Mechanically stabilized earth (MSE) systems are a popular and economical option to construct retaining walls for highways and railways, wingwalls and abutments for bridges, and numerous other applications for public and private development. An MSE wall is comprised of alternating layers of fill and soil reinforcements. The reinforcement materials are generally made of galvanized steel or polymers (e.g., geosynthetics) and take the form of straps or continuous elements. MSE walls have a vertical or near-vertical face typically comprised of concrete panels or blocks, segmental blocks, or a wrapped-earthen face with or without welded-wire forms. Design and construction guidance for MSE walls and backfill specifications for MSE backfill are dependent on the MSE wall application and location. Entities including the Federal Highway Administration’s National Highway Institute (NHI 2009) and the National Concrete Masonry Association (NCMA 2010) have published leading resources.
There are frequent instances where the subsurface conditions warrant the use of a lightweight backfill material such as ultralightweight foamed glass aggregate (UL-FGA) (Figure 1). The lightweight backfill may be needed because of soft, compressible soils or because of existing utilities or structures. UL-FGA has been available in Europe since the mid-1990s and is now also manufactured in the U.S. The manufacturing process consists of cleaning and milling postconsumer recycled glass, mixing the glass powder with a dry foaming agent, and then heating the mixture in a kiln to produce a foamed glass cake. Thermal stress breaks the cake into aggregate-sized pieces.
UL-FGA is manufactured from postconsumer recycled glass, which has inherent environmental advantages because it is not a material that is mined or petroleum-based. Because of the ultralightweight nature, a single truckload delivery of UL-FGA can be upward of 98 cubic yards (75 m3), which greatly reduces truck traffic on projects that require imported fill.
A more complete discussion of the physical and engineering properties of foamed glass aggregate is available in the literature. This article discusses the physical and engineering properties of UL-FGA that are needed for the design of MSE systems when using this material as backfill. Internal and external stability considerations of MSE walls with UL-FGA backfill are discussed. The case study shared in this paper utilizes polymer-based reinforcements with UL-FGA backfill.
UL-FGA MSE backfill properties
The typical dry bulk unit weight of closed-cell UL-FGA (Figure 2) as manufactured varies between 12 and 15 pounds per cubic foot (192 and 240 kg/m3). Moist bulk unit weights can vary based on moisture content but are typically between 15 and 18.75 pounds per cubic foot (240 and 300 kg/m3). Placement and compaction processes typically produce a volume reduction of up to 20% from the bulk to compacted state. This equates to a compaction factor of up to 1.25. Therefore, typical minimum and maximum in-place unit weights for UL-FGA are 15 and 23.5 pounds per cubic foot (240 and 375 kg/m3), respectively.
Because of the closed-cell nature and slight buoyancy of UL-FGA, a buoyancy check should be completed for projects near waterways. The buoyant unit weight of UL-FGA has been determined through testing to be -15 pounds per cubic foot (-240 kg/m3). Therefore, applying a factor of safety of 1.5, the design buoyant unit weight of UL-FGA is -22 pounds per cubic foot (-350 kg/m3). This buoyancy check will consider the design flood elevation and the thickness and unit weights of the UL-FGA and overlying layers.
The American Association of State Highway and Transportation Officials (AASHTO) has set forth requirements for MSE backfill material in the association’s Load Resistance Factor Design (LRFD) Bridge Construction Specifications (AASHTO 2017). Gradation, plasticity, shear strength, soundness and electrochemical testing are required to determine if a material complies with the AASHTO MSE backfill specification. Some test methods require modifications from the ASTM International or AASHTO test procedures to account for the large particle sizes and low density of UL-FGA. Both Loux et al. (2019) and Loux and Filshill (2021) (the latter the original source for this article) include more detailed information and test results that indicate UL-FGA meets the requirements set forth in the AASHTO MSE backfill specification.
The relatively uniform gradation of UL-FGA produces a layer with high porosity and, therefore, high permeability. To maintain the high porosity and low unit weight of the UL-FGA layer, it is recommended that a separation geotextile is used to encapsulate the UL-FGA layer wherever UL-FGA is in contact with adjacent granular materials (including foundation soil and retained soil), so intrusion of finer particles into the void space does not occur.
Internal stability of MSE walls is evaluated during the design process by checking the factor of safety against reinforcement pullout and reinforcement breakage. Internal stability calculations are used to assess or determine the reinforcement spacing, reinforcement lengths, reinforcement strength, and connection strength or overlap distance (the latter for wrapped-face walls). Laboratory scale pullout testing of UL-FGA with several reinforcement materials has been completed to generate a relationship for the coefficient of interaction with normal stress (Loux et al. 2019). External stability of MSE walls considers overturning, sliding and global failure of the MSE wall and often requires site-specific knowledge of the existing on-site soils. To complete the internal and external stability evaluations, earth pressures due to retained soil, surcharge loads and live loads must be considered.
Some projects using UL-FGA backfill may encounter longer than typical reinforcement lengths to satisfy internal and external stability factor of safety requirements. A full-height UL-FGA MSE wall is shown in Figure 3a. If an MSE wall with UL-FGA backfill has stability concerns, three potential layout adjustments to increase stability are possible, as shown in Figures 3b–3d. In Figure 3b, while the net surcharge load on the foundation soils will be higher versus a full-height MSE wall with UL-FGA backfill, this layout may still be competitive with other lightweight fill options that are available (i.e., expanded shale or clay, or cellular concrete). Figure 3c shows extending the reinforcements into typical unit weight reinforced fill. The approach in Figure 3d is to cut the existing soils back to a stable angle and fill the triangle formed between the reinforced fill and cut slope with additional UL-FGA. This method, while increasing the quantity of the UL-FGA, will greatly decrease the lateral earth pressures on the wall from the UL-FGA retained backfill.
Additional UL-FGA/MSE considerations
Other design details that are typically considered for MSE walls include drainage, foundation preparation and wall-facing details. Drainage piping may be included within the UL-FGA to outlet water from the retained soil, but the entire layer will be highly permeable. Piping may be used to outlet low spots within the UL-FGA backfill, so any collected water is allowed to outlet instead of infiltrate into the foundation soil.
On all walls with UL-FGA backfill that are under roadways, the pavement section should be analyzed to ensure that the distance between the top of pavement and the top of UL-FGA layer is sufficient for the pavement design requirements. It is not unusual for this distance to be 3–4 feet (914–1,219 mm) not only to address a flexible pavement design, but also to accommodate moment slabs for traffic barriers, guide rail, fencing or buoyancy concerns.
Standard compaction procedures for soils and traditional aggregates should not be applied to UL-FGA. The compaction procedures recommended by European manufacturers are method-based specifications that rely on tracked equipment or vibratory plate compaction only and are based upon more than 25 years of experience with UL-FGA. Standard installation procedures from various manufacturers of UL-FGA cite large maximum lift thicknesses of 2 to 3.3 feet (610 to 1,006 mm). For standard load-bearing applications, a compression ratio of between 1.15 and 1.25 is achieved. A compression ratio of 1.25 corresponds to a volume reduction of 20% from the bulk to compacted state.
The equipment that is typically used to compact UL-FGA is tracked equipment with ground pressures between 625 and 1,025 psi (4,309 and 7,067 kPa). This equipment, typically an excavator or dozer, will complete four passes over the UL-FGA layer for a standard installation. Compaction occurs by the static and dynamic forces imparted by the tracked equipment and will produce slight particle breakage but good particle interlock. In areas not accessible to tracked equipment, thinner lifts of 12 to 24 inches (305 to 610 mm) can be compacted using a 110-to-220-pound (50-to-100-kg) plate compactor. This method includes areas of MSE wall backfill close to the wall face. Unless higher degrees of compaction are required, static or vibratory rollers are not recommended for the compaction of UL-FGA.
A Light Detection and Ranging (LIDAR) study was completed to verify the efficacy of the method-based specification for compaction (McGuire et al. 2021). Because UL-FGA is an open-graded aggregate, typical construction quality assurance (CQA) procedures for soil compaction (i.e., nuclear density gauge testing) are not applicable. Instead, observance and verification of the method-based specification for UL-FGA compaction is necessary.
Case history: Route 6/10 interchange reconstruction
The Route 6/10 Interchange reconstruction project in Providence, R.I., is the largest design-build project to date in the history of the Rhode Island Department of Transportation (RIDOT). The project required embankment construction over buried utilities, including a large 100-year-old brick sewer, and challenging subsurface soils. The relocation or realignment of the existing utilities was not feasible, and the utility owner required that no additional loads be imposed by the embankment on the subsurface utilities.
A range of readily available conventional solutions for soft soils and buried utilities was evaluated during the design process, including soil-mixed rigid inclusions, expanded polystyrene (geofoam), foamed concrete and expanded shale aggregate. None of these more conventional solutions were found feasible because of uncertainty, time of construction or technical feasibility, and, in some cases, were cost prohibitive. RIDOT and the design-build team developed the concept of using UL-FGA as MSE backfill to protect the buried utilities against the weight of the embankment and to satisfy the zero-settlement constraint of the sewer. This approach has the benefits of reducing the number of trucks on the road required for delivery, saving more than $3 million versus the next cheapest alternative, and reducing time related to ramp construction on the project schedule.
The MSE walls with UL-FGA backfill on this project are flyover ramp retaining walls FO-1 and FO-2 and Ramp B retaining wall B-1 and B-2 (AECOM 2018). The section of the flyover ramp requiring UL-FGA fill is 450 feet (137 m) long, and the maximum height of UL-FGA fill in FO-1 and FO-2 is 40 feet (12 m). The depth of undercut of the existing soils to maintain a net zero load on the utility varies due to the differing height of the flyover ramp, but the maximum is 6.2 feet (1.9 m). At 330 feet (101 m) long and 26 feet (8 m) high, maximum, the UL-FGA fill in Ramp B is shorter in length and height than the flyover ramps. The depth of the undercut of the existing soils to maintain a net zero load also varied for Ramp B but maxed out at 4.2 feet (1.3 m).
A cross section of the flyover ramp is shown in Figure 4. The MSE wall system used with this project for both the flyover ramp and Ramp B consisted of 4 foot (1.2 m) tall precast concrete U-shaped panels with alternating layers of geogrid and polymer geostrap reinforcements extending between the retaining wall faces. UL-FGA installation occurred July through December 2019 for the flyover ramp construction (Figures 5a, 5b and 6) and October through December 2021 for the Ramp B construction (Figure 1).
This article discusses the conformance of UL-FGA with the requirements for MSE backfill per the LRFD Bridge Construction Specifications (AASHTO 2017). Considerations when using UL-FGA as MSE backfill are presented, including potential adjustments to UL-FGA layout to increase internal and external stability of the wall. There have been numerous projects where UL-FGA has been used as MSE backfill; this article includes the details of a highway project in the U.S. where UL-FGA has been used.
Theresa Andrejack Loux, Ph.D., P.E., is chief technical officer of Aero Aggregates of North America LLC in Eddystone, Pa.
Archie Filshill, Ph.D., ENV SP, is CEO of Aero Aggregates of North America LLC in Eddystone, Pa.
All figures courtesy of the authors except as noted.
AASHTO. (2017). LRFD bridge construction specifications, 4th ed. American Association of State and Highway Transportation Officials, Washington, D.C.
AECOM. (2018). “Retaining wall package 1_004_plans & profiles, Reconstruction of Route 6 & Route 10 interchange, Providence, Rhode Island.” AECOM, Dallas, Texas. Aug. 31.
Loux, T. A., and Filshill, A. (2021). “Ultra-lightweight foamed glass aggregate as MSE wall backfill: Properties and case studies.” Proc., Geosynthetics Virtual Conference 2021, Industrial Fabrics Association International, Roseville, Minn., 570–82.
Loux, T. A., Swan, R. H., Yuan, Z., and Filshill, A. F. (2019). “Pullout testing of geogrids, geostraps and steel strips embedded in foamed glass aggregate,” Proc., Geosynthetics Conference 2019, Industrial Fabrics Association International, Roseville, Minn., 728–37.
McGuire, M. P., Loux, T. A., and VandenBerge, D. R. (2021). “Field-scale tests to evaluate foamed glass aggregate compaction.” Proc., International Foundations Conference and Equipment Expo (IFCEE), American Society of Civil Engineers, Reston, Va., 157–68.
NCMA. (2010). Design manual for segmental retaining walls, 3rd ed. National Concrete Masonry Association, Herndon, Va.
NHI. (2009). Design and construction of mechanically stabilized earth walls and reinforced soil slopes, vol. 1., FHWA-NHI-10-024, Federal Highway Administration, National Highway Institute, Washington, D.C.
SIDEBAR: Project highlights
Route 6/10 Interchange reconstruction
Rhode Island Department of Transportation
6/10 Constructors Joint Venture
SFC Engineering Partnership Inc./AECOM
SF110 geogrid, SF65 geogrid, SF35 geogrid
ParaWeb 30 geogrid, ParaWeb 40 geogrid
Synteen Technical Fabrics Inc.