Strength and deformation behaviour of coarse-grained soil by true triaxial tests

In order to investigate the influence of intermediate principal stress on the stress-strain and strength behaviour of a coarse-grained soil, a series of true triaxial tests were performed. The tests were conducted in a recently developed true triaxial apparatus with constant minor principal stress σ3 and constant value of intermediate principal stress ratio b=（σ2-σ3）/（σ1-σ3） （al is the vertical stress, and % is the horizontal stress）. It is found that the intermediate principal strain, ε2, increases from negative to positive value with the increase of parameter b from zero to unity under a constant minor principal stress. The minor principal strain, ε3, is always negative. This implies that the specimen exhibits an evident anisotropy. The relationship between b and friction angle obtained from the tests is different from that predicted by LADE-DUNCAN and MATSUOKA-NAKAI criteria. Based on the test results, an empirical equation of g（b） that is the shape function of the failure surface on re-plane was presented. The proposed equation is verified to be reasonable by comparing the predicted results using the equation with true triaxial test results of soils, such as coarse-grained soils in this study, sands and gravels in other studies.

Mechanical and electrical properties of coarse-grained soilaffected by cyclic freeze-thaw in high cold regions

To evaluate the geotechnical properties of coarse-grained soil affected by cyclic freeze-thaw,the electrical resistivity and mechanical tests are conducted.The soil specimens are prepared under different water contents,dry densities and exposed to 0?20 freeze-thaw cycles.As a result,the stress?strain behavior of the specimen(w=14.0%andρd=1.90 g/cm^3)changes from strain-hardening into strain-softening due to the freeze-thaw effect.The electrical resistivity of test specimen increases with the freeze-thaw cycles change,but the mechanical parameters(the unconfined compressive strength qu and the deformation modulus E)and brittleness index decrease considerably at the same conditions.All of them tend to be stable after 7?9 cycles.Moreover,both the dry density and the water content have reciprocal effects on the freeze-thaw actions.The failure and pore characteristics of specimens affected by freeze-thaw cycles are discussed by using the image analysis method.Then,an exponential function equation is developed to assess the electrical resistivity of specimens affected by the cyclic freeze-thaw.Linear relations between the mechanical parameters and the electrical resistivity of specimens are established to evaluate the geotechnical properties of the soil exposed to freeze-thaw actions through the corresponding electrical resistivity.

Gradation equation of coarse-grained soil and its applicability

Gradation equation is one way to describe the gradation of coarse-grained soil conveniently,exactly and quantitatively.With the gradation equation,the influence of gradation on the mechanical behaviors of coarse-grained soil can be expressed quantitatively.A new gradation equation with a parameter is proposed.The basic properties and applicability of the new equation are studied.The results show that the proposed equation has the applicability to express coarse-grained soil gradation(CSG),and the range of the parameter β is found to be 0<β<1.The value ofbdetermines the gradation curve shape.If β>0.5,the gradation curve is sigmoidal,otherwise the gradation curve is hyperbolic.For well graded gradations,the parameter has the value of 0.13<β<1.Several CSGs used in domestic and foreign earth-rockfill dams are probed,and the value of the parameterbfalls in the range of 0.18 to 0.97.The investigation of the range of β is of value to guide the design for CSG of earth-rockfill dam.

Deformation characteristics of coarse-grained soil with various gradations

By using large scale triaxial shearing apparatus,consolidated-drained shear tests were conducted on coarse-grained soil with different gradations.In order to describe their deformation rules,three main characteristics of tangent Poisson ratio curves were summarized and the reason was revealed by dividing the movement of soil particles into two kinds: the movement of fine particles and the movement of coarse particles.Then,a volumetric strain expression and a tangent Poisson ratio expression were put forward,and two defects of widely used Duncan-Chang model were fixed.Results calculated from them agree well with test results.There are three parameters,namely L,G and F,in this new model.Parameter L reflects the dilatancy of a specimen and L=4 can be used as a criterion to estimate whether a certain kind of soil has dilatancy quality or not.Parameters G and F relate to the initial slope of tangent Poisson ratio curves,and G=F=0 indicates a special situation which happens in dense granular material of the same diameter.Influences of various gradations on volume deformation are mainly reflected in parameter L which is smaller when there are more gravels in specimens.

Influence of fines content on the anti-frost properties of coarse-grained soil

This paper aims to determine the optimal fines content of coarse-grained soil required to simultaneously achieve weaker frost susceptibility and better bearing capacity. We studied the frost susceptibility and strength properties of coarse-grained soil by means of frost heaving tests and static triaxial tests, and the results are as follows： （1） the freezing temperature of coarse-grained soil decreased gradually and then leveled off with incremental increases in the percent content of fines; （2） the fines content proved to be an important factor influencing the frost heave susceptibility and strength properties of coarse-grained soil. With incremental increases in the percent content of fines, the frost heave ratio increased gradually and the cohesion function of fines effectively enhanced the shear strength of coarse-grained soil before freeze-thaw, but the frost susceptibility of fines weakened the shear strength of coarse-grained soil after freeze-thaw; （3） with increasing numbers of freeze-thaw cycles, the shear strength of coarse-grained soil decreased and then stabilized after the ninth freeze-thaw cycle, and therefore the mechanical indexes of the ninth freeze-thaw cycle are recommended for the engi- neering design values; and （4） considering frost susceptibility and strength properties as a whole, the optimal fines content of 5% is recommended for railway sub,fade coarse-~rained soil fillings in frozen re^ions.

煤气化粗渣改良季节冻土区黄土填料的冻融特性及其微观机理研究

由于周期性的冻融循环作用,中国西北黄土区路基工程极易发生冻融病害,常用的解决方法是用改良黄土作为路基填料。目前工业废弃物改性不良路基土已成为工程地基改良处理的新趋向,鉴于西北地区煤气化渣堆存量巨大且资源化利用率低,尝试用煤气化粗渣改良季节冻土区黄土,以素黄土与单掺石灰改良黄土作为对比试验组,通过室内冻融循环试验、电镜扫描、CT扫描试验来探究在冻融循环条件下煤气化粗渣改良黄土的冻融特性及其微观机理,为季节冻土区黄土路基工程稳定性及煤气化渣资源化利用提出一些新的思考。试验结果表明:(1)经历1次与2次冻融循环后,单掺15%煤气化粗渣试验组的冻胀率为0.02%与0.29%,融沉系数为0%与0.05%,素黄土、单掺4%石灰试验组的冻胀率分别为0.43%与0.63%、0.38%与0.42%,融沉系数分别为0.26%与0.22%、0%与0.13%,但在经历3次冻融循环后,单掺15%煤气化粗渣试验组的冻胀率和融沉系数开始增大,5次冻融循环周期内的平均冻胀率和融沉系数分别为0.38%与0.17%,已接近甚至大于素黄土与单掺4%石灰试验组的数值。在前两次冻融循环周期内,平均冻胀率和融沉系数较素黄土组分别减小了70.8%和89.6%。(2)单掺15%煤气化粗渣试样各位置的温度梯度最小,其次是双掺煤气化粗渣与石灰试样,并且这两组试样正负温条件下的导热系数最小。(3)经过5次冻融循环后,单掺15%煤气化粗渣试样各高度的含水率最低。(4)单掺适量的煤气化粗渣在冻融循环的初期能明显地改善黄土的冻融特性,再掺入少量的石灰能使试样在冻融循环前后的孔隙率较素黄土分别降低41.5%与47.8%。总的来说,黄土中掺入煤气化粗渣在冻融循环次数少于3次时能改善黄土路基填料的寒区服役性,但其在更多次冻融循环条件下的冻融特性还需进一步研究。

Numerical and experimental direct shear tests for coarse-grained soils

The presence of particles larger than the permissible dimensions of conventional laboratory specimens causes difficulty in the determination of shear strength of coarse-grained soils. In this research, the influence of particle size on shear strength of coarse-grained soils was investigated by resorting to experimental tests in different scale and numerical simulations based on discrete element method （DEM）. Experimental tests on such soil specimens were based on using the techniques designated as ＂parallel＂ and ＂scalping＂ to prepare gradation of samples in view of the limitation of laboratory specimen size. As a second approach, the direct shear test was numerically simulated on assemblies of elliptical particles. The behaviors of samples under experimental and numerical tests are presented and compared, indicating that the modification of sample gradation has a significant influence on the mechanical properties of coarse-grained soils. It is noted that the shear strengths of samples produced by the scalping method are higher than samples by the parallel method. The scalping method for preparing specimens for direct shear test is therefore recommended. The micromechanical behavior of assemblies under direct shear test is also discussed and the effects of stress level on sample behavior are investigated.

基于原型监测的分离式加筋土桥台工作性能数值分析

依托明巢高速公路分离式加筋土桥台工程原型监测,用FLAC2D软件建立桥台数值模型,并将数值模拟得到的桥台受力及变形情况与桥台原型监测结果进行对比验证.考虑到填料类型及其压实工序与填料力学性能的相关性,模拟分析了填料内摩擦角对加筋土桥台工作性能的影响.结果表明:随着填料内摩擦角的增大,分离式桥台的水平土压力和竖向土压力都有所减少,并且竖向位移、水平位移及筋材应变也有所减少;当填料内摩擦角增大到一定程度时,其对承载性能的优化作用有所减少;工程中选取加筋土填料时,根据现场施工情况确定填料的内摩擦角,并权衡工作性能及经济效益,做出合理的选择.

Using a modified direct shear apparatus to explore gap and size effects on shear resistance of coarse-grained soil

This work used a modified direct shear apparatus, created newly by the authors, to explore effects of the gap between shear box halves and specimen size on the shear resistance of coarse-grained soil. The shear boxes of this apparatus were assembled from a series of steel structures capable of superimposition and nesting. Such characteristics facilitated variation of specimen size in both diameter and height. The new device can also maintain a constant gap during shearing. We performed a series of gap-effect and size-effect tests for two uniformly graded, coarse^grained soil samples. The test results showed that both the gap space and specimen size bad significant influences on shear resistance of the coarse-grained soil. Further, analysis of variations in shear strength indices led to a reasonable gap dimension and specimen size of the two soil samples.

泡沫轻质土在既有公路改扩建工程中的应用

依托京沪高速公路新沂至江都改扩建工程,在路桥过渡段及一般纵断面抬升路段大规模采用泡沫轻质土进行路基帮宽,阐述泡沫轻质土的性能及施工特点,结合数值模拟手段,探究高速公路改扩建工程中泡沫轻质土帮宽路基与软土地基处理后的沉降变形特性,验证帮宽路基及路桥过渡段采用泡沫轻质土作为填料的适用性。

水泥-碱渣固化高含水率海相软土力学性质试验研究

为了解决低剂量水泥单独固化高含水率海相软土早期强度不足及固废碱渣资源化利用率低等问题,采用水泥、碱渣联合固化高含水率软土。通过30种不同质量配比的无侧限抗压强度试验得出,水泥-碱渣固化高含水率海相软土试样的无侧限抗压强度不仅与水泥、碱渣的掺量有关,还受到软土含水率的控制。在无侧限抗压强度研究基础上,推荐优选配比(碱渣、软土、水泥质量比为30∶70∶6),其7 d无侧限抗压强度为682.7 kPa。优选配比固化土的加州承载比(CBR)和回弹模量满足各级公路路基填料的指标要求。通过对高含水率软土中水的影响以及固化土级配和微观结构分析得出,水泥-碱渣固化高含水率软土的力学强度主要来源于碱渣吸水作用、碱渣骨架作用以及各组分材料化学反应的作用。化学作用产生的C-S-H,C-A-H和Aft等凝胶物质共同密实结构,提高固化土强度。研究结果可为固废物固化高含水率软土在路基中的应用提供理论支持。

振动荷载下高铁路基粗粒土填料应变规律研究

为提高高铁路基土的安全服役性能,文中利用室内动三轴试验对高铁路基粗粒土填料进行了振动荷载作用下的动力特性试验,对比分析了不同细粒含量以及围压条件下的高铁路基粗粒土填料的累积应变曲线变化规律。结果表明,当动应力为30 kPa时,所有细粒土掺量及围压条件下土体的累积塑性应变均随着加载作用次数的增加呈现出先快速增加后缓慢增加的变化规律;当动应力为60、90 kPa时,所有细粒土掺量及围压条件下土体的累积塑性应变均随着加载作用次数的增加呈现出先快速增加后趋于稳定的变化规律。

浅谈轻质填料在路基中的应用

为减少路基沉降,通常采用轻质填料填筑路堤,其中常用的填料有粉煤灰、土工泡沫塑料和泡沫轻质土。通过工程实例,介绍了这3种填料在路堤中的填筑方法,并结合不同路堤高度、不同填料的工后沉降量及工程造价,对工程设计方案进行了比选。

高水位软土公路路基土石方填筑施工技术研究

由于土石方填筑施工缺乏与实际工况的结合,导致存在高水位软土公路填筑路基沉降值超标的情况,为此,文中对高水位软土公路路基土石方填筑施工技术进行研究。收集高水位软土公路路基各土层的厚度、力学参数(压缩模量、抗剪强度等)等基本地质信息,结合水位分布以及高度,确定其对地基沉降情况的影响,计算高水位软土公路路基沉降状态。在土石方填筑施工阶段,结合计算结果,针对不同土层的具体情况,从填筑松铺厚度、填料粒径、碾压速度、碾压次数,以及特殊工况的施工基础进行针对性设计。应用路基填筑技术后,60 d的最大沉降值仅为0.66 mm,在沉降值允许范围(≤1.0 mm)内,且在50 d后,沉降逐渐收敛,10 d内,沉降值变化幅度仅为0.03 mm。

生石灰与水泥处治过湿土路基填料性能试验研究

为了改善过湿土路基填料的性能,选用生石灰、水泥作为改良剂,对生石灰、水泥改良过湿土路基填料的性能进行了对比试验研究。研究结果表明:相比于素过湿土,石灰或水泥改良过湿土的最佳含水量、无侧限抗压强度、回弹模量值均得到了显著提升,且均随石灰或水泥掺量的增加而增加;石灰改良过湿土虽然一定程度上提高了过湿土的CBR值,但仍不满足相关规范对于一级公路、高速公路路基填料的要求,水泥改良过湿土的CBR值在掺入水泥后显著提升,能够较好满足规范要求;石灰改良过湿土的最佳含水量要优于水泥改良过湿土,水泥对于过湿土CBR值、无侧限抗压强度、回弹模量的改善效果要显著优于石灰。

工程渣土再生填料长期浸水性能试验研究

工程渣土再生填料浸水条件下的性能衰减是一个复杂过程,分析掌握其衰减规律具有重要意义。通过浸水试验,研究了工程渣土再生填料试件在室内静荷载作用下,浸水时长7~360d范围内的抗压强度和质量变化。试验数据表明,浸水条件下,工程渣土再生填料保持了前期强度增长较快、后期平缓的增长趋势,同时试件完整性较好。综合来看,在长时间浸水条件下,工程渣土再生填料可保持良好的路用性能,符合沿海地区道路工程使用要求。

南宁外环膨胀土路基处治技术及设计方案研究

拟建南宁外环路穿越膨胀土分布区,若实施刚性支挡难以保证开挖边坡稳定,沿用弃土换填方案填筑路堤至少需废240万方,使工程投资大增并严重破坏生态环境,为此,研究运用膨胀土处治新技术进行路基工程设计。通过系统土性、填料性质试验,论证了物理处治技术修建膨胀土路堤的可行性;采取FLAC和极限平衡分析,获得实施柔性支护前后典型膨胀土堑坡的稳定性。基于试验、理论分析并借鉴交通部西部膨胀土项目的成功经验,提出两种封闭包盖措施填筑南宁膨胀土下路堤、不同边界条件下的路堑边坡柔性支护设计方案。对比分析表明,采用新技术效益明显,能实现南宁外环高速路安全、经济、节能环保的修建目标。

戈壁粗粒土填料填筑铁路路基压实评价指标研究

选取兰新铁路第二双线4个具有代表性的路基试验段,进行戈壁粗粒土填筑路基的压实质量评价指标研究。研究表明:粗粒土填料填筑路基的孔隙率偏小,但现有规范规定的孔隙率标准偏大,起不到控制路基密实度的作用,因此提出用填料的细颗粒室内击实试验得到的孔隙率换算整体孔隙率的方法控制粗粒土填筑路基的密实度;K30能较好地反映填料的塑性变形,且对压实质量敏感,比Ev2更适合作为路基压实质量的评价指标;Ev2/Ev1控制路基压实质量的实质是Ev1在起作用;规范不应用统一的Ev2/Ev1标准,而应根据填料的粗颗粒含量制定相应的Ev2/Ev1标准,填料的粗颗粒含量越多,填筑的路基的弹性变形量越小,Ev2/Ev1的规范标准应越大;Ev1的测试深度比K30大,且塑性变形在总变形中的比例也大,因此可以考虑将Ev1设定为路基压实质量的评价指标。

土石混填料台背回填柔性加筋注浆技术加固试验研究

通过采用柔性加筋注浆技术处理台后回填土现场试验研究并结合室内试验的方式,研究了柔性加筋注浆新技术处理湘西红砂岩土石混填台背的施工工艺特性、加固实现机制和加固效果。现场试验及质量测试表明,水平柔性加筋注浆新技术处理湘西红砂岩土石混填台背施工工艺简单、加固效果良好,具有较好的开发应用价值。同时,现场试验过程取得大量施工参数和试验数据,为改进该新技术的设计计算及施工参数、完善施工工艺获得素材;对有效预防和减少桥头跳车现象的发生,保证行车的舒适和安全也提供了可供选择的新方式。

多级土壤渗滤系统填料的脱氮除磷性能研究

为筛选用于多级土壤渗滤系统的填料,选取活性炭、粉煤灰、膨润土、煤渣及红黏土作为土壤砖块材料,2种粒径(1~3 mm和3~5 mm)的生物陶粒和沸石作为渗滤层材料,进行渗透性、氮磷等温吸附以及硝化反硝化强度研究。结果表明,活性炭和煤渣的添加可以有效改良红黏土渗透性;Langmuir方程能更好地描述各填料对氮磷的吸附特征;通过拟合,红黏土、粉煤灰及活性炭对磷的理论饱和吸附量较大,沸石及红黏土对NH4+-N的吸附能力较强;室外接种2个月后,沸石的硝化能力明显高于其他填料,而活性炭、煤渣的反硝化能力显著高于其他填料。综合考虑,2种粒径的沸石均适宜作为多级土壤渗滤系统渗滤层材料,红黏土混合质量分数20%~40%的活性炭或煤渣更适合作为土壤砖块层填料。