ENVIRONMENTAL EVOLUTION OF ORDOS DESERT IN CHINA SINCE 1.1MA B. P. AS INDICATED BY YULIN STRATIGRAPHICAL SECTION AND ITS GRAIN-SIZE ANALYSIS RESULTS

1INTRODUCTIONOrdosDesertisageneral appellationof Qubqi Desert onthe north, Hedong Sandy Landonthesouthwest and MuUs Desert on the southeast (Fig. 1). It is situated on atransitional zone from arid desert steppe to semi-aridsteppe between ariddesertinNorth…     

GEOCHEMICAL RECORDS OF PALAEOCLIMATIC CHANGES IN WEINANSECTION SINCE THE LAST INTERGLACIAL

ChineselOeSSischaracterizedbythegreatthicknessandgabscontinuity.ItrererdsQuaternalyclimaticoscillations(Liu,1985).Inrecentyears,scientistshavetriedtOusephysicalorchemicalparameterstOrecostructthepastclimaticchanges.FOrinstance,theparticlesizeisUSedasproxyindicatorforthewintermo~intensity(Liu,1993),stableoxygenisotopicrecordofCarbenatenodulesisusedtoreflectthetemperaturevallationsOfsoilformation(Han,1995),andrnsgneticsuSCeptibilityandCarhanatecontentareughasproxyparametersforsuxnmermourn…     

Luminescence chronology and age model application for the upper part of the Chumbur-Kosa loess sequence in the Sea of Azov,Russia

A reliable chronology is essentially critical for correlating loess records with other paleoenvironmental time series, as well as for continuing improvements in the reconstruction of paleoenvironment and paleoclimate changes. It is exactly that the scarcity of chronologies across the Sea of Azov has limited the interpretation of climatic and environmental information in the East European Plain. In view of this, this paper conducted an exploratory study to investigate whether the optically stimulated luminescence (OSL) dating of mediumgrained quartz could be used to obtain a set of chronologies and the age models could be used to establish an independent time scale since the Late Pleistocene for the Sea of Azov loess. The results showed that an internally consistent set of optical ages for the Azov loess deposited up to ~76 ka. In addition, the ages developed based on magnetic susceptibility and grain size ages models showed good comparability with independent OSL ages at an acceptable range, suggesting that it might be practicable to establish an independent time scale using age models at the Sea of Azov loess, at least for the uppermost part of the Chumbur-Kosa section. Comparison with the ages based on two age models, the grain size ages using fine-grain fractions may provide a more reliable chronological sequence at the Azov loess since the Late Pleistocene. With the help of absolute ages and climate proxies (magnetic susceptibility and grain size), paleoclimatic change in the Sea of Azov have been traced for the Late Pleistocene.     

富县地区末次间冰期以来古气候环境的演变

研究剖面位于洛川源北部的富县县城东郊。剖面由第一层古土壤(S_1)、马兰黄土(L_1)和全新世古土壤(S_0)组成。孢粉分析结果表明,黄土地层中孢粉非常丰富,为恢复古植被与古气候提供了可靠依据。S_1的植被是以榛、菊、蒿为主的疏林草原,有亚热带树种出现,反映了末次冰期的温湿气候。L_1时主要为菊、蒿干草原,气候冷干,其间有四次乔木增多期,反映了末次间冰期中有四次间冰阶的相对温湿气候波动。近代耕土中为以松、藜、中华卷柏为主的森林草原,反映了全新世某个时期比较温凉干旱的气候。     

Extraction and analysis of gully head of Loess Plateau in China based on digital elevation model

In China′s Loess Plateau area, gully head is the most active zone of a drainage system in gully areas. The differentiation of loess gully head follows geospatial patterns and reflects the process of the loess landform development and evolution of its drainage system to some extent. In this study, the geomorphic meaning, basic characteristics, morphological structure and the basic types of loess gully heads were systematically analysed. Then, the loess gully head′s conceptual model was established, and an extraction method based on Digital Elevation Model(DEM) for loess gully head features and elements was proposed. Through analysing the achieved statistics of loess gully head features, loess gully heads have apparently similar and different characteristics depending on the different loess landforms where they are found. The loess head characteristics reflect their growth period and evolution tendency to a certain degree, and they indirectly represent evolutionary mechanisms. In addition, the loess gully developmental stages and the evolutionary processes can be deduced by using loess gully head characteristics. This study is of great significance for development and improvement of the theoretical system for describing loess gully landforms.     

Experimental study on improving collapsible loess with cement

The collapsibility of loess ground can directly affect stability of subgrade. Therefore, how to adopt practical technical measures to reduce or eliminate its collapse deformation is an important content in foundation design in collapsible loess zone. Selecting collapsible loess from Fuxin-Chaoyang highway in Liaoning, the authors conducted a series of tests for improving loess with cement. The loess in different water content was mixed with the cement in varying proportions, unconfined compression strength for the samples at four different curing periods were tested, and the relationships of improved soil strength among cement mixture ratio and curing periods were analyzed. When the curing periods are certain, the strength of loess increases along with the mixture ratio increases; when the cement mixture ratio is 5%-15%, the scope of increases is quite obvious; when the mixture ratio is greater than 15%, the tendency of intensity increases turns slow. When the mixture ratio for the specimen is certain, the intensity of the test specimen increases along with the curing period increases, the intensity grows obviously in 28 days, and the growth rate is small in 28-90 days, the intensity tends to be steady in the curing period of 90 days.     

Effects of site conditions on earthquake ground motion and their applications in seismic design in loess region

The Loess Plateau is an earthquake prone region of China, where the effects of loess deposit on ground motion were discovered during the 2008 Wenchuan earthquake (Ms8.0) and the 2013 Minxian-Zhangxian earthquake (Ms6.6). The field investigations, observations, and analyses indicated that large number of casualties and tremendous economic losses were caused not only by collapse and damage of houses with poor seismic performance, landslides, but also amplification effects of site conditions, topography and thickness of loess deposit, on ground motion. In this paper, we chose Dazhai Village and Majiagou Village as the typical loess site affected by the two earthquakes for intensity evaluation, borehole exploration, temporary strong motion array, micro tremor survey, and numerical analysis. The aim is to explore the relations between amplification factors and site conditions in terms of topography and thickness of loess deposit. We also developed site amplification factors of ground motion for engineering design consideration at loess sites. The results showed that the amplification effects are more predominant with increase in thickness of loess deposit and slope height. The amplification may increase seismic intensity by 1 degree, PGA and predominant period by 2 times, respectively.     

Variations of suction and suction stress of unsaturated loess due to changes in lignin content and sample preparation method

Unsaturated loess in natural sites loses stability as the overburden load continuously increases. Traditional soil modifiers such as cement and fly ash affect the surrounding environment. A new type of material, i.e., lignin, is environmentally friendly and able to increase the strength of loess. However, the engineering characteristics of the improved loess under unsaturated conditions are not yet clear. In this study, the soil-water characteristic curves(SWCCs) of lignin-improved loess samples were determined from 0 k Pa to 700 k Pa using a pressure plate instrument, and then, they were fitted using the van Genuchten(VG) model and the Fredlund and Xing(FX) model. In addition, the effects of the lignin content and sample preparation methods on the SWCCs were investigated to determine the optimal lignin content and a suitable sample preparation method for loess foundations. As the lignin content increases, the matric suction and residual water content of the improved loess increase. The suction stress increases with the increasing lignin contents of 1%-2%. At lignin contents of 3%-4%, the suction stress begins to decrease and the samples prepared using the slurry method has a lower suction stress than that prepared using the wet mixing method. The air entry value(AEV) increases with increasing lignin content. In addition, scanning electron microscopy(SEM) was used to investigate the microstructural variations. It was found that after the addition of lignin, the entrapment of the loess particles by the lignin fibers created some larger particles and smaller pore diameters, which in turn led to poor connectivity of the loess pores. These changes cause the matric suction of the modified loess to increase.     

黄土高原沟谷地貌发育演化研究进展与展望

黄土沟谷是黄土地貌中最有活力、最具变化、最富特色的对象单元,黄土高原千沟万壑的地貌形态以及触目惊心的侵蚀状态也让区域内沟谷地貌的形成、发育及演化问题成为研究中焦点及前沿性科学问题。近年来,诸多学者采用地学测年法、特征表达法、监测模拟法力图实现对黄土沟谷发育演化进程中“过去-现代-未来”的科学认知。这些研究在相当程度上丰富了黄土沟谷发育过程的认知。本文梳理了黄土高原沟谷地貌演化相关研究的现状,并从黄土高原地貌演化、黄土沟谷发育、基于DEM的沟谷信息提取与表达等研究进行了系统的回顾、梳理与分析。此外,本文提出“黄土沟道剖面群组”概念与方法,试图从新的视角审视黄土沟谷地貌发育演化过程。沟道剖面在黄土沟谷发育演化进程中传递物质能量和累积地形动力,并通过径流节点的串联实现剖面群的连接与组合,形成独特的剖面“群组”模式;该沟道剖面群组是集黄土沟谷地貌特征与过程于一体的综合信息集成体,其三维空间结构是对黄土沟谷地貌发育演化的高度抽象与映射,并可望进一步丰富黄土高原数字地形分析理论与方法体系,为黄土高原黄土地貌成因机理与空间分异格局带来创新的认识。     

基于地形剖面的黄土塬梁峁地形各向异性奇异强度研究

黄土高原不同地貌类型对应的地形起伏变化特征各不相同,地形起伏在不同方向上的变化特征具有很大差异性,即地形各向异性。本文引入多重分形谱参数描述不同地貌类型地形各向异性的变化规律,以反映不同黄土地貌地形各向异性的局部和整体变化格局。本研究以5 m分辨率的DEM数据为基础,选取淳化、甘泉、绥德3个典型黄土塬梁峁地貌类型研究样区,对其地形各向异性的变化特征进行分析。研究表明:①3种黄土地貌类型地形各向异性变化具有显著的多重分形特征,地形各向异性变化的奇异强度由强到弱依次为淳化、绥德、甘泉;②淳化样区地形各向异性奇异强度在1.4的概率最大,在奇异强度为2.2附近又有一个小的峰值,总体以奇异强度1.4为主;③甘泉样区的奇异强度在1.8有一个小的峰值,总体以奇异强度0.7为主;④绥德样区奇异强度为0.8的概率最大,且较为均匀地分布在0.8附近,地形各向异性的概率分布呈现为对称分布。研究结果为黄土塬梁峁地形各向异性变化奇异强度为黄土塬最大,黄土峁次之,黄土梁最小。该研究可为黄土地貌的精确分类提供定量信息支持,为区域水土侵蚀评价提供地形指标。     

原状及重塑黄土双轴试验微观力学特征离散元模拟

黄土作为一种特殊的颗粒材料,微观上颗粒组成的结构决定了其力学特性。原状及重塑黄土因结构的差异而具有不同的力学特性。针对黄土结构性如何影响其力学特征这一基本问题,开展基于电镜扫描获取细观颗粒信息,同时考虑颗粒形状、颗粒破碎可能性进行建模的离散单元法进行原状黄土和饱和重塑土在恒定应变速率双轴试验下的宏观力学和细观力学性能研究。研究结果显示:试样微观结构的差异对变形破坏过程产生显著影响。当轴向应力较低时原状黄土及重塑黄土力链多分布于大型骨架颗粒附近,随着轴向应力增加,原状黄土力链形成网状图案但仍具备主要传导区域,重塑黄土无明显主要传导区,呈现均匀网状。原状土及重塑土骨架颗粒簇周围多形成张拉裂隙,剪切裂隙多数形成于骨架颗粒簇内部,又以颗粒簇相互挤压接触时最为明显。使用该建模方法,可以有效反映原状及重塑黄土由于内部结构组成差异导致相同应力条件下产生的不同内部应力状态。基于以上研究结论,给出了黄土结构性对宏观强度影响的微观解释。研究成果可为黄土地区地质灾害防治提供一定依据。使用该建模方法,可以有效反映原状及重塑黄土由于内部结构组成差异导致相同应力条件下产生的不同内部应力状态。基于以上研究结论,给出了黄土结构性对宏观强度影响的微观解释。      

西宁的黄土研究

根据古地磁、磁化率、粒度和重矿物分析及电镜扫描石英砂表面特征等测试手段的综合研究,西宁地区的黄土可分出从S_0至L_(17)的连续黄土——古土壤序列。它包括全新世黄土、马兰黄土、离石黄土和午城黄土,形成的时间为1.2MaB·P.。黄土粒度粗,且含有两个风成细砂分层。重矿物以不透明矿物、角闪石为主,次为绿泥石、锆石和金红石等。石英砂颗粒以次棱角状为主,其次为棱角状。石英砂表面机械结构中有众多的冰川作用痕迹和风力作用痕迹。本区黄土主要来源于青藏高原第四纪局部冰盖、冰川及其外围广大冰缘地区所产生的粉砂物质。西宁大墩岭黄土剖面详细地记录了1.2MaB·P.的环境变化信息。其中1.1MaB·P.形成的L_(15)顶部的细砂分层代表1.2Ma以来最为干冷的严酷时期。L_1和L_2是末次冰期和倒数第二次冰期的产物。倒数第二次冰期比末次冰期更为干冷,极盛时期为0.155MaB·P.。S_1S_3和S_4可能代表1.2Ma以来本区最为温暖湿润的时期。     

三向应力及湿度状态改变对非饱和黄土力学特征的影响

通过对黄土常规三轴试验结果分析,给出了非饱和黄土的强度参数随含水量的指数型变化关系式,并验证了D-P屈服准则对不同湿度状态下黄土发生塑性屈服的适用性。不同三向应力条件和温度状态下的压缩试验表明,Q3黄土的应力应变关系随着三向应力和含水量而变化,这种本构关系可以用一个简单的指数函数来描述。与已有的模型相比,基于室内试验提出的半经验型黄土本构关系具有模型简单、参数少、易确定的优点。     

Centrifuge model test of an irrigation-induced loess landslide in the Heifangtai loess platform,Northwest China

The Heifangtai platform in Northwest China is famous for irrigation-induced loess landslides. This study conducted a centrifuge model test with reference to an irrigation-induced loess landslide that occurred in Heifangtai in 2011. The loess slope model was constructed by whittling a cubic loess block obtaining from the landslide site. The irrigation water was simulated by applying continuous infiltration from back of the slope. The deformation, earth pressure, and pore pressure were investigated during test by a series of transducers. For this particular study, the results showed that the failure processes were characterized by retrogressive landslides and cracks. The time dependent reductions of cohesion and internal friction angle at basal layer with increasing pore-water pressure were responsible for these failures. The foot part of slope is very important for slope instability and hazard prevention in the study area, where concentration of earth pressure and generation of high pore-water pressures would form before failures. The measurements of earth pressure and pore-water pressure might be effective for early warning in the study area.     

Tracing the provenance of aeolian loess in the Yangtze River Delta through zircon U–Pb age and geochemical investigations

Given the conflicts over the proposed formation mechanisms of Xiashu loess, the question of the provenance of sediments comprising the Xiashu loess in the Yangtze River Delta has not been satisfactorily resolved. In this study, the provenance of aeolian sediments of the Yangtze River Delta, China was examined by applying the detrial zircon U–Pb dating technique, Sr–Nd isotopic and trace element compositional analysis. U-Pb dating analysis was conducted on the Xiashu loess at three locations over the Yangtze River Delta, including Huangnishan(HNS) hill, Shengshan(SS) island and the Xuancheng(XC) area. The Xiashu loess and the sediments of the Yangtze River Valley share considerable similarity in their zircon U-Pb age spectra with the same main age peak and comparable age distribution. By contrast, significant differences in the age spectra, existbetween the Xiashu loess and loess deposits of Chinese Loess Plateau(CLP). Coarse grains of the Yangtze River Delta loess may have a proximal material source identical to the sediments from the Yangtze River valley. Sr–Nd isotopic values of the Xiashu loess match those from the northern margin of the Tibetan Plateau. Rare earth element ratios independent of grain size illustrate that the values from loess of the Yangtze River Delta mostly overlap with those of CLP loess. This feature implies that loess from the Yangtze River Delta has a dominant source of distant material similar as the CLP loess. As such, we conclude that multi-proxy analysis of sediments can shed new light on tracing the provenance of aeolian loess in the Yangtze River Delta.     

湿陷性黄土地区海绵城市建设研究进展

海绵城市作为城市雨洪管理、人居环境改善的重要手段,在水资源、水环境及水灾害问题突出的湿陷性黄土地区开展具有现实意义。为保证湿陷性黄土地区海绵城市建设的有效性与安全性,通过梳理黄土水分入渗规律、湿陷机理及湿陷性黄土地区海绵城市建设的研究趋势和不足,总结了湿陷性黄土地区海绵城市安全有效推进所面临的问题主要有:海绵城市建设需求旺盛但科技支撑薄弱;海绵设施“渗、滞、蓄”功能需求与黄土的水敏性和湿陷性矛盾突出;海绵城市建设冒进式开发与总体供给不足,规划水平有待提高。为了克服上述问题,提高海绵城市开发质量,当前面对的挑战体现在:差异化地层条件、渗流边界和应用场景下湿陷性黄土场地海绵城市建设的成套技术研发;湿陷性黄土场地海绵城市建设的适宜性评价体系和风险评估理论构建;黄土地区海绵城市建设规划的方法优化。     

模糊评判法在公路黄土洞穴发育程度分区中的应用

阐述了影响黄土洞穴形成的黄土湿陷性、崩解性、地质构造、节理裂隙、微地形地貌和水文及气候条件等工程地质特性和地质因素;在公路沿线地区,公路工程质量及人类活动也是黄土洞穴形成的影响因素。洞穴发育程度分区预测和评价极具复杂性、随机性和模糊性,为此,在对各影响因素分析的基础上,借助模糊综合评判法,对黄土区公路沿线洞穴发育程度分区进行了研究。与实际情况对比分析表明,预测评价结果合理可靠,可作为黄土区已建公路洞穴灾害的防治依据。     

山西乡宁—吉县地区黄土高边坡可靠度研究

在山西乡宁—吉县地区实测了7个具有代表性的自然极限状态黄土高边坡断面,建立边坡地层结构模型;选取研究区合理的黄土物理力学参数,并分地层年代对10个典型工点的黄土强度参数内聚力和内摩擦角的变异性进行分析;选取6组典型的强度参数变异系数组合,基于Monte Carlo法进行可靠度模拟,评价该区边坡稳定性;采用自然类比法进行边坡设计.结果表明:边坡失效概率随坡高的变化趋势一致,均是中等坡高(49.8m)的边坡失效概率较大,低坡和高坡的失效概率较低;当变异系数较小时,边坡失效概率对坡高和坡度的变化敏感,而当变异系数较大时,边坡失效概率对坡高和坡度的变化不敏感;当坡型一定、强度参数变异系数较小时,边坡的稳定系数基本不变,而当参数变异系数超过某一界限时,稳定系数随变异系数的增大而增大,二者存在非线性相关关系;研究区黄土边坡处于基本稳定状态,但其失效概率最大达69.3%,平均21.1%,介于20%~30%的比例为33.3%,大于30%的比例为14.3%,可靠度指标介于-0.5~8.5,其中小于2.7的比例为88.1%;对于坡高约为50m的黄土高边坡,若取30%作为可接受失效概率,边坡坡度需降至45°以下,如果期望可接受失效概率在10%以内,则坡度不宜超过34°.     

基于CT扫描的黄土孔隙结构特征研究

孔隙特征作为反映黄土微观结构的重要特征之一, 直接影响黄土的水敏性、渗透性和强度等物理力学性质。为了研究水力耦合作用下黄土微观孔隙结构特征, 使用CT技术对天然原状、原状饱和与重塑黄土的初始结构以及不固结不排水剪切试验后的土体结构进行了扫描, 通过建立黄土三维结构模型, 分析了剪切试验前后孔隙结构的演变特征。结果表明: 饱和与重塑作用使天然原状黄土的大孔隙减少, 剪切作用使天然原状黄土和重塑黄土发生剪切破坏, 原状饱和黄土发生压缩破坏, 局部孔隙率增加。天然原状黄土与原状饱和黄土在剪切前后均表现为微孔和小孔数量较多, 其孔隙倾角主要分布在50°~90°之间, 解释了黄土亚稳态结构形成的主要原因。扰动作用使重塑黄土的孔隙尺寸分布均匀, 且重塑黄土与原状饱和黄土在水力作用下更易失稳屈服。揭示了黄土剪切变形破坏的微观结构主要表现为粒间胶结物的溶解、孔隙的坍塌与填充以及颗粒旋转、破碎和滑移。试验结果可为黄土剪切强度降低和湿陷机理研究提供依据。      

宝鸡黄土中的风化壳

根据野外调查,在宝鸡黄土中确定了五个时期的风化壳,它们分别为第1～5层古土壤与其下的风化黄土构成。根据X-射线衍射得知,这些风化壳为水云母和绿泥石等构成的粘土型风化壳。CaCO_3淀积深度和Fe_2O_3、Al_2O_3迁移特征表明第1和3～5层古土壤与其下黄土构成的风化壳发育时处于酸性风化阶段,当时年平均降水量在900mm以上;第2层古土壤与其下风化黄土构成的风化壳发育时处于中性风化阶段,当时年平均降水量为800mm左右。