Pore size distribution of hybrid nonwoven geotextiles

Pore size distribution has become a prerequisite in determining the performance of geotextiles for various functions including filtration, separation and reinforcement. The pore structure and morphology in a nonwoven geotextile are known to be complex and it becomes further complicated in hybrid nonwoven geotextiles consisting of two types of fibers. In this study, a modified model of pore size distribution of hybrid nonwoven geotextiles has been proposed based on sieving-percolation pore network theory. A comparison has been made between theoretical and experimental pore size distributions of hybrid needlepunched nonwoven geotextiles consisting of predefined weight proportions of viscose and polyester fibers. The weight proportions of the constituent fibers have been theoretically analysed for obtaining the desired pore size distributions of hybrid nonwoven geotextiles.     

Stabilisation of soil using hybrid needlepunched nonwoven geotextiles

In the past, natural and synthetic fibre based geotextiles have been used for short- and long-term applications of soil erosion. It is well known that these geotextiles complement each other in terms of various physical and mechanical properties. In this study, an attempt has been made to study various properties of hybrid geotextiles. These hybrid geotextiles have been produced from the blend of polypropylene/viscose and polyester/viscose fibres in defined weight proportions (0%, 20%, 40%, 60%, 80% and 100%). Subsequently, a comparison has been made between various physical and mechanical properties of needlepunched nonwoven geotextiles. In this research work, it was found that hybrid geotextiles made of viscose (up to 40 wt.%) can replace 100% polypropylene or polyester based geotextiles.     

Microstructural investigation on mechanical behavior of soil-geosynthetic interface in direct shear test

Interface shear strength between soil and geosynthetics mainly depends on the mechanical and physical properties of soil, geosynthetics and the normal stress acting at the interface. This paper presents results of an extensive experimental investigation carried out on sand-geosynthetic interface using modified large direct shear box. The study focusses on the shearing mechanism at the sand-geosynthetic interface and the effect of different parameters on the shearing mechanism. Smooth HDPE geomembrane, nonwoven needle punched geotextile and two types of sand having different mean particle size, have been used in the present study. Microstructural investigation of deformed specimen through Field Emission Scanning Electron Microscope (FESEM) reveals the shearing mechanism which includes interlocking and fiber stretching for sand-geotextile while sliding, indentation and plowing for sand-geomembrane interface. The shearing mechanism for sand-geomembrane interface highly depends on the normal stress and degree of saturation of sand. The critical normal stress that demarcates the sliding and plowing mechanism for sand-geomembrane interface is different for dry and wet sand. The amount of scouring (or plowing) of the geomembrane surface reduces with increase in the mean particle size of sand. FESEM images revealed that the sand particles get adhered to the geotextile fibers for tests involving wet sands. The present microstructural study aided in understanding the shearing mechanism at sand-geosynthetic interface to a large extent.     

Large scale direct shear tests of soil/PET-yarn geogrid interfaces

The interface shear strength of soil against geosynthetic is of great interest among the researchers in geosynthetic properties. This study conducts a series of large scale direct shear tests to investigate the interface shear strength of different soils (sand, gravel, and laterite) against PET-yarn geogrids of various tensile strengths, percent open area, and aperture patterns. First, the appropriateness of different set-ups of a lower shearing box is examined in this study. It reveals that a lower box which is filled with the test soil and is of the same size as the upper box is more suitable for testing the soil/geogrid interface. The test results show that the soil/PET-yarn geotextile interface has significantly lower shear strength than soil strength. The ratio of shear strength soil/PET-yarn geotextile interface to internal shear strength of soil is about 0.7–0.8 for Ottawa sand and for laterite, and it is about 0.85–0.95 for gravel. On the other hand, the soil/geogrid interface has higher shear strength. The ratio of shear strength soil/PET-yarn geogrid interface to internal shear strength of soil is about 0.9–1.05. It is found that the shear strength ratio of soil/PET-yarn geogrid interface is positively correlated to the transverse tensile strength of the PET-yarn geogrid. However, it is negatively correlated with the aperture length and percent open area of the PET-yarn geogrid. The interface shear test results of PET-yarn geogrid against different soils are compared with the test results predicted by a classical model for analyzing the applicability of the classical model. Further, a simple model is proposed herein to estimate the bearing resistance provided by the transverse ribs of geogrid. It shows this component to be about 0–15% when PET-yarn geogrid is against Ottawa sand or laterite, while it is smaller when the PET-yarn geogrid is against gravel.     

直剪试验与三轴试验的对比探讨

介绍了直剪试验与三轴试验的仪器设备及方法，通过对两种不同试验方法及结果的对比，指出三轴试验结果更接近土的实际理论值，数据更安全可靠，为岩土工程师使用抗剪强度指标提供了依据。     

无纺织物单向拉伸孔径变化试验与理论预测对比分析

无纺织物作为反滤材料,常处于单向受拉工作状态。单向拉伸引起无纺织物孔径变化,易导致反滤工程失效。通过控制织物应变的干筛试验,定量测试了无侧限单向拉应变逐级增大的过程中,两种不同厚度短纤针刺无纺织物的孔径分布曲线变化。采用干筛试验结果,对现有两种体系的单向应变下无纺织物孔径预测理论解进行验证:一类是佘巍等效孔径O95理论解,一类是Rawal孔径分布曲线理论解。通过对比两种理论解对各级拉应变下的O95值预测,归纳二者的预测误差规律,从理论假设出发分析误差原因。同时采用前人图像法测得的热粘无纺织物O95变化验证两类理论解。两种理论解均能较好地预测无侧限单向拉应变下无纺织物O95的变化规律。O95随单向拉应变呈近似线性减小的规律。对于O95变化斜率的预测,佘巍解较准确,Rawal解偏大。对于O95数值预测可结合两类理论解给出变化范围。     

利用弯曲元测试砂土剪切模量的国际平行试验

首先介绍了弯曲元技术的基本原理和应用研究情况,然后详细报道了浙江大学岩土工程研究所参与此次国际平行试验的试验内容和初步分析结果。试验对象为饱和Toyoura砂,内容为排水条件下的多级加荷三轴(固结比Kc=1,2)和固结仪K0试验,在每级荷载下利用弯曲元测试土样的剪切波速并计算剪切模量Gmax。本文试验结果与弯曲元技术应用方法为TC-29的最终分析提供了有效的试验数据和参考依据。     

Relation between the critical state friction angle of sands and low vertical stresses in the direct shear test

In this study, the critical state friction angle in a direct shear box was tested at vertical effective stresses between 4?kPa and 150?kPa for seven sands of different origins. The results show a stress dependency on the angle of friction at low vertical stresses, and that the critical state friction angle decreases with increase in the vertical effective stress. The largest drop occurs for vertical effective stresses between 4?kPa and 50?kPa and varies between 4° and 10° depending mostly on the shape of the sand particles and the difference in the extreme void ratios.     

Effect of geotextile ageing and geomembrane surface roughness on the geomembrane-geotextile interfaces for heap leaching applications

A series of large scale direct shear experiments is used to investigate the effect of the geomembrane (GMB) surface roughness, geotextile (GTX) properties, and GTX ageing, on the GMB-GTX interface shear behaviour. Interfaces involving smooth, coextruded textured, and structured surface GMBs underlying four different nonwoven needle-punched staple fibres (GTXs) with mass per unit areas between 200 and 2400 g/m², and a geocomposite drain (GCD) are examined at normal stresses between 250 and 1000 kPa. The results showed that the interlocking between the GMB and GTX increased with increasing the GMB asperity height and/or decreasing the mass per unit area of the GTX. For the interfaces that involved GTXs preaged prior to the shear box experiments for up to 2 years at 85 °C, it was found that the 2400 g/m² heat bonded two-layered GTX exhibited internal shear failure at low shear displacements. However, all the highly aged single layered GTXs showed an increase in the peak interface friction angles with the increase in their ageing. For these single layered GTX, the results suggest that assessing the interface friction angles using unaged GTXs for the stability analysis is conservative as long as the GTX remains intact in the field.     

Effects of water saturation and loading rate on direct shear tests of andesite

For estimating the long-term stability of underground framework, it is vital to learn the mechanical and rheological characteristics of rock in multiple water saturation conditions. However, the majority of previous studies explored the rheological properties of rock in air-dried and water saturated conditions, as well as the water effects on compressive and tensile strengths. In this study, andesite was subjected to direct shear tests under five water saturation conditions, which were controlled by varying the wetting and drying time. The tests were conducted at alternating displacement rates under three vertical stresses. The results reveal that the shear strength decreases exponentially as water saturation increases, and that the increase in shear strength with a tenfold increase in displacement rate is nearly constant for each of the vertical stresses. Based on the findings of the shear tests in this study and the compression and tension tests in previous studies, the influences of both water saturation and loading rate on the Hoek-Brown failure criterion for the andesite was examined. These results indicate that the brittleness index of the andesite, which is defined as the ratio of uniaxial compressive strength to tensile strength, is independent of both water saturation and loading rate and that the influences of the water saturation dependence and the loading rate dependence of the failure criterion can be converted between each other.     

Design, operation and validation of a new fluid-sealed direct shear apparatus capable of monitoring fault-related fluid flow to large displacements

A new type of direct shear apparatus has been developed to allow for deformation of large and intact rock samples under fluid-sealed conditions. The sealed cell was specifically designed to monitor changes to fluid flow across the evolving rupture surface to large displacements (≤120 mm), and effective stress conditions up to 36 MPa. To maintain a seal for the pore fluid during deformation, a viscous and immiscible sealant fluid was injected at the interface between the rock specimen and the inner sample cell. Finite element modelling was conducted in parallel to verify the stress distribution in the specimen, notably the effect of geometrical constraints on the fault zone development and mechanical data. Results obtained from natural Castlegate sandstone, synthetically cemented sandstone and sandstone–clay composite blocks demonstrate the suitability of this equipment to investigating coupled fluid mechanical behaviour of a wide range of rock specimens. In particular, the test configuration is demonstrated to be capable of exploring fault-related fluid flow behaviour in reservoir–seal pair analogues with application for the hydrocarbon industry and the emerging field of carbon dioxide storage assessment.     

土工合成材料大型直剪界面作用宏细观研究

利用大型直剪模型试验设备,在不同竖向压力下进行一系列的土工合成材料直剪试验,应用数码可视化跟踪技术,结合土体变形无标点量测技术来研究双向土工格栅与砂土直剪界面作用的宏细观特性,同时分析界面附近土压力分布规律,并研究界面颗粒运动变化规律和细观组构演化特征与宏观特性的关联。分析结果表明,直剪筋土界面附近竖向压力分布从前端依次向后端减少;直剪界面位移达25 mm时,形成了稳定的剪应变集中带;在筋土界面（6～8）D₅₀粒径厚度范围内,界面颗粒以旋转和平动方式同时位移,该范围外颗粒以平动方式沿剪切方向位移,且位移较小;在剪切过程中,界面颗粒发生旋转,土体发生剪胀,孔隙率增大,平均接触数减小,颗粒重新被压密,孔隙率减小,平均接触数增多,颗粒长轴排列趋于水平方向,各细观组构处于相对稳定状态。     

立面开大洞短肢剪力墙-筒体结构的动力试验研究及有限元计算分析

高层住宅结构为满足建筑功能多样化、美观等要求,出现了底部大空间、立面大洞口等特殊效果的短肢剪力墙-筒体结构形式,对该复杂结构体系整体抗震性能的评估,需进行专门的试验研究和计算分析。本文针对具有上述结构特点的某高层结构整体模型,进行了模拟地震振动台试验和三维有限元分析,得到了结构的动力特性,以及在7度多遇、基本、罕遇烈度地震作用下的加速度反应、惯性力分布和位移反应等。结果表明,在地震作用下该结构底部墙肢中仅少数构件出现反弯点甚至不出现反弯点;除结构出屋面部分鞭梢效应明显外,其余部分均可以满足现行规范的抗震设防要求;工程设计中宜改善端部筒体以及转换大梁与筒体连接处的强度和延性。     

Interface shear strength between geosynthetic clay liner and covering soil on the embankment of an irrigation pond and stability evaluation of its widened sections

Geosynthetic clay liners (GCLs) are typically used for widening sections of an embankment. They are also used as low permeability liners to minimize water leakage from reservoirs such as irrigation ponds. However, few investigations have been carried out on the specific properties of GCLs, such as granulated bentonite sandwiched between geotextiles, their internal shear strength, and the shear strength at the interface between a GCL and an embankment body. In this study, a series of direct box shear tests were performed to determine the shear strength properties of bentonite and compacted soils as well as at the interface between a GCL and bentonite or compacted soil. In addition, a series of field-loading tests were conducted to investigate the failure behaviour of an embankment body containing a GCL when changes in the water content of the bentonite of the GCL in a real embankment occur. Furthermore, the stability of widened embankment bodies that incorporated GCLs were evaluated. The main conclusions of this study are as follows: (1) The shear strength of the interface between the covering soil and geotextiles varied according to the soil type, geotextile type, and the submergence period, (2) the maximum safety factor was observed at the interface between decomposed granite soil and the geotextiles, while the minimum safety factor was observed at the interface between the bentonite and the geotextiles, and (3) the influence of GCLs on the instability of a widened embankment was extremely small.     

Study on the mesostructure and mesomechanical characteristics of the soil–rock mixture using digital image processing based finite element method

The soil–rock mixture (SRM) is a kind of inhomogeneous geomaterial, which poses difficulties of in situ sample acquisition and in laboratory geomaterial tests; hence, the study of the SRM's mechanical properties is still at an early stage. In this paper, the technique of digital image processing based on the finite element method (DIP-FEM) is introduced to study SRMs in the Leaping Tiger Gorge Reservoir Area, China. Based on the DIP, the mesostructural characteristics of the SRM are analyzed statistically. The mesostructural concept model of SRM that can actually represent the inhomogeneity of SRM is built. By using geometry vectorizaiton transformation, the mesostructural model of SRM in the binary image format has been translated into a vector format (such as DWG or DXF format) which can be imported into the finite element software. By using the finite element method, two large-scale direct shear tests of inhomogeneous SRM and homogeneous soil are simulated. The numerical results indicate that the existence of “rock” blocks in SRM will greatly influence the distribution and the failure models of the internal stress field. As a result, three kinds of failure models of the SRM are put forward.