Soil-geosynthetic composites such as those used in Mechanically Stabilized Earth (MSE) retaining walls and embankments are experiencing widespread use, particularly in transportation applications. These structures offer substantial economic and, in some cases, performance advantages over traditional options such as reinforced concrete gravity or cantilever walls. Continued growth in the use of MSE walls, particularly in critical applications such as bridge abutments, is anticipated.
Several methods for designing these structures are currently in use. Two commonly used design guidelines are published by the National Concrete Masonry Association (NCMA, 1996) and the American Association of State Highway and Transportation Officials. The NCMA guidelines are followed primarily within the private sector; the AASHTO specifications are employed in the public sector. The successful application of these or any of the other design guidelines may be distilled to two concepts, (1) proper assessment of the anticipated loading conditions and (2) proper characterization of the load transfer mechanisms between the components of the MSE systems (backfill soil, reinforcing materials, and fascia units). This research project addresses issues related to the second concept. More specifically, the research examines the interactions and load transfer mechanisms between the backfill soil and reinforcing materials.
The economic advantage of MSE walls is markedly increased if on-site soils are used as the backfill material in the reinforced zone. Ideally, this backfill material is relatively clean (e.g., limited fines content) and cohesionless. Practically, this is not often available on-site. The potential economic benefit of using lower quality, on-site material in MSE retaining wall applications is substantial. Using on-site material would eliminate the time and expense associated with identifying and transporting select fills.
This research examined the suitability of lower quality backfill soil by studying the load transfer mechanisms between representative soils and geosynthetic reinforcing materials. The primary method of studying this interaction was via a series of pullout tests as described in subsequent sections of this report.
The following conclusions are based on the activities performed during this research program:
- It is important to note these conclusions are based on research performed with four specific geosynthetic reinforcing materials embedded in two types of Piedmont residual soil. While certainly representative of overall behavior, the extension to general conclusions for all geogrid products or reinforced soil types may not be appropriate.
- A large database of soil-geosynthetic reinforcement interaction behavior has been developed. This data is particularly relevant to the work conducted by the NCDOT as it uses local, Piedmont residuum.
- Parametric studies have been performed to examine the importance of soil type, reinforcement type, and confining pressure on the load transfer mechanisms of geosynthetic-reinforced soils.
- The results of this study indicate that geotextile reinforcing materials may be a better choice than geogrid materials, particularly if minimizing displacement is an important performance consideration.
- Based on measurements of load and displacement, the use of lower quality soils appears feasible.
Based on the results of this research program, the following recommendations are made:
- For temporary earth retaining structures, it is feasible to use lower quality backfill in the reinforced zone. Material that satisfies the Class II - Type 2 classification in Section 1016-3 of the NCDOT Standard Specifications for Roads and Structures may be used provided the material is placed and compacted properly. As with virtually all projects employing earth as an engineering material, proper placement is absolutely critical if desired performance is to be achieved.
- It appears that the geotextile products may be the better choice of geosynthetic reinforcing material, provided the confining pressure is sufficiently large.
The next appropriate step is to extend this laboratory-based study to the field. This may be achieved by constructing and monitoring prototype-scale temporary retaining walls either on the UNC Charlotte campus or at a more desirable location for the NCDOT personnel. These walls should be built using the same types of soil as used in this research program (Class II - Type 2) and be reinforced with, as a minimum, a representative variety of geotextiles. Performance monitoring should focus primarily on deformations (both horizontal and vertical) and should be made throughout the construction process and for at least 18 months afterward. At that point, the walls should be loaded to failure (destructive testing) to glean as much design and performance information as possible.