浅地下水位条件下的弱性土质基础施工

介绍输电线路工程中,在浅地下水位条件下,管井降水法在砂土、粉土等弱性土质基础上的施工应用。根据500kV漯河至开封输电线路的施工实践,对传统抽水开挖与管井降水法进行功效比较,并从安全质量、施工时间、人力物耗等角度予以分析总结。

考虑孔隙比的冻胀弱敏感性土修正PCHeave模型研究

针对我国严寒地区高速铁路路基填料和机场填方等工程具有严格的压实度要求,而考虑孔隙比的冻胀弱敏感性土冻胀影响研究尚未有系统性的理论分析,采用理论与经验分析相结合的方法,基于冻胀弱敏感性土的孔隙比与渗透系数和未冻水含量之间的关系,建立考虑孔隙比的修正PCHeave模型。基于该模型分析孔隙比与冻胀量、冻结深度和冻胀率之间的关系,并进行试验对比验证。研究结果表明:孔隙比与冻胀弱敏感性土的未冻水含量之间存在很好的线性关系;冻胀弱敏感性土的冻胀量和冻胀率均随孔隙比先增大再减小,即存在一个最不利孔隙比使得冻胀量和冻胀率均达到最大值;冻胀弱敏感性土的冻结深度随孔隙比增大而逐渐增大;若仅考虑孔隙比对饱和渗透系数的影响,冻胀量和冻胀率均随孔隙比显著增加,若仅考虑孔隙比对未冻水含量的影响,冻胀量和冻胀率均随孔隙比减小,但减小幅度很小。冻胀弱敏感性土通常被认为不能产生显著冻胀,在一定孔隙比下也能产生较大冻胀,同时考虑孔隙比对饱和渗透系数与未冻水含量的影响要更合理。冻胀弱敏感性土的冻胀不能忽略孔隙比的变化,与试验结果对比,该模型能够较好地描述孔隙比对冻胀弱敏感性土的冻胀影响。

地下水在弱膨胀性土边坡稳定中的影响

地下水是影响边坡稳定一个重要因素,主要体现为内、外两个方面的影响。经常考虑的地下水对边坡地层重度、强度等方面的影响属于内的方面;地下水还可能对边坡岩土体产生如浮托力、静水压力、动水压力、渗透力等附加力属于外的方面。通过对一弱膨胀性土边坡失稳各阶段、各影响因素下的分析,阐述了地下水在弱膨胀性土边坡稳定分析中内、外方面的不利影响。并得出结论:在边坡稳定分析时,仅考虑地下水对地层重度、抗剪强度等指标的影响,其分析结果偏于不安全;尤其是边坡地层为弱膨胀性土时,浸水后强度大幅下降,应采用浸水试验指标进行边坡稳定性复核;其次还应考虑地下水渗流排泄对边坡产生的附加荷载。

弱膨胀性土地区氧化沟施工技术

氧化沟是我国目前对污水进行二级生化处理的核心构筑物,但某设施平面面积大,对地基不均匀沉降十分敏感,施工措施稍欠缺,就有可能造成底板开裂。文章简要介绍了弱膨胀土地区如何进行地基处理、主体结构如何防渗漏,供设计和施工参考。

高铁弱胀缩性隧道底鼓病害及仰拱结构分析

高速铁路隧道穿越红粘土等弱膨胀地层,隧道仰拱底鼓病害是常见问题之一。为探索弱胀缩性土隧道仰拱底鼓病害形成原因及合理仰拱结构形式,以某弱胀缩性地层高铁隧道为依托工程,通过数值模拟正交试验,确定了弱胀缩性土隧道仰拱底鼓病害的敏感性因素;将仰拱结构单元简化为三铰拱模型,将其所受胀缩力简化为沿隧道结构的均布荷载,考虑膨胀力作用,建立仰拱-围岩的受力模型,确定胀缩性黏土地层中隧道仰拱的最优拱轴线线型;在此基础上,通过改变仰拱结构的矢跨比来对仰拱结构进行优化,得到了穿越红粘土等弱膨胀地层高铁隧道仰拱最优矢跨比。研究结果表明:隧道仰拱结构变形最受围岩等级及胀缩性控制,其次为衬砌厚度,最后为仰拱曲率半径、混凝土强度、隧道洞径;增大仰拱厚度可有效控制仰拱底鼓变形,但对隧道拱顶沉降的抑制效果欠佳;胀缩性黏土地层中隧道仰拱的最优拱轴线应为圆弧线;在合理拱轴线的条件下,仰拱结构所受最大轴力与其矢跨比呈反比,且矢跨比越大,仰拱与过渡圆弧接茬处所受轴力越大,对拱脚产生的影响越显著。研究成果为特殊土地层高铁隧道底鼓病害的致灾机理及控制技术提供参考。

昆明丘陵地区土体抗剪强度指标分析

现行规范用于支挡设计时规定，土体抗剪强度指标宜采用直剪固结快剪强度指标或按三轴试验确定的固结不排水（快）剪指标，而未明确选取时的适用状态（地下水位以上、以下）。呈贡新区地处昆明盆地的丘陵区，地下水位较低，土体具有一定的弱膨胀性（自由膨胀率〈40％，不属于膨胀土，但具有遇水软化的特点），在边坡、基坑设计时极易忽视水的作用，从而出现工程事故。本文以该地区某一工程事故为例，根据极限平衡理论，通过对土体指标进行反演分析，得出变形的主要原因为土体指标取用不当。提出该区域土体宜采用浸水后的固结快剪指标或采用折减后的固结快剪指标。

膨胀土地区空冷岛地基计算

分析了空冷岛地质情况及空冷岛自身特点,经计算分析,确定出在具有弱膨胀性地基土上的空冷岛能否采用天然地基。

Assessment of Vulnerability of Oxisols to Compaction in the Cerrado Region of Brazil

The Cerrado region of Brazil,one of the fastest-growing agricultural areas in the world,is undergoing very rapid mechanization;this development is good for the country's economy but may also snowball into substantial loss of agricultural land and other environmental degradation if the soil susceptibility and vulnerability to compaction is not well understood.In this study we explored different techniques for estimating soil intrinsic abilities to withstand applied pressure using the prevalent Oxisols of the Cerrado region under native vegetation.Undisturbed samples were collected from three sites within the region and subjected to 1) compressibility test;2) manual penetration resistance test;3) automated penetration resistance test;and 4) packing density analysis.The four methods produced analogous results.Our results showed the significant influence of soil structure on the inherent strength and compaction susceptibility of the soils.We also established the association of soil structure with the mineralogy of the soil.It is noteworthy that the packing density which was developed from very simple and easily obtained parameters could give a picture of the tendency of the soil to compact.Thus,this approach would be useful in developing compaction susceptibility maps for the region.It could be concluded that most of the Oxisols in the Cerrado region would support mechanized agricultural production very successfully.However,there is need for careful management of the mechanization process to avert degradation.The management techniques should include conservationist practices such as no till in order to maintain as much as possible the favorable structure and drainage of these very old soils.

Development of a multivariate empirical model for predicting weak rock mass modulus

Estimating weak rock mass modulus has historically proven difficult although this mechanical property is an important input to many types of geotechnical analyses. An empirical database of weak rock mass modulus with associated detailed geotechnical parameters was assembled from plate loading tests per- formed at underground mines in Nevada, the Bakhtiary Dam project, and Portugues Dam project. The database was used to assess the accuracy of published single-variate models and to develop a multivari- ate model for predicting in-situ weak rock mass modulus when limited geoteehnical data are available. Only two of the published models were adequate for predicting modulus of weak rock masses over lim- ited ranges of alteration intensities, and none of the models provided good estimates of modulus over a range of geotechnical properties. In light of this shortcoming, a multivariate model was developed from the weak rock mass modulus dataset, and the new model is exponential in form and has the following independent variables： （1） average block size or joint spacing, （2） field estimated rock strength, （3） dis- continuity roughness, and （4） discontinuity infilling hardness. The multivariate model provided better estimates of modulus for both hard-blocky rock masses and intensely-altered rock masses.

The status evaluation of the permafrost environment along the Chaida'er-Muli Railway in southern Qilian Mountains in northern Qinghai Province,China

Engineering construction has major influence on the permafrost environment.This paper analyzes the interaction between engineering construction and permafrost environment along the Chaidaer-Muli Railway(simply,CMR) based on the press-state-response(PSR) framework.The permafrost environmental system is divided into three subsystems,consisting of permafrost thermal stability,proneness to the freeze-thawing erosion and permafrost ecological fragility.Each subsystem considers its most important influencing factors.Catastrophe Progression Method(CPM) is applied to calculate the current environment condition along the railway.The result indicates that:(1) as far as the thermal stability is concerned,most sections along the CMR are mainly concentrated in rank Ⅲ(fair situation),and a few in Ⅱ(good situation) and Ⅳ(bad situation),respectively;(2) for the proneness tothe freeze-thawing erosion,the entire railway route falls largely in rank Ⅱ(good situation);(3) along the CMR,the ecological fragility of the permafrost environment is in rank Ⅱ(good situation),or slightly fragile;(4) overall,the permafrost environments along the CMR are in rank Ⅲ(fair situation) or Ⅱcondition(good situation).In general,the permafrost environment along the CMR is fair.It is mainly because a series of active measures of protecting permafrost were taken for stabilizing the CMR foundation soils.On the one hand,we should try our best to minimize the influences that engineering activities have exerted on ecology and environment,on the other hand,the positive measures have made improvements to prevent the permafrost environment from deterioration.

Spatial Analysis of Flood Vulnerability Levels in Port Harcourt Metropolis Using GIS

The study identified spatial variations in flood vulnerability levels in Port Harcourt metropolis with the use of GIS （geographic information systems）. This study considered four factors and these included landuse types, drainage, residential densities and elevation. The elevation data and drainage data were derived from the topographical map of scale 1：35,000, while the land use types were derived from the imagery of Port Harcourt metropolis downloaded from Google Earth, 2010 version. Both the topographical map and imagery were geo-referenced to geographic coordinates and geographic features were digitized in form of shapefiles using both ArcView GIS 3.3 and ArcGIS 9.2 versions. AHP （analytical hierarchical process） was adopted in this study whereby many flood factors were ranked and overlaid for decision making. The contour data was used to generate the DEM （digital elevation model） through the process called kriging in ArcGIS 9.2. Based on the ranking index, factors considered were reclassified to three levels of vulnerability namely highly vulnerable, moderately vulnerable and lowly vulnerable through ranking method and these reclassified factors were then overlaid using an addition operator. The analysis shows that communities like Eagle Island, Ojimbo, Kidney Island were highly vulnerable to flood while communities like Choba, Ogbogoro, Rumualogu were moderately vulnerable. Communities like Rumuigbo, Rumuodomaya etc. were lowly vulnerable to flood. The highly vulnerable places covered 98.18 km2, moderately vulnerable was 220.46 km2 and lowly vulnerable areas covered 330.77 km2.

Unsaturated Creep Behaviors of Weak Intercalated Soils in Soft Rock of Badong Formation

The cutting slopes in soft rock of redbed appeared in Yichang-Badong highway often suffer from the instability along weak intercalations, so the creep behaviors of weak intercalated soils are crucially important for the stability of cutting slopes. Because the deformation of weak intercalated soils is significantly affected by water content due to the strong water sensitivity, it is necessary to study the influence of matric suction on the creep behaviors of weak intercalated soils. In order to find out the unsaturated creep characters of weak intercalated soils, a GDS unsaturated triaxial apparatus was used. Then the triaxial creep experiments on weak intercalated soil samples under varying matric suction were conducted to obtain the unsaturated creep curves. The results show that the weak intercalated soils have obvious creep behaviors, and the creep strain is in nonlinear relationship with stress and time. When the matric suction is constant, a larger deviator stress will lead to a larger creep strain; When the deviator stress is constant, a smaller matric suction will lead to a larger creep strain. Based on the Mesri creep model, an improved creep model for weak intercalated soils under varying matric suction was established, in which the relationship of stress-strain was expressed with a hyperbolic function, and the relationship of strain-time was expressed with power functions in stages. Then an unsaturated creep model including stress-matric suction-strain-time for weak intercalated soils was established based on the power function relationship between matric suction and Ed(a parameter of the improved creep model). The comparison of the calculated values of creep model and the experimental values shows that the creep behaviors of weak intercalated soils can be predicted by the unsaturated creep model by and large.

Agronomic Potential of Partially Acidnlated Rock Phos－phates in Acid Soils of Subtropical China

A glasshouse experiment was conducted to evaluate the agronomic potential of four partially acidulatedrock phosphates (PARP) in three representative soils sampled from subtropical China. The PARPs weremanufactured by attacking a moderately reactive phosphate rock either with sulfuric acid alone or withcombination of sulfuric and phosphoric acids at 30 or 60 percent of acidulation. Shoot dry weight and Paccumulation of six successive cuttings of ryegrass were used to compare the agronomic potential of thesefertilizers with that of the raw rock phosphate (RP) and monocalcium phosphate (MCP). Results indicatedthat the effectiveness of various phosphates was determined both by the solubility of the phosphates andby the acidity and P-fixing capacity of the soils. The higher the watersoluble P contained, the better theeffectiveness of the fertilizer was. Although plant P accumulation of PARP treatments was constantly lowerthan that of MCP treatment, some PARPs could still get a dry matter production similar to that of MCPtreatment. PARP SP60, which was acidulated with a mixture of sulfuric acid and phosphoric acid at 60percent of acidulation and contained the highest soluble P, was as effective as MCP in terms of dry matterproduction on all the soils. 560 and C1, which were both acidulated with sulfuric acid with the formerat 60 percent of acidulation and the latter at 30 percent but with a further addition of monoarninoniumphosphate, were more than 80 percent as effective as MCP. Raw RP also showed a reasonable effectivenesswhich increased with soil acidity. It was suggested from the study that some of these PARPs could beexpected to have a comparable field performance as soluble P fertilizers in the acid soil regions.

Natural Barriers to Eco-environmental Vulnerability in a Complex Ecosystem

Natural features such as mountain ranges, steep slopes and vegetation prevent human movement from one habitat to another. They prevent the ecological harm from natural phenomenon like erosion and landslide. Forests destruction has brought about deterioration of ecological environment such as increasing soil and water losses. RS （Remote Sensing） and GIS （Geographic Information System） technology have enhanced the eco-environment assessment procedure using eco-environment quality index tool. This paper presents results of the research on the investigation of the potentials of different landscapes on the complex ecosystem of Makeng Village in Fuj Jan Province to act as natural barrier to eco-environmental vulnerability. Vulnerability factors analysed were soil erosion, vegetation cover, land use types, slope and elevation. To see how one factor acts as natural barrier eco-environment stressors, factor maps were overlaid in pairs using ArcGIS 9.2 software and the matrix statistics exported for analysis in Microsoft Excel. The results showed steep slopes naturally limit human activities, growth of big trees and increase soil erosion. Flat and gentle slopes are less vulnerable to erosion. Elevation is among natural barriers to human activities. Human activities decrease with increasing elevation, hence making the eco-environment naturally stable/undisturbed. In this study, eco-vulnerability to erosion decreases with increasing vegetation cover.