考虑土体性质的黄土震陷灾害区域性特征

本研究在通渭、兰州、西吉、宝鸡、西安等多个西北典型黄土震陷灾害场地获取原状土样,通过对密度、含水率、孔隙比、塑液限等物理指标的测试,分析其与土体震陷性的关系;通过动弹性模量和阻尼比、动强度、震陷等室内动力特性试验,并结合大量前人研究资料,分析土体性质对岩土震陷灾害的区域性影响。结果表明：1）震陷系数与单一土体物性参数呈正相关或负相关关系,六盘山以西和以东的黄土震陷特性反映了明显的地区变化规律,在相同地震荷载作用下震害东轻西重;2）随着时间的推移,人类的活动增加,改变了土的含水率,导致土体上覆压力增大,在上覆压力的作用下,土体的初始孔隙数量有所减少,其固结程度越来越好,宏观表现为土体的易损性即震陷性减小。     

湿陷性黄土场地灰土挤密桩间距设计初探

结合工程案例对比研究了湿陷性黄土物理力学指标间的关系,发现运用岩土工程勘察报告提供的湿陷性黄土物理力学参数值之间的统计关系,可以找到对应于某湿陷系数的孔隙比,然后依据土的天然孔隙比和挤密后设计拟达到的孔隙比目标值,进行灰土挤密桩间距设计。     

水域隧道地震响应分析

本文基于Biot动力固结理论和弹性动力学理论,考虑海床（土壤）的两相性、黏弹性人工边界及流（水）-固耦合作用,建立了隧道-土-流体相互作用的力学模型,讨论了P波作用下有无水的情况以及水深、水域隧道埋深、海床土性质和地震波入射角等因素对隧道及其周围海床应力的影响。结果表明:隧道周围海床土的孔隙水压力和隧道内应力随着水深的增加而增加;地震波特性和海床土特性对隧道的内应力和海床土的孔隙水压力均有较大的影响;海床土的渗透性和隧道埋深对隧道的内应力影响较小,而对隧道周围海床土的孔隙水压力影响较大;地震动的入射角对隧道的内应力和隧道附近土层的孔隙水压力均有较大影响。      

大厚度黄土地区单击夯击能影响强夯加固效果的试验研究

针对黄土丘陵地区高填方下大厚度黄土地基强夯加固参数及效果开展了系列试验研究,分析了强夯前、后各试验区平均夯沉量和土体主要物理力学指标的变化规律,并给出6 000、8 000、10 000、12 000 kN·m能级条件下强夯加固的夯点中心距、击数、有效加固深度等主要参数,在此基础上确定了强夯有效加固深度的估算方法。试验结果表明,加固后黄土的孔隙比、干密度和湿陷系数可作为强夯加固效果的评价指标;强夯处理后地基土的物理力学指标在满足设计要求时其下限深度即为有效处理深度;在湿陷性大厚度黄土地区,以夯点土和夯间土湿陷性均消除的地基土下限深度可作为有效处理深度。     

两种粉质黏土的静强度特征对比分析

以南京与无锡的两种粉质黏土为研究对象,通过使用TSZ-2全自动三轴仪对这两种土在饱和以及非饱和状态下的静强度进行测定。对比分析两种土在饱和状态下,其原状样与扰动样之间抗剪强度的差异,以及两种土在非饱和状态下不同的初始含水率对其抗剪强度产生的影响。得出无锡土样较南京土样具有含水率低、密度高、孔隙比小、压缩性低、静强度高的特点,但扰动作用对无锡饱和土造成的影响较大,而南京土样受扰动作用影响小。确定非饱和粘性土的抗剪强度与含水率以及颗粒级配存在关联,从而导致两种土非饱和状态的抗剪强度存在差异。     

制样方法对非饱和压实黄土强度变化影响的试验研究

为研究不同制样方法对非饱和压实黄土抗剪强度指标的影响规律,进行不同干密度压实土在预湿及加湿两种制样方法下的剪切试验研究。结果表明:预湿制样试验中,存在一个临界含水率影响着试验黄土的黏聚力、摩擦角,当试样的含水率低于临界含水率时,抗剪强度随含水率增加而提高,而当含水率高于临界含水率时,抗剪强度随含水率的增加而降低;抗剪强度指标总体随试样干密度的增大而增大。加湿制样试验中,随着试样含水率的增加,黏聚力连续降低,摩擦角随含水率的增加先提高后降低,对比预湿试样而言,含水率对摩擦角的折减程度降低。可以看出制样方法对非饱和压实黄土黏聚力的影响特征比较突出,当含水率低于临界含水率时加湿试样的黏聚力比预湿试样大,而当含水率高于临界含水率时预湿试样的黏聚力较高,认为加湿试样存在类似"木桶效应",而预湿试样则与试样整体水平有关。研究结果对于进一步加深非饱和黄土抗剪强度指标的适用性研究具有一定参考价值。     

砾性土剪切波速影响因素试验研究

剪切波速被认为是砾性土液化判别的有效指标,为探究砾性土剪切波速的影响因素,设计四因素三水准试验,利用GDS大型三轴-剪切波速设备研究含砾量、相对密度、固结应力和应力比等因素对砾性土剪切波速的影响。试验结果表明:砾性土的剪切波速与前述因素都有较好的相关性;当含砾量、相对密度、固结应力、应力比等增大,砾性土的剪切波速也随之增大;四个因素对砾性土剪切波速的影响程度与模式存在差异,其中固结应力是最为显著的影响因素。     

场地黄土扰动前后的湿陷性与其物性指标的关系

黄土物性指标可以近似但较为简便快速地评价其湿陷性。原状黄土在不同荷载和浸水的扰动下,随着其物性指标的改变,湿陷性也随之变化,故为了更好地评价这类扰动性黄土的湿陷性,在兰州东岗某场地上进行的两次相隔四十多年室内试验的基础上,采用相关分析的方法,研究扰动前后湿陷性系数与干密度、含水率、孔隙比、饱和度和塑性指数之间的关系。结果表明:(1)扰动前后湿陷性系数和干密度、含水率、饱和度和塑性均呈负相关,和孔隙比呈正相关,且扰动后湿陷性系数整体上低于扰动前,各指标对湿陷性系数的影响减弱。(2)以干密度、含水率、孔隙比、饱和度和塑性指数作为反映扰动前后黄土湿陷性的因子进行多元线性回归和因子权重分析,得出用以评价扰动前后黄土湿陷性的回归公式和影响湿陷性的因子排序。研究结果对于研判扰动黄土的湿陷性并选择合适的地基处理方法具有一定参考价值。     

兰州高坪湿陷性黄土地基处理方法分析

根据兰州地区3处大厚度高坪黄土的天然密度、含水量、土粒比重等物理性质指标沿深度变化的统计规律,分析了计算值-天然孔隙比随这些指标的变化规律,得出黄土天然孔隙比简化的计算公式。藉此分析强夯法、挤密桩法等地基处理方法的合理设计参数:对于强夯法,平均夯沉量可以作为初步设计参数和质量检测指标;对于挤密桩法,确定了加固深度和处理后的孔隙比,即可确定桩间距和桩径的比值。     

固原重塑黄土动力特性共振柱试验研究

利用GCTS共振柱测试仪研究了干密度和固结压力对固原黄土重塑土动剪切模量和阻尼比的影响及试验数据误差的基本规律。通过设计不同工况的试验条件,对比分析结果表明:相同含水率条件下,最大动剪切模量G_(dmax)随固结压力和干密度的增大而增大。相同干密度条件下,黄土的动剪切模量比随围压的增大而增大;阻尼比随围压的增大而减小;相同围压条件下,黄土动剪切模量比随干密度的增大而增大,阻尼比随干密度的增大而减小;不同剪应变特征点的动剪切模量比和阻尼比试验数据呈现一定的离散性,且离散程度阻尼比明显高于动剪切模量比。研究成果可为该地区场地地震反应分析及工程建设提供基础资料。     

Seismic performance analysis of underground structures based on random field model of soil mechanical parameters

Soils with spatial variability are the product of natural history. The mechanical properties tested by soil samples from boreholes in the same soil layer may be different. Underground structure service in surrounding soils, their seismic response is controlled by the deformation of the surrounding soils. The variability of soil mechanical parameters was not considered in the current research on the seismic response of underground structures. Therefore, a random field model was established to describe the spatial variability of surrounding soils based on the random field theory. Then the seismic response of underground structures in the random field was simulated based on the time-domain explicit global FEM analysis, and the soil mechanical parameters and earthquake intensity influencing the seismic response of surrounding soils and underground structures were studied. Numerical results presented that, the randomness of soil parameters does not change the plastic deformation mode of surrounding soils significantly. The variation coefficients of inter-story deformation of structures and lateral deformation of columns are much smaller than that of mechanical parameters, and the randomness of soil parameters has no obvious effect on the structural deformation response.     

中国软土及其动力学特性研究的概况

软土由于具有强度低、渗透性小、压缩性高、欠固结以及流变效应等不利于工程建设的特性,通常不被选做天然地基。但当软土以建筑环境或地基夹层存在时,对工程会带来一定的危害。在动力的作用下,软土表现特殊的力学行为,在地震作用有时会产生震陷;位于表层的软土放大效果显著;位于底层的软土有时具有隔震的作用。由于软土在动力学实验方面存在一定的困难,并且缺少软土场地的强震记录,使得这方面的研究受到一定影响。简要介绍了软土的成因及工程特性;总结和归纳了国内外软土的动剪切模量比和阻尼比以及剪切波速等动力学参数的特征,评述了软土动本构关系和地震反应特性等方面的研究进展,在此基础上提出了目前需要进一步开展研究的问题。该项工作对从事软土研究的科技工作者有一定的参考价值。     

小应变硬化土模型参数的确定与敏感性分析

地下工程施工引起的土体扰动区可分为剧烈扰动区、扰动区、微扰动区和未扰动区。为全面反映土体在扰动下的应力路径和力学响应,必须考虑全应变范围的土体特性,尤其是小应变范围内的力学响应,因此对小应变硬化土本构模型关键参数(初始剪切模量和剪应变阀值)的确定方法进行介绍。开展上海典型软土的三轴固结排水剪切试验和固结试验研究,给出确定上海软土小应变硬化土模型(HSSmall)参数的方法,建议采用原位测试的方法确定土体的初始弹性模量。基于土单元数值模拟进行初始弹性模量和剪应变阀值的参数敏感性分析。随着初始弹性模量的增大,初始压缩曲线与卸载-再压缩曲线的斜率均增大。由于对应的回弹模量不变,初始弹性模量与回弹模量的差值增大,应变与偏应力试验曲线的回滞环宽度也随之增大。随着剪应变阀值的增大,初始压缩曲线和再压缩曲线的近似直线段增长,在同样剪应力情况下,土体的应变值减小,土体保持初始弹性模量刚度的区间增大。     

基于支持向量机上海地区土体物理力学指标相关性研究

针对上海地区土体物理力学指标开展相关性分析,结合多个工程场地获取的土体室内试验数据,采用支持向量机算法构建了土体塑性指数、液性指数与压缩系数的相关性分析模型,并结合误差指标对模型参数进行优化。将支持向量机模型与传统的线性、多项式拟合方法结果对比分析,表明该模型预测结果与实际结果较为吻合,且该模型另一优势在于能够从更多的数据中进行更深度的挖掘来提升自身的鲁棒性。考虑到不同土体的工程性质差异较大,进一步研究该模型的预测性能与适用性,就每个测试样本点预测偏差与其物理指标建立二者的关系曲线,结果表明可塑性小的中压缩性土体相较于高压缩性土体的预测偏差更小,模型更加稳定与准确,可为上海地区土体压缩性相关研究提供参考。      

Modelling of stormwater biofilters under random hydrologic variability: a case study of a car park at Monash University,Victoria (Australia)

Biofiltration systems represent an effective technology for the management of urban stormwater runoff volumes and quality. The performance of these systems, although largely dependent on their physical characteristics, is also strongly affected by the natural variability of runoff occurrence and volumes. This article presents a model that describes the statistical behaviour of the main variables involved in the water balance of a biofiltration system, given its main physical properties (filter media and vegetation types) and accounting for the natural inflow variability in terms of occurrence and water volumes. The model permits the analytical derivation of the long‐term (e.g. annual) probability density function of the soil water content in the filter media and the estimation of the main statistics of water fluxes, that is, outflow, evapotranspiration and overflow. By relating the soil water content in the filter media before inflow events to the outflow total nitrogen concentrations, the model also gives estimates of the statistics of nitrogen removal performance as a function of inflow variability. The model was tested against field data collected at a stormwater biofiltration system in Melbourne, Australia. The model could be used to rapidly assess the hydrologic and nitrogen treatment performance of alternative applications of biofiltration for stormwater management across a range of climates. Copyright © 2011 John Wiley & Sons, Ltd.     

Experimental study of consolidation properties of unsaturated soils during draining

Changes in groundwater elevation may cause a change in the net normal stress and matric potential within the soil mass, which results in volume changes of unsaturated soil. This research investigated the relationship between the drawdown of groundwater and the characteristics of volumetric compressibility of unsaturated soil. Sand column experiments were designed and conducted to measure the volume changes of coarse and fine sands under different types of drainage conditions at fast and slow drainage rates. The finite element program FEMWATER was calibrated and used to simulate the distributions of stress, tension and moisture content within the sands. Finally, based on the changes of net normal stress and matric potential and the observed volume change of the sands, a least‐square method was applied to determine the volumetric consolidation parameters of the unsaturated soils. Copyright © 2004 John Wiley & Sons, Ltd.     

双向动荷载下重塑红黏土动变形特性研究

利用SDT-20型三轴仪探究双向动荷载下红黏土动变形特性。结果表明:相位差为0其他条件相同时红黏土动剪切模量随径向动荷载幅值增加而减小;双向动荷载下红黏土动剪应变与振动次数近似呈指数型关系增长,并存在一个临界循环次数;随含水率升高和固结应力增大,重塑红黏土破坏模式由受拉破坏变为受压破坏;径向动荷载幅值的增加使重塑红黏土更容易发生受拉破坏;双向动荷载下阻尼比随动剪应变增加无明显规律,动剪应变小于1%时阻尼比的变化无规律,动剪应变大于1%时随动剪应变增加阻尼比处于稳定平衡阶段。含水率对阻尼比变化规律有明显影响,含水率小于20%时阻尼比随含水率增大而增大,含水率大于20%时其对阻尼比影响可以忽略。     

Discriminating residual and colluvial soils using topographic analysis in a small,forested catchment in Japan

Soil moisture plays an important role in hydrology. Understanding factors (such as topography, vegetation, and meteorological conditions) that influence spatio‐temporal variability in soil moisture, and how this influence is manifested, is important for understanding hydrological processes. A number of distributed (quasi‐)physical hydrological models have been developed to investigate this subject. Previous studies have shown that the spatial differences in the distribution of soil types (residual and colluvial soils) dominantly reflect spatio‐temporal fluctuations in soil moisture and runoff. We present a methodology for assessing the spatial distribution of residual and colluvial soils, which differ with respect to their physical characteristics, in a 0·88 km² forested catchment with complex topography and a complex land‐use history. Our method is based on penetration resistance profile data; in this data set, each data point represents soil physical characteristics within an area of about 25 m². If the spatial distribution of soils under similar meteorological, geological, historical land use, and other conditions could be characterized on the basis of similarity in topographic features, then the spatial distribution of soil could be predicted based on relationships between various topographic indices (e.g. topographic index and local slope). We tested whether our model correctly assessed the reference data. The model's results were 90·5% correct for residual soils and 87·3% correct for colluvial soils. Further studies will quantify the relationships between topographic features of land covered by residual and colluvial soils and changes in spatio‐temporal variations in the catchment (e.g. vegetation and land use) as a function of geology or meteorology. Copyright © 2007 John Wiley & Sons, Ltd.     

Characterizing incipient motion of low fines content soils with varying compositions,water contents,and relative densities

Laboratory flume experiments were conducted to quantify the effects of the soil characteristics on the critical shear stress of low fines content soil samples collected from the Montauk shores in New York. The collected soils were reconstituted at five different fines contents, ranging between 0 and 20%. These soil mixtures were composed of two initial water contents, dry of optimum and optimum moistures, and two relative densities, one moderate dense and the other dense. The strength indices of the soils, including the effective cohesion and effective angle of internal friction, were measured using the consolidated undrained (CU) triaxial test. The initiation of erosion tests was conducted on the soil mixtures under a unidirectional steady current condition. The near-bed flow velocity, at the onset of erosion, was used to determine the critical velocity and shear stress for each soil sample. The results indicate that the critical shear stress increases with the fines content and effective cohesion. The soils with the optimum initial water contents demonstrate a higher erosion resistance than those with the initial water contents dry of optimum. The higher relative density appears to overshadow the effects of the fines content such that the critical shear stress of the denser soils remains relatively insensitive to the soil composition. The denser soils compacted at the optimum initial water content show the highest resistance against erosion. The critical Shields parameter is modified to include the fines content, relative density, and initial water content.     

The influence of Caragana korshinskii shrub on soil and hydrological properties in a revegetation-stabilized desert ecosystem

Abstract

Shrub-induced spatial heterogeneity of soil and hydrological properties are common in arid and semi-arid ecosystems. To examine the influence of shrubs on spatial patterns of soil physical, chemical and hydrological properties, the typical sand-fixation species, Caragana korshinskii, was studied in the Shapotou area of the Tengger Desert, China. Miniature cylinder infiltrometers were used to quantify the spatial variations of infiltration rate in the soils, and were installed at 20-cm intervals around the shrubs. Meanwhile, soil samples were collected at 0–5 cm depth every 10 cm to analyse their physical and chemical properties and soil moisture content. The results indicate that the various measured parameters showed a gradational change from sub-canopy to open space. The establishment of shrubs formed obvious “fertile islands” where more soil nutrients collected. The total nitrogen (TN), soil organic matter (SOM), electrical conductivity (EC) and surface soil moisture content decreased gradually from around shrub stems to the interspace. The sand content around shrub stems was significantly higher (p < 0.05), and decreased gradually from the centre towards the outside microsites. The silt and clay contents showed opposite variability characteristics. The variation of soil bulk density was less within 140 cm distance from the stem, and no abrupt change was found at the shrub’s drip line. No significant tendency was found for the soil pH values. The steady infiltration rates declined with increasing stem distance and then tended to be stable, and no abrupt change occurred at the position of the overhead canopy margin. The increase of infiltration rate was rapid nearer to the stem; the variability trend can be fitted by a log-log (power function) model. This study indicated the gradational change in soil and hydrological properties, which was not consistent with the binary division of shrubs into “canopy” and “interspace” zones.

Editor Z.W. Kundzewicz