、土壤类型、降雨量等级下的含量与空间分布特征,并讨论该地球化学走廊带上化学蚀变指数。结果表明:在兴蒙造山带—华北克拉通地球化学走廊带上,作为沉积物主体的SiO_2、Al_2O_3两者含量为明显负相关而空间分布表现出此消彼长的特征;CaO、MgO、CO_2高含量分布区则与碳酸盐岩地层或含碳酸盐矿物的土壤有关,而MgO的高含量还与走廊带上镁铁质基性-超基性岩有一定关系,表明了这三个指标的高含量受特定岩性或矿物的影响较大;Na_2O、K_2O含量除了受继承的基岩影响外,还受到后期的风化作用和气候及自身地球化学性质等复杂因素综合影响,对Na_2O来说尤为如此;TFe_2O_3、MnO、TiO_2、P_2O_5相对于在地质背景复杂地段,经过冲积平原的沉积物混匀后,含量差异更小;相对干冷的内蒙古半干旱草原的气候条件有利于FeO存在,使得其相对燕山地区含量差异较TFe_2O_3要小,在章丘以南相对温暖湿润地区更易被氧化而导致其含量整体较低;北方干冷气候条件下,沉积物H_2O~+含量普遍较低,局部高含量位于碳酸盐岩地层或第四系分布区;走廊带上的pH值反映了沉积物偏碱性的特征,而在章丘以南的地区随着降雨量的增加,相对温暖湿润的气候条件,沉积物pH值表现为中性-偏弱酸性;兴蒙造山带—华北克拉通地球化学走廊带沉积物的CIA值反映出在寒冷、干燥气候条件下低等的化学风化程度,兴蒙造山带沉积物平均风化程度相对华北克拉通沉积物的平均风化程度差异变化相对要小,显示兴蒙造山带的降雨量与温度等风化影响因素变化较小。作为反映元素的地球化学亲和性的量化指标,离子电位可以更好地帮助理解沉积物元素的含量和空间分布差异。     

构造物理模拟和PIV有限应变分析对构造裂缝预测的启示

油气储层中构造裂缝发育与有限应变状态关系密切,为了探索有限应变分析与构造裂缝预测的新技术方法,此次研究设计完成了一组单侧挤压收敛模型的物理模拟实验,并引入粒子图像测速(PIV,Particle Image Velocimetry)技术对实验过程进行了定量化分析。实验模型在垂向上为含粘性层的多层结构,实验结果形成了一个肉眼可见的箱状褶皱。通过PIV技术可以获取实验模型变形演化过程中各阶段的位移场数据,计算出各阶段的增量应变,实现从初始状态到褶皱形成之后整个变形过程的有限应变分析,探讨构造裂缝成因机制和分布规律,进行定量化裂缝预测。挤压变形过程初期,应变分布范围很广,有限应变较弱(约4%~8%),在挤压方向上的线应变表现为弱压应变,在垂向上的线应变表现为弱张应变,这种现象是褶皱和断层产生前平行层缩短和层增厚的纯剪变形结果,也是区域型张裂缝和剪裂缝形成的主要机制。褶皱和断层即将发育之时至发育之后,应变局限在断层发育的剪切带及附近区域,有限应变表现为较强(达20%)的剪切应变和剪切张应变,是断层面附近简单剪切变形作用的结果,也是局部型剪裂缝和张剪裂缝形成的主要机制。     

重度石漠化区不同土地利用方式下土壤养分特征

本文以云南省蒙自市西北勒乡碧色寨村石漠化土地为研究对象,基于主成分分析法（PCA）研究岩溶石漠化区不同土地利用方式下的土壤养分特征。结果表明:（1）研究区土壤各养分指标变异系数介于0.14~1.76,不同土地利用方式下,土壤养分状况有较大差异;（2）研究区土壤12项养分指标可归结为3个主成分,所提供的信息占全部信息的90.69%,其中PC1主要包含了全磷、有效磷、速效钾和有效锰,PC2主要综合了有机碳、全氮、全钙和有效锌,有效铁对PC3具有较大的贡献。（3）对不同用地类型土壤养分的主成分得分进行排序,由大到小依次为火龙果地>林地>封育草地>玉米地,表明植树造林及火龙果种植等方式能有效改善石漠化区的土壤养分状况。建议对样区的石漠化治理应以封山育土为前提,以生态恢复为目的,通过种植火龙果与修复林地改善样区土壤状况,逐步形成生态环境与社会经济协调发展的石漠化综合治理体系。