Mechanically Stabilized Earth Walls Research Articles

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°+ϕ/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.

Evaluation of abutment types on highway in terms on driving comfort

The inverted T-type abutments are generally used in highway bridges constructed in Korea. This type of abutment is used because it has greater stability, with more pile foundations embedded in the bedrock, while simultaneously providing support for lateral earth pressure and vertical loads of superstructures. However, the cross section of inverted T-type abutments is large compared with the piers, which makes them more expensive. In addition, a differential settlement between the abutment and embankment, as well as the expansion joints, causes driving discomfort. This study evaluated the driving comfort of several types of abutments to improve driving comfort on the abutment. To achieve this objective, a traditional T-type abutment and three types of candidate abutments, namely, mechanically stabilized earth wall (MSEW) abutment supported by a shallow foundation (called MSEW abutment), MSEW abutment supported by piles (called MSEW abutment), and pile bent and integral abutment with MSEW (called abutment), were selected to consider their design and economic feasibility. Finite element analysis was performed using the design section of the candidate abutments. Subsequently, the settlements of each candidate abutment, approach slabs, and paved surfaces of the bridges were reviewed. Finally, the driving comfort on each candidate abutment was evaluated using a vehicle dynamic simulation. The true MSEW abutment demonstrated the most excellent driving comfort. However, this abutment can cause problems with respect to serviceability and maintenance due to excessive settlements. After our overall review, we determined that the mixed MSEW and the MIP abutments are the most appropriate abutment types to improve driving comfort by taking the highway conditions in Korea into consideration.

Electrical resisitivity of mechancially stablized earth wall backfill

Mechanically stabilized earth (MSE) retaining walls utilized in transportation projects are typically backfilled with coarse aggregate. One of the current testing procedures to select backfill material for construction of MSE walls is the American Association of State Highway and Transportation Officials standard T 288: “Standard Method of Test for Determining Minimum Laboratory Soil Resistivity.” T 288 is designed to test a soil sample's electrical resistivity which correlates to its corrosive potential. The test is run on soil material passing the No. 10 sieve and believed to be inappropriate for coarse aggregate. Therefore, researchers have proposed new methods to measure the electrical resistivity of coarse aggregate samples in the laboratory. There is a need to verify that the proposed methods yield results representative of the in situ conditions; however, no in situ measurement of the electrical resistivity of MSE wall backfill is established. Electrical resistivity tomography (ERT) provides a two-dimensional (2D) profile of the bulk resistivity of backfill material in situ. The objective of this study was to characterize bulk resistivity of in-place MSE wall backfill aggregate using ERT.Five MSE walls were tested via ERT to determine the bulk resistivity of the backfill. Three of the walls were reinforced with polymeric geogrid, one wall was reinforced with metallic strips, and one wall was a gravity retaining wall with no reinforcement. Variability of the measured resistivity distribution within the backfill may be a result of non-uniform particle sizes, thoroughness of compaction, and the presence of water. A quantitative post processing algorithm was developed to calculate mean bulk resistivity of in-situ backfill. Recommendations of the study were that the ERT data be used to verify proposed testing methods for coarse aggregate that are designed to yield data representative of in situ conditions. A preliminary analysis suggests that ERT may be utilized as construction quality assurance for thoroughness of compaction in MSE construction; however more data are needed at this time.

FEM to investigate the effect of overlapping-reinforcement on the performance of back-to-back embankment bridge approaches under self-weight

Abstract Mechanically Stabilized Earth Walls (MSEW) are cost-effective soil-retaining structures that can tolerate much larger settlements than reinforced concrete walls. Geosynthetic reinforcement has been used successfully in MSEW, and has several advantages compared to other retaining structures. Several MSEW with simple geometries have been built throughout the world. Structures with more complex geometries or significant external loads are practical and require consideration including, bridge abutments, superimposed walls, walls with uneven length reinforcement and back-to-back walls commonly used for embankments approaching bridges to raise elevations. In the Federal Highway Administration (FHWA) guidelines, the distance between two opposing walls is a key parameter used for determining the analysis methods of back-to-back walls and two extreme cases are identified: (1) reinforcements from both sides meet in the middle or overlap, and (2) the walls are far apart, independent of each other. However, existing design methodologies do not provide a clear and justified answer how the required tensile strength of reinforcement and the external stability change with respect to the distance of the back-to-back walls. Recently, both limit equilibrium and numerical methods were used to evaluate the stability of back-to-back mechanically stabilized earth under static conditions when the walls are far apart. However, investigation of the overlapping length reinforcement is rarely discussed in the literature. The focus of this paper is to investigate the stability of embankment bridge approaches constructed by close back-to-back geosynthetic reinforced soil retaining walls self-weight. The finite element method incorporated in the PLAXIS computer software was chosen and parametric studies were carried out by varying the distance of the overlapping length reinforcement, L R , to investigate its effects on the critical failure surface, the factor of safety, the wall displacement and the required tensile strength of reinforcement, when by back-to-back geosynthetic reinforced soil retaining walls are very close. The effects of height and width on the performance of close back-to-back MSE walls were also investigated.

The use of polymeric and metallic geogrid on a full-scale MSE wall/embankment on hard foundation: a comparison of field data with simulation

A full-scale reinforced earth embankment was designed and constructed by the Department of Highways of Thailand on a hard foundation in Phitsanulok Province, Thailand. Two types of reinforcement were used in the embankment. One side was reinforced with polymeric reinforcement consisting of polyester (PET), polypropylene (PP) and high-density polyethylene (HDPE) and referred to as a reinforced steep slope (RSS), with an angle of 70° from horizontal. On the other side, the embankment was reinforced with metallic reinforcement consisting of metallic strips (MS) and steel wire grids (SWG) combined with vertical segmental concrete facing and referred to as a mechanically stabilised earth wall (MSEW). The behaviour of the reinforced soil slope and the mechanically stabilised earth wall on a hard foundation were observed and compared with predictions from the PLAXIS 3D software. The lateral displacements and settlements were very small in the case of the MSEW with inextensible reinforcement. The corresponding lateral and vertical deformations in the RSS were much larger due to its extensible reinforcing materials. The stiffnesses of the reinforcing materials decrease in the following order: MS, SWG, PP, HDPE and PET. The results obtained from three-dimensional (3D) finite element method simulations (using PLAXIS 3D) were in good agreement with the field measurements in terms of vertical and lateral deformations and strains in the reinforcement.

Influential factors affecting drainage design considerations for mechanical stabilised earth walls using geocomposites

Mechanically stabilised earth (MSE) walls are made of unsaturated soil that is strengthened with artificial materials. They have been widely used over the past three decades; however, occasional failures due to inadequate drainage during heavy rainfall have been encountered. Essentially, the reinforced zone of the MSE wall must be protected from water inflow for the system to work satisfactorily. Recent studies have reported on the alternative use of geocomposites to replace the conventional drainage system in MSE walls, typically comprising well-graded gravel. Geocomposites comprise a core material with a large flow channel, which is covered by a nonwoven geotextile. Although it is widely recognised that geotextiles possess a water retention characteristic (WRC), only a few studies have examined the effect of WRC on the seepage responses of MSE walls. In this research, large-scale physical model tests for seepage flow through an MSE wall were undertaken with and without an L-shaped geocomposite drain. Co...

Effects of interface connections on narrowed mechanically stabilized earth walls

In cases where the space available for the mechanically stabilized earth (MSE) walls is less than required, the “narrowed mechanically stabilized earth walls” are usually designed and constructed next to the existing stable slopes/walls. Although such walls have already been used and studied in some projects, the effects of different interface connections on complicated narrowed MSE walls are still not clear. The performance and mechanism of narrowed MSE walls still need further exploration, especially those with complex boundaries. Accordingly, two centrifuge model tests of narrowed MSE walls with thick soil mass above (like having an inside bench) were designed and carried out with full instrumentations. One of the models had mechanical interface connections to the boundary/stable slope and the other one without connections. The authors analyzed the experimental data including settlement, lateral displacement of the facing and earth pressure in the narrowed MSE walls and used limit equilibrium method to help locate critical failure planes. The results indicated that narrowed MSE walls had obvious differential settlement on crest, and around the inside bench the vertical earth pressure was observed to have concentrated and reduced distributions. The analyses indicated that the critical failure plane was bilinear, also different from conventional MSE walls. Mechanically connecting the narrowed MSE wall to the stable slope/wall is an efficient measure to help control the deformation and improve stability of the system, whereas the unconnected wall would experience larger displacement and was prone to collapse. Furthermore, the rupture of model reinforcements suggested that the extension of the upper reinforcements beyond the stable wall/slope face was not enough to ensure external stability.

Deterministic and probabilistic prediction of facing deformations of geosynthetic-reinforced MSE walls using a response surface approach

This paper describes a strategy to compute deterministic and probabilistic estimates of maximum outward deformation of mechanically stabilized earth (MSE) modular block wall structures of particular type, materials and boundary conditions. The reference walls in this study were a group of MSE modular block walls constructed under carefully controlled laboratory conditions. A numerical FLAC model was first demonstrated to give reasonable estimates of measured maximum wall deformations. The model was then used to carry out a sensitivity analysis using a modest number of simulations with different values of key input parameters. Six key control parameters were selected based on the results of the sensitivity analysis and a review of the literature on wall performance. A response surface of full quadratic form was used to predict maximum normalized wall deformations as a function of the six key parameters. The results of a second set of 729 numerical simulations corresponding to the maximum number of possible permutations of three different values for each of the six control parameters were used to find the coefficient terms for the response surface equation using a conventional least squares method. The response surface method (RSM) equation was then used to carry out a large number of Monte Carlo simulations using random sampling of distributions for soil friction angle, surcharge pressure, block–block interface shear stiffness and reinforcement stiffness input parameters. The resulting statistics were used to construct a cumulative distribution function (CDF) for maximum normalized wall deformations. The CDF is then used to link reliability performance-based design to a reasonable range of serviceability criteria for wall deformations generated under operational conditions.

Frictional Resistance of Closely Spaced Steel Reinforcement Strips Used in MSE Walls

AbstractFrictional resistance between soil and steel reinforcements develops as a result of relative displacement at the soil-reinforcement interface and is typically characterized using interface shear or reinforcement pullout tests. The soil-reinforcement interaction between uniformly-graded soils and metal reinforcements has been well-characterized. However, the interface response of well-graded gravelly soils and ribbed steel strips is primarily based on lower-bound estimates of pullout resistance using databases of single isolated reinforcement pullout tests. Increases in horizontal stresses that develop in tall mechanically stabilized earth (MSE) walls are often accounted for in design by reducing the reinforcement spacing; however, the effect of possible frictional interference between closely spaced inextensible reinforcements has not been explored. Two reinforcement pullout apparatuses have been developed to study the soil-reinforcement interaction and effect of reinforcement proximity in a well-...

Monitoring and numerical modelling of an instrumented mechanically stabilised earth wall

The use of mechanically stabilised earth (MSE) structures is becoming widespread, especially for transportation-related infrastructure. The present study is a contribution to a better understanding of MSE wall structural performance, through the analysis of measurements obtained during the construction of a MSE wall for the A4 Portuguese highway, between Amarante and Vila Real – and the development of a numerical model based on the finite-element method. The monitoring programme and equipment are described. The numerical model was developed using a commercially available finite-element software package together with the Mohr–Coulomb elastoplastic constitutive model for the soils. The numerical model was calibrated against measured data. Finally, a parametric analysis was performed to quantify some of the most significant parameters that influence the general behaviour of the wall.

Automated measurement of highway retaining wall displacements using terrestrial laser scanners

Abstract Highway retaining walls are continuously monitored during their construction to ensure the required performance criteria are met. Currently, inspection data are obtained manually by using geotechnical field instrumentations and surveying equipment. The data collection practice is often time-consuming, and erroneous. High precision and accuracy of laser scanners have made them valuable in construction, especially for structural health monitoring and change detection. This paper proposes a new method to (1) extract geometric features of highway retaining walls from the laser-scan data; and (2) use the extracted features as benchmarks for detecting wall displacements. The method is evaluated using Mechanically Stabilized Earth (MSE) walls, which are one of the most widely used highway retaining walls in the United States. Specifically, we investigated the hypothesis that the horizontal joints between the panels of an MSE wall could be used in point clouds to detect the wall's displacement with the required accuracy for condition assessment purposes. First, the proposed methodology was tested using the actual point cloud data of an MSE wall. The proposed feature extraction method was able to extract the horizontal joints with 94.26% accuracy. Then to test the generalizability of the method, we created 3D models of an MSE wall by using the wall's actual dimensions and geometric parameters. These models were used to examine the validity of the hypothesis for 36 laser scanner settings and 15 wall displacement scenarios. Laser scanner sensor parameters along with the 3D models were imported into a simulation environment to generate synthetic data to evaluate accuracy of the proposed method. The method was also tested with 3D models of two more types of MSE walls for a selected number of simulation scenarios. The results demonstrated that the proposed method is capable of measuring wall displacements with an average error of 0.9 mm.

Driving Discomfort of MSEW and Inverted T-type Abutments

Many highway bridges have bumps at the bridge approaches and expansion joints, which is a serious problem from the user point of view. Also, most of highway abutments are made to resist the vertical loads and lateral earth pressure, which induces massive abutment sections compared to the bridge pier. In order to solve these problems, a mechanically stabilized earth wall (MSEW) abutment was developed to reduce height differences at the bridge approach and to reduce the abutment section. The MSEW abutment was compared to the conventional inverted T-type abutment with regard to driving comfort using a vehicle dynamic simulation. Based on the .result, the MSEW abutment provides more driving comfort than the inverted T-type abutment.

Temperature Distribution in Mechanically Stabilized Earth Wall Soil Backfills for Design Under Elevated Temperature Conditions

Two-dimensional (2D) transient numerical thermal modeling was undertaken using ansys fluent v12.1 software to estimate distribution of soil backfill temperatures in a typical mechanically stabilized earth (MSE) wall. The modeling was calibrated using field-measured temperature data from the Tanque-Verde MSE wall in Tucson, Arizona (AZ) in which computed temperature data were found to be within ±5% of the field data. The calibrated model predictions for Las Vegas, Nevada (NV) showed an overall average soil backfill temperature of 34.3 °C relative to a maximum outside surface temperature of 51.6 °C. Such a high average soil backfill temperature calls for modification of design procedures since conventional designs are based on geosynthetic tensile strength determined at 20 °C.

Reinforcement Load and Compression of Reinforced Soil Mass under Surcharge Loading

AbstractReinforced soil composites and mechanically stabilized earth (MSE) walls are frequently employed to carry large surcharge or footing loads. For the safety and serviceability of these reinforced soil structures, it is necessary to analyze the reinforcement load and compression of reinforced soil mass subjected to surcharge loading. In the research reported in this paper, an analytical method proposed by the writer was extended to meet these needs. The extended analytical method explicitly considers soil nonlinearity, soil dilatancy, soil-reinforcement interaction, and end restrictions of reinforced soil mass. Both plane-strain and triaxial stress states can be considered in the method. The applicability of the method for reinforcement load was validated against eight large-scale tests of reinforced soil mass or MSE walls, and the method for reinforced soil compression was validated against two large-scale tests. The compressions of four reinforced soil minipiers under surcharge loading were also pr...

MSE walls as bridge abutments: Optimal reinforcement density

Geosynthetics have become a common means of soil reinforcement in earth retention structures; however, the efficacy of its application can be enhanced dependent on use. Geosynthetic design often involves uniform spacing of reinforcements throughout a retaining structure. However, optimization of reinforcement spacing can have a significant impact on stability, especially when subjected to a finite surcharge, as demonstrated by a series of numerical simulations using Limit Analysis (LA). Use of a rigorous LA tool that employs an algorithm to computationally determine the critical collapse state allows for the determination of optimal reinforcement placement in Mechanically Stabilized Earth (MSE) walls based on both spacing and concentration – specifically at the toe or crest of the structure. The effects on stability are evaluated for two scenarios: (1) a “top-down” approach where reinforcement spacing density is progressively increased from the top of the wall downwards and (2) a “bottom-up” approach where the spacing density is progressively increased from the toe of the wall upwards. Presented within this study are the results of comprehensive parametric analysis varying reinforcement spacing, reinforcement strength and footing location, highlighting the stability benefits of top-down and bottom-up reinforcement density approaches. Placement of dense reinforcements near the crest of surcharge-supporting walls had a notable benefit for stability, while placement of dense reinforcements at the toe improved stability for walls without a surcharge. Hence, reinforcement spacing tailored to specific wall functions may provide significant economic benefits when considering reduced reinforcement needs, lessened tensile strength requirements, or shortened bridge deck length based on surcharge placement – all by concentrating reinforcement density at a specific height of a structure.

Instrumented Mechanically Stabilized Earth Wall Reinforced with Polyester Straps

A wall of mechanically stabilized earth (MSE) with large concrete panel facing, reinforced with polyester straps, is an emerging technology in the United States. To enable the design of this wall system through AASHTO's methodology, the system's classification of extensibility must be established. Therefore, a section of such a wall was instrumented as part of a newly constructed interchange on I-95 at Christiana, Delaware. The section was one of several similar walls that allow access to bridges composing the interchange. Monitoring included tensile force distribution at selected straps and force at the connection of a few straps to the large concrete panels. The recorded data were from April 2012, the beginning of construction of the instrumented wall, to June 2014, about 20 months after construction ended. When this reinforced wall system was classified as extensible, the maximum load in the straps and connections was about one-fourth the predicted value from using the AASHTO method, and the lengths of the reinforcing straps were adequate to enable the wall to sustain the required loads. The indicated conservatism of the measured versus the predicted maximum load in the reinforcements was attributed to significant underestimation of the reinforced soil strength in AASHTO's design and, potentially, to a large toe resistance. The data suggest that AASHTO's design, which is based on extensible reinforcement for this family of MSE walls, is adequate.

Evaluation of Interface Shear Strength Properties of Geogrid Reinforced Foamed Recycled Glass Using a Large-Scale Direct Shear Testing Apparatus

The interface shear strength properties of geogrid reinforced recycled foamed glass (FG) were determined using a large-scale direct shear test (DST) apparatus. Triaxial geogrid was used as a geogrid reinforcement. The geogrid increases the confinement of FG particles during shear; consequently the geogrid reinforced FG exhibits smaller vertical displacement and dilatancy ratio than FG at the same normal stress. The failure envelope of geogrid reinforced FG, at peak and critical states, coincides and yields a unique linear line possibly attributed to the crushing of FG particles and the rearrangement of crushed FG after peak shear state. The interface shear strength coefficientαis approximately constant at 0.9. This value can be used as the interface parameter for designing a reinforced embankment and mechanically stabilized earth (MSE) wall when FG is used as a lightweight backfill and triaxial geogrid is used as an extensible earth reinforcement. This research will enable FG, recently assessed as suitable for lightweight backfills, to be used together with geogrids in a sustainable manner as a lightweight MSE wall. The geogrid carries tensile forces, while FG reduces bearing stresses imposed on the in situ soil. The use of geogrid reinforced FG is thus significant from engineering, economical, and environmental perspectives.

Laboratory evaluation of governing mechanism of frictionally connected MSEW face and implications on design

This paper presents a laboratory evaluation of purely frictionally connected geotextile and concrete facing block of Mechanically Stabilized Earth Wall (MSEW) systems. The study focuses on investigating the governing failure mechanism along the wall face, as determined from the pullout of reinforcement in between the facing blocks (herein referred as pullout mechanism) and sliding of the blocks over the geotextile, where the reinforcement stays stationery (herein referred as direct shear mechanism). A total of seventy-two tests were performed to investigate the effect of laboratory specimen size, difference in geotextile reinforcement, and repeatability of the test results. Overall, the results showed that at lower normal loads, sliding of the blocks over the geotextile reinforcement along the wall face is more likely to occur before the pullout of the geotextile in between the blocks. At higher normal loads, this order is reversed and pullout of the geotextile appears to occur first. The test results also indicated that the size of the specimen tested in the laboratory frictional connection evaluation has an effect on the measured connection strength.

Limit Analysis Optimization of Design Factors for Mechanically Stabilized Earth Wall-Supported Footings

Increasingly in the past few decades, mechanically stabilized earth (MSE) walls have been a common choice for earth retention solutions due to their ease of construction, cost-effective design, and flexibility. However, limit state capacity of footings atop a MSE structure, such as a bridge abutment, is a complex and non-intuitive stability problem dependent on factors like reinforcement layout and strength, location of footing, and failure mechanism. In order to evaluate the effects of these factors, a parametric study was performed utilizing a robust tool in the framework of limit analysis of plasticity. It demonstrates the effect of the parameters on bearing capacity and optimized reinforcement efficiency, all in context of an existing MSE wall-supported bridge abutment. It is shown that closely spaced reinforcement layouts allow for more efficient reinforcement behavior and improved bearing capacity nearer to the wall facing, while inverse, detrimental trends were observed for wider reinforcement spacing. The implications of this analysis illustrate that an optimized approach involving footing location, reinforcement strength, and reinforcement spacing allow for reduced bridge deck length requirements. This clearly has economic consequences. The optimization of these design factors is especially applicable to MSE wall-supported bridge abutments since the location of the footing affects expensive elements such as length of girders. Fundamentally, this work adds to the limited existing knowledge of bearing capacity of footings on top of MSE walls.

Strength and Creep Characteristics of Reclaimed Asphalt Pavement–Sand Blend Backfill in Mechanically Stabilized Earth Walls

Using reclaimed asphalt pavement (RAP) as a backfill for mechanically stabilized earth (MSE) walls provides both environmental and economic benefits by allowing in situ recycling for road widening projects. RAP, a well-drained granular material, has lower reinforcing strip pullout strength than A-3 sand and creeps under constant load. The objective of this research was to determine whether blending RAP with A-3 sand would improve RAP's performance in MSE applications. Laboratory specimens of 100% RAP, 100% A-3 sand, and 50% RAP–50% sand blends were compacted by the modified Proctor method and tested for vertical creep with one-dimensional oedometer compression tests at three stress levels to simulate different depths behind an MSE wall. Large-scale test pit testing was performed to determine reinforcing strip ultimate lateral pullout strength; pullout creep at 25%, 50%, and 75% of ultimate pullout; and vertical creep at the three overburden stress levels used in laboratory testing. RAP–sand blends had higher density, friction factors, and pullout strength and developed ultimate pullout strength at lower displacements than either 100% sand or 100% RAP. The RAP–sand blends exhibited more horizontal and vertical creep than 100% sand but significantly less creep than 100% RAP.