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.

Effect of Casuarina Crushed Nodules, Rhizospheric Soil and Leaves Compost on Salt Tolerance of <i>Casuarina equisetifolia</i>L. and <i>Casuarina obesa</i>Miq.

Soil salinization is one of the major causes of land degradation. In Senegal, this phenomenon continues to grow, making soils unsuitable for agriculture. To rehabilitate salty lands, one of the recommended strategies is the use of salt-tolerant plants. Among them, plants of Casuarinaceae family form a relationship with symbiotic microorganisms such as arbuscular mycorrhizal fungi and nitrogen fixing bacteria. It has been shown that symbiotic microorganisms play an important role in the establishment of tolerant plants in saline conditions (Djighaly et al., 2018). They improve plant performance and reduce transplant shock under salt stress conditions (Diagne et al., 2014). These microorganisms can be used as biofertilizers. However, inocula containing symbiotic microorganisms are either too expensive or unavailable in many developing countries. The aim of this study is to test alternatively affordable and low-tech solutions to promote symbiotic interactions such as Casuarina crushed nodule, Casuarina rhizosphere soil and Casuarina leaves compost that may contain symbiotic microorganisms and also nutrients such as N and phosphorus. Two species of Casuarina (Casuarina equisetifolia L. and Casuarina obesa Miq.) were grown in the greenhouse on sterile soil to which an amendment was added (Casuarina crushed nodules, Casuarina Rhizospheric soil or Casuarina leaves compost). Plants were subjected to saline stress. After four months of cultivation, they were harvested and morphological and physiological parameters were determined. Results showed that inoculation with Casuarina crushed nodules, Casuarina rhizospheric soil and Casuarina leaves compost improved growth, total dry biomass, total chlorophyll and proline contents of C. equisetifolia and C. obesa plants in salt stress condition. These positive effects were more important in C. obesa plants amended with Casuarina leaves compost. This study shows that Casuarina leaves compost can play an important role in the rehabilitation of saline soils by improving Casuarina trees performance in saline conditions.

阳江地区咸酸田土地改良的试验研究

通过对阳江地区沿海丢荒的咸酸田进行地力监测,通过“水改+肥改”措施对土壤改良,引入水稻-禾虫综合种养模式改善土壤生态,并持续地力监测,评估经济效益。以期达到沿海咸酸田生态环境改善、水稻提质增效和丢荒农田复耕的目的,为治理土壤污染、修复耕地提供有益参考。

中山咸水歌的代际传承困境及发展策略

自20世纪90年代开始,我国农村城镇化快速发展,传统的乡村场域逐步消失,传统民俗逐渐丧失了存在与延续的根基,具有独特魅力的传统民俗文化正在式微和消解。该文通过对中山咸水歌进行田野调研和相关资料收集,发现通过三代传承人的努力,中山咸水歌实现了复兴,但受到外部环境变化、流行文化冲击和政策保障缺乏的影响,以及第二代、第三代传承人老去或过世,年轻人逐渐失去了对咸水歌的兴趣,中山咸水歌面临断层的危机。在此基础上,该文提出了开发多元化产品,融入村落乡村旅游开发,加强政府、村落与学校之间的互动关系等建议,以促进中山咸水歌的可持续发展。

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.

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.

硫酸盐渍土盐胀特性试验及分析

硫酸盐渍土分布广泛,其盐胀特性给工程建设带来诸多困难,以往许多这方面的研究多是通过向非盐渍土中掺入不同比例的盐分,人工制备成盐渍土样进行,且仅侧重于盐胀率的测试和分析,对盐胀压力的研究很少。结合硫酸盐渍土地区工程现场出现的渠道面板胀裂变形等问题,取样天然硫酸盐渍土,对单因素影响下的硫酸盐渍土膨胀率进行测试和分析,对各因素作用下的盐胀机理加以分析,并对法向膨胀力进行测试,与非盐渍土的冻胀力进行对比后,发现硫酸盐渍土具有较大的法向盐胀压力,建议设计分析中需在试验的基础上考虑盐胀的影响。

气候变暖对宁夏引黄灌区土壤盐分及其灌水量的影响

气候变暖加剧了土壤水分蒸发,带动土壤盐分向上移动,引起土壤盐分增加,导致土壤盐碱危害加剧。采用近35a来的土壤盐分定位观测资料和气象资料,研究气候变暖对宁夏引黄灌区土壤盐分变化的影响。研究结果表明:近35a随着全球变暖,宁夏引黄灌区土壤全盐含量呈明显的增加趋势,轻盐化土壤、中盐化土壤和重盐化土壤全盐质量分数分别增加0.08、0.13和0.19g/kg;当温度增加0.5～3.0℃时,宁夏引黄灌区轻盐化土壤、中盐化土壤和重盐化土壤淋洗土壤增加盐分所需的灌水量分别增加8.2%～9.1%、8.2%～8.7%和8.3%～8.8%,总灌水量增加1.29～1.40亿m3。

广西海滩红树林与土壤的关系

不同红树林植物对生态环境和土壤条件的要求不同．因此，在潮间带上的生态分布不同；红树林通过旺盛的生物累积和循环、强烈的生物积盐和严重的酸化作用等．使其生长的基质土壤的理化性状受到较大影响，明显有别于无红树林生长的潮滩土壤。

新疆地区公路沥青路面弯沉综合修正系数

根据新疆地区公路沥青路面实测弯沉数据,分别对盐渍土和非盐渍土地区进行弯沉综合修正系数分析,得出修正系数计算公式,并与现行规范做了比较分析。结果表明,所得公式更符合新疆地区的实际情况。

含水量对冻结含盐粉土单轴抗压强度影响的试验研究

采用取自甘肃省白银市平川区黄河岸边的天然盐渍土,用蒸馏水洗去土中的盐分,配制成NaCl/Na2SO4含量为1.5%,含水量不同的试样,研究了冻结条件下含水量对冻结含盐粉土单轴抗压强度、破坏应变的影响.结果表明:当含水量较小时,随着含水量的增加,冰的胶结作用增强,并与土颗粒、盐晶体一起承受荷载,冻结含盐土的单轴抗压强度不断增大;当含水量超过某一值时,试样更多地呈现出冰的性质,而冰的强度远远小于矿物颗粒的强度,单轴抗压强度随含水量的增加而减小.随着含水量的变化,含盐土的破坏应变与单轴抗压强度有相似的变化规律.

铜在典型内陆盐土中的腐蚀特征

采用在两种西部内陆盐土中自然埋藏的试验方法,结合扫描电镜、EDAX能谱及X射线衍射分析研究了铜的产物及腐蚀特征.结果表明,铜在格尔木盐渍土中呈大面积斑坑腐蚀,在库尔勒荒漠土呈局部较深点蚀.土壤中氧气含量、氯离子和硫酸根离子含量、含水量及土壤质地是造成两种土壤中铜腐蚀差异的主要原因.

苏打盐渍土壤微咸水淋洗改良技术研究

研究了土壤粘土特性与土壤碱化的关系和淋洗水的矿化度对土壤渗透性的影响 ,提出了微咸水改良苏打盐渍土的技术思路。探讨了淋洗用微咸水的水质标准、作物耐盐碱标准、作物需水量及灌溉淋洗定额等参数及其确定方法 ,通过田间试验 。

基于均匀设计的砾类硫酸盐渍土盐胀特性试验研究

为了深入研究盐渍土地区粗颗粒盐渍土的工程特性及其作为路基填料的可用性指标，从盐胀的主要控制因素土、水、盐、温、力5个方面，采用均匀设计方法，通过一系列的室内单次降温盐胀试验，研究了砾类硫酸盐渍土的盐胀特性及其盐胀规律。试验结果表明，影响砾类硫酸盐渍土盐胀量大小的主要控制因素是含盐量、含水率及其初始干容重，且这3个因素互为联系彼此影响，上覆附加荷载对盐胀量大小的增长具有很强的抑制作用；粗颗粒盐渍土盐胀增长活跃温度区间主要为-5-5℃，其活跃温度区间主要控制因素是含水率；如果土样初始干客重较低，盐胀过程中土体会发生体缩现象。

中国公路盐渍土的分区方案

在研究中国公路自然区划和盐渍土地区公路修筑技术的基础上,论述了中国盐渍土的分布和分布规律,分析了原有公路盐渍土分区的不足。根据新的资料和工程实践经验,考虑公路发展的需要,采用综合自然区划的一般原则和方法,紧紧围绕为公路工程服务的目的,提出了新的公路盐渍土分区方案。将中国盐渍土分布区划分为滨海盐渍土大区、东部半干旱-半湿润盐渍土大区、中部干旱盐渍土大区、西部强干旱盐渍土大区与特干盆地盐渍土大区5个大区,并给出了各大区内盐渍土的特点和修筑公路所面临问题的主要差别。对各大区内部亚区的划分也提出了方案和意见。

长期咸水灌溉对小麦光合特性与土壤盐分的影响

于2013—2015年研究了不同咸水利用方式(CK,淡水;T1,咸水与淡水混配为1.8 g/L的混合水灌溉;T2,3.6 g/L咸水与淡水交替灌溉;T3,3.6 g/L咸水灌溉;T4,无灌溉)对冬小麦光合特征及土壤盐分的影响。结果表明:T3和T4处理的株高、叶面积指数、叶面积持续期、叶绿素含量、最大净光合速率(Pnmax)、表观光量子效率(φ)、暗呼吸速率(Rd)和产量较淡水处理显著下降,且连续灌溉3.6 g/L的咸水导致土壤发生积盐,不宜连续灌溉。T1和T2处理与CK的株高、光合特性无显著差异,土壤盐分虽有一定积累,但未影响作物的生长。可见,T1(咸淡混溉)和T2(咸淡水交替灌溉)处理的咸水利用方式对冬小麦生长无负调控效应。从土壤生态环境及小麦产量的影响角度考虑,混灌和轮灌既能保证作物产量较淡水灌溉不减产,土壤未发生次生盐渍化,同时节约淡水资源。