A laboratory evaluation of reinforcement loads induced by rainfall infiltration in geosynthetic mechanically stabilized earth walls

A laboratory testing that simulates the mechanisms of a geosynthetic-reinforced layer was used to assess the impact of rainwater infiltration on reinforcement loads and strains in mechanically stabilized earth (MSE) walls. The testing device allows measuring loads transferred from a backfill soil subjected simultaneously to surcharge loading and controlled irrigation. Load-strain responses of geosynthetic-reinforced layers constructed with three different geosynthetics under a moderate rainfall are related to suction captured along the depth of reinforced layers. Results show infiltration leading to increases on strains and tensile loads mobilized by reinforcements. Rates of increases of both parameters were found to be dependent of global suction, geosynthetic stiffness and hydraulic properties. In addition, increases in water content at soil-geotextile interfaces due to capillary breaks also had a significant effect on mobilized loads. The loss of interaction due to the interface wetting was observed to affect the stress transference from soil to geosynthetic reinforcement. An approach suggested for calculation of lateral earth pressures in unsaturated GMSE walls under working stress conditions and subjected to rainfall infiltration demonstrated a reasonable agreement with experimental data.     

A new apparatus for the study of pullout behaviour of soil-geosynthetic interfaces under sustained load over time

At present, the design of geosynthetic-reinforced soil structures is executed with reference to the tensile strength of the reinforcement obtained from in-air short-term tensile tests, decreasing this value by means of several factors. Among these, the creep effect resulting from in-air tensile creep tests reduces tensile strength the most. Consequently, this procedure does not take into account the effects of soil confinement and interaction on the tensile response of the reinforcements. This paper illustrates a new large-scale pullout prototype apparatus, with the capacity to investigate the behaviour of a geosynthetic reinforcement embedded in a compacted soil and subject to a tensile load kept constant over time. The apparatus allows the verification of how the soil can modify the prediction of the long-term behaviour of geosynthetics. Results in terms of confined tensile strains were analysed, and the comparison of those values with the strains obtained by in-air tensile creep tests has led to the conclusion that the creep reduction factor might be conservative. Moreover, the confined tensile strains were related to the apparent coefficients of friction to propose a new procedure capable of determining the design interaction parameter under long-term pullout load as a function of the allowable reinforcement strains.     

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.     

Long-term behavior of GRS retaining walls with marginal backfill soils

The creep of geosynthetics leads to the increase of Geosynthetic-Reinforced Soil (GRS) wall's deformation. More importantly, the influences of creep of geosynthetics are also affected by the creep properties of soils. In this paper, a Finite Element procedure was validated against a model test on the creep response of a clay–geotextile composite. An extensive parametric study was then carried out to investigate the long-term response of 8-meter-high model GRS walls with marginal backfill soils. The influences of backfill creep, reinforcement creep, reinforcement stiffness, reinforcement length and reinforcement spacing were analyzed. A long-term analysis was conducted for 5 years and the results at the end of construction (EOC) and 5 years afterwards were compared. It is found from the analysis that the relative creep rate between geosynthetic reinforcement and backfill soil influenced not only wall deformation but also reinforcement loads and stress states in the soils. The load distribution in backfill soil and reinforcement is the result of battling between their time-dependent properties. Large reinforcement creep can lead to large post-construction deformation and increase in soil stress; on the other hand, large soil creep can induce a significant increase in reinforcement load. It is hence necessary to take into account the relative creep rate of reinforcement and backfill soil in the design of GRS walls. It may not be adequate to consider only the long-term strength of reinforcement, which is the state-of-the-practice at present.     

土工合成材料与土界面作用特性的研究

土工合成材料加筋土工程中 ,土工合成材料与填料的界面作用特性是最关键的技术指标 ,因此利用直剪试验和拉拔试验研究土工合成材料与填料的界面作用特性是非常必要的。本文以 5种不同种类的国产土工合成材料为加筋材料 ,以砂和石灰粉煤灰为填料 ,通过直剪试验和拉拔试验比较各种国产土工合成材料与填料的界面作用特性 ,得到一些有益的结论 ,可指导土工合成材料的优选和研究加筋机理。     

Soil–geosynthetic interaction: Modelling and analysis

Interaction between soils and geosynthetics is of utmost importance in applications of these materials as reinforcement in geotechnical engineering. That is also the case for some applications of geosynthetics in environmental protection works. The mechanisms of soil–geosynthetic interaction can be very complex, depending on the type and properties of the geosynthetic and the soil. This paper presents and discusses some experimental, theoretical and numerical methods for the study and evaluation of interaction between soils and geosynthetics, with particular reference to the applications of these materials in soil reinforcement. The main advantages and limitations of some traditional experimental and theoretical methods for the study of soil–geosynthetics interaction are presented and new applications of these methods are addressed. The need for improvements in experimental and theoretical techniques for a better understanding of soil–geosynthetic interaction is highlighted.     

Shaking table tests on geosynthetic encased columns in soft clay

This paper presents the results of an experimental research on the behavior of geosynthetic encased stone columns and ordinary stone columns embedded in soft clay under dynamic base shaking. For this purpose, a novel laminar box is designed and developed to run a total of eight sets of 1-G shaking table tests on four different model soil profiles: Soft clay bed, ordinary stone column installed clay bed, and clay beds with geosynthetic encased columns with two different reinforcement stiffnesses. The geosynthetic encased columns are heavily instrumented with strain rosettes to quantify the reinforcement strains developing under the action of dynamic loads. The responses of the columns are studied through the deformation modes of the encased columns and the magnitude and distribution of reinforcement strains under dynamic loading. The response of the granular inclusion enhanced soft subsoil and embankment soil and the identification of the dynamic soil properties of the entire soil body are also discussed in this article. Finally, to determine the effect of dynamic loading on the vertical load carrying capacity, stress-controlled column load tests are undertaken both on seismically loaded and undisturbed columns.     

土工合成材料加筋砂土三轴试验研究

本文以 5种国产土工合成材料为加筋材料 ,它们分别是针刺无纺土工织物、涤纶纤维经编土工格栅、玻璃纤维土工格栅、聚丙烯双向土工格栅和聚乙烯土工网 ,用三轴试验比较各种土工合成材料对砂土的加筋效果 ,得到一些有益的结论 ,可指导土工合成材料的优选和研究加筋机理 ,同时指出部分国产土工合成材料产品的不足。     

An experimental evaluation of the behavior of footings on geosynthetic-reinforced sand

This research was performed to investigate the behavior of geosynthetic-reinforced sandy soil foundations and to study the effect of different parameters contributing to their performance using laboratory model tests. The parameters investigated in this study included top layer spacing, number of reinforcement layers, vertical spacing between layers, tensile modulus and type of geosynthetic reinforcement, embedment depth, and shape of footing. The effect of geosynthetic reinforcement on the vertical stress distribution in the sand and the strain distribution along the reinforcement were also investigated. The test results demonstrated the potential benefit of using geosynthetic-reinforced sand foundations. The test results also showed that the reinforcement configuration/layout has a very significant effect on the behavior of reinforced sand foundation. With two or more layers of reinforcement, the settlement can be reduced by 20% at all footing pressure levels. Sand reinforced by the composite of geogrid and geotextile performed better than those reinforced by geogrid or geotextile alone. The inclusion of reinforcement can redistribute the applied footing load to a more uniform pattern, hence reducing the stress concentration, which will result reduced settlement. Finally, the results of model tests were compared with the analytical solution developed by the authors in previous studies; and the analytical solution gave a good predication of the experimental results of footing on geosynthetic reinforced sand.     

新型可视土工拉拔试验仪的研发与应用

研制了一台新型可视自动采集数据的土工拉拔试验装置,可用于多种土工材料和填料作用下的拉拔试验。该装置改进了加载系统和反力系统,实现了拉拔界面的可视与数据采集的自动化,并可量测土工材料不同嵌固长度处的位移,获取土工材料变形值,探索筋土作用过程中筋材受力机理及界面土体位移变化规律。使用新研制的试验装置开展了以砾类粗粒土为填料的格栅拉拔试验,结果表明:上覆荷载增大,土中格栅的应变变小,土体与格栅的界面摩擦和嵌固作用越显著;筋土界面处土体颗粒存在平移及转动两种运动模式,且界面处土体形成稳定的位移集中带。     

Evaluation of disturbance function for geosynthetic–soil interface considering chemical reactions based on cyclic direct shear tests

Various geosynthetics for reinforcement, protection and encapsulation are widely applied to civil structures and waste landfill sites. The use of geosynthetics inevitably involves the coupled behaviors of different materials which include large displacement and strain-softening behaviors, etc. Current research indicates that the behavior of geosynthetic–soil systems depend on the shear strength of the interface governed by several intrinsic and environmental factors, such as moisture, normal stress, chemical conditions, and thermal components, etc.In this study, the effects of acidity and basicity from leachate and waste are intensively considered in order to build up the chemical reaction mechanism of the shear strength of the interface under cyclic loading based on an experimental inspection. The Multi-Purpose Interface Apparatus (M-PIA) has been newly manufactured, and cyclic direct shear tests for submerged geosynthetic–soil specimens under different chemical conditions have been performed. A Focused Ion Beam (FIB) analysis has also been performed to induce the reason for the variation in the disturbance function and to verify the hypothesis on the decay-proof ability of geosynthetics.Consequently, a new approach to reflect the chemical effect of geosynthetics has been applied by suggesting the use of new disturbance function parameters in the Disturbed State Concept. The basic schematic of the Disturbed State Concept (DSC) constitutive model is employed; then, new disturbance function parameters are proposed to describe the chemical degradation of the geosynthetic–soil interface under dynamic conditions. Furthermore, based on the FIB results, it is be deduced that the variation in the disturbance function mainly results from the different types of decay in the soil minerals.     

土工合成材料加筋的试验研究

以 6种不同种类的国产土工合成材料为加筋材料 ,即针刺无纺土工织物、涤纶纤维经编土工格栅、玻璃纤维土工格栅、单向塑料拉伸土工格栅、双向塑料拉伸土工格栅和土工网 ,进行三轴试验比较各种土工合成材料对砂的加筋效果 ;进行直剪试验和拉拔试验比较各种土工合成材料与填料 (砂和石灰粉煤灰 )的界面作用特性 ,得到一些有益的结论 ,可指导土工合成材料的优选和研究土工合成材料的加筋机理。     

Geotextile filtration opening size under tension and confinement

Nonwoven geotextiles have been used as filters in geotechnical and geoenvironmental works for half a century. They are easy to install and can be specified to meet the requirements for proper filter performance. There are situations where a geotextile filter may be subjected to tensile loads, which may alter relevant filter properties, such as its filtration opening size. Examples of such situations are silty fence applications, geotextile separators, geotextile tubes and geotextiles under embankments on soft soils. This paper investigates the effects of tensile strains on geotextile pore dimensions. A special equipment and testing technique allowed tests to be carried out on geotextile specimens subjected to tension and confinement. The results obtained showed that the variation in filtration opening size depends on the type of strain state the geotextile is subjected, under which the geotextile pore diameter may remain rather constant or increase significantly. However, confinement reduces the geotextile filtration opening size independent on the strain mobilised. An upper bound for the filtration opening size of strained nonwoven geotextiles is introduced and was satisfactory for the geotextile products tested.     

土工合成材料用于道路防裂的模式探讨

通过对聚酯长丝土工布和玻纤格栅两种土工合成材料的试验,说明在受力条件下,土工布和沥青混凝土基本是同步变形,可以共同起作用。所以土工布不仅仅是隔离,也起到加筋的作用。再经薄面强基+土工合成材料结构模式的可行性分析和实践,说明在防治反射裂缝作用方面,土工布比玻纤格栅的效果会更好一些。     

Experimental study on the load bearing behavior of geosynthetic reinforced soil bridge abutments with different facing conditions

This paper presents an experimental study on reduced-scale model tests of geosynthetic reinforced soil (GRS) bridge abutments with modular block facing, full-height panel facing, and geosynthetic wrapped facing to investigate the influence of facing conditions on the load bearing behavior. The GRS abutment models were constructed using sand backfill and geogrid reinforcement. Test results indicate that footing settlements and facing displacements under the same applied vertical stress generally increase from full-height panel facing abutment, to modular block facing abutment, to geosynthetic wrapped facing abutment. Measured incremental vertical and lateral soil stresses for the two GRS abutments with flexible facing are generally similar, while the GRS abutment with rigid facing has larger stresses. For the GRS abutments with flexible facing, maximum reinforcement tensile strain in each layer typically occurs under the footing for the upper reinforcement layers and near the facing connections for the lower layers. For the full-height panel facing abutment, maximum reinforcement tensile strains generally occur near the facing connections.     

Shear strength behavior of geotextile/geomembrane interfaces

This paper aims to study the shear interaction mechanism of one of the critical geosynthetic interfaces,the geotextile/geomembrane,typically used for lined containment facilities such as landfills.A large direct shear machine is used to carry out 90 geosynthetic interface tests.The test results show a strain softening behavior with a very small dilatancy(0.5 mm) and nonlinear failure envelopes at a normal stress range of 25-450 kPa.The influences of the micro-level structure of these geosynthetics on the macro-level interface shear behavior are discussed in detail.This study has generated several practical recommendations to help professionals to choose what materials are more adequate.From the three geotextiles tested,the thermally bonded monofilament exhibits the best interface shear strength under high normal stress.For low normal stress,however,needle-punched monofilaments are recommended.For the regular textured geomembranes tested,the space between the asperities is an important factor.The closer these asperities are,the better the result achieves.For the irregular textured geomembranes tested,the nonwoven geotextiles made of monofilaments produce the largest interface shear strength.     

Clay—Geotextile interaction in large retaining wall models

Large geotextile reinforced clay wall models were built to investigate the mechanism of clay—geotextile interaction and the effects of the geotextile reinforcement on the load-bearing capacity of the clay. A silty clay soil (CL) with an undrained strength of 25 kPa was used as backfill and a low-cost, non-woven, needle-punched geotextile as the reinforcement. No face panels were used. The wrapped back geotextile reinforcement provided the face of the wall. The wall models were tested under uniformly distributed and discrete strip loads. Vertical and horizontal displacements as well as geotextile strains were monitored. The load-bearing capacity of the clay was increased nearly two times with the geotextile reinforcement. For the interpretation of the test results total stress analysis was carried out on the active failure plane taking into account the tensile forces acting in the geotextiles reinforcing layers intersecting the failure plane. Good agreement was found between the measured and the calculated failure loads. The results of the testing programme are promising and encourage further research into the applicability of cohesive soils in geotextile-reinforced soil structures which might have great economic significance in areas where good-quality granular backfill is not readily available.     

基础局部沉降下土工格栅加筋衬垫系统变形特性的试验研究

基础局部沉降会引起垃圾填埋场衬垫系统中的土工膜产生较大的拉应变,有可能导致衬垫系统性能下降,因此正确评价衬垫系统的应变就显得非常重要。通过模拟试验,采用应变片和位移计对基础发生局部沉降后土工格栅加筋衬垫系统的变形进行试验研究。试验结果表明:环境温度对衬垫系统的变形影响较大;相同组成材料下土工格栅和土工膜叠放在一起比其他方案更能降低土工膜的应变;衬垫系统刚度对沉陷范围影响不大,但对最大应变值影响较大。所得结果对垃圾填埋场衬垫系统的设计具有一定的指导意义。     

Experimental studies of the geosynthetic anchorage – Effect of geometric parameters and efficiency of anchorages

The soil reinforcement by geosynthetic is widely used in civil engineering structures: embankments on compressible soil, slope on stable foundations, embankments on cavities and retaining structures. The stability of these structures specially depends on the efficiency of the anchors holding the geosynthetic sheets. Simple run-out and wrap around anchorages are two most commonly used approaches. In order to improve the available knowledge of the anchorage system behaviour, experimental studies were carried out. This paper focuses on a three-dimensional physical modelling of the geosynthetics behaviour for two types of anchors (simple run-out and wrap around). The pull-out tests were performed with an anchorage bench under laboratory controlled conditions with three types of geosynthetic (two geotextiles and one geogrid) and in the presence of two types of soil (gravel and sand).The results show that there is an optimum length for the upper part of the geosynthetic for the wrap around anchorage.     

Investigation of geotextile–soil interaction under a cyclic vertical load using the discrete element method

Geotextiles are often used in roadway construction as separation, filtration, and reinforcement. Their performance as reinforcement in geotextile-reinforced bases depends on geotextile–soil interaction. This paper investigates the geotextile–soil interaction under a cyclic wheel load using the Discrete Element Method (DEM). In this study, soil was modeled as unbonded particles using the linear contact stiffness model, and the geotextile was modeled as bonded particles. The micro-parameters of the soil and the geotextile were determined using biaxial tests and a tensile test, respectively. The influence of the placement depth and the stiffness of the geotextile on the performance of the reinforced base was investigated. The DEM results show that the depth of the geotextile significantly affected the degree of interaction between the geotextile and the soil. Under the applied cyclic vertical load, the geotextile developed a low tensile strain. The effect of the stiffness of the geotextile on the deformation was more significant when the geotextile was placed at a shallower location than when placed at a deeper location.