海相淤泥软土地基强夯置换砂桩试验分析

针对某海相淤泥软土的处理工程,选取了一强夯砂桩置换试验区,监测了强夯置换过程中夯坑及其周围土体的变形,超孔隙水压力的增长和消散情况以及深层土体水平位移。分析结果表明,单个置换砂桩夯击8次较为合理;强夯置换后静置一段时间,可以有效减小强夯过程中的超静孔隙水压力,且一遍强夯后静置7 d后超孔隙水压力基本消散完;深层土体的水平位移主要发生在地表以下3～6.5 m深度范围内。     

Microbiotic crusts as biomarkers for surface stability and wetness duration in the Negev Desert

Microbiotic crusts play an important role in arid and semi‐arid regions. Yet, very little information exists regarding the factors that impact their development. In an attempt to assess the main factors that may determine their growth, measurements of the amount of fines (silt and clay), rain, moisture content, wetness duration and wind erosion and deposition were carried out along a 12 station transect within a partially crusted dune field in the western Negev Desert and compared to the crust cover and chlorophyll content. Surface stability was the only variable that exhibited significant relationship with crust cover while daylight wetness duration exhibited strong positive relationship (r² = 0·92–0·99) with the crust's chlorophyll content. The data point out that microbiotic crusts may serve as a useful biomarker for surface stability. While wetness duration and wind will control crust cover and the crust chlorophyll content in semi‐stable habitats (with absolute annual change in sand level of 2–3 mm), stable habitats (absolute change <1 mm) will be controlled primarily by moisture, while habitats with low surface stability (absolute change of tens and hundreds of millimeters) will be primarily controlled by wind. Furthermore, owing to the strong positive relationship between daylight wetness duration and the crust's chlorophyll content, the crust may serve as a useful biomarker for the quantification of surface wetness duration. Copyright © 2009 John Wiley & Sons, Ltd.     

Holocene evolution of a rock‐bounded barrier‐lagoon system,Cíes Islands,northwest Iberia

Coastal geomorphology results from the combined effects of contemporary dynamics, sea‐level rise and the inherited geological framework, yet the relative importance of these driving mechanisms may change throughout the evolutionary history of coastal deposits. In this contribution, we analyse the depositional history of the Cíes Islands barrier‐lagoon system, based on lithofacies, radiocarbon ages, and pollen analysis. Our results reveal a sedimentary sequence that provides evidence for striking changes in the dynamical functioning of this complex since the mid‐Holocene. The sedimentary sequence commenced about 7700 cal years bp by fresh‐water ponding of an upland depression located about 4 m below present mean sea‐level. Fresh‐water ponds were infilled by aeolian sediments following a gradual lowering of the water‐table 4000 cal years bp . Post‐3600 cal years bp sea‐level rise allowed water oscillations to reach the elevation of the bedrock causing the inundation of fresh‐water ponds and subsequent lagoonal and marine sedimentation. Subsequently, landward and upward migration of a sand‐barrier led to overwash and deposition of sand in the newly formed lagoon. The resultant sedimentary sequence suggests that climatic conditions played an important role controlling the sedimentation regime during the entire history of the basin; changing water‐table levels during early stages of evolution and increasing storminess during more recent times. In addition, background sea‐level rise related to the Holocene transgression was a key factor in controlling the evolution of the system, yet its influence depended to an extent on the relative elevation of the bedrock topography. Copyright © 2009 John Wiley & Sons, Ltd.     

昆明盆地砂土液化灾害评价

系统收集整理昆明盆地238个钻孔资料,根据<建筑抗震设计规范>(CB50011-2001)中砂土液化的判别方法,对其中81个钻孔进行了液化判别和等级划分.结果显示,昆明盆地地形平坦,由北向南高程渐低,自然坡降为1‰～2‰,地层由第四纪的冲积、湖积及湖沼相沉积之砂质粘土、淤泥、草煤及粉砂组成,其液化判定结果与盆地地貌和沉积物的分布规律一致.在Ⅷ度以上地震烈度影响下,滇池北岸到昆明机场的范围内存在砂土液化的可能.     

循环荷载下南京片状细砂的动应力——应变关系

以片状颗粒成分为主的片状结构砂与常用的圆形颗粒标准石英砂相比,在物理力学特性上有显著的差异。循环荷载作用下,饱和砂土振动孔压上升会导致土体刚度发生软化,当振动孔压累积达到一定水平时,会产生液化现象,从而引起土体结构发生破坏。采用英国WFI动三轴仪,研究了南京片状细砂在循环荷载作用下,静偏应力水平、循环应力比水平和循环次数对其动应力一应变关系的影响,考虑每一次循环过程中动应力—应变关系滞回曲线的卸载及再加载割线动剪切模量Gsec和最大割线模量Gmax的变化特性,建立了动剪模量软化的经验公式;静偏应力水平对动剪模量软化有显著影响,随着循环次数的增加,动应力—应变滞回圈逐渐向应变累积方向滑移和向应变轴方向倾斜,且彼此分离;考虑循环软化特性,采用修正的Masing准则,描述了循环荷载下南京片状细砂的动应力—应变关系。     

Morphodynamic evolution of a lower Mississippi River channel bar after sand mining

In‐channel sand mining by dredge removes large quantities of bed sediment and alters channel morphodynamic processes. While the reach‐scale impacts of dredging are well documented, the effects of the dredged borrow pit on the local flow and sediment transport are poorly understood. These local effects are important because they control the post‐dredge evolution of the borrow pit, setting the pit lifespan and affecting reach‐scale channel morphology. This study documents the observed morphological evolution of a large (1·46 million m³) borrow pit mined on a lateral sandbar in the lower Mississippi River using a time‐series of multibeam bathymetric surveys. During the 2·5 year time‐series, 53% of the initial pit volume infilled with sediment, decreasing pit depth by an average of 0·88 m yr^?1. To explore the controls of the observed infilling, a morphodynamic model (Delft3D) was used to simulate flow and sediment transport within the affected river reach. The model indicated that infilling rates were primarily related to the riverine sediment supply and pit geometry. The pit depth and length influenced the predicted magnitude of the pit bed shear stress relative to its pre‐dredged value, i.e. the bed‐stress reduction ratio (R*), a metric that was correlated with the magnitude and spatial distribution of infilling. A one‐dimensional reduced‐complexity model was derived using predicted sediment supply and R* to simulate patterns of pit infilling. This simplified model of borrow‐pit evolution was able to closely approximate the amount and patterns of sediment deposition during the study period. Additional model experiments indicate that, for a borrow pit of a set volume, creating deep, longitudinally‐shorter borrow pits significantly increased infilling rates relative to elongated pits. Study results provide insight into the resilience of alluvial river channels after a disturbance and the sustainability of sand mining as a sediment source for coastal restoration. Copyright © 2015 John Wiley & Sons, Ltd.     

列车振动荷载作用下南京细砂动强度变化规律研究

以南京细砂为研究对象,采用空心圆柱扭剪仪模拟列车振动荷载作用对应的土体单元所受应力路径,考虑试样围压、加载幅值和排水条件,研究列车振动荷载作用下土体动强度的变化规律。试验结果分析表明:在模拟列车振动荷载作用对土体竖向应变的影响时,采用椭圆应力路径来代替心形应力路径是切实可行的,能克服试验仪器高频加载时无法有效模拟心形应力路径的缺点;其次,当围压和加载幅值都较小时南京细砂主要表现为在振动初期强度强化特征,当振动次数达到一定数量后强度也达到一个稳定阶段;当试验围压较大时,随振动次数增加,南京细砂的强度变化主要以强度弱化阶段为主;同时,排水条件对其强度变化的主要影响表现为对其振动前期强度强化阶段的影响,对其强度弱化阶段的影响并不明显。     

含黏粒砂土的循环弹塑性本构模型及其动力数值研究

首先完善上下负荷面中超固结状态参量R与结构性状态参量R*的定义表达,并以此重新推导了应力诱导各向异性上下负荷面模型,并在数值程序中再现该模型,实现土的统一本构的数值化模拟。分析了黏土以及砂土的力学特性,及其在数值模型状态变量中的差异,为模拟含黏粒砂土的实现进行理论分析。通过参数试算找出含黏粒砂土的不同黏粒含量分别所对应的模型参数。     

Site characterizations for the Tehran network (TDMMO) in Tehran Region Using Micro-Earthquake,Microtremor and Quarry Blast Data

We did a Study of Horizontal-to-Vertical Component Spectral Ratio in the Tehran seismic zone. Micro-earthquakes, microtremors and quarry blasts data were used as an estimation of the site response in the Tehran zone. Site effects were studied based on horizontal to vertical ratios by the Nakamura׳s technique. Also, we used the spectra of signals for three components with the lowest noise levels for spectral slope studies. The analysis used seismic events from a network of 13 seismic stations by the permanent local seismological network of the Tehran Disaster Mitigation and Management Organization (TDMMO) from 2004 to 2007. The number of events used were different for each station. Quarry blast events were with 1.2≤M_L≤2.2 and micro-earthquakes were with 1.1≤M_L≤4.1.By comparing results for earthquake, microtremor and quarry blast, we could see that there is a significant difference between them. The data showed clear observations, especially in high-frequencies. The H/V spectral ratios indicate dominant frequency for rock/soft site with a higher ratio level for quarry blast ratios, which are comparable to the earthquake results due to their difference sources. The results derived by spectral H/V ratios and spectral analysis may be used to distinguish between local earthquakes and quarry blasts.     

爆炸荷载作用下钢柱的动力响应与影响因素分析

采用有限元软件ANSYS/LS-DYNA,建立钢柱有限元模型,模拟爆炸荷载作用下钢柱的动力响应,并对影响钢柱动力响应的主要因素进行数值分析。考虑了不同爆炸荷载、材料失效应变、单元网格密度、柱高、柱截面尺寸和柱承担的轴向压力等参数的影响。通过对钢柱动力响应时程曲线进行分析,研究爆炸荷载作用下钢柱响应特性及其破坏机理;通过分析,得到各参数对其动力响应的影响规律。分析结果表明：增大柱的截面尺寸,能够降低柱跨中水平位移;增大柱截面高度,能有效地提高钢框架柱的抗爆承载力;在钢柱抗爆设计中,应控制其所承受的轴向压力大小,轴压比值不宜超过0.3。