A unified plasticity model for large post-liquefaction shear deformation of sand

Based on previous experimental findings and theoretical developments, this paper presents the formulation and numerical algorithms of a novel constitutive model for sand with special considerations for cyclic behaviour and accumulation of large post-liquefaction shear deformation. Appropriate formulation for three volumetric strain components enables the model to accurately predict loading and load reversal behaviour of sand, fully capturing the features of cyclic mobility. Compliance with the volumetric compatibility condition, along with reversible and irreversible dilatancy, allows for physically based simulation of the generation and accumulation of shear strain at zero effective stress after initial liquefaction. A state parameter was incorporated for compatibility with critical state soil mechanics, enabling the unified simulation of sand at various densities and confining pressures with a same set of parameters. The determination methods for the 14 model parameters are outlined in the paper. The model was implemented into the open source finite-element framework OpenSees using a cutting-plane stress integration scheme with substepping. The potentials of the model and its numerical implementation were explored via simulations of classical drained and undrained triaxial experiments, undrained cyclic torsional experiments, and a dynamic centrifuge experiment on a single pile in liquefiable soil. The results showed the model’s great capabilities in simulating small to large deformation in the pre- to post-liquefaction regime of sand.     

On the influence of elastic–plastic coupling on sands response

A sand constitutive model accounting for elastic–plastic coupling is presented. To this aim, general constitutive equations describing an elastic–plastic coupling effect are developed first. Afterwards, a modified critical state plasticity model for granular soils is introduced accordingly. Several examples are presented to show the achieved improvements compared to the existing approaches. Comparing directly with experimental data, it is shown that the proposed model provides realistic simulations for pore pressure built-up under undrained cyclic loadings.     

Research Progresses and Prospect of Chimney Effect about Carbon Cycle in the Karst Cave System

There is a long history of several decades for the systemic study of global carbon cycles, and many important achievements have been obtained. However, these studies of carbon cycles in the karst area that accounts for about 10% of the entire Earth’s land area were paid attention gradually only in the past twenty. Recent researches suggest that karstification has important effects of the global carbon cycle. At present, scientists have had a very comprehensive and systemic understanding of various factors affecting the carbon cycle in the karst area, and estimated the carbon source or sink. However, unlike non karst area, there are generally many caves and fractures that have various sizes and morphologies in the karst area. These caves and fractures provide gas exchange between epikarst zone and atmosphere with a channel (as a window) in the karst area. There have been few reports about the role of the channel in the carbon cycle in the karst area. Previous studies have shown there is an obvious chimney effect in the karst cave system, which cannot be ignored. However, through the overview of the previous studies on carbon cycle of chimney effect in the karst cave system, we found that there were still many problems need to be further studied, such as the contribution of chimney effect to regional and global atmospheric CO₂; the regional differences, the seasonal changes and its reason (driving force) of chimney effect; the influences of chimney effect on the local ecosystem. With the development of modern science and technology (such as eddy covariance, large aperture scintillation and isotope), it becomes possible to solve the above problems. Therefore, it is necessary to use these advanced means for automatic and continuous monitoring and researching the carbon cycles in the karst cave system, which are of important significance for the researches of carbon cycles in the karst area, even for the global carbon cycle studies.     

江汉平原地球关键带监测网建设进展

地球关键带是地球科学领域的热点研究主题,我国地球关键带研究尚处于起步阶段,如何形成符合我国实情的地球关键带调查与研究范式,仍需进一步探索。江汉平原地处长江中游,是长江经济带的重要组成部分之一,其主要特点在于受不同级次水利工程的强烈影响。基于多年调查研究,在江汉平原建立了一套较为完善的地球关键带监测网络,涉及水、土、气、生物等各地球关键带要素。监测网络设置上,既考虑了不同级次水利工程的影响,也考虑了区域营养元素/重金属的富集特征; 监测技术上,部分指标实现了原位监测,大部分指标实现了野外现场测试,未来将进一步加强在线监测并实现数据远程传输。目前,江汉平原地球关键带监测网已成功纳入全球地球关键带研究网络,这是我国地球关键带调查与研究的初步探索,不仅可为长江流域的地球关键带调查研究提供示范,也可为长江大保护战略提供有力的理论与方法支撑。     

Transitions to adulthood among young entrepreneurs in the informal mobile telephony sector in Accra,Ghana

The rapid expansion of the mobile telephony sector in African countries has been accompanied by the establishment of a wide range of informal support businesses, mostly run by young people. Little is known, however, about the lived experiences of young entrepreneurs working in this rapidly changing, technologically-driven sector. Drawing on qualitative research conducted in Accra, this paper explores young people's experiences of running informal businesses within the mobile telephony sector, including the sale of mobile phones and accessories, repair and technical support services, and the sale of airtime and mobile money services. Fateful and critical moments relating to personal and family events, as well as social networks and structural factors, are shown to mediate young entrepreneurs' chances of success in this new ‘niche' economic sub-sector. Despite the challenges they face, the paper illustrates how many of these young people have been able to achieve financial independence, afford rental accommodation, provide support for family members, and establish and sustain households. The mobile telephony sector is shown to be offering young people the opportunity to carve out a living, facilitate transitions into adulthood, and even enable some to move up the social ladder. By highlighting the agency of this group of young people, and for some their success in achieving the status of adulthood through their hard work and ingenuity, this study offers an important counter balance to images of young people in sub-Saharan Africa as being ‘stuck' or in ‘waithood'.     

Effect of Rheology and Thickness Variation of Basal Detachment on Fold-and-Thrust Belts: Numerical SimulationEI北大核心CSCD

Detachment is the prime factor affecting the fold-and-thrust belts. In order to investigate the effect of rheology and thickness variation of basal detachment on fold-and-thrust belts, numerical simulations are carried out by using two dimensional plane strain mechanical models with elastoplastic or viscoelastoplastic material behavior solved with the finite difference software FLAC. Universal mudstone detachment has frictional property. Such fold-and-thrust belts are simulated with elastoplastic constitutive model. In contrast, salt detachment has creep property, so viscoelastoplastic constitutive models are employed for these fold-and-thrust belts. The results show that rheology and thickness of basal detachment influence the fold-and-thrust belts greatly. When the detachment is mudstone, the typical structural style of imbricate fan will develop, also the propagating forward piggy-back thrust sequence; however, when the detachment is salt, structural style will show Jura-type consisted of a series of anticlines and synclines, and out-of-sequence thrusts will appear. Varieties of thickness of basal mudstone detachment will change eventual surface slope of the fold-and-thrust belts, but have little effect on the structural styles and thrust sequence. For salt detachment, the effect is significant, no matter where the thickest detachment distributes, most strongly deformation and the highest surface will be located there. Thickening of salt detachment combined with slight thinning of overburden strata from hinterland to foreland will result in uncommon structural style and thrust sequence: back thrusts and backward propagating. These differences are mainly due to the much smaller shear strength of salt than that of mudstone. © 2017, Science Press. All right reserved.     

基于FloodArea模型的呼和浩特市城区大青山南麓山洪致灾临界雨量研究

以呼和浩特市城区北部大青山南麓山洪沟流域为研究区域,利用流域内国家气象站以及区域气象站逐小时降水数据、数字高程模型DEM数据、土地利用数据等资料,采用FloodArea淹没模型,对1998年7月12日和2020年8月1日暴雨时段进行洪水动态淹没模拟和效果检验。结果表明：对于1998年7月12日暴雨过程,F1oodArea水动力模型模拟结果与实际情况较为吻合,该流域内9条山洪沟洪水淹没深度与累计9小时滑动面雨量的相关性最好,并构建了面雨量与淹没深度的回归关系方程,基于隐患点3个风险等级,最终确定了不同等级下的临界面雨量,以红山口沟为例,分别为69.9mm(1级),39.3mm(2级),17.2mm(3级)。对于2020年8月1日暴雨过程,模型模拟结果基本与山洪过程一致,也证明了上述线性回归模型的适用性,为今后进一步建立全市山洪灾害预报服务奠定了基础。     

基于CI指数的黑龙江省作物生长关键期干旱变化

利用1961—2008年黑龙江省逐日气象观测资料,计算综合气象干旱指数（CI）,分析黑龙江省作物生长关键期（5—8月）气象干旱频率和气象干旱强度变化。结果表明：黑龙江省作物生长关键期干旱频率1960年最大为79%,1980年最小为58%,干旱频发中心位置变化不大,主要出现在松嫩平原西南部,但集中区向北转移。黑龙江省作物生长关键期干旱强度1960年为最强,其次为2000年,干旱强度最大区域基本稳定在松嫩平原西南部和三江平原中东部。2000年以来,作物生长关键期干旱频发且强度加大,北部渐频、渐强,原来较少发生干旱的三江平原干旱也时有发生且强度较大。     

2008年1月末九江雨凇转大雪天气过程成因分析

对2008年1月25—29日九江市一次雨凇转大到暴雪天气过程的大气垂直结构进行了分析。结果表明,在雨凇和降雪区域中层有暖层,低层有冷层,且存在对称不稳定层结。垂直风切变明显,有利于对称不稳定能量积累和上升气流的形成。高、低空急流辐散、辐合相互垂直,引发的较强上升运动是雨雪天气得以维持的重要条件。东西风向切变使冷、暖气流在雨雪区域上空交汇,形成水平气旋性辐合,产生垂直运动,且与垂直风切变相互作用,促动上、下层动量传递,使高、低空急流维持。雨凇转大到暴雪时,暖层气温降低,垂直风切变向上增强,对称不稳定能量增大,风向切变辐合加大,上升气流和水汽输送加大。     

2011年7月31日黑龙江省暴雨天气诊断分析

应用常规观测资料、NCEP再分析资料和卫星云图产品,对2011年7月31日黑龙江省西部暴雨天气成因进行诊断分析。讨论了产生暴雨的天气系统特征,大气不稳定条件及产生暴雨的水汽条件和动力触发机制。结果表明：暴雨是由低涡、低涡槽前暖湿气流与冷空气的共同影响产生的。低层强盛的偏南气流建立起水汽通道,将水汽源源不断地向暴雨区输送。低层增温增湿使得大气层结不稳定。低层较强的西北气流与强盛的东南暖湿气流汇合,产生强切变,辐合上升运动增强,为暴雨的产生提供了动力条件,有利于不稳定能量释放。高层辐散与低层辐合相配合,有利于上升运动发展和维持。地面中尺度低压和中尺度辐合线为中尺度云团的发展和维持提供了条件;中尺度云团在暴雨区旋转停留近21 h,这是暴雨发生的主要原因。