Filtration performance of non- woven geotextiles with internally-stable and -unstable soils under dynamic loading

In many applications, geotextiles are subjected to dynamic loading conditions, for example, below roads and railways, for which a Gradient Ratio (GR) test is often used to assess filtration compatibility of soil-geotextile systems. This paper presents results from GR filtration tests with internally-stable and -unstable soils under dynamic loading conditions. In the tests, four non-woven geotextiles were used with varying types of soils under a hydraulic gradient of 5. Test results were interpreted in terms of GR values, permeability values, and mass and gradation characteristics of the soil before/after testing as well as the particles passing through the geotextiles. The test results show that the dynamic loading resulted in an increase of soil migration within the soil as well as an increase in the quantity of soil passing through the geotextiles. The available criteria for evaluating the internal stability of soils are evaluated based on the experimental data. Based on the test results, improvements to filter retention design criteria are suggested which take into account the internal stability of soils under dynamic loading.     

结合微孔隙水压力计改良型GR试验系统分析研究(英文)

本研究针对结合微孔隙水压力计改良型GR试验系统与一般GR试验系统进行了一系列的试验分析比较,研究中所使用的试验材料包含4种不同针轧不织布及5种不同比例渥太华砂与风化泥岩混合土壤。试验结果发现,由改良型GR试验系统所得的GR试验值都高于一般GR试验系统所得结果,同时也验证了微孔隙水压力计在GR试验系统中可提供较好的孔隙水压力量测与抗阻塞潜势评估。     

A new laboratory apparatus for studying dynamic compaction grouting into granular soils

As granular soils may be compressible or have inadequate strength, compaction is particularly useful when soils are subjected to dynamic loading or cyclic loading. A new laboratory apparatus for investigating dynamic compaction has been designed and fabricated. The basic principle of this new technique is to introduce vibrations during the expansion process in static compaction grouting. In these tests, the injection pressure, the excess pore water pressure, and the change in void ratio of the specimens are measured. The main focus is to investigate the development of the injection pressure, the void ratio, and the excess pore water pressure due to dynamic compaction and the subsequent consolidation of the soils. In addition, the relative density of the soils is used to evaluate the dynamic compaction efficiency. Scaled laboratory experiments are conducted to study the effect of this dynamic compaction frequency on compaction efficiency. The experimental results show that the change in void ratio in the dynamic compaction tests is about four times greater than that in the static compaction tests. Dynamic compaction frequency plays an important role in soil densification due to dynamic compaction.     

动三轴试验研究土的动剪切模量和阻尼比

对不同土样的动剪切模量和阻尼比进行试验研究,最后采用曲线拟合得到动剪切模量以及阻尼比与剪应变的关系曲线,可供岩土和其他土木工程的地震安全性评价工作参考和借鉴.     

A laboratory investigation on the impact resistance of a woven geotextile

This paper focuses on the impact resistance of geotextiles when subjected to impact loadings induced by dropping of stones. Such scenarios occur when geotextiles are used as a protective measure for fine granular material where is prone to be washed away. Usually, these geotextiles are restrained by placement of stones on top of them. A laboratory testing program is performed to expose a woven geotextile under dropping of a concrete block with various dropping energies and geometries. The induced damage on the geotextiles is inspected after the drop. Results indicate that as the drop energy increases, not only the possibility of puncturing of geotextiles increases but, in case of puncturing, the punctured area of geotextile expands as well. In addition, it is found that the geometry of the concrete block, where it collides on the geotextile, plays an important role on the survivability of geotextiles. In addition, PIV analysis has been performed to better understand the deformation pattern of the geotextile under impact loading. Based on the PIV results a simple scheme is suggested to estimate the drop energy threshold that the geotextile can survive under certain block geometry.     

A pilot test on a membraneless vacuum preloading method

A membraneless vacuum preloading method is proposed in this paper for soft soil improvement. The method offers several advantages over the conventional vacuum preloading in which membrane is used to create the airtight condition and sand blanket layer to distribute vacuum. To assess the effectiveness of the proposed method, a pilot test was conducted at a land reclamation site in Tianjin, China. The ground settlement and the pore water pressure (PWP) at different elevations in soil were measured. After vacuum preloading, the average water content of the soft soils reduced by approximately 12% and the undrained shear strength increased twofold. The average degree of consolidation at the end of the vacuum preloading achieved 85.1% based on the settlement data and 84.5% based on the PWP data. The pilot test data have shown that the proposed method exhibits similar efficiencies to the conventional vacuum preloading method.     

Filtration behaviour of soil-nonwoven geotextile combinations subjected to various loads

Geotextiles are often subject to different load types in their filtration applications. The load action can cause changes in soil density, geotextile stretching and flow interaction at the soil-geotextile interface. All of these load-induced changes to a geotextile may affect the filtration behaviour of the soil-geotextile system. The impact of load type on the filtration behaviour of soil-nonwoven geotextile combinations has been studied through a series of tests using an experimental apparatus designed specially for the laboratory tests. In these tests, the soil-geotextile combination was fabricated by inserting a piece of nonwoven geotextile between a 50 mm thick soil layer and a layer of steel beads. Two chemical-bonded nonwoven geotextiles were employed in this study. One of the three load types, namely sustained, pulsatory and a combination of both was applied to the combination prior to each filtration test. The frequency of the pulsatory load was 0.1 Hz and a total of 5000 cycles of repeated load applied to the combination for each load type test. After applying this specific type of load on a soil-geotextile combination, water was allowed to flow down through the combination from the soil into a drainage layer set at various hydraulic gradients. The flow rates corresponding to elapsed times were measured and the average hydraulic conductivity value was extracted by using Darcy’s law to characterize the filtration performance of the entire soil-geotextile combination. Variations in the average hydraulic conductivity value with respect to the soil void ratio, magnitude and type of normal load were examined.The experimental results revealed that the void ratio of soil decreased with the increase of total load. Although two parent geotextiles under study, namely GT1 and GT2, have similar filtration characteristics, soil-geotextile combinations composed of these two geotextiles exhibited different filtration responses to the normal load. Soil-GT1 combinations exhibited a normal relationship between the average hydraulic conductivity and the normal load applied; the average hydraulic conductivity increased with an increase in the total load. Soil-GT2 combinations exhibited different load-dependent responses to a normal load with the average hydraulic conductivity depending on the magnitude and type of load. Such load-dependent hydraulic conductivity changes are attributed mainly to the geotextile in-plane strain and the pumping action in the combination.     

Tension force analysis of geotextile tubes by half cross-section test

A simplified equation, which allows for the calculation of the tension force using the actual tube height and pumping pressure with the flexibility to use a coefficient of lateral pressure (K), is proposed and validated theoretically by comparing the proposed method with two well-known methods in the literature, and experimentally, by conducting several half cross-section tests. The half cross-section test proposed in this study is unique and configured in such a way that the top and bottom of the geotextile tube is supported by load cells to be able to quantitatively measure the maximum tension force, as well as the stress and strain of the geotextile tube. With the use of the simplified equation, the actual field conditions can be exceptionally represented, making it more advantageous over the previous methods.     

A closed-form solution for column-supported embankments with geosynthetic reinforcement

Soil arching effect results from the non-uniform stiffness in a geosynthetic-reinforced and column-supported embankment system. However, most theoretical models ignore the impact of modulus difference on the calculation of load transfer. In this study, a generalized mathematical model is presented to investigate the soil arching effect, with consideration given to the modulus ratio between columns and the surrounding soil. For simplification, a cylindrical unit cell is drawn to study the deformation compatibility among embankment fills, geosynthetics, columns, and subsoils. A deformed shape function is introduced to describe the relationship between the column and the adjacent soil. The measured data gained from a full-scale test are applied to demonstrate the application of this model. In the parametric study, certain influencing factors, such as column spacing, column length, embankment height, modulus ratio, and tensile strength of geosynthetic reinforcement, are analyzed to investigate the performance of the embankment system. This demonstrates that the inclusion of a geosynthetic reinforcement or enlargement of the modulus ratio can increase the load transfer efficiency. When enhancing the embankment height or applying an additional loading, the height of the load transfer platform tends to be reduced. However, a relatively long column has little impact on the load transfer platform.     

不同桩土刚度比桩体复合地基模型试验研究

通过对不同桩体材料的复合地基及不同含水率地基土的复合地基竖向静载的模型试验,分析了在不同桩体刚度和不同地基土含水率的地基情况中,地基沉阵、桩土应力比及桩身应变的变化规律,对常规复合地基的适用性具有指导意义。     

南京新近沉积土动剪切模量比与阻尼比的试验研究

通过对南京淤泥质粉质粘土、粉质粘土、粉质粘土与粉砂互层土、粉土、粉砂和细砂等6类新近沉积土120个原状土样、历时4 a的试验研究,给出了南京6类新近沉积土的动剪切模量比G/Gmax和阻尼比D随剪应变幅值γ变化的平均曲线的拟合曲线及其参数的推荐值。为便于一般工程借鉴,给出了该6类新近沉积土的G/Gmax–γ和D–γ试验曲线的典型值。该成果有利于加深对南京新近沉积土动力特性的认识,并对该地区一般工程建设场地的地震安全性评价工作具有借鉴和参考价值。     

A numerical modelling technique for geosynthetics validated on a cavity model test

Numerical modelling approaches can aid in designing geotechnical constructions involving geosynthetics. However, the reliability of numerical results depends on how the model is developed, the constitutive model, and the set of parameters used. By comparing the numerical results with experiment, the present work verifies a numerical modelling technique developed to model multilayered geosynthetic lining systems for landfills. The numerical modelling technique involves strain softening at interfaces and allows the axial stiffness of the geosynthetics to evolve as a function of strain. This work focuses on a two-dimensional finite-difference model, which is used to simulate three types of experimental tests: conventional uniaxial tensile tests, direct shear tests, and a large-scale test that was used to assess the overall mechanical behaviour of a reinforced geosynthetic system that spanned over a cavity. This reinforced geosynthetic system consisted of a 50 kN/m polyvinyl alcohol geogrid reinforcement embedded in a layer of sand, a geosynthetic clay liner, a high-density polyethylene geomembrane, and a non-woven needle-punched geotextile. The uniaxial tensile tests, direct shear tests, and the large-scale test were numerically modelled and the numerical results were compared with experimental results. The results of the numerical modelling technique presented very closely match the results of the three experimental tests, which indicates that the numerical model correctly predicted the measured data.     

Stress-strain behavior of geotextile: A proposed new indirect calculation using the static puncture test (CBR test)

Some of the main applications of geosynthetics include use as a hydraulic barrier in sanitary landfills, as a reinforcement element and in pavement engineering. In most cases, these materials are subject to the overlapping effects of tensile strength and puncture. This paper presents a review of indirect methods for calculation of stress and strain averages by means of the California Bearing Ratio (CBR) puncture strength test. In addition, a new calculation method is proposed based on the Kirchhoff plate theory, which interprets the behavior of thin circular plates subjected to a uniform normal loading. This new method enables analysis of the stress-strain in each stretch of the geosynthetic. The methodology is applied to four woven geotextiles of different weights. The results of the new calculation method yielded a better stress-strain correlation with direct tensile strength tests, presenting the smallest relative errors compared to the other indirect calculations reviewed. With the aid of a disk and pins, vertical displacement values at different points in the geotextiles were measured and showed good agreement with analytical predictions. Therefore, the static puncture test combined with the new proposed calculation method is a good alternative for determining the stress-strain parameters of geotextile.     

A multi-camera based photogrammetric method for three-dimensional full-field displacement measurements of geosynthetics during tensile test

Conventional methods for measuring the deformational response of geosynthetics, such as the linear variable differential transformers (LVDTs), strain gauges, and extensometers have several limitations in fully determining the complete strain distribution in geosynthetics. This paper presents a multi-camera based photogrammetric method to track the 3D full-field displacements of geosynthetics during tensile tests. The proposed method extends the conventional one-camera based photogrammetry for static object measurements to multi-camera-based photogrammetry for object measurements with continuous movements or deformations. It is non-contact, cost-effective, highly accurate, and capable of measuring the 3D full-field displacements of the geosynthetics. A tensile test on a geogrid specimen was performed to verify the effectiveness and accuracy of the proposed photogrammetric method. The results from the tensile tests using both the proposed method and conventional methods, such as machine-controlled displacement measurements and extensometer, were presented and compared. It was observed that the average absolute difference between the proposed photogrammetric method and the machine-controlled movements of the bottom clamp was 0.25%, and the average absolute error was 0.038 mm. The average difference in measurements made by the proposed method and extensometer was 0.07%. It was further found that the proposed method can provide more comprehensive input, such as the complete strain and modulus distributions in the geosynthetics, for a probability-based geosynthetics design.     

固结应力比对土样动强度和动孔压发展规律的影响

回顾了初始剪应力对土样动强度和和动孔压发展规律的影响,使用动三轴对某土石坝坝基饱和砂砾石料进行动强度试验,研究固结应力比Kc对土样动力特性的影响。分析试验结果如下:所有土料都存在一个转折点Kc′:KcKc′时动强度随Kc的增大而降低,动变形和动孔隙水压力随Kc的增大有稍许加大趋势。转折点Kc′的具体数值与土料性质及所受应力状态有关。本次试验Kc=0.5～3.0,转折点Kc′为2.8左右。Kc过大后,初始剪应力较高、剩余强度较低而导致动强度降低。偏压固结Kc=0.7～1.3时,试验土料最终孔压值均可达到围压;偏压固结Kc≥1.5后,试样最终孔压值是否可达到围压取决于动应力σd是否大于主应力差(σ1c-σ3c),即σd是否形成拉应力使试件伸长。但当Kc很大后,σ1c和σ3c相差太大,需施加非常大的动应力σd才能使其应力应变反向,试样不可能达到初始液化。     

水泥基灌浆材料塑性生膨胀率测式方法的比较与分析

介绍国际上常用的4种水泥基灌浆料塑性膨胀率的测试方法:ASTM C827非接触式测量法、PTGS量筒法、GB/T 50448-2008架百分表法及橡胶袋法,并对测试方法、测试结果及其相关性进行对比分析.结果表明,非接触式测量法和橡胶袋法能够准确、全面地反映出浆体塑性阶段的体积变化;架百分表法无法反映出浆体入模后1h内的体积变化;量筒法难以定量评定,且易受观察者主观性的影响.非接触式测量法、架百分表法及橡胶袋法相关性很高,各因素间存在很好的相关性.     

Study on dynamic response of large and small aspect ratio isolated buildings

An experimental study of a large aspect structure, 1:4 scale five‐storey steel frame, is presented. Seismic responses of the test model to the unilateral and bilateral excitations are compared. The effects of aspect ratio on the maximum acceleration, the storey drift of the superstructure, the maximum displacement and the vertical load of the isolation system are investigated. A comparison of the large aspect ratio isolated building with the small aspect ratio isolated building showed significant difference of the effects of aspect ratio subjected to disparate earthquake ground motions. Experimental results reveal that the aspect ratio is an important factor influencing the axial load action on isolators and the tension stress of the lead‐rubber bearings. The superstructure flexibility of the large aspect ratio building‐isolation system and the effects of the axial force variation of the lead‐rubber bearings should be carefully considered for design. Copyright © 2013 John Wiley & Sons, Ltd.     

负温对土动剪切模量阻尼比的影响规律

 以我国引进的首台低温共振柱仪为依托,研究不同负温下冻结土的初始剪切模量、剪切模量比和阻尼比非线性曲线的变化规律和模式。对低温共振柱仪试验技术进行研究,结果显示,采用常温试验方法进行低温共振柱试验,会导致结果离散过大,通过试验确定围压介质、冻结时间等试验条件和参数。同时,采用更符合客观实际的围压、固结和冻结方式,以粉土为样本,在室温和不同负温下进行对比试验。结果表明：负温对土的初始模量和剪切模量比影响显著,0 ℃～－4.0 ℃为敏感段,－4.0 ℃后为平稳段;敏感段内,随负温增加,初始剪切模量呈Boltzmann函数形式急速增加,参考剪应变则呈Boltzmann函数形式迅速下降,平稳段上,二者随负温增加分别平稳上升和下降;负温下阻尼比曲线在剪应变小于一定值时高于常温曲线,而在大于该应变时常温试验曲线则超过负温曲线;负温对最大阻尼比有较大影响,随负温增大,最大阻尼比呈指数形式降低。该试验采用专门的低温共振柱仪完成,围压、固结和冻结条件更接近客观实际,得到的负温对土动剪切模量阻尼比的影响模式更真实可靠,可为季冻土区抗震规范中设计谱修正以及季冻土区重大工程抗震设计中获得合理地震动输入提供依据。     

Model test study on the dynamic response of the portal section of two parallel tunnels in a seismically active area

A model test of the portals of two parallel tunnels is carried out to learn about the dynamic response of tunnel liner and the interaction between surrounding rock and liner in earthquakes. The experiment results show that: first, when the seismic acceleration traverses the model material, the low-frequency segment of seismic acceleration is magnified and the high-frequency segment of seismic acceleration is attenuated; second, the horizontal shear failure of the surrounding rock is caused by the interaction between the surrounding rock and the tunnel liner, and the cracks in the surrounding rock grow nearly in the same direction, however, because of the different constraints on the tunnel liner by the surrounding rock outside the tunnel, the destruction degree is different; third, the liner cracks of the left tunnel with short length appear mainly at the left tunnel entrance, the cracks of right tunnel with large length appear mainly at the right tunnel entrance and the tunnel cross-section nearly which is in the same vertical plane with the left tunnel portal, and the liner cracks are distributed mainly on the closer side of two liners between the two holes; finally, in the same vertical testing cross-section, the liner maximal strain at the inner sides between two tunnels is greater than outer sides. In addition, the cross-section maximal strain on the right tunnel decreases with the increasing distance between the tested cross-section and a reference vertical plane containing the left tunnel portal.