Molecular fossil and paleovegetation records of paleosol S4 and adjacent loess layers in the Luochuan loess section, NW China

Using gas chromatography-mass spectrometry (GC-MS) technique, a series of biomarkers were identi- fied, including n-alkanes, n-alkane-2-ones, isoprenoid etc. from the loess-paleosol samples collected from the S4 and adjacent L5, L4 of the Luochuan loess section, Northwestern China. Based on these data, especially n-alkanes and high-resolution magnetic susceptibility and grain size data, the pa- leoenvironment and paleovegetation history during S4 was reconstructed. The CPI (Carbon Predomi- nance Index) and correlation between n-alkanes and magnetic susceptibility and grain size data dem- onstrated that the molecular fossils in paleosol and loess layers can reflect the vegetation condition during the loess-paleosol formation, if the allochthonous organic inputs could be excluded reasonably. The ACL (average chain length) index is correlated well with paleomagnetic susceptibility and grain size variations, displaying their good synchrony with warm and humid climate. However, it relatively lagged behind the paleomagnetic susceptibility and the grain size variations when the climate began to deteriorate. During the formation period of paleosol, the n-alkanes was dominated by C31 homologue, indicating that the primary organic input originated from herbs. Our study also demonstrated that the herbs were more flourish than wood plants in Loess Plateau, especially in the Luochuan area during the warm and humid phase, and there was no typical forest vegetation developed in the studied period.

Instability characteristics of the East Asian Monsoon recorded by high-resolution loess sections from the last interglacial (MIS5)

The selection of high-resolution loess sections is needed in order to determine the climatic variability of the East Asian Monsoon during the last interglacial. Two sequences of S1 on the eastern and west-ern sides of the Liupan Mountain were both composed of five paleosol layers and four loess layers,indicating that there were five strong summer monsoon events and four strong winter monsoon events in MIS5. This corresponds with other records of the East Asian Monsoon,along with NGRIP and the North Atlantic records,implying that the climate of the Northern Hemisphere was very instable during the last interglacial. Two layers of paleosols and one layer of loess had developed during MIS5a and MIS5c. Compared with MIS5e,the climate in MIS5a and MIS5c fluctuated more intensively on a millen-nial scale,whereas the climate was relatively stable in MIS5e.

The elemental carbon record in Weinan loess section since the last 21 ka

We studied the records of elemental carbon (EC) of the last 21 ka in the Weinan loess section, Shanxi Province. The variations of EC abundance and δ13CEC value along with depth (or age) were presented. There are four large peaks of EC abundance around the following years: 20.16 ka, 17.76 ka, 11.97 ka and 4.49 ka. Climatic situation was changed rapidly during these periods. The peaks around 11.97 ka and 20.16 ka are particularly sharp, occurring over intervals of tens to hundreds of years, which could represent short-duration intense events. δ13CEc values in the upper 4 m of the Weinan loess section vary between -11.71‰ and -21.34‰, which suggests that the vegetation pattern of the last 21 ka on the Loess Plateau is G4-dominated grasses.

Characteristics of magnetotactic bacteria in Duanjiapo loess section, Shaanxi Province and their environment significance

Magnetotactic bacteria (MB) have been isolated from 61 samples which have been collected from S0, L1, S1 and L2 layers in the Duanjiapo loess section. A few MB (【25 cell counts per sieve mesh) have been found in loess layers (L1, L2), bow-shaped, each cell containing only two magnetosomes. while much more MB (】125 cells counts per sieve mesh) have been found from paleosol layers (S0, S1), rod-shaped, each cell containing 8-26 magnetosomes arranged in irregular chains. Magnetosomes with Fe and Co as the main metallic elements are spheroid-shaped, and mainly round in cross-section. Normal saturated fatty acids in MB ranged from C14 to C28; almost no monounsaturated fatty acids have been identified. Most suitable oxygen content for MB growing is 10%, and low concentration of organic salt (0.02mmol/L qumsic iron) is beneficial to the forming of magnetosomes. Results suggest that the paleosol development stage is suitable for the growing of MB and the climate fluctuation periods for magnetosomes formation.

A PRELIMINARY STUDY ON THE B/M POLARITY TRANSITION IN THE DUANJIAPO LOESS SECTION

The study on the polarity transition is a hot point subject in current geophysical researches. One of its principal aspects is to observe the variations in direction and intensity of the geomagnetic field during a polarity transition recorded in various sediments. The

凤凰山特大断面黄土隧道五步单侧壁导坑法研究

为解决特大断面黄土隧道采用双侧壁导坑法施工进度慢、成本高的问题,以G30连霍高速公路清水驿至忠和段扩容改造工程凤凰山隧道为背景,提出五步单侧壁导坑法。通过数值模拟对比分析双侧壁导坑法、五步单侧壁导坑法、传统单侧壁导坑法和台阶法在施工过程中的围岩变形及钢拱架应力,并结合现场试验段监测数据验证特大断面凤凰山隧道采用五步单侧壁导坑法施工的可行性。结果表明:1)五步单侧壁导坑法拱顶沉降和水平收敛比传统单侧壁导坑法分别减小3.59 cm和1.31 cm,比双侧壁导坑法分别增大1.4 cm和0.59 cm;钢拱架应力除拱肩外均小于双侧壁导坑法,最大处减少了70 MPa,钢拱架应力最大值与钢材屈服强度的比值比双侧壁导坑法增大0.04,比单侧壁导坑法减小0.08。2)传统单侧壁导坑法的安全性小于五步单侧壁导坑法,改进后的五步单侧壁导坑法相比双侧壁导坑法能在控制围岩变形的同时加快施工进度,降低施工成本。3)五步单侧壁导坑法中隔壁的最优位置应结合具体工程情况进行单独优化计算,针对本工程而言,其最优位置为45%洞宽。

黄土区大跨度隧道开挖技术探讨

临洮(安家咀)至临夏一级公路百和隧道均为Ⅴ级围岩,主要有黄土浅埋段、岩质地段、黄土深埋段、小净距段以及黄土深埋段等情况,针对不同的围岩情况和地质条件,有针对性的采用CD法、上下台阶留核心土法及三台阶分步法进行开挖。

大断面黄土隧道初期支护变形控制技术

结合某隧道工程实例,提出三台阶大拱脚临时仰拱法和设置围岩应力释放装置两种方法,根据现场试验结果对比分析两种方法的初期支护变形控制效果。结果表明,两种方法均能够控制初期支护变形,但从综合应用效果来看,设置围岩应力释放装置更为可行,其优势在于充分发挥出隧址区围岩的自承能力,与初期支护结构共同作用,提升支护效果,且机械化程度高,施工成本低,施工速度快,取得了良好的应用效果。

大断面黄土隧道近距离下穿桥梁桩基变形控制技术

以西延铁路洛川隧道下穿S304省道黑木沟中桥为例,针对高速铁路大断面黄土隧道近距离下穿桥梁桩基时,施工变形控制难、施工风险大等特点,提出对通行道路的导改、隧道开挖采用3台阶临时仰拱法(腰梁式)、内支护和锁脚锚管的加强等措施,现场监控量测数据表明均未出现一级预警状态,地表、桥梁和桩基均处于正常状态,解决了高速铁路大断面黄土隧道近距离下穿既有公路桥梁桩基施工安全,形成了一套大断面黄土隧道近距离下穿桥梁桩基变形控制技术,可为类似工程施工提供技术借鉴。

大断面黄土隧道施工变形控制技术研究

常规的黄土隧道施工变形控制技术主要使用单点临时仰拱法控制隧道开挖步距,易受隧道断面含水量变化影响使得隧道拱顶变形沉降过高,故设计一种新的大断面黄土隧道施工变形控制技术。首先开挖大断面黄土隧道施工导坑,再支护大断面黄土隧道施工钢拱架,最后注浆加固隧道围岩,从而实现大断面黄土隧道施工变形控制。实例分析结果表明:使用设计的大断面黄土隧道施工变形控制技术展开施工后,各个隧道监测点的拱顶变形沉降较低,能够满足大断面黄土隧道施工变形控制要求。

GEOCHEMICAL RECORDS OF PALAEOCLIMATIC CHANGES IN WEINANSECTION SINCE THE LAST INTERGLACIAL

Weinan section consists of Holocene soil SO, last glacial loess L1, and last interglacial soil S1, furtherly L1 can be subdivided into three layers of loess (L1-1, L1-3, and L1-5), and two layerS of weakly developed soil (L1-2 and L1-4). Based on studying the content variations of Carbonate, free Fe2O3, and the total organic content in Weinan section, the environmental evolution of this region has been discussed over the last glacial-interglacial cycle. Our results indicate that the chemical parameters can be ed as climatic proxy data, and the variations of these indexes reflect the cyclic nature of the Quaternary climatic change. In addition, the climatic records of Weinan section can also be correlated with that of the marine oxygen isotope records, so the environmental evolution of the Loess Plateau is also consistent with the global climatic changes. The unexpected finding is that the climatic curve of L1-5, which can be correlated with stage 4 of marine oxygen isotope records, consists of three troughs and two sandwiched peaks, and may suggest the existence of the second-order warmcold oscillations of this period.

ORGANIC GEOCHEMISTRY MARKERSOF CLIMATIC EVOLUTION IN LOESSREGION OF CHINA

Two hundred and thirty-five samples collected from Weinan loess section (L2 - S0)have analyzed by TOC (total organic carbon) and pyrolysis hydrocarbons. The curve of TOC with depth has ho set up and compared well with other climatic markers. The main periods of TOC variation of the last 0. 14 Ma are 67.08 ka and 18.29 ka, which is near with the periods of the variation of sunshine, showing that the TOC variation may be controlled by the variation of 65°N sunshine. The distribution of pyrolysis hydrocarbon and its parameters shows that there are two kinds of hydrocarbon associated with climatic change. One is called hydrocarbon pyrolysted directly from organism (pHC2-1) which is in the range of Tmax350℃ - 500℃, and associated with atmospheric water. The other is called carbonate inclusion inclusion (pHC2-2) which is in the range of Tmax 500℃ - 600℃ and formed by evaporation and associated with dry climate. The formula calculating humidity has been deduced Og = pHC2-1/(pHC2-1 + pHC2-2). The range of humidity of the section is 47% - 77% and the reducing is S1-3, S1-2, S0, S1-1, S1-4, S1-2, L1-1, L1-3, and L1-5. And the trend of the humidity is still increasing today.

Methods Development for the Optical Determination of the Black Carbon Content of Loess Samples

We evaluate the applicability of an optical transmission measurement method commonly used for the analysis of the Black Carbon (BC) content of aerosol sample, to determine the BC content of loess sediments. A number of different sample pretreatment procedures are developed and compared, leading to an optimum preparation process. The results include: 1) Subtraction of the optical attenuation values before and after heating of the sample filters (“ΔATN”) varies linearly with the sample mass. The slope of the regression line provides the best determination of BC concentration. 2) When the sample mass is small, (NaPO3)6 pretreatment is best for BC measurement, and the BC concentration results are given by the slope of the regression between ΔATN and sample mass, for a series of samples of varying mass. 3) HF pretreatment accompanied by centrifugation and rinsing may produce a negative bias on the result. 4) Replicate measurements of BC for loess samples showed a maximum deviation less than 5.6%, suggesting that measurements of the BC concentration of a sequence of loess samples could determine variations to this degree of significance. 5) The overall trends of BC concentration in loess section sequences were similar for all chemical pretreatments. The BC concentration result for replicate samples is comparable when pretreated by the same procedure.

湿陷性黄土地区过河段地下综合管廊施工关键技术

地下综合管廊可充分利用地下空间,提升安全运转品质,提高防灾抗灾能力,改善交通状况,创造和谐生态环境,实现城市可持续发展。但地下综合管廊建造距离长、复杂环境地段较多,尤其在我国分布范围很广的湿陷性黄土地区的施工较为困难,且施工工程中会遇到河流、涵洞等一些恶劣环境,因此,需要研究新的施工技术以适应施工中遇到的各种难题。以湿陷性黄土地区的某管廊项目工程为背景,结合湿陷性黄土的地质特点和过河段的复杂环境,提出了围堰-架设引流管、钢板桩+压密注浆复合支护、加腋区现浇结构建造和回填注浆等一系列湿陷性黄土地区过河段施工技术,为后续类似复杂环境下过河段管廊工程建设提供技术支撑和建造经验。

基于黄土隧道洞口浅埋段的Peck公式修正

黄土隧道洞口施工地表沉降开裂影响施工安全,以往采用Peck公式进行研究,存在预测精度等问题。为更准确地预测黄土隧道洞口浅埋段地表沉降,以临夏-大河家高速公路胡林家隧道为例,基于进口段实测数据,采用线性回归法对Peck公式进行优化。引入最大沉降量Smax修正系数α、降槽宽度i的修正系数β和隧道开挖进尺对最大沉降量影响的系数α1,确定修正系数得出Peck修正公式。基于洞口浅埋段开挖进尺与最大沉降间的影响关系,发现埋深对Smax修正系数与沉降槽宽度的作用规律,构建黄土隧道洞口浅埋段施工参数对地表沉降关系模型。实测数据样本点分布在修正后的Peck公式上下限之间,修正后的Peck公式误差减少,对临夏等地区黄土隧道洞口浅埋段的适用性较好。

高速公路大断面隧道在黄土地区施工技术研究

本文以G30连霍高速公路忠和隧道施工为背景,对三车道大跨度隧道下穿黄土地区所遇扰动后变形大、易坍塌等重难点问题进行探讨分析,并提出针对性施工措施,通过详细分析超前支护、导坑开挖等施工技术要点及技术参数,辅以对施工过程中压力及应力应变进行实时监测,结合监控量测的双控手段,为大断面黄土隧道施工保驾护航,有效实现对大断面黄土隧道围岩支护结构的动态监测管理,科学指导施工,确保大断面隧道安全穿越黄土地区。该高速公路大断面隧道在黄土地区施工的相关技术研究及数据、资料积累也为后续类似施工提供了借鉴和参考。

湿陷性黄土地区轨道交通桥隧过渡段处理措施研究

在湿陷性黄土地层中建设轨道交通工程在国内已有大量研究成果及工程案例,但轨道交通桥隧过渡段穿越大厚度湿陷性地层目前暂无工程经验可以借鉴。文章以西安北至机场城际轨道交通项目空港新城—机场(T5)区间桥隧过渡段工程实例为背景,对穿越大厚度湿陷性黄土的轨道交通桥隧过渡段的地基及结构处理开展方案研究,提出安全、可靠、经济的设计方案及处理措施,可为类似工程提供一定的参考和借鉴。

超大断面富水黄土隧道拱墙二次衬砌混凝土灌注施工技术研究

混凝土灌浆是隧道拱墙二次衬砌施工的重要环节,超大断面富水黄土隧道拱墙二次衬砌施工难度较高,因此,可通过超前支护、防水混凝土配合比设计与制备、混凝土喷注完成二次衬砌混凝土灌注施工。分析结果表明,混凝土抗拉强度与强度安全系数均大于最小限值,符合相关规范要求,设计技术能够保证施工质量,可为同类工程提供借鉴。

末次间冰期以来渭南黄土剖面地球化学指标所反映的古气候变化

渭南剖面由全新世土壤Ｓ０、末次冰期黄土Ｌ１及末次间冰期古土壤Ｓ１组成。其中，Ｌ１可以进一步划分为三层黄土（Ｌ１－１、Ｌ１－３、Ｌ１－５）和两层弱发育的土壤（Ｌ１－２、Ｌ１－４）；Ｓ１则由三层土壤组成（Ｓ１－１、Ｓ１－２、Ｓ１－３）。碳酸盐、游离铁及有机质含量在渭南剖面上的变化表明上述化学成分可以用作气候的替代性指标，并能反映古气候的变化。此外，渭南剖面的气候记录可以与深海氧同位素曲线很好对比，由此揭示出黄土高原的环境变迁也是对全球气候变化的响应。渭南剖面Ｌ１－５层位的气候曲线还可进一步划分为两个峰和三个谷。

Mastersize 2000型激光粒度仪分析数据可靠性检验及意义——以洛川剖面S4层古土壤为例

黄土、古土壤剖面中物质粒度是恢复古环境的重要指标之一。精确地测定粒径的大小,是准确而可靠获取气候变化信息的基本前提。通过对洛川典型黄土剖面古土壤S 4及相临黄土层高密度、连续样品多次重复粒度测试结果经统计分析表明:粒径测量的误差主要来自于两方面,即仪器测量与前处理过程,其中前处理过程的影响是粒度测量中误差的主要来源。同时证明,单次测量结果具有不确定性,因此可靠的粒度参数需经多次测量平均获得。但是,此过程会导致某些快速变化信息的丢失和因峰态的平滑而造成粒度变化所反映的变化幅度弱化。