基于有限元数值模拟的穿越影响范围分析

北京地区对于施工下穿(侧穿)桥梁工况,一般是参考北京市地方标准划分主、次要影响区.该标准在使用中存在两个主要问题:1)拟建工程与桥梁下部结构接近关系判别定义不清;2)拟建工程对桩周土扰动范围按无桩情况套用的适宜性评价.以北京地区某实际工程为依托,采用有限元数值模拟方法和北京市地方标准中规定的方法,分析穿越工程主、次要影...     

12 kaBP lake environment on the Tibetan Plateau

The systemic analyses have been carried out in this paper to the paleolake shorelines, paleolake sediments and paleoclimatic proxies of 20 lakes, in which there were 12 kaBP dating data on the Tibetan Plateau. The results showed that the paleolake level had apparently risen during 14-11 kaBP, the glaciers melting period, in the Tibetan area and Northwest China. Especially, much more increasing amplitude supplied by thawy glaciers water occurred than in the best period of Holocene. The temperate-humid climate around 12 kaBP appeared in the Tibetan area and even in the whole China. This event may be compared with the Bolling/Allerod warm period which was reflected by Europe and Greenland ice core records. It showed that the B/A event was not a regional one in the North Atlantic area.

     

用单站铅直能量廓线图作局地暴雨预报辅助工具的初步尝试

本文通过对2001年至2003年4次暴雨个例的单站铅直能量廓线图的各种参数进行分析，发现用能量学方法作为常规天气学方法的辅助工具，在暴雨预报中有很好的应用价值。     

矿井底板破碎带温度场模型推导及模拟分析

为了研究矿井底板破碎带温度场对地下水的响应机制,运用热传导理论,结合数学物理方法,建立了垂向水流作用下岩体温度场模型和地下水、空气等多热源作用下岩体过余温度分布模型,并求出了相应模型的温度、温度梯度及过余温度解析解。结果表明：沿着地下水径流方向,起始段的岩体温度梯度值一般较小,而终止段的温度梯度值则一般较大;当温度梯度与地下水流动方向相反时,岩体温度梯度变化幅度较小;回采工作面接近含导水构造时,距离导水构造越近,同一时刻岩体过余温度增大越迅速,体现在温度曲线的曲率相应增大。在一定范围内,地下水对矿井岩体温度梯度及过余温度影响显著。     

天津地区地层热物性特征及影响因素分析

为分析天津地区地层热物性参数的地区分布特征和平面分布规律,布置88个勘查孔、现场采集1 076个岩土样、室内分析热导率、比热容、热扩散系数等地层热物性实验,对测试数据进行统计分析。结果表明:天津市岩土体热导率在1.26~1.70 W/(m·K),比热容在2 050~2 090 J/(kg·K),热扩散系数在0.45×10-6~0.74×10-6 m2/s。同一地区不同岩性的比热容,黏土最大、粉砂最小,热导率刚好相反,热扩散系数与比热容规律相同;不同地区同一岩性的热物性参数差别不大。天津市比热容与热导率呈现大致相反的趋势,热导率高值区位于蓟县、宝坻和宁河的东部、武清西部、静海南部以及滨海新区的中部地区;比热容高值区位于蓟县、宝坻和宁河西部、武清东部、静海北部以及滨海新区大部分地区。     

若尔盖RM孔有机碳、总氮和碳氮比值的垂直分布

分析了ＲＭ孔岩心样５９５个．孔内总有机碳含量ＴＯＣ变动范围较大，低者１—２％，高者１０—２０％；总氮含量ＴＮ多０．１—０．２％，少数０．４—０．８％；碳氮比值Ｃ／Ｎ较低（≤８）段属浅湖沉积，Ｃ／Ｎ较高（＞８）段属湖滨沼泽沉积；ＴＯＣ，ＴＮ和Ｃ／Ｎ明显有四个波动段．     

Weak chemical weathering during the Little Ice Age recorded by lake sediments

Low magnetic susceptibility, low Sr content and hence high Rb/Sr ratio in the lake sediment sequence indicate a weak chemical weathering process under arid and cold climate of the Little Ice Age in a single closed lake watershed. According to different geochemical behavior between rubidium and strontium in earth surface processes, variation of Rb/Sr ratios in the lake sediment sequence can be used as an effective geochemical proxy with definite climatic significance of chemical weathering in watershed. Unlike chemical weathering process in tropic zone and modern temperate-humid climate, concordant changes in both Sr content and magnetic susceptibility with δ¹⁸O values of Dunde ice core suggest that the weak chemical weathering was controlled by air temperature during the Little Ice Age maximum. After the Little Ice Age, chemical weathering intensity was controlled also gradually by precipitation with increasing in temperature.     

Environmental variation in central Tibetan Plateau in the last 200 years

Through the analyses on sediment lithology,²¹⁰Pb and¹³⁷Cs dating, carbonate content, Sr/Ca and Mg/Ca of Ostrocod, and carbon and oxygen stable isotope of carbonate of gravity core from Cuoe Lake in the Naqu area, Tibetan Plateau, the paleolimnology was studied to investigate the environmental variation in the central Tibetan Plateau in the last 200 years. It is inferred that the environmental variation has undergone two stages. The climate was arid in the early 100 years, while it became warmer and more humid in the later 100 years, with a period of 20 years for humidity change.     

Chemical weathering since the Little Ice Age recorded in lake sediments: a high‐resolution proxy of past climate

Because of the different geochemical behaviour of rubidium and strontium in earth surface processes, variations of the Rb/Sr ratios in lake sediments were used as a geochemical proxy of chemical weathering and past climate in a single watershed. Low magnetic susceptibility, low CaCO₃, low Sr concentration and, hence, high Rb/Sr ratio in the lake sediments indicate weak chemical weathering under a cold but wet climate during the Little Ice Age (LIA) in the closed Daihai Lake watershed. The concordant change in both Sr and CaCO₃ concentrations with δ¹⁸O values in the Dunde ice core suggests that weak chemical weathering during the wet LIA was controlled by air temperature. After the LIA, however, precipitation played a dominant role in chemical weathering. Copyright © 2001 John Wiley & Sons, Ltd.