Examining The Reinforced Earth Composite System Construction Essay

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Stability and economy of earth structures including the soil-structure interaction systems could be achieved through incorporation of the reinforcement in the earth mass developed properly. Through incorporation of reinforcement, the fill could stand with a vertical face without the support of a retaining wall. This has revolutionized the design and construction of earth works, such as are needed for the approaches to the bridges, the roadway embankments etc.

Design of reinforced earth retaining wall, in essence constitutes a soil-structure interaction system. A retaining wall without any kind of reinforcement constitutes one extreme, whereas absence of retaining wall, with the provision of reinforcement represents another extreme. It is likely, that in a given situation, an optimum solution lies in between these extremes. The search for the optimum design should be governed by this kind of consideration.

The construction of the reinforced earth retaining wall has to be over naturally available soil strata. Hence the compressibility of the same enters as yet another parameter affecting significantly the behavior of structural system. A foundation stabilizing treatment of soil strata may be needed for the safe design of the system. In a given situation the search for appropriate foundation system needs to be directed amongst possible alternatives.

Finally, the earth mass encountered in the foundation strata, and the soil mass compacted through layers of selected soils to build up the required height needs to be subjected to a non-linear deformation analysis. This is because, in general, the soil masses have stress-strain dependent modulus of elasticity and poisons ratio. To this, the technique of sequential construction adopted for raising the embankment, imparts a need for the non-linear finite element deformation analysis. Due to all these, the design problem acquires a sufficiently complicated character.

In the present paper, an attempt has been made to study the different conventional and advanced methods of analysis and design of reinforced earth walls with their features and limitations with numerical case studies. In view of the above investigation the detailed literature review for last 20 years was carried out. Such study will be effective tool and helpful to those who are engaged in research in this area.

Literature Review:

An extensive literature survey on various methods as applied to analysis and design of reinforced earth retaining wall has been carried out referring various journals, conference proceedings, standard codes and text books.

Romstad K. M., et.al., [01] have stated that in composite finite element analysis, the reinforced soil is characterized as a "homogeneous" composite structure with the properties of the composite material. Superposition of the stiffness of the soil and reinforcement forms the composite element stiffness. There are several shortcomings associated with the composite approach. e.g., information about the interaction between the soil and reinforcement, such as bond stress, stress concentration in the soil due to geometric discontinuities as a result of the presence of the reinforcement, and the edge effect due to the local transfer of stresses between the soil and the reinforcement at the boundaries, are usually not available. Therefore in a composite finite element analysis at the edge, the strains and forces in the reinforcements are predicted to be larger than actual measured values of the structure, thus overestimating the effectiveness of the reinforcement.

They further pointed out that in the discrete approach, the reinforcement system is considered as a heterogeneous body in which the soil element and reinforcing elements are separately represented by different material properties. The important advantage of this type of model is that detailed information is directly obtained about the behavior of interface between the soil and the reinforcement, and stress concentration in the soil due to reinforcing members.

Collin J. G., [02] in his doctoral research, studied the design of earth wall and stated that in reality reinforced soil wall represent a composite system including the facing, backfill material, reinforcement and foundation. The performance of the reinforced soil wall will rely on the interaction between its components and to a large extent; this interaction will arise from relative motions at the interface between the soil and the reinforcement. Also, relative strains and deformations occur under working conditions. These requirements can be established by construction and monitoring of a large number of full-scale test walls.

Unfortunately, the cost of performing and suitable monitoring of a sufficiently large number of full scale walls can be so large that this is often not practical. To overcome this, finite element analysis has been commonly employed for improved analysis and parametric studies. For performing a realistic finite element analysis of a reinforced soil structure, the computer procedure should have the capability of modeling the construction sequences, structural elements, soil elements, and interface elements that allow nonlinear behavior and relative motions between the soil and reinforcement.

Bathurst R. J., [03] has of the opinion that reinforced soil retaining structures have been widely used because they offer economic benefits compared to conventional retaining systems. In designing reinforced soil structures, there are two main approaches: Limit equilibrium methods and numerical (finite) element methods. The limit equilibrium-based approaches include two groups; force equilibrium analysis and strain compatibility analysis. Numerous simple design methods based on the concept of limit equilibrium do not provide information concerning deformations or stress distributions in either the soil or the reinforcement. Also, boundary conditions as well as stress equilibrium at each point within the reinforced mass and along the slippage surface are not involved in the formulation of the conventional limit equilibrium methods.

Swami Saran, et.al., [04] in their study, have analyzed the case of rigid wall retaining a reinforced cohesionless fill that carries a uniform surcharge load based on the limit equilibrium approach. The reinforcement may be in the form of strips or mats that are not connected to the wall. This analysis considers the stability of an element of the failure wedge, which is assumed to develop in the reinforced earth mass adjoining the back face of the wall. Non-dimensional design charts have been developed for computing the resulting lateral earth pressure on the wall and the height of its point of application above the base of the wall. The following major conclusions have been determined from this study:

1) Unattached reinforcing strips, embedded in the cohesionless backfill behind a rigid retaining wall, are effective in reducing the lateral earth pressure on the wall.

2) The extent of reduction in the resultant pressure will depend on the amount of reinforcement present in the backfill.

3) The optimum length of reinforcing strips is found to be around 0.6-0.8 times the height of wall for most practical cases.

Rowe and Hoe, [05] have studied compared predictions of 12 different limit equilibrium design procedures with performance of four experimental reinforced soil retaining structures. They concluded that "None of these methods can distinguish the differences that existed between the walls." The above conclusion indicates that conventional procedures do not include significant elements characterizing the physical systems. This situation is a result of the fact that existing codes and design procedures for reinforced soils are mixes of semi-theoretical and empirical rules, which do not combine into a coherent rational framework.

Hoe S. K., et.all., [06] have studied the finite element (FE) procedure for simulating the performance of geosynthetic reinforced soil retaining walls. Analysis were performed using a FE procedure, in which the material properties of the wall i.e. backfill, foundation, geosynthetic reinforcement, and fascia wall were expressed using non-linear elastic models. A series of parametric studies was conducted to identify the effects of geosynthetic length and facing and geosynthetic stiffness on performance.

It is observed that increased stiffness of wall facing and geosynthetic improve the performance by restraining the deformations. A wall face of small stiffness triggers excessive deformations in the wall and leads to shifting the point of maximum lateral deflection at the wall face from the top to bottom.

Hoe and Row, [07] have performed the numerical simulations to provide insight concerning the effects of variation of wall geometry on the behavior of reinforced soil walls. The geometric parameters included in the reinforcement length, number of layer of reinforcement, distribution of reinforcement and wall height.

It has been demonstrated that the most important geometric parameter is the reinforcement length to wall height (L/H) ratio. For a ratio equal to or greater than 0.7, there is generally little variation in the normalized stresses in the reinforced soil and force in the reinforcement. However, for a ratio less than 0.7 the effect of lateral thrust behind the reinforcement soil becomes significant and should not be overlooked since this greatly increases the force in the reinforcement.

Sridevi Jade, et.al., [08] have carried out a two dimensional (2-D) finite element analysis of the reinforced earth wall to obtain the deformation behavior of the reinforced earth retaining structure. The retaining wall facing and the backfill soil have been discretized using 2-D four node isoperimetric plane strain quadrilateral element. The reinforcing elements have been modeled as two-dimensional line element. The interfaces between the reinforcing strips and soil material have been modeled by two dimensional interface elements.

The FEM analysis has been carried out for the self weight of the retaining wall system, i.e. wall facing + reinforcing elements + backfill earth. The finite element analysis concluded that, the maximum vertical settlement of the retaining wall system obtained by the FEM analysis as well as in the field is very close to the wall facing. The settlement of the backfill decreases with increasing stiffness (k) of reinforcement and finally becomes constant for values of k grater than 1 x 106 KN/m. Also the lateral displacement of the retaining wall facing and the settlement of the back fill decreases as 'E' of the backfill increases.

Helwany S. M. B., et al., [9] have studied the effect of backfill on the performance of GRS retaining walls. In this investigation three different geosynthetic reinforcement and sixteen different backfills were implemented in the analysis of three different wall configurations to produce 144 analysis combinations. It was shown that the type of backfill had the most profound effect on the behavior of the geosynthetic reinforced soil (GRS) retaining wall. It was also shown that the stiffness of the geosynthetic reinforcement had a considerable effect on the behavior of the GRS retaining wall when the backfill was of lower stiffness and shear strength.

Parametric charts were established for GRS retaining walls based on the finite element analysis. These charts are useful to the design engineer in choosing the appropriate backfill and the appropriate geosynthetic reinforcement for GRS retaining walls in order to satisfy the prescribed requirements of maximum lateral displacement, maximum axial strain in the reinforcement, and /or average safety factors.

Row, R.,et.al., [10] have conducted research on retaining walls on layered soil foundations. The behavior of geosynthetic reinforced soil walls constructed on a rigid foundation has been extensively investigated both experimentally and theoretically in the past and many current design criteria's are based partially on this research. However, the behavior of these reinforced soil walls constructed on soft or yielding foundations has received only limited attention and many questions still remain as to the performance and response of these soil structures. This work investigates the short term behavior of a reinforced soil wall constructed on a yielding foundation and analyses the key factors influencing the wall behavior. The response calculated using a finite element (FE) analysis is compared to the observed behavior.

It has been shown that for the case of geosynthetic reinforced soil wall constructed on a yielding foundation, the stiffness and strength of the foundation can have significant effect on the wall's behavior. A slightly compressible and weak foundation layer can significantly increase the lateral displacement of wall's face and base, the strains in the reinforcement layers near the bottom of wall and, a lesser extent, the vertical stresses at the toe of the wall, compared to rigid foundation.

Hatami K., et.al., [11] have investigated the structural response of reinforced soil wall system with more than one reinforcement type (nouniform reinforcement) using a numerical approach. The selected reinforcement types and mechanical properties represent actual polyester geogrid and woven wire mesh products. The model walls are mainly of wrapped-face type and have different reinforcement lengths, arrangements, and stiffness values. Additional wall model with tired and vertical gabion facings are included for comparison purposes. The numerical simulation of wall models has been carried out using a finite difference-based program and includes sequential construction of the wall and placement of reinforcement at uniform vertical spacing followed by a sloped surcharge.

The wall lateral displacement and back calculated lateral earth pressure coefficient behind the facing in all nonuniform reinforcement wall models show a clear dependence on the relative stiffness values of reinforcement layers at different elevations.

Satyendra Mittal S., et.al., [12] stated that, in the concept of reinforced earth, the soil is reinforced by the elements, which can take tension. These reinforcing elements may be in different forms e.g. Metal sheets, Strips, nets, mats, synthetic fabric or fiber reinforced plastics etc. Their incorporation in the soil mass is aimed at either reducing or suppressing the tensile strain, which might develop under gravity and boundary forces.

There can be two ways in which concept of earth reinforcement can be made use of in the construction of retaining walls. These are (1) reinforced earth wall and (2) wall with reinforced backfill. The reinforced earth walls are suitable for the places with poor subsoil conditions. These walls require sufficient space for construction as the width of wall is determined by length of reinforcement used. Thus, there may be situations where construction of these walls is not feasible. In such situations wall with reinforced backfill may prove to be and ideal solution and that has been attempted in the present investigation.

The construction of wall with geogrid-reinforced backfill has shown that there is a considerable saving in cost, space and construction time. Further, bottom ash which is presently a waste material can be used as the backfill material. Thus, on the sites, which are close to thermal power stations or paper mills, bottom ash can successfully be used as the backfill material.

Dov Leshchinsky, et.al.,[13] stated that current design of geosynthetic reinforced segmental retaining walls consider a prior limitless length for reinforcement installation. Such length is typically 0.5-0.7 times the height of the wall. However, often there are constraints on such space, e.g. bedrock formation located at a small distance behind the facing. The general procedure is introduced in this investigation for assessing the required long term strength of the reinforcement, while considering its limited length. Predictions by a conventional slope stability analysis were first checked against a continuum mechanics based numerical analysis. Upon obtaining good agreement, a design chart was developed. The chart enables the determination of the reduction in the lateral earth pressure coefficient due to the constrained space. The revised earth pressure coefficient can be used with current analytical method to account for the limited space. The result appears to be valid for conventional walls retaining a limited volume of soil.

Chungsik Yoo, [14] has presented the results of an investigation of a geosynthetic reinforced segmental retaining wall (SRW), which exhibited a sign of distress and excessive movements six years after the wall completed. The investigation comprised of wall profiling and limit-equilibrium-based stability analysis. Finite element analysis was also performed to gain insights into the states of stress and strain within the components of the wall system.

The results of this investigation indicate that large post construction movements can occur for walls not adequately designed to meet the current design criteria. Also confirmed is the ability of geosynthetic-reinforced walls to accommodate large movements without a structural failure. More importantly, it was demonstrated that good construction quality control is of prime importance for ensuring the short and long term stability of a geosynthetic reinforced retaining wall system.

Chungsik Yoo, et.al., [15] observed the behavior of a geosynthetic-reinforced segmental retaining wall (SRW). A 5.6 m. high full scale SRW in a tied configuration was constructed and instrumented in an attempt to examine the mechanical behavior and to collect relevant data that will help to improve the current design approaches. The horizontal deformations at the wall face and strain in the reinforcement are reported.

The results show that the interaction between the upper and lower tiers not only influences the performance of the lower tier, but also that of the upper tier, resulting in large horizontal deformations in the upper tier and strains in the reinforcement that can depart significantly from what might be anticipated. It is also shown that for walls on a less competent foundation, significant post-construction wall movements have been occurred.

Chandra S., et.al., [16] have performed numerical simulation of the reinforced soil wall using a discrete finite element code. In the discrete approach, the reinforcement system is considered as a heterogeneous body in which the soil and reinforcing elements are separately represented by different properties. The important advantage of this type of model is that detailed information is directly obtained about the behavior of interface between the soil and the reinforcement, and stress concentration in the soil due to reinforcing members. The wall was modeled as a plane strain, two-dimensional problem including simulation of construction sequence.

The measured behavior of the wall at the end of construction and after opening to traffic has been compared with the prediction from the finite elements analysis with respect to lateral stresses on the wall facing, soil strains, geogrid strains, horizontal and vertical soil stress, lateral stresses, carried by geogrid and wall displacements. Overall the nonlinear finite element procedure provides very good correlations between measured and predicted results of all quantities.

Taeson Park, et.al., [17] adopted finite element method to analyze the effect of the inclusion of short fiber in sandy silt (SM) soil on the performance of reinforced soil walls. The inclusion of short fiber in soil is expected to increase soil strength and improve stability, when it is used as the backfill material. Short fiber of 60mm length was used and the mixing ratio of the fiber was 0.2% by weight of the soil. The finite element method was used to examine the influence of the reinforced short fiber or reinforced walls. The vertical and horizontal earth pressure, displacement and settlement of the wall face were analyzed

It is shown that use of short fiber reinforced soil increases the stability of the wall and decreases the earth pressures and displacements of the wall. This effect is more significant when short fiber soil is used in combination with geogrid.

Krystyna Kazimierowez-Frankowska, [18] has conducted the experimental investigations to record the shape and magnitude of deformations on the surface of the reinforced soil wall. The simple model of reinforced soil wall was used in experiment. The reinforcement in each layer was wrapped around the back fill. The wall deformations were monitored for nearly 3 years. During this period the structure was exposed to natural weather conditions.

The experimental result showed the influence of reinforcement creep on the wall faces deformations. The influence of creep occurred 3 months after the completion of the experimental wall and, it was visible during later series of measurement. The greatest elongation of the reinforcement was indicated in the bottom layer (4%) the smallest (05%) in the top one. Both results were obtained 33 months after completion of the wall.

The specimens of geosynthetic reinforcement taken from inside the experimental wall after 33 months of working as reinforcement were in very good condition. Their basic mechanical parameters were nearly the same as initially. There is no reduction of tensile strength for samples taken from top and middle layers.

Kianoosh Hatami K., et.al., [19] have simulated the construction and surcharge loading response of full- scale reinforced soil segmental retaining wall using non linear finite element procedure. The numerical model implementation is described and constitutive models for the component materials i.e., modular block facing units, back fill, and four different reinforcement materials are presented. The influence of back fill compaction and reinforcement type of end-of-construction and surcharge loading response is investigated. Predicted responses features of each test wall are compared against measured boundary loads, wall displacements, and reinforcement strain values. Predictions capture important qualitative features of each of the four walls and in many instances the qualitative predictions are within measurement accuracy.

Zang, M.X., et.all., [20] have stated that reinforced soil with geosynthetic as a composite material represents orthogonally anisotropic properties. However, current analytical methods usually treat the soil and reinforcement separately, which is not true of practical situations. Therefore, it is difficult to use these methods to study the real effects of the reinforcement, Hence an analytical model based on the theory of elasticity of orthogonally an isotropic materials that can be used in analyzing reinforced soil structures with geosynthetic is presented in this work. The results of the model prediction are compared with those obtained from the model test as well as finite element analysis.

It is observed that the results of the analytical solution with respect to the stress, deformations, lateral displacement at facing, and settlement at the surface are in the good agreement with those of the physical model test and the finite element analysis.

Concluding remarks:

The literature review highlights different approaches of rational design of reinforced earth retaining walls (REW). It is also found that the investigations with respect to the behavior of REW are carried out by considering rigid foundation. Usually REW are constructed on moderate foundation and very little work is carried out in this area.

It reveals the scope to investigate the behavior of REW by varying properties of reinforcement, backfill, and foundation material, considering foundation as an integral part of the REW.