Analytical model for predicting time-dependent lateral deformation of geosynthetics-reinforced soil walls with modular block facing

To date,few models are available in the literature to consider the creep behavior of geosynthetics when predicting the lateral deformation(d)of geosynthetics-reinforced soil(GRS)retaining walls.In this study,a general hyperbolic creep model was first introduced to describe the long-term deformation of geosynthetics,which is a function of elapsed time and two empirical parameters a and b.The conventional creep tests with three different tensile loads(Pr)were conducted on two uniaxial geogrids to determine their creep behavior,as well as the a-Pr and b-Pr relationships.The test results show that increasing Pr accelerates the development of creep deformation for both geogrids.Meanwhile,a and b respectively show exponential and negatively linear relationships with Pr,which were confirmed by abundant experimental data available in other studies.Based on the above creep model and relationships,an accurate and reliable analytical model was then proposed for predicting the time-dependent d of GRS walls with modular block facing,which was further validated using a relevant numerical investigation from the previous literature.Performance evaluation and comparison of the proposed model with six available prediction models were performed.Then a parametric study was carried out to evaluate the effects of wall height,vertical spacing of geogrids,unit weight and internal friction angle of backfills,and factor of safety against pullout on d at the end of construction and 5 years afterwards.The findings show that the creep effect not only promotes d but also raises the elevation of the maximum d along the wall height.Finally,the limitations and application prospects of the proposed model were discussed and analyzed.

Novel experimental techniques to assess the time-dependent deformations of geosynthetics under soil confinement

A new experimental approach to assess the impact of soil confinement on the long-term behavior of geosynthetics is presented in this paper.The experimental technique described herein includes a novel laboratory apparatus and the use of different types of tests that allow generation of experimental data suitable for evaluation of the time-dependent behavior of geosynthetics under soil confinement.The soil-geosynthetic interaction equipment involves a rigid box capable of accommodating a cubic soil mass under plane strain conditions.A geosynthetic specimen placed horizontally at the mid-height of the soil mass is subjected to sustained vertical pressures that,in turn,induce reinforcement axial loads applied from the soil to the geosynthetic.Unlike previously reported studies on geosynthetic behavior under soil confinement,the equipment was found to be particularly versatile.With minor setup modifications,not only interaction tests but also in-isolation geosynthetic stress relaxation tests and soil-only tests under a constant strain rate can be conducted using the same device.Also,the time histories of the reinforcement loads and corresponding strains are generated throughout the test.Results from typical tests conducted using sand and a polypropylene woven geotextile are presented to illustrate the proposed experimental approach.The testing procedure was found to provide adequate measurements during tests,including good repeatability of test results.The soilegeosynthetic interaction tests were found to lead to increasing geotextile strains with time and decreasing reinforcement tension with time.The test results highlighted the importance of measuring not only the time history of displacements but also that of reinforcement loads during testing.The approach of using different types of tests to analyze the soilegeosynthetic interaction behavior is an innovation that provides relevant insight into the impact of soil confinement on the time-dependent deformations of geosynthetics.

Use of geosynthetics for performance enhancement of earth structures in cold regions

Earth structures, such as roadways, embankments and slopes, and earth retaining walls, have been commonly used in cold regions for transportation and other applications. In addition to typical design considerations for earth structures at normal temperature, a design must also consider the unique problems associated with low temperature, such as frost heave, lateral expansion, thaw settlement and weakening, and degradation of material properties. Geosynthetics have been used in cold regions to stabilize earth structures during construction and mitigate potential problems during their service at low tem- perature. This paper provides a state of practice review of the use of geosynthetics for performance enhancement of earth structures in cold regions. This paper starts with basic information on available geosynthetic products and their functions, evaluates properties and behavior of geosynthetics and soil-geosynthetic systems at low temperature, and discusses past studies and their key results on the use of geosynthetics to enhance the performance of roadways, embankments, and earth retaining walls in cold regions. This review reveals that geosynthetics at low temperature have higher tensile strength and stiffness, lower creep rate, and lower elongation at failure. The effect of temperature becomes significant when nonwoven geotextiles are subjected to moistening and soil intrusion at subfreezing temperature. Freeze-thaw cycles may degrade hydraulic and mechanical properties of geosynthetic-soil systems. The inclusion of geosynthetics in soil provides drainage and/or barrier to water flow, retains mechanical properties, and reduces frost heave during and after freeze-thaw cycles. Effectiveness of geosynthetics has been confirmed in the field in bridging over voids, stabilizing roadways over temper- ature-susceptible soils during thaw, and proving drainage and barrier to temperature-susceptible soils before freeze. To avoid frost heave and lateral expansion of backfill in earth retaining walls, granular fill without fines should be used. When backfill with fines is used for earth retaining walls, additional lateral earth pressure induced by soil freeze and thaw set- tlement should be considered in the design.

Geosynthetics used to stabilize vegetated surfaces for environmental sustainability in civil engineering

Geosynthetics, factory-manufactured polymer materials, have been successfully used to solve many geotechnical problems in civil engineering. Two common applications are earth stabilization and erosion control. Geosynthetics used for earth stabilization include but are not limited to stabilized slopes, walls, embankments, and roads. Geosynthetics used for erosion control are mostly related to slopes, river channels and banks, and pond spillways. To enhance environmental sustainability, vegetation has been increasingly planted on the facing or surfaces of these earth structures. Under such a condition, geosynthetics mainly function as surficial soil stabilization while vegetation provides green appearance and erosion protection of earth surfaces. Recently, geosynthetic or geosynthetic-like material has been used to form green walls outside or inside buildings to enhance sustainability. Geosynthetics and vegetation are often integrated to provide combined benefits. The interaction between geosynthetics and vegetation is important for the sustainability of the earth and building wall surfaces. This paper provides a review of the current practice and research in the geosynthetic stabilization of vegetated earth and building surfaces for environmental sustainability in civil engineering with the emphases on geosynthetic used for erosion protection, geosynthetic-stabilized slopes, geosynthetic-stabilized unpaved shoulders and parking lots, and geosynthetic-stabilized vegetated building surfaces.

The Efect of Rich Synthetic Copper-rich Solution on Antiseepage Dense Pre-hydration Geosynthetic Clay Liner

Dense pre-hydrated geosynthetic clay liners(DPH GCLs)were manufactured as innovative materials accompanied by the advantage of lower hydraulic conductivity(k).The k of DPH GCLs permeated with de-ionized water(DIW)was 9.8×10^(−12) m/s.The effect of Cu^(2+)synthetic solution on DPH GCLs was discussed.Furthermore,the effect mechanism was studied on the basis of test technologies.A significant adverse impact on hydraulic performance of DPH GCLs is found when the concentration of Cu^(2+)is greater than 1 g/L.SEM,XRD,XRF,FTIR,and XPS analyses show that the effect of Cu^(2+)on DPH GCLs includes two steps.Firstly,Cu^(2+)interacts with hydrophobic organic matter(HOM),and the adhesion of bentonite is destroyed,and some holes appear.The Cu^(2+)contacts with bentonite directly,and Cu^(2+)interacts with bentonite through ion exchange.Passivated phenomenon occurs on the surface of the bentonite,and swelling ability of bentonite is reduced,which causes permeable DPH GCLs.

Controlling strain in geosynthetic liner systems used in vertically expanded landfills

According to relevant new regulations in China,a composite liner system involving geosynthetic materials must be installed at the bottom of an expanded landfill.The deformation and integrity of the composite liner under a variety of factors are important issue to be considered in the design of a landfill expansion.In this paper,we investigate the strain distribution in geosynthetic materials within the composite liner system of expanded landfills,including strains in geosynthetic materials resulting from overall settlement and lateral movement of landfills,localized subsidence in landfills,and differential settlement around gas venting wells.The allowable strains of geosynthetic materials are discussed based on the results of tensile tests,and the corresponding design criteria for composite liner systems are proposed.Meanwhile,practical measures allowing strain control in geosynthetic materials used in landfill engineering are proposed.

Pseudo-static/dynamic solutions of required reinforcement force for steep slopes using discretization-based kinematic analysis

This paper presents a procedure for assessing the reinforcement force of geosynthetics required for maintaining dynamic stability of a steep soil slope. Such a procedure is achieved with the use of the discretization technique and kinematic analysis of plasticity theory, i.e. discretization-based kinematic analysis. The discretization technique allows discretization of the analyzed slope into various components and generation of a kinematically admissible failure mechanism based on an associated flow rule.Accordingly, variations in soil properties including soil cohesion, internal friction angle and unit weight are accounted for with ease, while the conventional kinematic analysis fails to consider the changes in soil properties. The spatialetemporal effects of dynamic accelerations represented by primary and shear seismic waves are considered using the pseudo-dynamic approach. In the presence of geosynthetic reinforcement, tensile failure is discussed providing that the geosynthetics are installed with sufficient length. Equating the total rates of work done by external forces to the internal rates of work yields the upper bound solution of required reinforcement force, below which slopes fail. The reinforcement force is sought by optimizing the objective function with regard to independent variables, and presented in a normalized form. Pseudo-static analysis is a special case and hence readily transformed from pseudodynamic analysis. Comparisons of the pseudo-static/dynamic solutions calculated in this study are highlighted. Although the pseudo-static approach yields a conservative solution, its ability to give a reasonable result is substantiated for steep slopes. In order to provide a more meaningful solution to a stability analysis, the pseudo-dynamic approach is recommended due to considerations of spatial etemporal effect of earthquake input.

Experimental study on repeatedly loaded foundation soil strengthened by wraparound geosynthetic reinforcement technique

In the recent past,the potential benefits of wraparound geosynthetic reinforcement technique for constructing the reinforced soil foundations have been reported.This paper presents the experimental study on the behaviour of model strip footing resting on sandy soil bed reinforced with geosynthetic in wraparound and planar forms under monotonic and repeated loadings.The geosynthetic layers were laid according to the reinforcement ratio to minimise the scale effect.It is found that for the same amount of reinforcement material,the wraparound reinforced model resulted in less settlement in comparison to planar reinforced models.The efficiency of wraparound reinforced model increased with the increase in load amplitude and the rate of total cumulative settlement substantially decreased with the increase in number of load cycles.The wraparound reinforced model has shown about 45% lower average total settlement in comparison to unreinforced model,while the double-layer reinforced model has about 41% lower average total settlement at the cost of approximately twice the material and 1.5 times the occupied land width ratio.Moreover,wraparound models have shown much greater stability in comparison to their counterpart models when subjected to incremental repeated loading.

Pullout behavior of polymeric strip in compacted dry granular soil under cyclic tensile load conditions

Assessment of the reinforcement behavior of soil under cyclic and monotonic loads is of great impor- tance in the safe design of mechanically stabilized earth walls, In this article, the method of conducting a multistage pullout （MSP） test on the polymeric strip （PS） is presented, The post-cyclic behavior of the reinforcement can be evaluated using a large-scale pullout apparatus adopting MSP test and one-stage pullout （OSP） test procedures, This research investigates the effects of various factors including load amplitude, load frequency, number of load cycles and vertical effective stress on the peak apparent coefficient of friction mobilized at the soil-PS interface and the pullout resistance of the PS buried in dry sandy soil. The results illustrate that changing the cyclic tensile load frequency from 0,1 Hz to 0,5 Hz does not affect the pullout resistance. Moreover, the influence of increasing the number of load cycles from 30 to 250 on the peak pullout resistance is negligible. Finally, the effect of increasing the cyclic tensile load amplitude from 20% to 40% on the monotonic pullout resistance can be ignored. The peak apparent coefficient of friction mobilized at the soil-PS interface under monotonic and cyclic load conditions decreases with the increase in vertical effective stress.

Compaction-induced stress in geosynthetic-reinforced granular base course--A discrete element model

A discrete element method(DEM) model was used to simulate the development of compaction-induced stress in a granular base course, with and without geogrid reinforcement. The granular base course was modeled as a mixture of uniformly sized triangular particles. The geogrid was modeled as a series of equally spaced balls that interact with each other through long-range interaction contacts. The longrange interaction contact was also used to simulate a deformable subgrade. The compactor was modeled as a solid cylinder rolling at a constant speed. The DEM model shows that the geogridreinforced granular base course gains additional compaction-induced stress due to the residual tensile stress developed in the geogrid. The residual tensile stress in the geogrid increases with the number of compaction passes. Parametric analyses were also conducted to assess the effects of geogrid stiffness and subgrade modulus on the compaction-induced stress.

Application of Geotechnical Synthetic Materials in Sponge City Construction

Along with the overall progress and development of market economy,Sponge City has received extensive attention,in order to establish a more compatible with the market development trend and environmental protection requirements of the control mechanism,it is necessary to scientifically plan synthetic materials,to a certain extent,to maintain the basic level of sponge city supervision work.In this paper,the research background of Geosynthetics applied in Sponge City construction is briefly analyzed,and the basic principles and specific application paths are discussed,which are for reference only.

Impact of repeated loading on mechanical response of a reinforced sand

Pavements constructed over loosely compacted subgrades may not possess adequate California bearing ratio (CBR) to meet the requirements of pavement design codes,which may lead to a thicker pavement design for addressing the required strength.Geosynthetics have been proven to be effective for mitigating the adverse mechanical behaviors of weak soils as integrated constituents of base and sub-base layers in road construction.This study investigated the behaviors of unreinforced and reinforced sand with nonwoven geotextile using repeated CBR loading test (followed by unloading and reloading).The depth and number of geotextile reinforcement layers,as well as the compaction ratio of the soil above and below the reinforcement layer(s) and the compaction ratio of the sand bed,were set as variables in this context.Geotextile layers were placed at upper thickness ratios of 0.3,0.6 and 0.9 and the lower thickness ratio of 0.3.The compaction ratios of the upper layer and the sand bed varied between 85% and 97% to simulate a dense layer on a medium dense sand bed for all unreinforced and reinforced testing scenarios.Repeated CBR loading tests were conducted to the target loads of 100 kgf,150 kgf,200 kgf and 400 kgf,respectively (1 kgf=9.8 N).The results indicated that placing one layer of reinforcement with an upper thickness ratio of 0.3 and compacting the soil above the reinforcement to compaction ratio of 97% significantly reduced the penetration of the CBR piston for all target repeated load levels.However,using two layers of reinforcement sandwiched between two dense soil layers with a compaction ratio of 97% with upper and lower thickness ratios of 0.3 resulted in the lowest penetration.

Enhancing mechanical behavior of micaceous soil with jute fibers and lime additives

Micaceous soils are common in many tropical countries and regions,and in some locations with moderate climate.The soils are spongy and unstable when loaded and are not considered suitable as construction material in earth structures.To resolve the issue,this work examined performance of micaceous soil reinforced with a combination of jute fibers,hydrated lime or slag-lime.A total of 28 sample sets were prepared at various dosages.Unconfined compression tests were conducted on the samples cured for 7 d and 28 d,respectively.The test results suggested that the unconfined compressive strength(UCS)and material stiffness were increased with the inclusion of up to 1%fiber and decreased if additional fibers were used.The ductility was improved consistently with up to 1.5%fiber content.The inclusions of fibers combined with hydrated lime or slag-lime further enhanced strength and stiffness of micaceous soil,and the improvement depended on the dosages used.For the dosages examined,jute fibers outweighed lime and slag in gaining ductility,and the optimal fiber content was 1%where strength and ductility were considered.

Performance of geosynthetic cementitious composite mat and vetiver on soil erosion control

An understanding of how different land covers affect soil erosion caused by rainfall is necessary in mountainous areas.The land cover usually plays an important role in controlling landslide hazards associated with these terrains.This paper presents the results of a field experiment where several types of land covers were placed on a full-scale embankment as erosion control.An 8 m wide,21 m long,and 3 m high embankment with a 45°side-slope was built with lateric soil.The soil was compacted under a relative compaction of 70%to simulate a natural soil slope.Two sides of the embankment were divided into six land cover areas,with three different areas of bare soil,and one each of a geosynthetic cementitious composite mat(GCCM),vetiver grass,and a combination of GCCM and vegetation.Soil erosion and moisture levels were monitored for each land cover area during six natural rainfall events encountered over the experimental period.Field results were compared with a numerical simulation and empirical soil loss equation.The results revealed that the GCCM gave the best erosion control immediately after installation,but vetiver grass also exhibited good erosion control six months postconstruction.

Railroad bed bearing strength in the period of thawing and methods of its enhancement

The practice of building and operating of railroad beds shows that the greatest attenuation of soils occurs in the spring, during their Iransifion from the frozen to thawed state. The geatest influences on the properties of clay soils that form the railway are from hydration, fieeze-thaw cycles and vibrodynamic impact of Wains. The increase in soil moisture is due to infillration of water into the ground, as well as the rise in water level due to soil redistribution during winter freezes. This can dramatically alter the basic characteristics of the soil, such as shear resistance and bulk density, on which strength and stability of soil mass depend primarily. Therefore, the degree of railway bed stability is not constant, but varies with time.

A comparative study for the performance of encased granular columns

This paper describes a three-dimensional(3D) numerical analysis of a test embankment on geotextileencased columns(GECs), in comparison with two-dimensional(2D) axisymmetric and plane strain analyses. The 3D numerical analysis was performed considering a rectangular strip under the embankment centerline. The 2D analysis was also carried out using axisymmetric unit cell and plane strain approaches.Numerical results indicated that the adopted 3D strip model represented well the measured deformations and pore pressure evolution during embankment construction and post-contraction periods.Unlike the unit cell model, both plane strain and 3D analyses could properly determine the settlement profile along the embankment base as well as the profile of the horizontal soil deformation beneath the embankment toes. The plane strain analysis, however, was not able to compute the geotextile ring force which might be simply calculated using the axisymmetric unit cell approach. The paper also showed that, due to horizontal boundary fixities applied to the embankment borders, the unit cell model clearly underestimated the tension developed in the basal geogrid.

Interaction analysis of back-to-back mechanically stabilized earth walls

Back-to-back mechanically stabilized earth walls （BBMSEWs） are encountered in bridge approaches, ramp ways, rockfall protection systems, earth dams, levees and noise barriers. However, available design guidelines for BBMSEWs are limited and not applicable to numerical modeling when back-to-back walls interact with each other. The objective of this paper is to investigate, using PLAXIS code, the effects of the reduction in the distance between BBMSEW, the reinforcement length, the quality of backfill material and the connection of reinforcements in the middle, when the back-to-back walls are close. The results indicate that each of the BBMSEWs behaves independently if the width of the embankment between mechanically stabilized earth walls is greater than that of the active zone. This is in good agreement with the result of FHWA design guideline. However, the results show that the FHWA design guideline underestimates the lateral earth pressure when back-to-back walls interact with each other. Moreover, for closer BBMSEWs, FHWA design guideline strongly overestimates the maximum tensile force in the reinforcement. The investigation of the quality of backfill material shows that the minor increase in embankment cohesion can lead to significant reductions in both the lateral earth pressure and the maximum tensile force in geosynthetic. When the distance between the two earth walls is close to zero, the connection of reinforcement between back-to-back walls significantly improves the factor of safety.

Consolidation of soft clay foundation improved by geosyntheticreinforced granular columns:Numerical evaluation

Soft clays are problematic soils as they present high compressibility and low shear strength.There are several methods for improving in situ conditions of soft clays.Based on the geotechnical problem’s geometry and characteristics,the in situ conditions may require reinforcement to restrain instability and construction settlements.Granular columns reinforced by geosynthetic material are widely used to reduce settlements of embankments on soft clays.They also accelerate the consolidation rate by reducing the drainage path’s length and increasing the foundation soil’s bearing capacity.In this study,the performance of encased and layered granular columns in soft clay is investigated and discussed.The numerical results show the significance of geosynthetic stiffness and the column length on the embankment settlements.Furthermore,the results show that granular columns may play an important role in dissipating the excess pore water pressures and accelerating the consolidation settlements of embankments on soft clays.

Laboratory large-scale pullout investigation of a new reinforcement ofcomposite geosynthetic strip

In this paper,more than 70 large-scale pullout tests were performed to evaluate the performance of an innovative composite geosynthetic strip(CGS)reinforcement in sandy backfill.The CGS reinforcement is composed of a geosynthetic strip(GS)and parts of a scrap truck tire as transverse members.The experimental pullout results for the CGS reinforcement were compared with the suggested theoretical equations and ordinary reinforcements,including the GS,the steel strip(SS),and the steel strip with rib(SSR).The pullout test results show that adding three transverse members to the GS reinforcement(CGS3)with S/H?6.6(where S and H are the space and height of the transverse members,respectively)increases pullout resistance by more than 120%,170%,and 50%compared to the GS,the SS,and the SSR,respectively.This result shows that the CGS3(CGS with three transverse members)reinforcement needs at least 55.5%,63%,and 33.3%smaller length compared to the GS,the SS,and the SSR,respectively.In general,implementation of mechanically stabilized earth wall(MSEW)with the proposed strip may help geotechnical engineers prevent costly designs and solve the problem of MSEW implementation in cases where there are limitations of space.