【作者】 袁克阔；

【导师】 党进谦；

【作者基本信息】 西北农林科技大学 ， 岩土工程， 2010， 硕士

【摘要】 黄土是干旱半干旱地区的第四纪沉积物,其特殊的变形特性和对水的特殊敏感性直接影响着黄土地基的长久稳定。在黄土区各级公路的建设中,探索路基、路面病害的产生机理与发展规律以及路基黄土强度和变形特性成为一项急需解决的问题。本文研究了路基黄土在荷载和水分作用下的强度和变形特点以及渗透和崩解特性,主要工作和成果如下。在阅读和分析大量文献资料的基础上,总结了路基病害的主要影响因素,分析推算了路基土工作区深度和车辆荷载作用大小。通过三轴试验测定了压实黄土的抗剪强度指标随含水率、干密度的变化规律,同时分析了CD试验过程中土样的体积变化规律。结果表明:重塑黄土的粘聚力和内摩擦角随含水率的增大而减小;随土体密实度的增大而增长;两指标和土的含水率与干密度之比具有很好的负线性关系。含水率较高时土体剪切过程中出现剪缩现象,含水率较低或围压较小时黄土的三轴剪切出现剪胀现象,该现象具体到公路工程时表现为在车辆荷载作用下浅层部位与存在临空面的路基土将被挤压发生侧向滑移变形,路基产生侧向挤出沉降问题和填方路基路肩宽度的设计问题。应用常规三轴仪对土样进行三轴单向循环加卸载作用,考察了不同物性的土体在循环加卸载过程中的变形和循环作用前后抗剪强度的变化特点:不论含水率和循环荷载作用大小,土体轴向应变在循环作用下总表现为弹塑性变形。在最初几次荷载循环中,土体在加卸载循环中的残余(塑性)变形较大,在小应力水平下随着循环次数的增加,残余变形逐渐减小并趋于稳定,加载压缩曲线和卸载回弹曲线逐渐相互靠近而使得应力应变曲线显得很密集,土体逐步表现出残余塑性形变基本恒定仅发生弹性变形的特点;循环应力水平较高时土体残余变形随循环次数的增加而持续发展。随含水率或荷载值的增大,土体的变形特点表现为塑性应变的累积增长和弹性变形的减小,总体上仍然表现为总变形增长的特点。可见严格控制水分的浸入和车辆超重对公路路基病害防护具有重要的意义。通过对循环作用前后的强度比较发现:围压较小时,循环作用和静荷固结作用后土体应力应变曲线均为软化型;但土体强度随静荷固结应力的增大而增大,而随循环应力的增大而减小,而且循环作用所引起的变形要远大于静止荷载固结作用引起的变形。在较大围压下,循环作用和静荷固结后应力应变曲线均为硬化型;静荷固结作用引起强度值小幅增大,随循环加卸荷载作用的增大,土体强度却大幅减小。小含水率和较小压实度下循环作用后土体抗剪强度减小;压实度较大时循环作用对残余强度的影响比较显著而对土体强度峰值的影响较小;较大含水情况下,循环作用和静荷压缩作用对土体的影响主要表现在变形上,表明水份浸入土体后无论哪类荷载作用都会引起变形的产生和发展采用常规三轴仪进行了压实黄土浸水湿化变形试验,得出了不同初始含水率、压实度、围压、主应力差及湿化水头等条件下黄土浸水湿化变形特点。结果表明:土体湿化变形过程曲线及湿化轴变、体变大小均与上述初条件有明确的关系,土样湿化轴向应变随主应力差、湿化水头的增大而增大,随围压、初始含水率,压实度的增大而减小;而体变则随湿化水头增大而增大,随主应力差、围压、初始含水率、压实度的增大而减小;土样固结稳定后进行浸水湿化再次达稳定状态所需时间随主应力差和初始含水率的增大而增大,随压实度、围压及湿化水头的增大而减小。对压实黄土进行了不同初始状态下的三轴渗透试验与崩解试验,并对三轴试验破坏前后的圆柱样的崩解特性进行了对比分析。得出了压实黄土的渗透系数与干密度和周围压力的定性关系以及压实黄土的崩解特性与压实度、含水率的关系;对实际工程的防护提出了一些建议。更多还原

【Abstract】 Loess is a kind of quaternary period deposit in the arid and semi-arid areas of northwest China,its special distortion characteristic and water sensitivity seriously affect the running of loess road construction.So apprehending and searching for the nature and development rules of subgrade and pavement disease become the strategic issues.Therefore,besides loess strength and deformation under static and cyclic load as well as water penetration were investigated,the seepage and disintegration properties were also discussed.Details as follows.The main factors influencing subgrade diseases were summarized, the depth of roadbed work area and vehicles wheel load were calculated,which were based on the observation and analysis of a large number of literature references.By the triaxial shear tests of disturbed and compacted loess,the variation laws of shear strength index changed with water content and dry density were analyzed,the variation law of loess sample’s volume was also studied by CD test. The results shows:the cohesion of compacted loess gradually decreases as the water content is increased,the variation law of internal friction angle of loess is much the same as the cohesion.The variation trend of cohesion and internal friction angle is increscent as the dry density is increased. Both of cohesion and internal friction angle have a significant negative linear correlation with the ratio of water content to dry density.When the loess sample is at high water content level, the volume of samples are all minished and exhibiting shearing contraction.And the volume of samples appear shear dilatancy when the loess sample is at low water content level or under low confining pressure,as we narrow these phenomenon down to the issue of highway engineering, this is reflected in the subgrade soil that the shallow subgrade soil and exposed slope pattern will be squeezed lateral slip deformation.It directly raises two serious questions:the settlement problems of highway foundation in the case of lateral extrusion and the design problems of the width of embankment of fill subgrade.Experiments of uniaxial cyclic loading on compacted loess samples were carried out on general triaxial shear equipment,and the the variation law of the deformation under cyclic loading/unloading and shear strength before and after cyclic loading/unloading in accompany with different compaction degree and water content were studied.Regardless of the quantity of water content and cyclic loading, the axial deformations of compacted loess samples characterize a synthesis of elastic and plastic deformation under the action of cyclic loading.The residual (plastic) deformations of those samples are larger in the first few load cycles than after several roles’,and the stress-strain curves of loading and unloading gradualy close to each other with the increase of the number of cycles at low stress levels,in this case the residual deformation decreased and stabilized,and the loess samples appear elastic deformation only.Moreover, the residual deformation grows continuously with the increase of the number of cycles at high stress levels.As the moisture content or the load value increases, the deformation of soil characterized by the cumulative growth of the plastic strain’increase and elastic deformation’decrease,and the total deformation decrease on the whole.It can clearly be seen that it is vital significant to strictly control water immersion and overweight vehicles in roadbed construction and other foundation projects.Comparison of shear strength before and after cyclic loading/unloading shows: The stress-strain curves are basically softing model under cycle and static loading at low confining pressure, the strength is increases with the increase of static consolidation loading while decrease with the increase of cyclic loading,and the deformation caused by cyclic loading is much larger than that caused by the static consolidation loading.The stress-strain curves are hardening model under cycle and static loading at high confining pressure,and the shear strength has a slight increase under static loading,while the shear strength is significantly reduced with the increase of cyclic loading/unloading. The shear strength is diminished when both of compaction degree and water content are in a low state,and the compaction degree has a significantly impact on the residual strength while has little effect on the peak strength of the loess sample when it’s compaction degree is fairly high.Through the triaxial test and analysis of compacted loess wetting deformation by the way of taking loess roadbed as simulation object,the wetting distortion characteristics and possible reasons were tentatively discussed. The results showed that: The required times of wetting deformations re-steady after stabilization by consolidation are related with initial water content and compaction degree, applied confining pressure and principal stress difference as well as wetting waterhead.The soil samples wetting distortion process curves and the wetting axial strain and body strain also have the explicit relations with the above conditions.For sake of gaining a deep understanding of the water immersion characteristics of compacted roadbed loess,recognizing the relation between roadbed damages and disintegration, the experiments were performed with the application of triaxial permeability test and disintegration test,also a comparative analysis of the disintegration properties between pre and post-shear failure in different water contents was conducted.And obtained some results about the qualitative relationship involving the permeability coefficient of compacted loess and compactness or confining pressure as well as the relation relation between disintegration properties and compactness or moisture content.Finally,some suggestions for practical engineering protection were put forward.更多还原