冻融循环作用下宽级配砾质土的渗透特性

针对西北地区特殊的气候条件和砾质土料源丰富的实际情况,提出采用宽级配砾质土代替黏土作为土工合成材料膨润土垫(GCls)的保护层共同构成垃圾填埋场复合防渗系统的构想。文中选取宁夏银川地区冲-洪积作用形成的天然砾质土料,经人工掺和制配成满足规范对GCls防渗垫保护层渗透系数要求的宽级配砾质土样,在实验室对该土样进行了冻融循环作用下的渗透性能试验研究。结果显示,随着冻融次数的逐渐增加,土样的冻胀率逐渐变大,渗透系数也相应增大,经过12次冻融循环后,渗透系数约增大1～2个数量级;冻融循环初期,冻融作用对土样的影响最为剧烈,随着冻融循环次数的增加以及时间的延续,土样性状逐渐趋于稳定。由于宽级配砾质土对冻融循环作用的敏感性小于粉质黏性土,故采用宽级配砾质土作为Cls/GM的保护层共同组成填埋场复合防渗系统是值得期待的。     

准噶尔盆地北缘山前带沉积物源及相模式研究--以哈拉阿拉特山前缘春晖探区为例

准噶尔盆地北缘山前带是我国目前油气勘探的一个重点领域.为了深入探讨山前带沉积研究中存在的疑问,以准北缘哈拉阿拉特山山前春晖探区为对象,综合利用岩芯、钻测井资料及大量沉积学实验手段,对其侏罗系八道湾组沉积特征及物源方向进行研究,证实了该区粗碎屑沉积物主要来自北西方向的哈山山区,属于典型的近源沉积.通过构造、沉积演化史分析与现代沉积考察相结合的方法,总结出了山前带近源砾石高磨圆度的两类成因,分别对应于砾石形成期的滨海环境以及再搬运沉积期的湿地扇环境,最终建立了春晖探区八道湾组湿润型冲积扇沉积相模式.     

Medium‐term fluvial island evolution in a disturbed gravel‐bed river (Piave River,Northeastern Italian Alps)

River islands are defined as discrete areas of woodland vegetation surrounded by either water‐filled channels or exposed gravel. They exhibit some stability and are not submerged during bank‐full flows. The aim of the study is to analyze the dynamics of established, building, and pioneer islands in a 30‐km‐long reach of the gravel‐bed Piave River, which has suffered from intense and multiple human impacts. Plan‐form changes of river features since 1960 were analyzed using aerial photographs, and a LiDAR was used to derive the maximum, minimum and mean elevation of island surfaces, and maximum and mean height of their vegetation. The results suggest that established islands lie at a higher elevation than building and pioneer islands, and have a thicker layer of fine sediments deposited on their surface after big floods. After the exceptional flood in 1966 (RI > 200 years) there was a moderate increase in island numbers and extension, followed by a further increase from 1991, due to a succession of flood events in 1993 and 2002 with RI > 10 years, as well as a change in the human management relating to the control of gravel‐mining activities. The narrowing trend (1960–1999) of the morphological plan form certainly enhanced the chance of islands becoming established and this explains the reduction of the active channel, the increase in established islands and reduction of pioneer islands.     

Relationships between magnetic susceptibility and heavy metals in urban topsoils in the arid region of Isfahan, central Iran

Electromagnetic induction (EMI) method results are shown for vertical magnetic dipole (VMD) configuration by using the EM38 equipment. Performance in the location of metallic pipes and electrical cables is compared as a function of instrumental drift correction by linear and quadratic adjusting under controlled conditions. Metallic pipes and electrical cables are buried at the IAG/USP shallow geophysical test site in São Paulo City, Brazil. Results show that apparent electrical conductivity and magnetic susceptibility data were affected by ambient temperature variation. In order to obtain better contrast between background and metallic targets it was necessary to correct the drift. This correction was accomplished by using linear and quadratic relation between conductivity/susceptibility and temperature intending comparative studies. The correction of temperature drift by using a quadratic relation was effective, showing that all metallic targets were located as well deeper targets were also improved.     

来自东南极西福尔丘陵附近冰碛物中沉积岩砾石的碎屑锆石LA-ICP-MS U-Pb年龄及其意义

东南极西福尔丘陵东南侧分布着长约20 km带状冰碛物,这些冰碛物成分复杂,其中含有少量与该地区高级片麻岩的基岩显著不同的沉积岩砾石.根据冰川流动方向可以推测它们来自西福尔丘陵的东南侧的冰盖之下.对其中8个具有代表性的沉积岩砾石样品进行碎屑锆石LA-ICP-MS U-Pb年龄测试,其U-Pb表面年龄主要集中在2410～2...     

A two‐dimensional morphodynamic model of gravel‐bed river with floodplain vegetation

Artificially straight river channels tend to be unstable, and ultimately develop into river meanders through bank erosion and point‐bar deposition. In this paper account is taken of the effects of riparian and floodplain vegetation on bank strength, floodplain flow resistance, shear stress partitioning, and bedload transport. This is incorporated into an existing 2D hydrodynamic‐morphological model. By applying the new model to an initially straight and single‐threaded channel, the way that its planform and cross‐sectional geometry evolve for different hydraulic and floodplain vegetation conditions is demonstrated. The results show the formation and upstream migration of gravel bars, confluence scouring and the development of meandering and braiding channel patterns. In cases where the channel becomes unstable, the instability grows out of bar formation. The resulting braiding patterns are similar to analytical results. The formation of a transition configuration requires a strong influence from vegetation. Copyright © 2010 John Wiley & Sons, Ltd.     

Study of desiccation crack initiation and development at ground surface

Cracks are common in clayey or expansive soils and provide preferential pathways for water infiltration into soils. A field study was conducted to investigate the mechanisms of initiation and development of desiccation cracks on two soils. Based on results of the field study and measurements of soil-water retention curves and soil shrinkage properties in the laboratory, the conditions of crack initiation and the development of crack geometric parameters (i.e., crack porosity, crack aperture, and crack density) with water content or drying time were quantified. The results show that desiccation cracks developed in three stages: initial stage, primary stage, and steady state stage. In the initial stage, few cracks developed with gradually decreasing water content. When the water content reached a critical value for crack initiation, cracks developed quickly and this was the start of the primary stage. The critical suction at crack initiation was calculated using a stress criterion, which is in the range of 5.3–21.3 times the preconsolidation pressure of the soil. As the water content approached the shrinkage limit of the soil, cracks developed slowly and approached a steady state. The cracks were found to be repeatable during three drying–wetting cycles.     

Performance of quasilinear elastic constitutive models in simulation of geosynthetic encased columns

Past numerical simulations of geosynthetic encased columns (GECs) using different versions of the quasilinear elastic hyperbolic model for the encased granular material have, in certain cases, yielded unrealistic results. In this paper the cause of such results is investigated by performing three-dimensional finite element analyses of GECs in soft clay, utilizing three common functional forms of the hyperbolic model for the encased granular material. Results indicate that one form of the hyperbolic model can predict an unrealistic lateral response for GECs during application of load to the column. In addition, the inability of hyperbolic models to properly account for soil behavior near failure compromises their ability to realistically capture the behavior of encased granular soil in GECs. Modeling the behavior of soil near failure is essential for properly simulating the behavior of GECs, as soil shear failure is necessary to mobilize the tensile stresses in the encasement and improve the stress–displacement response of the GEC. Although this type of hyperbolic model behavior was demonstrated for the specific case of encased soil in a GEC, the limitations of the hyperbolic model described herein apply equally to other geotechnical problems in which some portion of the soil mass is at or near failure.     

Background levels and baseline values of available heavy metals in Mediterranean greenhouse soils (Spain)

This study determines extractable levels of Cd, Cu, Pb, Zn, Ni and Co in western Almería (Spain) greenhouse surface soil horizons using EDTA solution, which is identified as the fraction available for organisms and plants. It also establishes background levels, geochemical baseline concentration and reference values (RV), and investigates the possible relationships between soil properties and elemental concentrations. The results show that the soil concentration of these extractable heavy metals was high as those reported by other authors for Spanish agricultural soils. The available RV concentrations obtained (mg kg⁻¹) were: Cd 0.17, Cu 1.6, Pb 13.8, Zn 5.0 Ni 1.7 and Co 2.9. Using the upper baseline criterion, 95% of greenhouse soils present a relatively higher content of extractable heavy metals given their Cd and Cu concentration. Significant correlations between total and EDTA-extractable metal levels were found for Zn, Pb, Cu, Cd and Ni. Soil properties also related to heavy metals content, suggesting that Cd, Cu, Pb and Zn are of similar origin and relate to anthropic activities, and implies the same interactions and/or relationships among these metals.     

大王北洼陷湖相滩坝砂体风暴沉积特征

通过对大王北洼陷钻井岩心精细观察,及对洼陷资料重新分析,发现大王北洼陷沙二段为盆地断坳期,盆地呈"碟"状,地势平缓,广泛发育浅水漫湖砂质滩坝沉积.浅水环境下原有的正常砂质滩坝沉积极易受风暴流作用,接受风暴搬运的沉积物,并受风暴流后期改造,形成风暴沉积.大王北洼陷沙二段整体呈细质沉积的滩坝砂体中粗粒泥质沉积,是由风暴沉积作用形成的,这一认识为解释滩坝砂体沉积成因提供了新线索,对该区岩相古地理恢复和油气勘探具重要意义.