第四纪环境研究新理论

On the basis of extensive survey to the Quatemary paleosols, soils and weathering layers in the vast northern China and the Loess Plateau, we found some unusually special phenomena of chemical components such as unsuccessive illuvium, twin illuviums, unusually thick illuvium and multi-illuviums etc. According to the analysis on the content of CaCO3 and the data of penetrating experiment, a new theory of illuvial depth or removal depth of CaCO3 in weathering and leaching belt was constructed, which indicates that the time of CaCO3 removing to the illuvial depth is very short,and the effect of time factor on illuvial depth can be ignored. The theory can be taken as a credible foundation for studying many geological and geographical problems in weathering and leaching belt.When the illuvial depth of CaCO3 iS bigger than the thickness of developing belt of soil or paleosol, it can be determined that the paleosol has turned into weathering crust. When the illuvial depth of CaCO3 is bigger than the thickness of paleosol, paleosol is leaching moderately acid soil. When two, three layers or unusually thick CaCO3 illuviums exist in the same weathering section or at the bottom of the same paleosol, there were two or more periods forming paleosol and corresponding climatic stages at that time. On the basis of the equation of relationship between mean annual rainfall (y) and illuvial depth of CaCO3 (x) (y = 305.5x 168.5) determined in the paper, mean annual precipitation during the development of paleosol can be calculated.     

Liquefaction mitigation in silty soils using composite stone columns and dynamic compaction

The objective of this study is to develop an analytical methodology to evaluate the effectiveness ofvibro stone column (S. C.) and dynamic compaction (D.C.) techniques supplemented with wick drains to densify and mitigate liquethctionin saturated sands and non-plastic silty soils. It includes the following: (i) develop numerical models to simulate and analyze soil densification during S.C. installation and D.C. process, and (ii) identify parameters controlling post-improvement soil density in both cases, and (iii) develop design guidelines for densification of silty soils using the above techniques. An analytical procedure was developed and used to simulate soil response during S.C. and D.C. installations, and the results were compared with available case history data. Important construction design parameters and soil properties that affect the effectiveness of these techniques, and construction design choices suitable for sands and non-plastic silty soils were identified. The methodology is expected to advance the use of S.C. and DC. in silty soils reducing the reliance on expensive field trials as a design tool. The ultimate outcome of this research will be design charts and design guidelines for using composite stone columns and composite dynamic compaction techniques in liquefaction mitigation of saturated silty soils.     

强夯-降水联合加固饱和软粘土地基试验研究

针对强夯法加固饱和软粘土地基时超静孔压消散难和易出现“橡皮土”的问题,提出将强夯和井点降水技术结合起来,利用井点降水技术来加速夯后超静孔压的消散和软土固结,并结合上海市某集装箱堆场工程中土部为吹填细砂、下部为饱和软粘土的地基进行了现场试验研究,对加固过程以及加固后的土体变形、超静孔压变化、工艺参数、加固效果和加固后土体的时间效应以及强夯的加固机理等进行了分析和讨论。强夯和井点降水技术的合理结合进一步拓宽了井点降水和强夯法的应用范围,为我国沿海地区广为分布的软土地基的加固问题提供了新的途径。     

Geotechnical characterisation of pyroclastic soils involved in huge flowslides

Landslides of the flow type involving granular geo-materials frequently result in casualties and damage to property because of the long travel distance and the high velocities that these may attain. This was true for the events that took place in Campania Region (Southern Italy) in May 1998, involving pyroclastic soils originating from explosive activities of the Somma-Vesuvius volcano. Although these phenomena have frequently affected various areas of the Campania region over the last few centuries, there were no useful geological and geotechnical references available in the aftermath of the May 1998 events. For this reason Salerno University, which was involved in the scientific management of the emergency, addressed the issue of acquiring data on the geological, geomorphological and hydrogeological features of the slopes where the landslides had taken place. The information acquired made it possible to set up a slope evolution model that is able to interpret, from a geological point of view, past and more recent landslides that had occurred in the same area. As preliminary geotechnical analyses had already validated the above model, more detailed investigations were performed both on the pore pressure regimen of the covers still in place as well as on the physical and mechanical properties of pyroclastic soils, in saturated and unsaturated conditions. The present paper begins by discussing the data acquired during the .rst phase of the studies and then goes on to illustrate the laboratory results so far obtained with the aid of approximate procedures. These help advance our knowledge of pyroclastic soils within a reasonable time frame, thus improving landslide triggering analysis.     

Relationship between pedogenetic manifestations in some arid soil and age of the landforms

Arid soils with slight variations in parent material, vegetation and climate were studied for their morphological, chemical and mineralogical characteristics. Results show that the soils are at different stages of profile development. Varying degrees of pedogenic features have been tied up with data on palaeoenvironment to develop a relationship between the soil and age of the landform. Studies reveal that Dune and Shergarh sandy soils with least degree of manifestation belong to early Holocene to latest Pleistocene, whereas Chirai sandy soil with weakly developed B horizon belongs to the upper Holocene. Soils (Khajwana and Gajsinghpura) with well-developed B horizon and a calcic layer with lime segregations belong to mid-Pleistocene whereas Pali and Pipar soils with illuvial clay in B horizon belong to early Pleistocene.     

Effect of rod friction on vane shear tests in very soft organic harbour mud

Very soft organic harbour mud is increasingly used as a filling and construction material in harbour construction and reorganization. The undrained shear strength of such soft sediments is the critical geotechnical soil parameter with regard to any specific construction design. Field and laboratory vane shear testing is a standard method to quickly determine this important parameter. So far, the effect of rod friction on vane shear tests in very soft organic soils is unclear. In this study we present results from laboratory experiments on harbour mud from a construction site in northern Germany. Relations among vane and rod geometry, penetration depth, water content, rod friction and undrained shear strength are derived. Based on these relations the influence of rod friction on vane shear test results is investigated. The results indicate that field and laboratory vane shear test measurements may be significantly influenced by rod friction. Methods are proposed to correct for the rod influence, which is shown to increase with rising water contents.     

Hydraulic and compaction characteristics of leachate-contaminated lateritic soil

Large quantities of leachate-contaminated lateritic soil results from dump yards in the southwest coast of India. These dump yards receive large quantities of municipal solid waste which includes chemical, industrial and biomedical wastes. Large areas of land are currently being used for this purpose. An extensive laboratory testing program was carried out to determine the compaction characteristics and hydraulic conductivity of clean and contaminated lateritic soil. Batch tests were used to study the immediate effect of leachate contamination on the properties of lateritic soil. Contaminated specimens were prepared by mixing the lateritic soil with leachate in the amount of 5%, 10% and 20% by weight to vary the degree of contamination. The results indicated a small reduction in maximum dry density and an increase in hydraulic conductivity due to leachate-contamination. The change induced by chemical reaction in the microstructure of the soil was studied by scanning electron microscope before and after contamination of soil with leachate. The structure of the leachate contaminated soil sample appeared to be aggregated in scanning electron microscope analysis. The aggregated structure increases the effective pore space and thus increases the hydraulic conductivity. Fifty percent increase in hydraulic conductivity was observed for specimens prepared at standard Proctor density and mixed with 20% leachate. Compaction characteristics did not change much with the presence of leachate up to 10%. With 20% leachate the maximum dry density decreased slightly indicating excess leachate in the soil. However the changes are not significant.     

基质吸力变化引起的体积应变研究

与降雨有关的边坡表层变形位移,其根源是基质吸力降低引起的非饱和土体积应变。利用某滑坡体的原状样及其重塑样,分别在100kPa、200kPa围压作用下,采用单纯增加基质吸力的应力路径,对基质吸力引起的体积应变,进行试验测试。结果显示: (1)试样的体应变,随基质吸力的增大,单调增大,服从对数函数关系;(2)围压由100kPa增加到200kPa的试样体积模量,随基质吸力的增大,单调增大,服从幂函数关系;(3)与基质吸力有关的体积变化系数,随基质吸力的增大,单调减小,服从幂函数关系;并且当基质吸力大于200kPa以后,随基质吸力增大,体积变化系数接近于常数,约为0.0059左右。在此基础上,本文针对这种由单纯基质吸力变化引起的非饱和土体积应变规律及机理,进行了探讨。     

考虑3种非线性关系的径向排水固结解析解

大部分砂井及排水板固结解析解理论结合了太沙基一维固结理论, 把土体考虑成线弹性问题, 而研究发现, 软土在等向固结条件下平均有效应力和水平渗透系数与孔隙比成半对数线性关系。同时, 实验表明, 在塑料排水板周围的涂抹区中水平渗透系数成抛物线状分布。因此, 为了充分考虑这些非线性的影响, 推导出了该问题的解析解, 并通过退化验证了该解析解的正确性。同时, 还对参数Cc/Ck和对固结速率的影响进行了分析, 结果表明:当Cc/Ck1时, 固结速率较快; 当Cc/Ck1时, 固结速率较慢; 参数增加, 固结速率逐渐变小。