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。

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.

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。

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。

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。

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。

渗流作用下多级配土石混合填料的结构演化规律

渗流是引起土石混合填料中细颗粒流失,致使土体结构变化引起路基变形及失稳的关键因素。采用自行研发的颗粒流失试验装置,对不同级配的土石混合填料进行渗流试验,监测渗流作用下填料的透水质量、细颗粒流失量和沉降量的变化过程,分析土体结构演变过程;采用PFC^(3D)(Particle Flow Code in 3D)对细颗粒流失过程进行数值模拟,分析填料孔隙和粒径动态变化特性。研究结果表明:透水质量和细颗粒流失率可以反映土石混合填料结构对水分的敏感程度。填料骨架结构对水分的敏感程度越低,可及时有效地排出水分,渗流对结构的破坏越小。渗流条件下土石混合填料的结构演化过程可分为细颗粒快速流失、骨架重塑和相对稳定3个阶段,其中骨架重塑阶段是引起其结构演化的关键阶段。细颗粒快速流失阶段,每小时透水量增长速率较快,细颗粒大量流失,沉降量很小;骨架重塑阶段,每小时透水量变化速率减缓,细颗粒流失量减少,但骨架结构重组导致颗粒产生明显的相对位移,沉降量增长较快;相对稳定阶段,每小时透水量变化缓慢,沉降量基本保持不变。颗粒流失数值模拟的过程表明细颗粒组分的迁移引起填料孔隙率产生变化,致使土体结构发生改变。上述结果表明,在所研究的几种填料中,n=0.55级配下土石混合填料的骨架结构最稳定,对水分的敏感性最弱。

颗粒细长比对粗粒土填料振动压实特性的影响

本文旨在探究粗颗粒(粒径d>5 mm)细长比对粗粒土填料振动压实特性的影响机制,深入揭示粗粒土填料振动压实机理。首先,基于优化Viola-Jones算法建立颗粒形状采集系统,通过几何算法量化粗颗粒细长比;其次,基于自研智能振动压实仪,选择3组不同细长比的粗粒土填料,开展大型振动压实试验,并通过物理-力学指标揭示细长比与粗粒土压实特性之间的相关性;最后,建立振动压实离散元数值模型,并通过孔隙率、颗粒应力以及颗粒运动特性等细观指标揭示细长比对粗粒土填料压实特性的影响机制。研究结果表明:在振动压实过程中,存在最优细长比使粗粒土填料密度最大,粗粒土填料承载力随细长比增加而增加;在数值模型中,粗粒土填料内部孔隙率随细长比增加而减小;细长比越小,颗粒应力越集中,导致颗粒破碎现象越显著,同时,颗粒运动也越剧烈,其中,细颗粒运动位移比粗颗粒的大。

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.

水泥粉煤灰改良粗粒土路基填料力学与收缩特性

河北省西北地区特殊气候环境下,经上部荷载作用基床底层收缩变形产生的裂隙向上扩展使得路基表层产生裂缝。针对这一问题,本文以该地区路基改良粗粒土填料为研究对象,分析其力学特性与收缩特性,并提出新的抗裂评价指标,综合评价填料工程适用性。结果表明:掺入粉煤灰后水泥改良填料力学性能降低,但填料拉压比增大;水泥掺量越低填料抗干缩性能越好,掺入粉煤灰填料抗干缩性能进一步改善,但粉煤灰存在最佳掺量;填料最大温缩应变随温度的循环逐渐增加,第3次温度循环后趋于平稳,填料对温度变化敏感度降低;从能量角度提出综合抗裂系数,得出水泥粉煤灰改良填料综合抗裂性优于水泥改良填料。

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.

铁尾矿砂改良膨胀土填料工程特性研究

为保证膨胀土填料路用工程性质良好,选用铁尾矿砂处治膨胀土填料,并通过室内试验研究了铁尾矿砂改良膨胀土填料击实特性、膨胀特性、力学特性和水稳定性。结果表明,随掺砂率增大,铁尾矿砂改良膨胀土最大干密度逐渐提高,最佳含水率逐渐降低;随掺砂率增加,铁尾矿砂改良膨胀土膨胀性减弱,掺砂率增加10%,改良膨胀土自由膨胀率和膨胀力分别平均降低17.2%、22.2%;在掺砂率30%时,铁尾矿砂改良膨胀土力学特性和水稳定性最优,较素膨胀土水稳系数约提高16.1%。建议铁尾矿砂改良膨胀土最佳掺砂率为30%。

基于连续压实及动态变形模量检测技术的土石混填路基压实质量研究

以静乐丰润至兴县黑峪口高速公路高填方路基填筑为背景,通过现场试验,对不同碾压遍数及松铺厚度条件下,高填方土石混填路基的连续压实值VCV及动态变形模量值进行研究,结果表明:(1)当土石混填路基的松铺厚度一定时,随着碾压遍数的增加,连续压实值VCV呈现增加的趋势,且连续压实值VCV在碾压前期增加速度较快,后期缓慢;(2)松铺厚度为50~60 cm,碾压遍数为5遍时可达到较好的经济效果;(3)路基松铺厚度为50 cm及60 cm时,连续压实值VCV与动态变形模量相关性系数分别为0.714、0.760,松铺厚度为70 cm时,两者离散性较大。