Effects of initial static shear on cyclic resistance and pore pressure generation of saturated sand

In practical engineering, cyclic shear stresses induced by earthquakes, traffic, and waves are superimposed on the initial static shear stress in sand fills or deposits, leading to complex responses of soils such as their deformation characteristics, pore pressure generation, and susceptibility (or cyclic resistance) to liquefaction. To experimentally investigate the undrained cyclic response of saturated sand, a series of triaxial tests were performed, covering a broad range of initial static and cyclic deviatoric stress levels. The results indicate that different stress conditions lead to two types of cyclic behavior: cyclic mobility and residual deformation accumulation. The compressional static stress is beneficial to the cyclic resistance of the dense sand, whereas the extensional static stress is regarded as detrimental as it tended to reduce the cyclic strength. Moreover, by comparing the available test data obtained from the same sand with varying initial densities and confining pressures, the static shear effect on cyclic resistance was found to be dependent on the state of the sand. Compared to the interpretation made using the limiting pore pressure-based criterion, the conventional failure criterion using a cyclic axial strain of 5% may lead to a substantial overestimation of the cyclic resistance, thus resulting in unsafe assessment and design. Hence, by employing the pore pressure criterion, the pore pressure generated in the cyclic tests was investigated and was found to be significantly influenced by the static shear stress. A pore pressure generation model is proposed to obtain the pore pressure characteristics of sand under various static shear stress conditions.     

Mechanism of sinkhole formation during groundwater-level recovery in karst mining area,Dachengqiao, Hunan province,China

The Meitanba Coal Mine area in Hunan province, China, had been impacted by severe cover collapse sinkholes since 1982 due to mine dewatering. After the coal mine was closed in February 2015, the groundwater level has increased significantly. A series of sinkholes were recorded in the study area during groundwater-level recovery. Analysis of monitoring results and in-situ investigation indicated that 13 sinkhole collapses were more likely induced by abrupt change of groundwater–air pressure in response to heavy rainfall from March 2015 to July 2016 when the groundwater level increased by as much as 76 m. When the karst conduit was flooded, a relatively sealed environment was formed between saturated sediments and flooded karst conduit. Implosion of entrapped air might have caused the cave roof to collapse followed by surface collapses in a short time. On the other hand, four sinkholes occurred in November 2016 when the groundwater levels were near the soil–bedrock interface at elevations between 52.5 and 58.9 m amsl and the groundwater-level increase was at slower paces. Field measurements indicate that the groundwater-level fluctuation at the soil–bedrock interface could enlarge the soil cavity and accelerate the subsoil erosion process.     

准噶尔盆地南缘小渠子背斜T/P 同构造不整合几何学、运动学特征及地质意义

为探讨准噶尔盆地南缘二叠纪-三叠纪盆地构造性质及构造演化过程,笔者对盆地南缘小渠子背斜保存较完整的T/P 不整合进行了几何学、运动学和沉积韵律旋回特征的分析。T/P 不整合具有同构造不整合的特点,表现为不整合之下削蚀、之上超覆,是由于盆地南缘经历晚二叠世-早三叠世区域性挤压作用造成的。通过对小渠子地区深层地质结构的分析,认为晚二叠世-早三叠世的构造演化过程与早石炭世伸展断陷的反转密切相关。     

水平双向荷载耦合对饱和软黏土动力特性的影响

地震过程中剪切波可用大小和方向上相互耦合的水平两向剪切来模拟。以温州典型软黏土为对象,采用多向动单剪系统(VDDCSS)进行不排水应力控制的循环单剪试验,重点研究了水平双向耦合荷载作用下循环剪应力比(SSR)和相位差?对饱和软黏土动力特性的影响,试验组数共计19组。试验结果表明:在单向循环剪切荷载作用下温州正常固结饱和软黏土的门槛循环应力比SSR_(x,menkan)为0.096,临界循环应力比SSR_(x,limit)为0.15;水平双向耦合荷载作用下,双向循环应力比SSR(SSR_x(28)SSR_y)的提高以及循环次数N的增加会促进剪应变?的增加和孔压u的发展;当SSR稍微大于SSR_(x,menkan)时,?的增加也会阻碍?和u发展,在临界循环应力比SSR_(x,limit)附近,?的存在会急剧加快土样总应变和孔压的发展,当SSRSSR_(x,limit)时,相位差?的存在对土样总应变和孔压发展的影响基本可以忽略。     

核磁共振找水方法的现状和发展趋势

简要介绍了利用核磁共振（ｎｕｃｌｅａｒ ｍａｇｎｅｔｉｃ ｒｅｓｏｎａｎｃｅ,缩写为ＮＭＲ）技术找水方法的发展状况、原理和方法特点,重点介绍了中国地质大学（武汉）ＮＭＲ科研组,利用ＮＭＲ找水方法在中国一些地区找水取得的成效,最后指出了ＮＭＲ找水方法的发展趋势。     

The Formation and Evolution of the Lujiang-Zongyang Volcano-Structural Depression in Anhui and Its Relation to Mineralization

The Lujiang-Zongyang volcano-structural depression resulted from the combined action of tectonism and volcanism. Its evolution progressed through five stages from 190 to 105 Ma B.P., during which nine types of metal and pyrite deposits were formed. These deposits show obvious regularities in the temporal-spatial distribution and have certain genetic relation, forming a minerogenetic series related to shoshonite within or around the volcano-structural depression on the basement that sank for a long period of time. This paper suggests a four-dimensional model of the formation and evolution of a typical volcano-structural depression and its related mineralizalion in a transition belt from the active margins to the inner part of a plate under alternating shear-compressive and shear-extensional stress conditions, but predominantly shear-extensional stress conditions.     

氟碳钙铈矿的42R、48R的16H新多型

采用CM12透射电镜,对我国四川昌北M稀土矿床中的氟碳钙铈矿微结构特征作了电子衍射分析和晶格象观察,揭示出氟碳钙铈矿晶体中混合多型体的堆垛序列,在不同部分观测到不同的多型体,其中以氟碳钙铈矿6R多型为常见普通多型体。本次工作发现了42R、48R和16H三种新多型,并进一步证明,氟碳钙铈矿晶体中存在的多型结构相当复杂,但以长周期堆垛形成的多型体出现的几率要比短周期堆垛的多型体少得多。     

安徽庐枞火山—构造洼地的形成,演化及成矿

庐枞火山-构造洼地是在构造和火山岩浆活动共同作用下产生的。从190Ma至105Ma其演化过程可分为五个阶段,共出现了9种金属和硫铁矿矿床类型,这些矿床在时-空分布上具有明显的规律性,在成因上具有一定的内在联系,构成了在长期沉降基底士的火山-构造洼地内、外与橄榄安粗岩系有关的成矿系列。本文提供了一个在剪切-挤压与剪切-拉张交替作用,而以剪切-拉张为主的构造背景下,在活动大陆边缘与板内过渡地带,一个火山-构造洼地及其成矿作用演化的四维模型。