颗粒特性对材料小应变动力特性的影响

土体的小应变剪切模量和阻尼比是表征土体动力学特性的重要参数.针对文献中关于砂土颗粒形状与粒径大小对小应变动力特性的影响仍存在争议,利用能量注入式虚拟质量共振柱,系统测量了特定形状与粒径的聚碳酸酯颗粒试样的小应变剪切模量和阻尼比.试验结果表明,颗粒粒径大小对试样的小应变动力学特性影响不大,而颗粒形状则有显著影响.球形颗粒...     

A unified approach to link small-strain shear modulus and liquefaction resistance of pumiceous sand

Natural pumiceous (NP) sands containing pumice particles, a type of volcanic soil, are commonly found in the central part of the North Island in New Zealand. The pumice particles are highly crushable, compressible, lightweight and angular, making engineering assessment of their properties problematic. In this paper, several series of bender element and undrained cyclic triaxial tests were performed on reconstituted and undisturbed NP sands to determine their small-strain shear modulus (G_max) and cyclic resistance ratio (CRR). Furthermore, similar tests were also conducted on normal hard-grained sands (e.g., Toyoura sand) for the purpose of comparison. The results showed that the NP sands have considerably lower G_max compared to normal sands, resulting in their higher deformability during the initial stages of the cyclic loading test. The high angularity of NP sands play an important role toward the end of the cyclic loading and contributed to their higher CRR. Next, the ratio of CRR/G_max for each sample was correlated to a level of strain denoted as cyclic yield strain (ε_ay), which was found to be significantly dependent on the percentages of pumice particles present in the natural soils. On the other hand, the ε_ay was found to be less sensitive to the consolidation stress (σ′_c) and the relative density (Dr) of the materials. For example, over different values of σ′_c and Dr, NP sands have substantially higher values of cyclic yield strain due to their lower G_max and higher CRR when compared with those of ordinary sands.     

Microscopic investigation into liquefaction resistance of pre-sheared sand: Effects of particle shape and initial anisotropy

Liquefaction resistance of sand can be either increased or reduced due to an undrained cyclic pre-shearing depending on the degree of pre-shearing, which hinders a better prediction of liquefaction potential to be established. The mechanism of such changes in liquefaction resistance has been poorly understood. This contribution aims to gain micromechanical insights into pre-shearing effects on liquefaction resistance of sand using discrete element method (DEM) simulations. In particular, effects of particle shape and initial anisotropy on liquefaction resistance are investigated. The simulation results from samples consisting of non-spherical particles with an initial anisotropy can qualitatively capture the mechanical responses observed in equivalent laboratory experiments. The samples which yielded a qualitative agreement with the laboratory results are further analyzed micromechanically, and the relationships between liquefaction resistance and some microscopic parameters before cyclic loading are discussed. Microscopic analyses reveal that mean mechanical coordination number is well correlated with liquefaction resistance, whereas liquefaction resistance is less sensitive to anisotropy in particle orientation induced by pre-shearing.     

干砂饱和砂小应变剪切模量共振柱弯曲元对比试验研究

通过对2种洁净的干砂与饱和砂进行弯曲元与共振柱对比试验研究,得出了可靠的弯曲元剪切波传播时间确定方法,并对饱和砂土弯曲元与共振柱试验结果的偏差进行解释。不同频率激发信号对弯曲元剪切波速传播时间的研究证实,剪切波速随激发频率的增大而增大,采用激发频率为10kHz的初达波法与共振柱试验得到的结果具有较好的一致性。饱和砂土弯曲元与共振柱试验剪切波速结果相差6%～10%,可通过饱和砂土中剪切波速的弥散性对其进行解释。大部分洁净砂在弯曲元试验中激发频率一般大于特征频率,均需考虑剪切波速的弥散性。而对于天然砂土或粉土一般具有较高的特征频率,其弯曲元试验中一般不考虑剪切波速的弥散性。为洁净砂和天然砂土饱和土样的弯曲元测试提供有效方法。     

Role of particle shape on the shear strength of sand-GCL interfaces under dry and wet conditions

Interface shear strength of geosynthetic clay liners (GCL) with the sand particles is predominantly influenced by the surface characteristics of the GCL, size and shape of the sand particles and their interaction mechanisms. This study brings out the quantitative effects of particle shape on the interaction mechanisms and shear strength of GCL-sand interfaces. Interface direct shear tests are conducted on GCL in contact with a natural sand and a manufactured sand of identical gradation, eliminating the particle size effects. Results showed that manufactured sand provides effective particle-fiber interlocking compared to river sand, due to the favorable shape of its grains. Further, the role of particle shape on the hydration of GCL is investigated through interface shear tests on GCL-sand interfaces at different water contents. Bentonite hydration is found to be less in tests with manufactured sand, leading to better interface shear strength. Grain shape parameters of sands, surface changes related to hydration and particle entrapment in GCL are quantified through image analysis on sands and tested GCL surfaces. It is observed that the manufactured sand provides higher interface shear strength and causes lesser hydration related damages to GCL, owing to its angular particles and low permeability.     

动剪切模量比与剪切波速对地震动影响及等量关系研究

采用数值模拟方法,研究动剪切模量比和剪切波速对地震动的影响问题,并以对地表加速度反应谱影响为标准研究动剪切模量比变化与剪切波速变化间的对等关系,同时建立了等价关系式,分析中考虑了场地类别、场地遭遇地震强度和地震输入波型的影响。结果表明:①动剪切模量比误差和剪切波速误差对地震动都有着重要影响,在土体强非线性和软弱场地中影响更为强烈,实测的动剪切模量比和剪切波速比真实值低时较比真实值高时对地表反应谱影响更大,三类场地0.1g地震输入时动剪切模量比6%误差或剪切波速25%误差的影响已经不可忽视;②对不同的场地类别、场地地震烈度和地震输入波情况,动剪切模量比单独变化与剪切波速单独变化对反应谱的影响都具有对等关系;③动剪切模量比误差对地震动的影响程度要大于剪切波速误差,二者对地表反应谱的敏感性相差3～5倍,绝对误差相同时动剪切模量比降低情况下与剪切波速的敏感程度相差更大。     

Effect of vertical stress rest period on deformation behaviour of unbound granular materials: Experimental and numerical investigations

Repeated load triaxial test is used to assess the deformation behaviour of unbound granular materials (UGMs) in flexible road pavements. Repeated load pulse characteristics (i.e. shape, loading period and rest period) are the stress configurations used in the experimental set-up to simulate the passing axle loads. Some researchers and standard testing protocols suggest a rest period of varying durations after a loading phase. A thorough review of existing literature and practises has revealed that there is no agreement about the effect of the rest period of vertical stress pulse on the deformation behaviour of the UGMs. Therefore, the main objective of this study is to investigate the effect of repeated stress rest period on the deformation behaviour of UGMs experimentally. Experiments are conducted, both with and without rest period, using basalt and granite crushed rocks from Victoria, Australia. Furthermore, in order to gain insight into the effect of the rest period, finite element modelling is also developed. Both the experimental and modelling results show that the rest period has a noticeable effect on both resilient and permanent deformation behaviours of UGMs. It is, therefore, recommended to take extra precautions while adopting a particular standard testing protocol and to supplement the results by additional tests with different loading configurations.     

页岩陶粒沥青混合料配制及其在不同时间和温度条件下的力学性能试验研究

经过选材与材料结构分析，制备出一种新的缓冲耗能材料——页岩陶粒沥青混合料；其中页岩陶粒密实堆积，改性沥青填充间隙。为了研究这种材料衰减地冲击的性能，采用万能试验机与霍普金森杆试验设备，通过控制室温的方法，进行了不同时温条件下的力学性能试验研究。试验结果表明：该材料具有明显的时温效应；准静态时，力学性能受温度影响较大；动态时，应变率效应明显，且在不同温度条件下材料动态破坏形式基本相同；在18℃时，应力-应变曲线出现明显的应力平台。     

Effects of the climatic conditions of the southeastern Brazil on degradation the fibers of coir-geotextile: Evaluation of mechanical and structural properties

In tropical countries where rainfall rates are high, and especially in deforested areas in the Atlantic Rainforest and Cerrado in southeastern Brazil, water is the dominant driving force of erosion. The most common method used to restore degraded tropical lands is to plant nursery-raised tree seedlings, but this method is not always practical and a variety of newer reforestation techniques have become available. Biodegradable coir geotextiles combined with native seeds can be used to restore degraded forest areas. The effects of the climatic conditions during a seasonal cycle of rain and drought were evaluated on the structural and mechanical properties of coir geotextile fibers that were treated, or not, with lime. Analyses of the tensile strength of coir fibers showed that after 12 months of exposure untreated fiber had retained 23% and treated fiber 19% of their initial strength. Two principal factors were considered in evaluating the structural properties of the coir fibers after environmental exposure: (i) initial cellulose retention and its stability after lime-treatment; (ii) lignin degradation and/or its loss to the environment. The structural changes seen by thermogravimetry (TGA) and Fourier Transforms in Infrared spectroscopy (FTIR) analyses explained the changes seen in coir mechanical properties. The greater cellulose contents of fiber structures treated with lime explained their greater tensile strength and high Young's modulus measures after the first three months of exposure in local weather conditions. Considering that lime treatment improved coir fiber properties, lime applications are indicated when coir geotextiles are to be used in acidic Brazilian Cerrado soils.     

Survival of manure-borne E. coli in streambed sediment: Effects of temperature and sediment properties

Escherichia coli bacteria are commonly used as indicator organisms to designate of impaired surface waters and to guide the design of management practices to prevent fecal contamination of water. Stream sediments are known to serve as a reservoir and potential source of fecal bacteria (E. coli) for stream water. In agricultural watersheds, substantial numbers of E. coli may reach surface waters, and subsequently be deposited into sediments, along with fecal material in runoff from land-applied manures, grazing lands, or wildlife excreta. The objectives of this work were (a) to test the hypothesis that E. coli survival in streambed sediment in the presence of manure material will be affected by sediment texture and organic carbon content and (b) to evaluate applicability of the exponential die-off equation to the E. coli survival data in the presence of manure material. Experiments were conducted at three temperatures (4 °C, 14 °C, and 24 °C) in flow-through chambers using sediment from three locations at the Beaverdam Creek Tributary in Beltsville, Maryland mixed with dairy manure slurry in the proportion of 1000:1. Indigenous E. coli populations in sediments ranged from ca. 10¹ to 10³ MPN g⁻¹ while approx 10³ manure-borne E. coli MPN g⁻¹ were added. E. coli survived in sediments much longer than in the overlaying water. The exponential inactivation model gave an excellent approximation of data after 6-16 days from the beginning of the experiment. Slower inactivation was observed with the increase in organic carbon content in sediments with identical granulometric composition. The increase in the content of fine particles and organic carbon in sediments led not only to the slower inactivation but also to lower sensitivity of the inactivation to temperature. Streambed sediment properties have to be documented to better evaluate the role of sediments as reservoirs of E. coli that can affect microbiological stream water quality during high flow events.