Effects of Reinforcement type and Loading History on the Deformation of Reinforced Sand in Plane Strain Compression

To evaluate the effects of reinforcement type in terms of stiffness, viscous property, rupture strength, shape and loading history on the stress-strain behaviour during primary, sustained and cyclic loading of reinforced sand, a series of drained plane strain compression tests were performed on Toyoura sand. The sand specimens were reinforced with two types of polymer geogrid as well as two types of metal grid, having largely different stiffness values and surface conditions. Despite that the effects of reinforcement type on the overall stress-strain characteristics of reinforced sand and their rate-dependency are significant during primary loading, the effects are much smaller than the difference in the stiffness of reinforcement. The effects of reinforcement type on the global unloading behaviour and the residual strain by cyclic loading during otherwise global unloading are generally insignificant or negligible. The residual strains by cyclic loading of reinforced sand became substantially small by preloading as well as pre-sustained loading and pre-cyclic loading at higher load levels. With this procedure, polymer geosynthetic reinforcement, which is much more extensible and viscous than metal reinforcement, can be used to reinforce soil structures allowing very limited residual deformation.     

Geosynthetic reinforcement stiffness for analytical and numerical modelling of reinforced soil structures

Many analytical and numerical analysis and design methods for geosynthetic-reinforced soil structures require a single-value (constant) estimate of reinforcement stiffness. However, geosynthetic reinforcement products are rate-dependent polymeric materials meaning that they exhibit time and strain-dependent behaviour under load. Hence, the appropriate selection of a constant (elastic) stiffness value requires careful consideration. A simple hyperbolic stiffness model is shown to be a useful approximation to the constant-load isochronous creep-strain behaviour of these materials at low load levels applicable to operational (serviceability) conditions of geosynthetic-reinforced soil structures. A large database of 606 creep tests on 89 different geosynthetic reinforcement products falling within seven different product categories was collected. From these data, isochronous stiffness values were determined for different combinations of duration of loading and strain level. Data from products falling within the same category were collected together to provide approximations linking the isochronous load-strain (creep) stiffness to the ultimate tensile strength of the material. These approximations are useful for analytical and numerical modelling particularly when parametric studies are undertaken to identify the sensitivity of model outcomes to reinforcement stiffness. Finally, three different geosynthetic-reinforced soil application examples are provided to demonstrate the important role of tensile stiffness on analysis and design outcomes.     

Rate-Dependent Load-Strain Behaviour of Geogrid Arranged in Sand Under Plane Strain Compression

A number of previous experimental studies showed that polymer geogrid reinforcement as well as sand exhibit significantly rate-dependent behaviour. The viscous properties of polymer geogrids and Toyoura sand were independently evaluated by changing stepwise the strain rate as well as performing sustained loading and load/stress relaxation tests during otherwise monotonic loading in, respectively, tensile loading tests and drained plane strain compression (PSC) tests. The viscous properties of the two types of material were separately formulated in the same framework of non-linear three-component rheology model. The viscous response of geogrid-reinforced sand in PSC is significant, controlled by viscous properties of geogrid and sand. Local strain distributions in the reinforced sand specimen were evaluated by photogrametric analysis and used to determine the time history of the tensile strain in the geogrid. The time history of tensile load activated in the geogrid during sustained loading of reinforced sand specimen was deduced by analysing the measured time history of geogrid strain by the non-linear three-component model. It was found that the tensile load in the geogrid reinforcement arranged in a sand specimen subjected to fixed boundary loads could decrease with time. In that case, the possibility of creep rupture of geogrid is very low.     

Viscous Behaviour of Geogrids; Experiment and Simulation

The viscous properties of three types of geogrid polymer were evaluated by sustained loading tests lasting for 30 days at a load level about a half of its nominal rupture strength. The sustained loading tests were performed during otherwise monotonic loading (ML) at constant strain or load rate, unlike the conventional creep tests, in which the strain rate immediately before the start of sustained loading, which controls the creep strain rate, is not controlled or even not recorded. The following are presented in this study. The tensile rupture strength measured by ML that was started following a 30 day-long sustained loading was essentially the same as the one at the same strain rate at rupture obtained by continuous ML without any intermission of sustained loading. This fact indicates that creep is not a degrading phenomenon. Then, if free from chemical and mechanical degrading effects, the strength of a geosynthetic reinforcement (for a given strain rate at rupture) can be maintained until late in its service life. A non-linear three-component model is used to simulate the experimental results from the previous and present studies. The model can simulate very well not only the load-strain behaviour during ML with and without step changes in the strain rate and the one after sustained loading, but also the time histories of creep strain during sustained loading for short (one hour) and long (30 days) periods.     

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.     

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.     

Reinforcement load and deformation mode of geosynthetic-reinforced soil walls subject to seismic loading during service life

A Finite Element procedure was used to investigate the reinforcement load and the deformation mode for geosynthetic-reinforced soil (GRS) walls subject to seismic loading during their service life, focusing on those with marginal backfill soils. Marginal backfill soils are hereby defined as filled materials containing cohesive fines with plasticity index (PI) >6, which may exhibit substantial creep under constant static loading before subjected to earthquake. It was found that under strong seismic loading reinforced soil walls with marginal backfills exhibited a distinctive “two-wedge” deformation mode. The surface of maximum reinforcement load was the combined effect of the internal potential failure surface and the outer surface that extended into the retained earth. In the range investigated, which is believed to cover general backfill soils and geosynthetic reinforcements, the creep rates of soils and reinforcements had small influence on the reinforcement load and the “two-wedge” deformation mode, but reinforcement stiffness played a critical role on these two responses of GRS walls. It was also found that the “two-wedge” deformation mode could be restricted if sufficiently long reinforcement was used. The study shows that it is rational to investigate the reinforcement load of reinforced soil walls subject to seismic loading without considering the previous long-term creep.     

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.     

关于土工合成材料加筋设计的若干问题

目前土工合成材料加筋技术被广泛应用,但人们对于加筋土中筋材与土间的相互作用的机理的认识还不够深入,因而在设计中总体上趋于保守。结合岩土工程的设计理论,指出土工合成材料在设计方法方面的不合理性;对于加筋挡土墙、加筋土坡、加筋软土地基上的土堤和桩网结构的设计分别进行了讨论;结合一些案例中的实测和预计的筋材应变和应力,进一步指出目前设计的保守性。最后指出,目前基于极限平衡法的设计不尽合理,而通过变形协调的筋土共同作用的研究,采用更能反映其相互作用机理的设计方法是非常必要的。     

A Simple Pneumatic Loading System Controlling Stress and Strain Rates for One-Dimensional Compression of Clay

A simple but automated pneumatic loading system that can control the stress and strain rates for one-dimensional (1D) compression of clay was developed. The rate-dependency of stress-strain behaviour due to the viscous property of clay was investigated by 1D compression tests on standard-size specimens by various loading methods: 1) Standard Consolidation Tests (SCTs), stepwise increasing the axial stress two times every one day; 2) ordinary Constant-Rate-of-Strain (CRS) tests at different strain rates; 3) special CRS tests including unloading and reloading cycles with different stress amplitudes at strain rates of which the absolute value was either kept constant throughout respective tests or changed at the start of reloading; and 4) special CRS tests including a number of sustained loading (SL) during otherwise primary loading or unloading or reloading at constant strain rate. Sufficiently low strain rates were employed to ensure essentially fully drained condition. The followings were found. Despite that the newly developed pneumatic loading system is rather simple, 1D compression tests following such various loading histories as above can be performed on four types of clay rather accurately. The stress-strain behaviour of clay is significantly rate-dependent, exhibiting significant creep strains at SL stages. The creep strain rate is significantly different whether SL starts during otherwise primary loading or unloading or reloading, controlled by the magnitude and sign of the initial strain rate at the start of SL. The whole observed trends of rate-dependent stress-strain behaviour can be qualitatively explained by the non-linear three-component elasto-viscoplastic model extended to cyclic loading conditions.     

A novel 2D-3D conversion method for calculating maximum strain of geosynthetic reinforcement in pile-supported embankments

For design of a geosynthetic-reinforced pile-supported (GRPS) embankment over soft soil, the methods used to calculate strains in geosynthetic reinforcement at a vertical stress were mostly developed based on a plane-strain or two-dimensional (2-D) condition or a strip between two pile caps. These 2-D-based methods cannot accurately predict the strain of geosynthetic reinforcement under a three-dimensional (3-D) condition. In this paper, a series of numerical models were established to compare the maximum strains and vertical deflections (also called sags) of geosynthetic reinforcement under the 2-D and 3-D conditions, considering the following influence factors: soil support, cap shape and pattern, and a cushion layer between cap and reinforcement. The numerical results show that the maximum strain in the geosynthetic reinforcement decreased with an increase of the modulus of subgrade reaction. The 2-D model underestimated the maximum strain and sag in the geosynthetic reinforcement as compared with the 3-D model. The cap shape and pattern had significant influences on the maximum strains in the geosynthetic reinforcements. An empirical method involving the geometric factors of cap shape and pattern, and the soil support was developed to convert the calculated strains of geosynthetic reinforcement in piled embankments under the 2-D condition to those under the 3-D condition and verified through a comparison with the results in the literature.     

Combined effect of PVDs and reinforcement on embankments over rate-sensitive soils

A numerical study of the behavior of geosynthetic-reinforced embankments constructed on soft rate-sensitive soil with and without prefabricated vertical drains (PVDs) is described. The time-dependent stress–strain-strength characteristic of rate-sensitive soil is taken into account using an elasto-viscoplastic constitutive model. The effects of reinforcement stiffness, construction rate, soil viscosity as well as PVD spacing are examined both during and following construction. A sensitivity analysis shows the effect of construction rate and PVD spacing on the short-term and long-term stability of reinforced embankments and the mobilized reinforcement strain. For rate-sensitive soils, the critical period with respect to the stability of the embankment occurs after the end of the construction due to a delayed, creep-induced, build-up of excess pore pressure in the viscous foundation soil. PVDs substantially reduce the effect of creep-induced excess pore pressure, and hence not only allow a faster rate of consolidation but also improve the long-term stability of the reinforced embankment. Furthermore, PVDs work together with geosynthetic reinforcement to minimize the differential settlement and lateral deformation of the foundation. The combined use of the geosynthetic reinforcement and PVDs enhances embankment performance substantially more than the use of either method of soil improvement alone.     

Strain distribution along geogrid-reinforced asphalt overlays under traffic loading

While there is significant field evidence of the benefits of geosynthetic-reinforced asphalt overlays, their use has focused on minimizing the development of reflective cracks. Yet, geogrids in asphalt overlays are also expected to develop reinforcement mechanisms that contribute to the pavement structural capacity. Specifically, the use of geosynthetics in asphalt overlays may also improve the mechanical behavior of paved roads by controlling permanent displacements and reducing strains in the pavement layers. While relevant advances have been made towards identifying the mechanisms in geosynthetic stabilization of base courses, such mechanisms may differ from those that develop in geosynthetic-reinforced asphalt overlays. This paper investigates the development and distribution of tensile strains along geogrids used to reinforce asphaltic layers. Experimental data was collected from large-scale paved road models subjected to the repeated loading imparted by wheel traffic. Specifically, the study examines both the elastic and permanent components of displacements induced in geogrids by using mechanical extensometers attached to the geogrids. The testing program includes a number of geosynthetic-reinforced paved road models, as well as a control (unreinforced) section that was also instrumented for comparison purposes. Asphalt strain gauges were used to measure strains within the asphalt concrete layer, providing an additional source of information that proved to be highly consistent with the results obtained from the extensometers. The experimental results showed a progressive mobilization of permanent geogrid strains that reached a final profile beyond which additional traffic loading did not result in additional straining. In comparison, higher strains developed in the unreinforced model, which showed a continuously increasing trend. Elastic tensile strains in the asphalt mixture and rutting under the wheel load were comparatively smaller when using geogrids. Overall, the results generated in this study indicate that the presence of geogrids in asphalt overlays results in a lateral restraining mechanism that influences on the mechanical behavior of flexible pavements.     

Soil-reinforcement interaction: Stress regime evolution in geosynthetic-reinforced soils

Understanding the stress regime that develops in the vicinity of reinforcements in reinforced soil masses may prove crucial to understanding, quantifying, and modeling the behavior of a reinforced soil structures. This paper presents analyses conducted to describe the evolution of stress and strain fields in a reinforced soil unit cell, which occur as shear stresses are induced at the soil-reinforcement interface. The analyses were carried out based on thorough measurements obtained when conducting soil-reinforcement interaction tests using a new large-scale device developed to specifically assess geosynthetic-reinforced soil behavior considering varying reinforcement vertical spacings. These experiments involved testing a geosynthetic-reinforced mass with three reinforcement layers: an actively tensioned layer and two passively tensioned neighboring layers. Shear stresses from the actively tensioned reinforcement were conveyed to the passively tensioned reinforcement layers through the intermediate soil medium. The experimental measurements considered in the analyses presented herein include tensile strains developed in the reinforcement layers and the displacement field of soil particles adjacent to the reinforcement layers. The analyses provided insights into the lateral confining effect of geosynthetic reinforcements on reinforced soils. It was concluded that the change in the lateral earth pressure increases with increasing reinforcement tensile strain and reinforcement vertical spacing, and it decreases with increasing vertical stress.     

土工合成材料加筋边坡宏细观机理模型试验研究

通过包裹式加筋边坡模型试验对加筋边坡宏观变形模式、宏观力学性状、筋土界面细观作用和剪切破坏带处土颗粒运动进行研究,分析土工合成材料加筋边坡宏细观机理。砂土加筋边坡由于加筋体的存在,宏观变形上呈现为类似黏性土边坡的整体滑移破坏模式,出现圆弧状剪切破坏带;坡顶上部基础 p – s 曲线类似基础整体剪切破坏弹性、弹塑性和塑性破坏三阶段,曲线各阶段变化与主剪切破坏带整体发展有密切关系;细观上筋土界面处筋材与土颗粒摩擦和咬合作用提供“似黏聚力”;“主剪切破坏带”内颗粒的滚动摩擦、“过渡区”内颗粒的滚动摩擦和滑动摩擦和“稳定区”内颗粒的咬合作用构成整个剪切破坏带抗滑阻力。筋土界面处筋材与土颗粒相互作用所提供的“似黏聚力”和主剪切破坏带的发展和贯通所提供的滚动摩擦和滑动摩擦是形成加筋砂土边坡整体滑移、改变整个边坡破坏模式的内在原因。     

考虑蠕变、地震效应的土工格栅砂性土加筋挡墙弹塑性有限元分析

土工合成材料加筋土挡土墙具有优良的抗震性能,但是由于加筋用的土工合成材料具有显著的蠕变及应力松弛特性,需要深入研究加筋土挡墙在经历蠕变后的地震动力行为及震后的进一步蠕变以理解其全面的静动力学性能。在已有研究的基础上,应用笔者提出的土工合成材料循环受载、蠕变和应力松弛统一本构模型模拟土工格栅的力学行为,考虑到土工格栅加筋土挡土墙的填土一般为砂性土,而砂性土一般蠕变变形较小,本文应用可以模拟砂土述砂性土非线性静动力性能的广义塑性模型模拟填土,未考虑其蠕变变形。结果表明,在正常加筋长度和密度情况下,加筋土挡墙在经受地震前的蠕变变形会趋于稳定,但筋材内力重分布明显;地震作用使得加筋土挡墙产生较大变形,加筋内力出现较大增长,但结构并未破坏;地震后加筋土挡墙蠕变变形继续发展,而土工格栅加筋会出现内力松弛现象。     

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.     

Viscous Property of Toyoura Sand Over a Wide Range of Shear Deformation Rate and its Model Simulation

As part of a long-term research program to evaluate the rate effects on the stress-strain behaviour of geomaterials, the viscous properties of a poorly-graded relatively angular quartz-rich sand, Toyoura sand, under air-dried conditions, were investigated by performing a comprehensive series of direct shear (DS) tests at a fixed normal stress equal to 50 kPa. The tests were performed on loose and dense specimens subjected to the following different loading histories: a) monotonic loading (ML) at constant shear displacement rate (s) differing by a factor up to 100,000; b) ML at constant s including otherwise a number of step changes in s by a factor of 100; and c) a number of sustained loading (SL) stages during otherwise ML at constant values of s differing by a factor up to 1,000. Tests a) revealed that, with dense specimens, the peak shear strength is remarkably independent of s while the residual shear strength noticeably decreases with an increase in s. That is, the viscous property is the so-called TESRA type at the peak stress state, while it is the so-called Positive & Negative (P&N) type at the residual state. With loose specimens, both peak and residual shear strengths decrease with an increase in s, indicating that the viscous property is already the P&N type at the peak stress state and definitely so at the residual state. These results are qualitatively and quantitatively consistent with those from tests b), by which the viscosity properties were quantified in terms of the rate-sensitivity coefficient. The results from tests c) showed that creep shear displacement, Δs, increases with a decrease in the tangent stiffness at the immediately preceding ML phase or with an increase in the shear stress level during the SL stage. The value of Δs for a given period steadily increases with an increase in s during the immediately preceding ML phase. These trends of viscous behaviour are simulated all very well by a non-linear three-component model incorporating a general expression of viscous stress.     

A reinforcing method for steep clay slopes using a non-woven geotextile

The effect of non-woven geotextile reinforcement on the stability and deformation of two clay test embankments is examined based on their performance for about 3 years for the first embankment and about years for the other. Horizontal planar sheets of a non-woven geotextile are expected to work in three ways: for compaction control; for drainage; for tensile reinforcement. The degree of stability of the steep slopes of the test embankments decreased during heavy rainfall. It is found that the use of non-woven geotextile reinforcement may effectively improve embankment performance. Only the stability analysis in terms of effective stresses can explain the performance of the test embankments. The horizontal creep deformation of the embankments during 2–3 years, which is partly attributed to the creep deformation of the non-woven geotextile, was found to be small. The results of both laboratory bearing capacity tests of a strip footing on a model sand ground reinforced with the non-woven geotextile and plane strain compression tests on sand specimens reinforced with the non-woven geotextile show that the non-woven geotextile gives tensile reinforcement to soils.     

Influence of asphalt thickness on performance of geosynthetic-reinforced asphalt: Full-scale field study

In this study, a series of controlled traffic loadings was conducted on unreinforced and geosynthetic-reinforced full-scale asphalt overlays. Unlike the common objective of using paving interlayers to mitigate the development of reflective cracks, the main purpose of adopting geosynthetics for this study was to render an increased roadway structural capacity. The project involved instrumented test sections constructed during the rehabilitation of an in-service roadway in Texas, USA. The rehabilitation involved repairing the pre-existing pavement, placing tack coat, installing a geosynthetic interlayer (except in the unreinforced section), and finally constructing a 75 mm-thick asphalt overlay. This overlay comprised a 50 mm-thick, dense-graded (TY-D) layer overlain by a 25 mm-thick, thin-overlay mixture (TOM) layer. Controlled traffic loadings were conducted, which involved driving standard and light axle loads directly above asphalt strain gauges that had been installed at mid-depth of the pre-existing asphalt layer. Comparison of tensile strains among the different test sections revealed significantly smaller tensile strains in the geosynthetic-reinforced sections compared to those obtained in the unreinforced section. Consequently, and even though geosynthetic interlayers have often been adopted to minimize reflective cracking in asphalt overlays, the field monitoring results generated in this study demonstrate that they also provide added roadway structural capacity.