Unreinforced Backfill Research Articles

Effects of Geogrid Reinforcement on the Backfill of Integral Bridge Abutments

The construction of integral bridges is one of the most effective methods to reduce bridges’ construction and in-service costs. However, there are associated geotechnical problems with their abutments backfill due to the integrated abutments. The main goal of this study is to evaluate and quantify the benefits of geogrid reinforcement for reducing the backfill’s geotechnical problems. For this purpose, using small-scale physical modeling, the benefits of geogrid reinforcing of the backfill of an integral abutment bridge subjected to cyclic movements are evaluated. The results are then compared with a previous study performed on unreinforced backfill and two types of geocells. In this study, 120 loading cycles are applied to geogrid-reinforced soil to simulate the cyclic loadings on integral abutment backfill due to seasonal abutment displacement. The horizontal reaction load at the top of the wall, changes in pressure behind the wall, and deformation in backfill soil are measured during the test. Then the results are discussed in terms of equivalent peak lateral soil coefficient (Kpeak), lateral earth pressure coefficient (K*), and normalized settlement behind the wall (Sg/H). The derived lateral soil coefficients and settlement behind the abutment show that geogrid substantially reduces pressure and settlements after 120 cyclic loads. Based on the results, Kpeak and K* of the geogrid-reinforced backfill decrease by up to 36%, and Sg/H behind the wall decreases by 62%. In addition, the comparison of the results for geogrid with two geocell types shows that geogrid is more efficient in terms of lateral soil coefficients.

Shaking table tests on a cantilever retaining wall with reinforced and unreinforced backfill

Shaking table tests on a cantilever retaining wall with reinforced and unreinforced backfill

Evaluating the Applicability of Geocell-Reinforced Dredged Sand Using Plate and Wheel Load Testing

Mismanagement of large amounts of sediment from dredged harbors has resulted in major environmental and economic problems in some coastal areas. The beneficial use of dredged material can reduce the volume of these materials, but their insufficient strength for engineering applications makes them more appropriate for use in soil improvement. The geocell-reinforcement technique can be implemented to increase the strength and stiffness of weak soil. This paper investigates reuse of dredged sand as backfill in a road embankment by employing the full-scale wheel and plate loading technique. In order to achieve a suitable layered system, which satisfies deflection criteria and rutting limitations, the backfill was reinforced with geocells having different cover layers (well-graded gravel and dredged sand). The results revealed that the load-bearing capacity can be doubled with the use of geocell reinforcement accompanied by well-graded gravel as the cover layer. In this regard, the wheel rut depth was reduced to one third of that of the unreinforced backfill.

Numerical parametric study on behavior of bearing reinforcement earth walls with different backfill material properties

This paper presents a numerical parametric study on behavior of bearing reinforcement earth (BRE) walls with different backfill properties using the finite-element method software PLAXIS 2D. The primary objective of this study was to improve the understanding of bearing stress, settlement, lateral earth pressure, and horizontal wall movement of BRE walls with different backfill materials. The second objective of this study was to evaluate the effects of various soil–structure interactions, foundations, and stiffness of reinforcements on horizontal wall deformations. The backfill materials consisted of four types of soil, which were mixtures of silty clay and sand at different fine contents of 2, 20, 40, and 80% by dry weight. The model parameters for the numerical simulation were obtained from the conventional laboratory tests and back-calculated from the laboratory pullout tests of the bearing reinforcement. The geotextile elements were used to model the bearing reinforcements by converting the contribution of friction and bearing resistances to the equivalent friction resistance, which was represented by the soil–bearing reinforcement interaction ratio, Rinter. The values of Rinter decreased following a polynomial function as an increase of fine content in the ranges of 0.65–0.38 and 0.75–0.40 for the numbers of transverse members, n = 2 and 3, respectively. The simulated bearing stress in the reinforced zone decreased from the front to the back of the wall because the BRE wall behaved as a rigid body built on the relatively firm foundation retaining the unreinforced backfill. The foundation settlement decreased from the facing of the wall to the unreinforced zone for all backfill properties due to the slight rotation of the wall. The relationship between the maximum horizontal wall movement and the fine content can be expressed by a polynomial function. The maximum horizontal wall movement significantly increased as the fine content increased. The excessive movement was realized when the fine content was greater than 45%. The increase of the fine content moved the location of the maximum wall movement higher up from the mid to the top of the wall. A numerical parametric study was conducted to investigate the soil–structure interaction, foundation, and stiffness of reinforcement. These parameters affected the horizontal wall deformation, which is especially important for serviceability of BRE walls. The knowledge gained from this study provides a preliminary guideline in predicting the behavior of BRE walls and may be used to investigate other BRE walls with different wall heights and features of bearing reinforcements.

Research and construction of geosynthetic-reinforced soil integral bridges

Geosynthetic-reinforced soil (GRS) integral bridge was developed to overcome several inherent serious problems with conventional type bridges comprising a simple-supported girder (or multiple girders) supported via bearings typically by RC abutments retaining unreinforced backfill (and a pier or piers for multiple girders). The problems include: (a) relatively high construction and maintenance costs with relatively long construction time resulting from the use of bearings and massive abutment structures usually supported by piles; (b) bumps immediately behind the abutments; and (c) a relatively low stability of the girders supported by roller bearings and the approach embankment against seismic and tsunami loads. For a GRS integral bridge, a pair of GRS walls (and an intermediate pier or piers if necessary for a long span) are first constructed. After the deformation of the supporting ground and the backfill of the GRS walls has taken place sufficiently, steel-reinforced full-height-rigid (FHR) facings are constructed by casting-in-place concrete on the wall face wrapped-around with the geogrid reinforcement. Finally a continuous girder is constructed with both ends integrated to the top of the FHR facings. The girder is also connected to the top of an intermediate pier, or piers, if constructed. The background and history of the development of GRS integral bridge is described. The first four case histories, one completed in 2012 for a new high-speed train line and the other three completed in 2014 to restore a railway damaged by a great tsunami of the 2011 Great East Japan Earthquake, are reported.

Post-construction performance of a two-tiered geogrid reinforced soil wall backfilled with soil-rock mixture

There have been very few studies on the application of soil-rock mixtures as the backfills of geogrid reinforced soil retaining walls with due concern for their long-term performance and safety. In this study, a 17-m high two-tiered reinforced soil wall backfilled with soil-rock mixture was instrumented for its performance under gravity load after construction. The instrumentation continued for 15 months. It is found that soil-rock mixtures with small rock content (<30%) have the potential to be used as the backfill materials of geogrid-reinforced retaining walls, but special attentions should be given to compaction quality, backfill–geogrid interaction, and installation damage to geogrids. Reinforcement slippage is possible because of the large particles, but it was small in this case and ceased to develop nine months after the end of construction. Compressibility difference between reinforced and unreinforced backfill might led to rotation of the upper tier. Using the estimated soil strength, the predictions of reinforcement loads by the FHWA methods were 100% higher than the estimated ones from measured strains.

Stability of existing bridges improved by structural integration and nailing

To examine whether and how the seismic stability of existing bridges can be substantially improved by integrating the girder, the abutments and the backfill, a series of shaking table tests were performed in 1 g. The tested small bridge models are (1) a conventional-type comprising a girder, supported by a pair of gravity-type abutments (without pile foundation) via bearings (fixed and movable), and unreinforced backfill, (2) the girder and the abutments of the above are integrated (without using bearings), (3) the backfill of the above is reinforced with two layers of large-diameter nails connected to the abutment top and the toe or the heel of the abutment footing and (4) the bottom nails of the above are replaced with longer ones connected to the toe of the abutment footing. Their dynamic behavior was analyzed as a damped single-degree-of-freedom system. The dynamic stability of the bridge was found to increase with an increase in (i) the dynamic strength against the response acceleration, (ii) the initial stiffness, (iii) the dynamic ductility (i.e., a smaller decreasing rate of stiffness during dynamic loading) and (iv) the damping ratio. When factors (ii) and (iii) are high enough, the natural frequency of a bridge can be kept much higher than the input frequency, and thus, the response acceleration can be kept low. All these factors can be improved by integrating the girder, the abutments and the backfill together with part of the supporting ground. In a series of static model tests, lateral cyclic displacements, caused by the seasonal thermal deformation of the girders with prototypes, were applied to the top of a small abutment model. The active failure in the backfill and the detrimental effects of large passive pressure, both developing due to the dual ratchet mechanism, can be effectively restrained by reinforcing the backfill and supporting the ground with nails connected to the top and the bottom of the abutments.

Numerical analysis of bearing reinforcement earth (BRE) wall

This paper presents a numerical simulation of the bearing reinforcement earth wall by PLAXIS 2D. The bearing reinforcement was regarded as a cost-effective earth reinforcement. The model parameters for the simulation were obtained from the conventional laboratory tests and back analyses from the laboratory pullout tests of the bearing reinforcement. The simplified method for modeling the bearing reinforcement, which converts the contribution of friction and bearing resistance to the equivalent friction resistance, is introduced. This method is considered to be acceptable and practical in working state with sufficient factor of safety and small pullout displacement. The bearing reinforcement is modeled as the geotextile and the equivalent friction resistance is represented by the soil/reinforcement interface parameter, R, which was obtained from a back analysis of the laboratory pullout test results. The R values are 0.65 and 0.75 for the bearing reinforcement with 2 and 3 transverse members, respectively. The change in bearing stresses, settlements, lateral earth pressures and tensions in the reinforcements during and after construction is simulated. Overall, the simulated test results are in good agreement with the measured ones. The simulated results show that the BRE wall behaves as a rigid body, retaining the unreinforced backfill. The simulated bearing stress presents a trapezoid distribution shape as generally assumed by the conventional method of examination of the external stability of MSE walls. The simulated settlement is almost uniform due to a high stiffness of the rigid foundation and the bearing reinforcements. The maximum lateral movement occurs at about the mid-height of the wall, resulting in the bi-linear maximum tension plane. The knowledge gained from this study can be applied to other BRE walls with different wall heights, foundations and features of bearing reinforcements.

Seismic stability of geosynthetic-reinforced soil integral bridge

The seismic stability of the conventional-type bridge, comprising a pair of cantilever-type abutments (e.g., gravity-type or L-shaped RC structure) supporting a girder on their top via fixed and movable bearings with the unreinforced backfill, is rather low. With Integral Bridges, the girder is integrated to a pair of RC abutments. A new type, called the GRS Integral Bridge, comprising a girder integrated to the top of a pair of RC facings with the backfill reinforced with geogrid layers connected to the facings, has a much higher seismic stability. The seismic stability increases by cement-mixing part of the reinforced backfill. By analyzing the results of shaking table tests on three Integral Bridge types by the one-degree-of-freedom dynamic theory, it is shown that the seismic stability increases with an increase in the dynamic strength, the dynamic ductility and the damping capacity.

Field Behavior of a Geogrid Reinforced Soil Retaining Wall with a Wrap-Around Facing

Abstract The earth pressure and deformation characteristics of a geogrid reinforced soil retaining wall were monitored during and for a period of 1.5 years after construction. Both vertical and lateral soil stresses were recorded with vibrating-wire earth pressure cells, and the reinforcement deformations were measured using flexible displacement sensors. The maximum vertical foundation pressure along the wall’s reinforcements occurred at the center of the reinforcement, gradually decreasing towards the front and back ends. The measured lateral earth pressure within the reinforced soil wall is non-linear along the wall height, and the value is less than the theoretical active lateral earth pressure. The distribution of tensile strain along the reinforcements within the lower portion of the wall has two peak values. The potential failure surface of the wall closely follows the theoretical Coulomb failure surface of an unreinforced backfill.

New procedure for active earth pressure calculation in retaining walls with reinforced cohesive-frictional backfill

A new approach is suggested to determine the active earth pressure on retaining walls with reinforced and unreinforced cohesive-frictional backfill based on the horizontal slices method. A 4 n formulation for unreinforced backfill and a 5 n formulation for reinforced backfill are introduced and the tensile forces of the reinforcements and angle of failure wedge are calculated. The proposed method shows that the variation of active earth pressure by the depth of the wall in cohesive-frictional soils has a non-linear distribution. Also, the point of application of the pressure always shifts to the lower two-thirds of the wall height. The angle of failure wedge for cohesive-frictional soils increases linearly with an increase in the cohesive strength of the soil. A comparison of the analytical results obtained from the proposed method with those of previous research and AASHTO method results shows a negligible difference. The analytical method presented can be used to calculate the active earth pressure, tensile force of reinforcements and angle of failure wedge for unreinforced and reinforced walls in cohesive-frictional soil.

ブロック積み擁壁土圧の模型実験と数値解析

This paper discusses active earth pressure acting on concrete block retaining wall considering deformation and stiffness of wall and backfill material. Many laboratory model tests are performed, subjected to block retaining walls which have reinforced or unreinforced backfill, and which have different inclination. Earth pressure distribution and active thrust, etc. are monitored according to the displacement of retaining wall. The monitored results are simulated fairly well by proposed FE analysis which employs Mohr-Coulomb yield criterion, a simple non-associated flow rule, and initial stress method for nonlinear analysis. By applying the proposed procedure to the result of full-scale model test, it is shown that the procedure duplicates fairly well the results observed in the test.

Limited reinforced space in segmental retaining walls

Current design of geosynthetic reinforced segmental retaining walls considers an a priori 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 objective of this note is to introduce a procedure 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 methods to account for the limited space. The results appear to be valid for conventional walls retaining a limited volume of soil. Comparison with limited experimental results for unreinforced backfill shows reasonably good agreement.

Mechanically stabilized walls over soft clay: Geosynthetic to prevent deep-seated failures

Mechanically stabilized walls can sometimes be economically constructed over poor foundations such as a soft clay. A major design concern in such a case, however, is the stability against deep-seated failure where the potential slip surface passes through the unreinforced backfill and the clayey foundation. To attain a prescribed safety margin against deep-seated instability, a high-strength geosynthetic sheet, placed at the backfill-clay interface, can be used. The objective of this paper is to present an analysis method which enables the user to determine the required distribution of tensile resistance of the reinforcing sheet and therefore can be used as a design tool. The analysis is based on limit-equilibrium and by virtue of the mechanism used it is rigorous in the sense that no statical assumptions are utilized. Its predictions compare favorably with some conventional methods; its application, however, is easier. It is demonstrated how the analysis results can be condensed into a useful format of design charts. Since the direction of the reinforcement's tensile resistance is not known, a zone is suggested within which is the probably required distribution of this force.

Single-shell In-situ concrete tunnel lining: Experience in the Federal Republic of Germany

Abstract Although traffic tunnel construction in West Germany typically has used in-situ concrete as a double-shell lining, this method is both time-consuming and costly to apply. Recent research efforts have focused on the possibilities of single-shell in-situ concrete tunnel lining. This paper describes tests performed recently in West Germany on the use of (1) shotcrete and (2) steel-fibre-reinforced pumped concrete. The discussion deals with problems involved in these types of construction, the site and laboratory tests performed, and recommendations/prospects for application of these types of tunnel linings. The article also reports on two recent research projects, involving: (1) tunnel lining for large cross-sections with steel plate skin and unreinforced backfill concrete; and (2) construction and economic possibilities for creating tunnels in monocoque design.