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.     

Study of the behavior of mechanically stabilized earth (MSE) walls subjected to differential settlements

The limit equilibrium (LE) analysis has been used to design MSE walls. Presumably, the deflection of MSE walls can be limited to an acceptable range by ensuring sufficient factors of safety (FOSs) for both external and internal stabilities. However, unexpected ground movements, such as movements induced by excavations, volume changes of expansive soils, collapse of sinkholes, and consolidations of underlying soils, can induce excessive differential settlements that may influence both the stability and the serviceability of MSE walls. In this study, a numerical model, which was calibrated by triaxial tests and further by a specially-designed MSE wall tests, investigated the behavior of an MSE wall as well as the influence of various factors on the performance of the MSE wall when the wall facing settled relatively to the reinforced zone. The numerical results showed that the differential settlement would cause substantial vertical and horizontal movements for the MSE wall, as well as an increase in lateral earth pressure and geosynthetic reinforcement strain. The maximum horizontal movement and increase of the lateral earth pressure occurred at about 1.0 m above the toe. The differential settlement resulted in a critical plane that coincided with the plane of 45°+?/2. The maximum increase of the strain for each geogrid layer occurred in that plane, and the bottom layer had the greatest strain increase among all layers of reinforcement. The study further indicated that the surcharge, backfill friction angle, tensile stiffness of geogrid, reinforcement length and MSE wall height had noticeable influences on horizontal and vertical movements, and strain in geosynthetics. According to the results, the MSE wall that had a higher factor of safety would have less movements and geosynthetic strain increase. In contrast, only the friction angle, tensile stiffness and MSE wall height showed some degree of influence on the lateral earth pressure due to differential settlements.     

Full-scale mechanically stabilized earth (MSE) walls under strip footing load

This study analyses two full-scale model tests on mechanically stabilized earth (MSE) walls. One test was conducted with a rigid and one with a flexible wall face. Other parameters were the same in these two tests, like the number and type of geogrid layers, the vertical distance between the layers and the soil type. The loads and strains on the reinforcement are measured as function of the horizontal and vertical earth pressure and compared with analytical models. Specifics regarding the behavior of the geogrids under the compaction load during the construction of the model and under strip footing load are included in the study. Results are compared with AASHTO and the empirical K-stiffness method. In this study, an analytical method is developed for the MSE walls taking into account the facing panel rigidity both after backfill construction and after strip footing load. There is good agreement between the proposed analytical method and the experimental results considering the facing panel rigidity. The results indicate that the tensile force on reinforcement layers for rigid facing is less than the flexible facing. The maximum strains in the reinforcement layers occurred in the upper layers right below the strip footing load. The maximum wall deflection for the flexible facing is more than for the rigid facing. The maximum deflection was at the top of the wall for the rigid facing and occurred at z/H?=?0.81 from top of the wall for the flexible facing.     

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.     

Effect of earth reinforcement,soil properties and wall properties on bridge MSE walls

Mechanically stabilized earth (MSE) retaining walls are popular for highway bridge structures. They have precast concrete panels attached to earth reinforcement. The panels are designed to have some lateral movement. However, in some cases, excessive movement and even complete dislocation of the panels have been observed. In this study, 3-D numerical modeling involving an existing MSE wall was undertaken to investigate various wall parameters. The effects of pore pressure, soil cohesion, earth reinforcement type and length, breakage/slippage of reinforcement and concrete strength, were examined. Results showed that the wall movement is affected by soil pore pressure and reinforcement integrity and length, and unaffected by concrete strength. Soil cohesion has a minor effect, while the movement increased by 13–20 mm for flexible geogrid reinforced walls compared with the steel grid walls. The steel grid stresses were below yielding, while the geogrid experienced significant stresses without rupture. Geogrid reinforcement may be used taking account of slippage resistance and wall movement. If steel grid is used, non-cohesive soil is recommended to minimize corrosion. Proper soil drainage is important for control of pore pressure.     

Deterministic and probabilistic assessment of margins of safety for internal stability of as-built PET strap reinforced soil walls

The paper demonstrates deterministic and reliability-based assessment of strength limit states (tensile resistance and pullout) and the service limit state for soil failure for mechanically stabilized earth (MSE) walls constructed with polyester (PET) strap reinforcement. The general approach considers the accuracy of the load and resistance models that appear in each limit state equation plus uncertainty in the estimate of nominal load and resistance values at time of design. Reliability index is computed using a closed-form solution that is easily implemented in a spreadsheet. Three PET strap MSE wall case studies are used to demonstrate the reliability-based assessment approach and to compare margins of safety using different load and resistance model combinations. In some walls using the Coherent Gravity Method to compute loads, the recommended nominal factors of safety for tensile strength and pullout limit states were not satisfied. However, reliability analyses showed that the walls satisfy recommended minimum target reliability index values for the limit states investigated, usually by large amounts. The most critical limit state is the soil failure limit state which is used in the Simplified Stiffness Method to keep the reinforced soil zone at working stress conditions assumed for geosynthetic MSE walls under operational conditions.     

Physical and analytical modelling of geosynthetic strip pull-out behaviour

The design methods used for soil mass structures, such as mechanically stabilised earth (MSE) structures, are based on soil/reinforcement anchorage models which require the knowledge of the soil/reinforcement interface friction capacity. However, different types of reinforcements are used in these structures and present different behaviour. This study concerns two types of strips reinforcements. The first one is metallic and is classically designed using elasto-plastic models (21 and 22). The second type is geosynthetic. The classical anchorage models do not take into account the extensibility of this materiel and do not reproduce its complex behaviour.     

Influence of uncertainty in geosynthetic stiffness on deterministic and probabilistic analyses using analytical solutions for three reinforced soil problems

The paper examines the quantitative influence of uncertainty in the estimate of geosynthetic reinforcement stiffness on numerical outcomes using analytical solutions for a) the maximum outward facing deformation in mechanically stabilized earth (MSE) walls, b) maximum reinforcement tensile loads and strain in MSE walls under operational conditions, and c) the mobilized reinforcement stiffness in a geosynthetic layer used to reinforce a fill over a void. The stiffness of the reinforcement is modelled using an isochronous two-parameter hyperbolic load-strain model. A linear relationship between isochronous stiffness and the ultimate tensile strength of the reinforcement is used to estimate reinforcement stiffness when product-specific creep data are not available at time of design. Solution outcomes are presented deterministically and probabilistically. The quantitative link between nominal factor of safety used in deterministic working stress design practice and reliability index is provided. The latter is preferred in modern performance-based design to quantify margins of safety within a probabilistic framework. Finally, the paper highlights the practical benefit of using product-specific isochronous secant stiffness data when available, rather than estimates of isochronous stiffness values based on reinforcement type or pooled 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。     

Characterization of geogrid mechanical and chemical properties from a thirty-six year old mechanically-stabilized earth wall

The mechanical properties of geosynthetic reinforcements are known to be time-, environment- and stress-dependent. Characterization of these reinforcement properties is often assessed under controlled laboratory settings and extrapolated to the design life of geosynthetic-reinforced soil structures. However, despite the wide application of geosynthetic reinforcement in earth retaining structures, there is limited evaluation of how mechanical properties of geosynthetic materials change in situ on constructed works; and primarily limited to case studies within the first decade following construction. This study describes the change in mechanical properties of geogrids retrieved from the facing of the wrapped-face of one of the oldest geosynthetic-reinforced mechanically-stabilized earth (MSE) walls in the United States, constructed in 1983 in a relatively harsh, coastal environment. Laboratory characterization of mechanical and chemical properties of the geogrid are presented, and compared to properties of archived samples, as well as samples from another structure exhumed 8 and 11 years after its respective construction. The laboratory test results demonstrate that the geogrid mechanical and chemical properties have not significantly changed in the 35+ years of service. While the data from this study represents a limited set of conditions, these results demonstrate that geogrids may perform well long after construction.     

One-step analytical method for required reinforcement stiffness of vertical reinforced soil wall with given factor of safety on backfill soil

The selection of geosynthetic reinforcements in the design of geosynthetic-reinforced soil (GRS) retaining walls has been based on the requirement on the long-term strength. However, the mobilized loads in the reinforcements are related to both the reinforcement stiffness and soil deformation, and the desired factor of safety may not exist in the earth structure if they are not properly considered. Therefore, it is also important to take into account the long-term reinforcement stiffness when designing GRS retaining walls. In this study, a simplistic analytical method is proposed to determine the required reinforcement stiffness with given factor of safety on the backfill soil. The method takes into account soil-reinforcement interaction, nonlinear stress-strain behavior of soil, and soil dilatancy. The reinforcement strains predicted by the proposed method were compared to those analyzed by validated nonlinear Finite Element analyses, and close agreement was obtained.     

横向地震作用下土工合成材料加筋土挡墙筋材拉力分析

土工合成材料加筋土挡墙具备优良的抗震性能,但是,国内外现行的加筋土挡墙筋材动拉力计算方法存在地震动参数选用不尽合理的问题,一方面可能带来结构安全隐患,另一方面也造成了工程界的疑虑.基于此,在前期工作的基础上应用非线性动力有限元法分析了高加筋土挡墙在不同地震激励作用下的地震响应,重点讨论了强震作用下筋材拉力的影响因素.分...     

Recent advances in geosynthetic-reinforced retaining walls for highway applications

Geosynthetic-reinforced retaining (GRR) walls have been increasingly used to support roadways and bridge abutments in highway projects. In recent years, advances have been made in construction and design of GRR walls for highway applications. For example, piles have been installed inside GRR walls to support bridge abutments and sound barrier walls. Geosynthetic layers at closer spacing are used in GRR walls to form a composite mass to support an integrated bridge system. This system is referred to as a geosynthetic-reinforced soil (GRS)-integrated bridge systems (IBS) or GRS-IBS. In addition, short geosynthetic layers have been used as secondary reinforcement in a GRR wall to form a hybrid GRR wall (HGRR wall) and reduce tension in primary reinforcement and facing deflections. These new technologies have improved performance of GRR walls and created more economic solutions; however, they have also created more complicated problems for analysis and design. This paper reviews recent studies on these new GRR wall systems, summarizes key results and findings including but not limited to vertical and lateral earth pressures, wall facing deflections, and strains in geosynthetic layers, discusses design aspects, and presents field applications for these new GRR wall systems.     

Ultimate bearing capacity of strip footing resting on soil bed strengthened by wraparound geosynthetic reinforcement technique

In the recent past, the wraparound geosynthetic reinforcement technique has been recommended for constructing the geosynthetic-reinforced soil foundations. This paper presents the development of an analytical expression for estimating the ultimate bearing capacity of strip footing resting on soil bed reinforced with geosynthetic reinforcement having the wraparound ends. The wraparound ends of the geosynthetic reinforcement are considered to provide the shearing resistance at the soil-geosynthetic interface as well as the passive resistance due to confinement of soil by the geosynthetic reinforcement. The values of ultimate load-bearing capacity determined by using the developed analytical expression agree well with the model footing load test values as reported in the literature.     

Reinforcement loads in geosynthetic walls and the case for a new working stress design method

The paper provides a synthesis of work by the writers that has the objective of developing a new working stress method for the calculation of reinforcement loads in geosynthetic reinforced soil walls. As a precursor to this objective, careful back-analyses of a database of instrumented and monitored full-scale field and laboratory walls are used to demonstrate that the current American Association of State Highway and Transportation Officials (AASHTO) Simplified Method used in North America results in excessively conservative estimates of the volume of reinforcement required to generate satisfactory long-term wall performance. The new design method captures the essential contributions of the different wall components and properties to reinforcement loads. The method is calibrated against measured in situ wall reinforcement loads using a careful interpretation of reinforcement strains and the conversion of strain to load using a suitably selected reinforcement stiffness value. A novel feature of the method is to design the wall reinforcement so that the soil within the wall backfill is prevented from reaching a failure limit state, consistent with the notion of working stress conditions.     

加筋形式对桩承式路堤工作性状影响的试验研究

对无加筋和采用不同加筋材料、加筋层数下桩承式路堤的工作性状进行了三维模型试验研究,侧重分析了桩土应力比、应力折减系数、填土中竖向应力分布、地基沉降等内容。结果表明加筋材料的设置有利于荷载向桩顶的转移,可有效减小沉降,但不同加筋形式下桩承式路堤的工作性状有所不同。使用单层或双层土工布时,路堤的荷载传递机理主要是填土的土拱效应和加筋材料的拉膜效应,但拉膜效应发挥相对较晚。使用双层格栅时,加筋材料与周围砂土形成半刚性平台。单层格栅的作用介于两者之间。试验结果与常规拉膜效应设计方法的对比表明,若假设荷载只由相邻桩间的加筋材料条带承担,计算的拉力将偏大,过于保守。     

Remedial treatment of soil structures using geosynthetic-reinforcing technology

The advantages of geosynthetic-reinforcing technology to construct new soil structures including; (a) a relatively short construction period; (b) small construction machines necessary; and (c) a higher stability of completed structures, all contributing to a higher cost-effectiveness, are addressed. A number of case successful histories of geosynthetic-reinforced soil retaining walls have been reported in the literature (e.g., [Tatsuoka, F., Koseki, J., Tateyama, M., 1997a. Performance of Earth Reinforcement Structures during the Great Hanshin Earthquake, Special Lecture. In: Proceedings of the International Symposium on Earth Reinforcement, IS Kyushu ‘96, Balkema, vol. 2, pp. 973–1008; Tatsuoka, F., Tateyama, M, Uchimura, T., Koseki, J., 1997b. Geosynthetic-reinforced soil retaining walls as important permanent structures, 1996–1997 Mercer Lecture. Geosynthetics International 4(2), 81–136; Tatsuoka, F., Koseki, J., Tateyama, M., Munaf, Y., Horii, N., 1998. Seismic stability against high seismic loads of geosynthetic-reinforced soil retaining structures, Keynote Lecture. In: Proceedings of the 6th International Conference on Geosynthetics, Atlanta, vol. 1, pp.103–142; Helwany, S.M.B., Wu, J.T.H., Froessl, B., 2003. GRS bridge abutments—an effective means to alleviate bridge approach settlement. Geotextiles and Geomembranes 21(3), 177–196; Lee, K.Z.Z., Wu, J.T.H., 2004. A synthesis of case histories on GRS bridge-supporting structures with flexible facing. Geotextiles and Geomembranes 22(4), 181–204; Yoo, C., Jung, H.-S., 2004. Measured behavior of a geosynthetic-reinforced segmental retaining wall in a tiered configuration. Geotextiles and Geomembranes 22(5), 359–376; Kazimierowicz-Frankowska, K., 2005. A case study of a geosynthetic reinforced wall with wrap-around facing. Geotextiles and Geomembranes 23(1), 107–115; Skinner, G.D., Rowe, R.K., 2005. Design and behaviour of a geosynthetic reinforced retaining wall and bridge abutment on a yielding foundation. Geotextiles and Geomembranes 23(3), 234–260]). Techniques for analyzing the seismic response of reinforced walls and slopes have also been developed (e.g. Nouri, H. Fakher, A., Jones, C.J.F.P., 2006. Development of horizontal slice method for seismic stability analysis of reinforced slopes and walls. Geotextiles and Geomembranes 24(2),175–187). Several typical cases in which embankments having a gentle slope and conventional-type soil retaining walls that were seriously damaged or failed were reconstructed to geosynthetic-reinforced steepened slopes or geosynthetic-reinforced soil retaining walls are also reported in this paper. It has been reported that the reconstruction of damaged or failed conventional soil structures to geosynthetic-reinforced soil structures was highly cost-effective in these cases. Rehabilitation of an old earth-fill dam in Tokyo to increase its seismic stability by constructing a counter-balance fill reinforced with geosynthetic reinforcement is described. Finally, a new technology proposed to stabilize the downstream slope of earth-fill dams against overflowing flood water while ensuring a high seismic stability by protecting the slope with soil bags anchored with geosynthetic reinforcement layers arranged in the slope is described.     

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.     

Laboratory large-scale pullout investigation of a new reinforcement of composite 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(CGS_3) 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 CGS_3(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.     

Dynamic response of Mechanically Stabilised Earth (MSE) structures: A numerical study

With increasing construction feasibility, lower costs and proven performance throughout past major seismic events, MSE retention systems have become one of the more preferred retention systems. To study the dynamic performance of MSE walls, the 2D FE simulation using the OpenSees programme with the Manzari and Dafalias constitutive relationship has been utilised. A series of one-dimensional (1D) and 2D site response analyses subjected to sinusoidal inputs at various frequencies have been conducted to find the natural period of the soil medium. Then, using three earthquake time-histories recorded on engineering bedrock (Vs > 700 m/s), the behaviour of MSE walls with geogrid length to wall height ratios of 0.50 and 0.75 has been investigated. Multi-pulse Ricker wavelets have been deployed for a closer inspection of possible failure mechanisms of these MSE walls. Finally, the possibility of simulating an elastic orthotropic block instead of the reinforced soil with geogrids has been examined.