Geosynthetic-reinforced Soil Abutment Research Articles

Applications and Performance of Geosynthetic-Reinforced Soil Abutments on Soft Subsurface Soil Conditions

The use of geosynthetic-reinforced soil (GRS) as a bridge abutment foundation has been adapted by the Eastern Federal Lands Highway Division in several projects. This adaptation is considered ideal and appropriate because of the nature of these projects. The design, construction, and performance of a GRS abutment system to support two bridges in the Mattamuskeet National Wildlife Refuge in Hyde County, North Carolina, are illustrated. The bridge foundations were constructed over soft, silty fat clay (A-7-6[CH]) soils. The GRS abutment alternative was favored over a deep foundation system of driven prestressed concrete piles for economic and constructability reasons. A cellular confinement system (CCS) filled with gravel was used for the GRS abutment facing instead of the types of flexible facings typically used with GRS abutments. CCS facing has the advantage of providing abutment face protection against erosion and shallow scour, and the outer cell on both sides of the abutment can be filled with topsoil and seed to provide an aesthetically pleasing vegetated face. A geotechnical instrumentation monitoring program consisting of settlement plates and piezometers was established to evaluate the GRS abutment system performance during and after completion of construction. The foundation soils supporting the GRS abutments were preloaded before construction of the bridge footings to reduce anticipated long-term settlement and improve bridge performance. Design procedures and initial performance monitoring results indicated that the GRS abutments were performing satisfactorily.

Two full-scale loading experiments of geosynthetic-reinforced soil (GRS) abutment wall

Two full-scale segmental facing Geosynthetic-Reinforced Soil (GRS) abutment walls, referred to as "the NCHRP test abutments", were constructed and load-tested at the Turner-Fairbank Highway Research Center in McLean, Virginia. The two experiments were conducted to (1) examine the behavior of segmental facing GRS abutment walls under increasing vertical loads on a bridge sill, and (2) furnish a set of well-defined data for verification of analytical models. This paper describes the two experiments and the measured and observed behavior due to the load applications. The behavior of the test walls were discussed and compared with existing performance criteria that were developed based on experience with real bridges. The comparisons were made in terms of maximum sill settlement, maximum angular distortion, and maximum lateral wall movement. In addition, the safety factors and failure loads of the test walls were evaluated by the prevailing design method. The predicted failure loads obtained from the prevailing design method, even with all safety factors and strength reduction factors being set equal to one, were found to be much lower than the measured values, suggesting that the prevailing design method is likely to be overly conservative.

Simulating the Behavior of GRS Bridge Abutments

Geosynthetic-reinforced soil (GRS) bridge-supporting abutments are similar in principle to GRS retaining walls, except that GRS abutments are typically subjected to a much higher area load, and that the loads are close to the wall face. The GRS abutment technology is relatively new, but it has great potential, and it has been gaining some popularity in recent years. This paper describes the finite element analyses of two full-scale loading tests of GRS bridge abutments referred to as the Cooperative Highway Research Program (NCHRP) experiment. The analysis was carried out using the computer program Dyna3d, developed at the Lawrence Livermore National Laboratory. The finite element analysis of the NCHRP experiment will help with the understanding of the complex behavior of GRS structures in general, and the behavior of GRS bridge abutments with modular block facing in particular. The analysis of the two full-scale loading tests allows the loading conditions that are of greatest concern in the design of the bridge abutments to be examined rationally. The analysis shows that the performance of a GRS abutment, resulting from the complex interaction among the various components, while subject to a service load or a limiting failure load can be simulated in a reasonably accurate manner. In addition, a parametric study was conducted to investigate the performance of the modular block facing GRS bridge abutments subjected to live and dead loads from a bridge superstructure. This study investigated the performance of the GRS bridge abutments as they are affected by backfill properties, reinforcement stiffness properties, and reinforcement vertical spacing.

Allowable Bearing Pressures of Bridge Sills on GRS Abutments with Flexible Facing

Compared to geosynthetic-reinforced soil (GRS) retaining walls, GRS abutment walls are generally subjected to much greater intensity surface loads that are fairly close to the wall face. A major issue with the design of GRS abutments is the allowable bearing pressure of the bridge sill on the abutments. The allowable bearing pressure of a bridge sill over reinforced soil retaining walls has been limited to 200 kPa in the current NHI and Demo 82 design guidelines. A study was undertaken to investigate the allowable bearing pressures of bridge sills over GRS abutments with flexible facing. The study was conducted by the finite element method of analysis. The capability of the finite element computer code for analyzing the performance of GRS bridge abutments with modular block facing has been evaluated extensively prior to this study. A series of finite element analyses were carried out to examine the effect of sill type, sill width, soil stiffness/strength, reinforcement spacing, and foundation stiffness on the load-carrying capacity of GRS abutment sills. Based on the results of the analytical study, allowable bearing pressures of GRS abutments were determined based on two performance criteria: A limiting displacement criterion and a limiting shear strain criterion, as well as the writers' experiences with GRS walls and abutments. In addition, a recommended design procedure for determining the allowable bearing pressure is provided.

GRS bridge abutments—an effective means to alleviate bridge approach settlement

Field tests of segmental block-faced geosynthetic-reinforced soil (GRS) bridge abutments and piers have demonstrated excellent performance characteristics and very high load carrying capacity. One important feature of GRS abutment is that it can potentially eliminate the use of piling when situated over a weak foundation. This will not only reduce the costs but also reduce “bridge bumps” often experienced at the ends of a bridge resting on a pile-supported abutment. This study was undertaken to investigate the potential of GRS bridge abutments to alleviate bridge approach settlements. The study was conducted by the finite element method of analysis using the computer program DACSAR. The program was first calibrated by comparing its results with the measured data of the Founders/Meadows bridge abutment recently constructed by the Colorado Department of Transportation. A parametric study was then conducted to examine the effects of different foundation soils, ranging from loose sand to stiff clay, on the performance of a GRS abutment. Special attention was placed on the maximum vertical and horizontal movements of the abutment as well as the approach settlement characteristics. The study indicated that the finite element computer code DACSAR is a reliable analytical tool for analyzing the performance of GRS bridge abutments and that the GRS abutment is an effective means to reduce differential settlements between the abutment and the approach embankment.

Geosynthetic-Reinforced Soil Bridge Abutments

Geosynthetic-reinforced soil (GRS) bridge abutments have been used on a number of bridge projects over the past decade. This adaptation of reinforced soil technology to bridge structures and their approach fills offers an excellent opportunity to simplify construction, reduce construction time, and reduce cost on structures for which this technology is appropriate. This design concept, in which the actual bridge superstructure rests upon the GRS abutment wall, minimizes differential settlement and eliminates the problematic “bridge bump” found on many structures. The technology has been adapted to both road and trail bridges. The basic design concept of GRS used in bridge abutment applications was evaluated, along with its advantages and disadvantages. Some selected case histories of GRS bridge abutments on low-volume roads and trails in Alaska and California were considered. In addition, the Mammoth bridges, in the mountains of northern California, with high design snow loads and high horizontal peak ground accelerations, afforded an opportunity to design, construct, and monitor GRS-supported spread-footing abutments under difficult service conditions.