Geosynthetic-reinforced Soil Research Articles

Design and construction of geosynthetic-reinforced soil structures for Hokkaido high-speed train line

The southern part between Shin-Aomori Station (at the north end of Main Island) and Shin-Hakodate Station (at the south end of Hokkaido Island) of a new high-speed train line called Hokkaido Shinkansen is nearly completed as of the end of 2013 and will be opened in 2015. In a range of 37.3 km at the south end of Hokkaido Island, a number of various types of geosynthetic-reinforced soil (GRS) structure were constructed: i.e., (1) GRS retaining walls (RWs) having full-height rigid facing for a length of about 3.5 km, having fully replaced the conventional type RWs; (2) in total 29 GRS bridge abutments, having fully replaced the conventional type bridge abutments; (3) a GRS integral bridge, the world-first one at Kikonai; (4) three GRS box culvert structures integrated to adjacent GRS RWs; and (5) nine GRS protection structures at the tunnel entrance. These GRS structures are those that have been constructed most densely ever for railways, which is definitely so for high speed trains. In this paper, the design and construction of these GRS structures is described while several lessons learned from this project are summarized.

Geosynthetic-Reinforced Soil Structures for Railways in Japan

Geosynthetic-reinforced soil retaining walls (GRS RWs) have been constructed for a total length of about 150 km as of June 2013 mainly for railways, including high-speed train lines. After a full-height wrapped-around GRS wall has been constructed and the major residual deformation of the backfill and supporting ground has taken place, a full-height rigid (FHR) facing is constructed by casting-in-place concrete on wrapped-around wall face in such that it is firmly connected to the reinforcement layers. A number of this type GRS RWs performed very well during the 1995 Great Kobe and the 2011 Great East Japan Earthquakes. The seismic design code for railway soil structures has been revised taking into account such high-level seismic loads as experienced during the 1995 Kobe EQ. A number of conventional-type RWs and embankments collapsed during these and other earthquakes, heavy rains, floods, and storm wave actions. Many of them were reconstructed to this type GRS RWs and geosynthetic-reinforced embankments. Among a couple of new bridge types that have been developed, GRS integral bridge comprises a continuous girder of which both ends are structurally integrated without using bearings to the top of the facings of a pair of GRS RWs. The first prototype was constructed for a high-speed train line in 2011 and three more were constructed to restore bridges that fully collapsed by great tsunami during the 2011 Great East Japan EQ.

Full-Scale Investigation of Differential Settlements beneath a Geosynthetic-Reinforced Soil Bridge Abutment

Geosynthetic-reinforced soil bridge abutments can be constructed quickly and at lower cost than traditional alternatives. However, the lack of deep foundation elements contributes to a perceived vulnerability to differential settlements at the foundation level, either because of scour undermining or compressible soils beneath the foundation. A full-scale abutment was constructed, instrumented, and subjected to carefully controlled differential settlements. Data collected during construction indicate that the abutment adequately supported the surcharge load. Subsequent large differential settlements imposed at the foundation level resulted in small surface expression of differential settlements, redistribution of stresses within the reinforced fill, and adequate support at the level of the superstructure. However, the facing units appeared vulnerable to removal, potentially exposing the reinforced fill to erosion. Three measures to help mitigate this risk are discussed: pinning the concrete masonry units (CMUs) near the corner of the abutment, adding a protective wrap behind the CMUs to encapsulate the fill, and increasing the reinforcement length at the base of the abutment.

한계평형해석과 수치해석에 의한 붕괴된 보강토 옹벽 복구 사례에 관한 연구

보강토 옹벽의 적용사례가 날로 증가하고, 일반화됨에 따라 그에 따른 피해도 종종 발생하고 있다. 최근 들어 게릴라성 집중강우 등으로 인해 보강토 옹벽이 붕괴되는 사례가 종종 발생되면서 붕괴된 옹벽의 복구방법에 대한 관심이 커지고 있다. 이와 같은 배경 하에 이 논문에서는 (1) 피해상태로 쏘일네일링을 사용하여 복구하는 안과 (2) 붕괴된 보강토 옹벽에 대해 보강토체 제거 후 재시공하는 안에 대해 한계평형해석과 수치해석을 수행하여 안정성과 거동을 분석하여 설계하고 시공한 사례를 소개하였다. 검토결과, 쏘일네일링 복구 옹벽의 경우 보강콘크리트 전면벽체에서 수평변위와 전단변형률 등은 거의 발생하지 않았으나, 재시공 옹벽은 수평변위의 경우 벽체 중앙부에서 발달하고 전단변형률은 옹벽의 저부를 중심으로 나선 형태로 발달하는 것으로 나타났다. 따라서, 피해옹벽은 쏘일네일링으로 복구하였으며, 일련의 시공과정을 소개하였다. Geosynthetic reinforced soil (GRS) walls have been increasingly applied recently due to its numerous geotechnical engineering applications. However failure occurs in some cases of constructed GRS walls. These GRS wall failures are mostly due to the unpredictable characteristics of intensive rainfall. Hence, the need for new and innovative ideas for rehabilitation methods has been getting attention. This paper introduces a case study for the design and restoration method of collapsed GRS wall using Limit equilibrium and Numerical Analyses. Restoration method includes: (1) soil nailing without backfill excavation and (2) reconstruction with GRS wall after collapsed backfill excavation. Analyses results show minimal horizontal displacements and shear strain on the reinforced concrete facing for the restoration case with soil nailing. On the other hand, horizontal displacements are developed in the middle of the mortar block facing and shear strains are developed at the bottom facing with spiral curves for the reconstructed GRS wall after collapsed backfill excavation. Therefore, the collapsed GRS wall was restored with the soil nailing without backfill excavation and its construction procedures are discussed in this paper.

Limit equilibrium analyses of geosynthetic-reinforced two-tiered walls: Calibration from centrifuge tests

The use of limit equilibrium (LE) for predicting performance at failure in centrifuge two-tiered geosynthetic-reinforced soil (GRS) wall models was evaluated. The variables considered in the centrifuge testing program were offset distance, D, and reinforcement length. Parametric studies were first performed to evaluate the effects of modeling assumptions of reinforcement force on LE results, including reinforcement force orientation, and reinforcement tensile load distribution with depth. The suitability of LE for the analysis of two-tiered GRS walls and design implications were then discussed. According to LE results, good agreement existed between LE and centrifuge models in locating failure surfaces. The LE results also indicate that offset distance correlated negatively with the effective overburden pressure on the reinforcement and the resulting confined Tult of the reinforcement. The critical offset distance of 0.7 times the lower tier height was determined when the decrease in confined Tult value as D increases reached a constant value. The LE analyses show that minimum reinforcement length of the upper tier (L1,min = 0.7H1), according to the compound design in FHWA design guidelines, is insufficient, such that failure surfaces do not pass through all reinforcement layers in the upper tier. Last, the effect of offset distance on the normalized reinforcement tension summation coefficient, KT, indicates that single and independent wall models yielded a single consistent KT value. For compound wall models, the KT value decreases as offset distance D increases.

External seismic stability of vertical geosynthetic-reinforced soil walls using pseudo-static method

Analysis of external stability of vertical geosynthetic-reinforced soil (GRS) walls is very important in the seismic prone zone. The scope of this paper is to obtain required minimum reinforcement length, L min, for external seismic stability of vertical GRS walls by pseudo-static limit equilibrium method. Then, L min can be calculated to resist sliding, eccentricity, and bearing capacity failure modes. The parameters considered include both horizontal and vertical seismic coefficients (k h and k v ), surcharge load (q), wall height (H) and the properties of retained backfill, GRS, and foundation soil. Results show that L min against sliding failure mode, L min,S , increases more quickly than that against the other two failure modes with the increase in k h , q, or unit weight of retained backfill, γ b , while L min,S decreases more quickly than that against the other two failure modes with increase in friction angle of retained backfill, ϕ b , or unit weight of GRS, γ r . For the different failure modes, the effect of k v on L min is not identical with the change of k h , and in addition, L min/H will tend to remain unchanged with the increase in H. In general, L min against bearing capacity failure mode, L min,BC, is larger than L min against the other two failure modes. However, L min,BC will be less than L min against eccentricity failure mode, L min,E , for k h exceeding 0.35, or friction angle of foundation soil, ϕ f , exceeding 37°, and L min,BC will also be less than L min,S for friction angle of GRS, ϕ r , being no more than 26°.

Role of strain magnitude on the deformation response of geosynthetic-reinforced soil layers

ABSTRACT: This paper reports an investigation into the deformation response of geosynthetic-reinforced soil layers to evaluate the level of strain required to mobilize reinforcement mechanisms during surface loading. Specifically, an assessment of data from previous studies reporting surface settlements of reinforced and unreinforced soil layers during monotonic or cyclic loading to failure is complemented with new dynamic tests performed at small- and medium-strain magnitudes. A series of test sections with and without geosynthetic reinforcement were characterized using a Vibroseis shaker truck as a dynamic loading source. The behavior of uniform sand layers under relatively small strain magnitudes (shear strain magnitudes less than 0.2%) were characterized using embedded geophones. Shear and normal strain distributions within the test sections were measured as a function of depth during application of surface shear loading. The presence of geosynthetic reinforcement (either geogrid or geotextile) at a depth of 254 mm below the surface of the sand layer does not significantly alter the distributions of shear strain or vertical normal strain relative to an unreinforced control section for the magnitudes of shear and compressive strains applied to the soil surface. The behavior of aggregate base layers overlying sand layers under medium strain magnitudes (surface deflections up to 25 mm) were characterized using an array of LVDTs on the soil surface. Surface deflection basins were measured during application of several thousand cycles of compressive loading. The presence of a geogrid within the aggregate base layer or a woven geotextile beneath the aggregate base layer were observed not to lead to a change in the surface deflection basins up to a maximum deflection of 25 mm. These observations, together with results from the literature, indicate that reinforcement mechanisms such as lateral restraint and the tensioned-membrane effect may not be mobilized until reaching relatively large displacements in some soil layers.

“DESIGN OF REINFORCED SOIL WALLS BY LRFD APPROACH”

In this paper, the detail of Load and Resistance Factor Design (LRFD) approach which is based on reliability concept for the design of Reinforced Soil (RS) walls is presented. For conventional methods i.e Allowable Stress Design (ASD) Method, the factor of safety is applied only to resistance and loads are considered without variations. For LRFD method, factor of safeties are applied for both load and resistance. Due to availability of large statistical data and economy, this method is preferred. An attempt is made to solve one numerical example of geosynthetic RS walls due to soil selfweight plus permanent uniform surcharge using LRFD as well as other conventional methods (ASD) viz. FHWA, Modified Rankine, NCMA and B.S Code Methods and the results of the LRFD methods are compared with conventional design methods and concluding remarks are presented. The various equations are obtained based on various curves plotted by using ASD and LRFD approaches. From these equations it is clear that if FOS against tensile rupture is known for any RS wall having 7m height and same properties and environmental conditions as mentioned in current study then FOS against pullout failure and pullout capacity can be computed for these walls.

Sand-geotextile interface characterisation through monotonic and cyclic direct shear tests

ABSTRACT: For the design and performance analyses of geosynthetic-reinforced soil structures under repeated loading, such as those induced by compaction, traffic and earthquakes, the understanding of cyclic soil–geosynthetic interface behaviour is of great interest. Nevertheless, experimental data concerning this type of behaviour are very scarce. A laboratory study was carried out and is described in this paper. This paper presents the behaviour of an interface between a silica sand and a high-strength geotextile under monotonic and cyclic loading conditions. A large-scale direct shear test device able to perform load- or displacement-controlled cyclic tests was used. The results obtained are presented and discussed, especially in terms of interface shear stiffness and damping ratio. Monotonic direct shear tests indicated that the coefficients of interaction for the sand–geotextile interface depend on the confining pressure. Cyclic direct shear tests indicated that the interface stiffness tends to increase during the first loading cycles, exhibiting slight variation after 10 cycles. Slightly higher values of shear stiffness and damping ratio were reached with displacement-controlled cyclic direct shear tests. The cyclic loading of the interface did not lead to the degradation of the post-cyclic peak shear strength; however, the post-cyclic shear strength for large displacements exhibited an important decrease.

Load-Carrying Capacity and Required Reinforcement Strength of Closely Spaced Soil-Geosynthetic Composites

AbstractIn current design methods for reinforced soil walls, it has been tacitly assumed that reinforcement strength and reinforcement spacing play an equal role. This fundamental design assumption has led to the use of larger reinforcement spacing (0.3–1.0 m) in conjunction with stronger reinforcement to reduce construction time. Recent studies, however, have clearly indicated that the role of reinforcement spacing is much more significant than that of reinforcement strength. With closely spaced (reinforcement spacing ≤0.3 m) reinforcement, the beneficial effects of geosynthetic inclusion is significantly enhanced, and the load-deformation behavior can be characterized as that of a composite material. A reinforced soil mass with closely spaced geosynthetic reinforcement is referred to as geosynthetic-reinforced soil (GRS). In this study, an analytical model is developed for predicting the ultimate load-carrying capacity and required reinforcement strength of a GRS mass. The model was developed based on a...

GRS防潮堤の耐越流侵食性に関する実験的・理論的検討

We recently applied a geosynthetic-reinforced soil (GRS) technology to increase the resistance of tsunami barrier to an overflow erosion. In the present study, we performed a series of model tests in the laboratory to evaluate the resistance of previous and new tsunami barriers on the strength against the over-flowing tsunami. We examined dynamical balance of concrete panels covering on the crest and its outflow conditions. The test results indicated that the GRS seawall has a function of tsunami barrier that has a high resistance to erosion due to tsunami overflow. The length of geogrid layers required to prevent outflow of concrete panels was evaluated quantitatively under various tsunami heights.

Seismic testing of model-scale geosynthetic-reinforced soil walls

This paper presents an experimental study on a series of reduced-scale model GRS walls with Full-Height-Rigid facings conducted on a shake-table at the University of Canterbury. Each model was 900 mm high, reinforced by five layers of stiff Microgrid reinforcement and constructed of dry dense Albany sand. The ratio of geogrid length L to wall height H, L/H, was varied from 0.6 to 0.9, while the wall inclination was generally vertical (90° to horizontal) with 70° for one test. During sinusoidal shaking, facing displacements and accelerations within the backfill were recorded. Failure for all models was predominantly by overturning, with some small sliding component generated in the final shaking step. An increase in L/H resulted in a decrease in wall displacement, while a decrease in wall inclination from the vertical resulted in similar benefits. Detailed analysis of the deformation of one of the tests is presented. During testing, global and local deformations within the backfill were investigated using two methods: the first utilised coloured horizontal and vertical sand markers placed within the backfill; the second utilised high-speed camera imaging for subsequent analysis using Geotechnical Particle Image Velocimetry (GeoPIV) software. GeoPIV enabled strains to be identified within the soil at far smaller strain levels than that rendered visible using the coloured sand markers. These complementary methods allowed the spatial and temporal progressive development of deformation within the reinforced and retained backfill to be examined.

Soil-structure interaction analysis of beams resting on multilayered geosynthetic-reinforced soil

In this paper, soil-structure interaction analysis has been presented for beams resting on multilayered geosynthetic-reinforced granular fill-soft soil system. The soft soil and geosynthetic reinforcements are idealized as nonlinear springs and elastic membranes, respectively. The governing differential equations are solved by finite difference technique and the results are presented in non-dimensional form. It is observed from the study that use of geosynthetic reinforcement is not very effective for maximum settlement reduction in case of very rigid beam. Similarly the reinforcements are not effective for shear force reduction if the granular fill has very high shear modulus value. However, multilayered reinforced system is very effective for bending moment and differential settlement reduction.

FEM Simulation of Earth Pressure on Geosynthetic-Reinforced Soil Retaining Wall under Variable Rate Loading

On the basis of the Dynamic Relaxation method, a nonlinear finite element method (FEM) analysis procedure was developed for the geosynthetic-reinforced soil retaining wall. The FEM procedure technique incorporated the unified three-component elasto-viscoplastic constitutive model which can consider the rate-dependent behavior of both the backfill soil and the geosynthitic reinforcement. A simulation was performed on a physical model test on geosynthetic-reinforced soil retaining wall to validate the presented FEM. Extensive finite-element analyses were carried out to investigate the earth pressure distributions from the back of retaining wall under variable rate loading. It is shown that this FEM can well simulate the rate-dependent behavior and the earth pressure of geosynthetic-reinforced soil retaining wall.

Predictive modeling on seismic performances of geosynthetic-reinforced soil walls

This paper presents the results of numerical parametric study of free-standing simple geosynthetic-reinforced soil (GRS) walls under real multidirectional ground motion shaking. The predictions were made using a validated finite element computer program. Design parameters, such as (1) wall height, (2) wall batter angle, (3) soil friction angle, (4) reinforcement spacing, and (5) reinforcement stiffness, were evaluated in the study. Prior to the parametric study, the extent of finite element model (FEM) boundary was verified in order to minimize the boundary effect. Results of parametric study were compared against the values determined using the Federal Highway Administration (FHWA) allowable stress design methodology. It was found that the FHWA methodology overestimates the reinforcement tensile load as compared to the FEM results. Multivariate regression equations were developed using FEM results for the various seismic performances based on multiple design parameters that are essential in the design of GRS walls. In particular, the prediction equations for wall facing horizontal displacement, wall crest settlement, and reinforcement tensile load are presented. The prediction equations can provide first-order estimates of the seismic performances of free-standing simple GRS walls.

Evaluation of the effect of compaction on the behavior of geosynthetic-reinforced soil walls

This paper presents a physical model study of the influence of compaction on the behavior of geogrid-reinforced soil walls. Experiments were accomplished in a facility at the Geotechnical Laboratory of COPPE/UFRJ. For soil compaction, two different types of hand-operated compactors were used: a vibrating plate and a vibratory tamper. Equivalent vertical stresses for the vibrating plate (referred to as the “light compactor”) were much lower than the corresponding value of the vibratory tamper (referred to as the “heavy compactor”). Mobilized tension along the reinforcements and external and internal displacements of the wall were monitored. The results showed that the effect of soil compaction is not limited to a reduction of the soil void ratio. Compaction has led to a significant increase in the horizontal stress inside the reinforced soil mass and generates a kind of pre-consolidated material. Analyses of results showed that compaction has played a decisive factor in terms of the reinforcement tensions and post-construction displacements. The connection load was much less in a wall with heavy compaction than that in a wall with light compaction. Results also showed that the position of maximum tensile force mobilized in the reinforcements was nearer to the face in the wall with heavy compaction. On the other hand, the mobilized tension measured along the reinforcement layers at the end of construction in the wall where heavy soil compaction was used was much higher than the values of tension measured in the wall where light soil compaction was applied. Nevertheless, it was observed that the difference in the mobilized tensions in the reinforcements of these two walls decreased with increase in the value of the external surcharge load.

FEM Analysis on Tensile Force of Geosynthetic Reinforcement Arranged in GRS-RW Considering Variable Loading Rate

The combined effects of the rate-dependent behavior of both the backfill soil and the geosynthetic reinforcement have been investigated, which should be attributed to the viscous property of material. A nonlinear finite element method (FEM) analysis procedure based on the Dynamic Relaxation method was developed for the geosynthetic-reinforced soil retaining wall (GRS-RW). In the numerical analysis, both the viscous properties of the backfill and the reinforcement were considered through the unified nonlinear three-component elastic-viscoplastic model. The FEM procedure was validated against a physical model test on geosynthetic-reinforced soil retaining wall with granular backfill. Extensive finite-element analyses were carried out to investigate the tensile force distributions in geosynthetic reinforcement of geosynthetic-reinforced soil retaining wall under the change of loading rate. It is found from the analyses that the presented FEM can well simulate the rate-dependent behavior of geosynthetic-reinforced soil retaining wall and the tensile force of geosynthetic reinforcement arranged in retaining wall.

Use of geosynthetics to improve seismic performance of earth structures

After reviewing seismic performance of earth structures based on case histories in Japan and relevant model test results, advantages of using geosynthetics in improving their seismic performance are demonstrated. As one of the successful applications, geosynthetic-reinforced soil retaining walls are highlighted, focusing on several influential factors such as facing rigidity, arrangement and properties of reinforcements, and backfill and subsoil conditions. In addition, further applications of the reinforcement method using geosynthetics are introduced, which include combination with other reinforcement methods, application to bridge abutments and piers, and application to ballasted railway tracks.

Stress distribution and development within geosynthetic-reinforced soil slopes

ABSTRACT: Numerical methods combined with centrifuge tests are used to investigate the distribution and development of soil stresses and reinforcement tensile loads in geosynthetic-reinforced soil (GRS) structures. In this study, system stability indicated by the factor of safety (FS) of GRS slopes is calculated by limit equilibrium analysis. Stress information under various stress states is evaluated using finite element analysis. Advanced models and an integration algorithm are implemented in finite element code to enhance the simulation results. The proposed numerical models are validated by centrifuge tests of two GRS slopes with different backfill densities. Numerical results indicate that soil stress mobilisation can be described by the soil stress level S, which is defined as the ratio of the current stress status to peak failure criteria. For both slope models, as loading increases, backfill stresses develop and propagate along the potential failure surface. Mobilisation of soil stress was non-uniform along the failure surface. Immediately after the stress level reaches peak (S = 1), strength softening initiates at the top and toe of the slope at approximately FS = 1.2. The slope settlement rate and reinforcement tensile load increase significantly when soil softening begins. The softening occurs randomly and irregularly along the failure surface, and the formation of the soil-softening band completes at approximately FS = 1.1. The failure surface corresponds to the locus of intense soil strains and the maximum tensile loads at each reinforcement layer. Additionally, the numerical results show that the initiation of soil softening and the failure of the slope occurred earlier in the slope model with low backfill density. The numerical results support the view that peak shear strength, not residual shear strength, governs system stability. Last, the distribution of maximum reinforcement tensile loads with depth was highly uniform at low g-level and became trapezoidal at high g-level. The peak value was located at approximately mid-height of the reinforced slopes. This observation contradicted the triangular distribution with depth assumed in current design methodologies for geosynthetic structures.

Dynamic stability of geosynthetic-reinforced soil integral bridge

ABSTRACT: To evaluate the dynamic stability of different bridge types, the results from a series of shaking-table tests on small-scale models of the following bridge types were analysed within the framework of the single-degree-of-freedom theory: (1) a conventional bridge (CB), comprising a girder (i.e. deck) supported via a pair of movable and fixed bearings (i.e. shoes) by gravity-type abutments (without a pile foundation) having unreinforced backfill; (2) a GRS-RW bridge, comprising a girder supported via a pair of movable and fixed bearings by a pair of sill beams placed on the crest of a pair of geosynthetic-reinforced soil-retaining walls (GRS-RWs) having a stage-constructed full-height rigid facing; (3) an integral bridge (IB), comprising a girder integrated to a pair of abutments (without bearings) and unreinforced backfill; (4) a GRS integral bridge, comprising a girder integrated to the abutments (in the same way as the IB bridge) while the backfill is reinforced with geosynthetic layers connected to the facings (in the same way as the GRS-RW bridge); and (5) a GRS integral bridge with a cement-mixed soil zone of rectangular prismatic or trapezoidal shape immediately behind the facing. The following is shown: the stability of the bridge against dynamic excitations increases: (1) with an increase in the initial natural frequency via an increase in the initial stiffness; (2) with a decrease in the decreasing rate of stiffness during cyclic loading (i.e. an increase in the dynamic ductility); (3) with an increase in the damping energy dissipation capacity near and at failure; and (4) with an increase in the dynamic strength. With the GRS integral bridge, the structural integration and geosynthetic-reinforcing of the backfill, as well as cement-mixing of the backfill immediately behind the facings, all contribute to the evolution of these four factors. The natural frequency can then always be kept much higher than the predominant frequency of ordinary design earthquake motion, the response acceleration is kept sufficiently low, and the dynamic stability can be kept very high.