Revised December 2025
INTRODUCTION
Segmental retaining walls (SRWs) are integral for creating usable spaces on challenging sites. They are gravity structures that rely on the weight of the units and the reinforced fill/backfill (if applicable) to resist the destabilizing forces of the retained soil and loads. Typically, specified backfill aggregates are compactable manufactured or, less commonly, river rock coarse aggregates chosen based on industry guidelines (REF. 3), project requirements and local availability. While construction with traditional backfills is standard, designers may have compelling reasons to consider alternative options on special projects. These include limited availability of traditional aggregates, poor site conditions that necessitate lightweight alternatives, impracticality of replacing extensive areas with poor soils, and the desire to utilize demolition debris or nontraditional industrial by-products like fly ash or steel slag for environmental or practical reasons.
Some of the most commonly available alternative materials are lightweight aggregates, recycled concrete aggregate (RCA) and recycled asphalt pavement (RAP), industrial by-products like fly ash, bottom ash or others.
Designers must carefully evaluate these alternatives to ensure they meet the needs of the project and that all environmental and design requirements are also met. This document explores examples of commonly used alternative backfill materials for SRWs and the considerations associated with their application.
FIGURE 1 — Example of Pumice Rocks (Source: Abdulrasool et al. 2022, REF. 1)
1.0 LIGHTWEIGHT BACKFILL
Lightweight fill materials are selected when the soil underneath the wall cannot resist the wall weight without failing or settling too much. Lightweight aggregates, utilized in SRWs, offer a range of advantages and disadvantages, influencing their suitability for various construction applications. One primary advantage is their low density, contributing to reduced structural load, smaller bearing loads and smaller settlements on the foundation soil. The decreased weight also facilitates ease of handling and transportation (big volume can be shipped), enhancing overall construction efficiency. On the other hand, their lower strength compared to traditional aggregates might limit their use in load-bearing applications and complicate the compaction operations, as they can be easily crushed. Something to also consider is that some of these aggregates are lighter than water, and in water, applications (wall submerged in water), may float and be unsuitable. Some of the most common lightweight materials used on SRWs are pumice and expanded clay, shale or slate aggregates, expanded foamed glass aggregates, and geofoam blocks.
FIGURE 2 — A) Example of Expanded Shale Clay or Slate Compared to Regular Aggregates B) Expanded Shale Clay or Slate Close-up (Photo courtesy of the Expanded Shale, Clay and Slate Institute (ESCSI), escsi.org, and Stalite Lightweight Aggregates, stalite.com)
1.1 Pumice and Expanded Clay, Shale orSlate Aggregates
If the designer is considering using the naturally occurring pumice (FIG. 1) or the manufactured expanded clay, shale or slate aggregates (FIG. 2), he/she needs to be aware that the materials do not have expansion problems; they are relatively expensive compared to regular sand and gravel, but can be economical when combined with a reinforced retaining wall on poor foundations compared to other retaining wall solutions (FIG. 3). Also, like most lightweight backfills, the particles will crush under compaction forces, so the maximum weight of the compactor, maximum height of lift and using only vibratory compactors will need to be carefully specified. These materials have low unit weight (around 60 pcf/961.1 kg/m3 ) and high friction angles (over 40°).
FIGURE 3 — Geosynthetic reinforced SRW, Hampden County Sheriff’s Department (Photo courtesy of Stalite Lightweight Aggregates, stalite.com)
1.2 Expanded foamed glass
The use of expanded foamed glass is becoming more common.The material is made from grounded post-consumer recycled glass mixed with a foaming agent that is heated and crushed to meet the size and gradation specifications (FIG. 4). The material is very light, so a large volume of the material can be transported and delivered to the site. Compaction requires the same care as pumice and expanded clay, shale and slate. This material has a very low unit weight (20 pcf/320.4 kg/m3 ) and high friction angles (over 50°), having the same issue in water applications, as it will float on water. An application can be seen in FIG. 5.
FIGURE 4 — Example of Expanded Foam Aggregates (Photo courtesy of Aero Aggregates of North America, LLC)
FIGURE 5 — SRW with Expanded Foam Aggregates, Gunnel Oval Project (Photo courtesy of Aero Aggregates of North America, LLC)
1.3 Geofoam Blocks
Geofoam is an alternative when lightweight fill is needed; the material is a closed-cell expanded polystyrene (EPS) manufactured in the form of giant blocks. The materials weigh approximately 1% of the weight of soil and less than 10% the weight of other lightweight fill alternatives (12 pcf/192.2 kg/m3 ) (FIG. 6A). The material is used in construction where conditions will not allow for conventional soil, sand or stone fill materials (FIG. 6B). It is highly susceptible to degradation from gasoline and diesel, and the care during installation has to be clearly specified so the blocks are not exposed to them. To install the blocks, they are cut to size and piled in the field as the soil backfill. When installing this material, protection with geomembranes is highly recommended, especially for roadway embankments.
FIGURE 6 — A) Example of Geofoam Block in the Field B) Retaining Wall Example (Photo courtesy of Geofoam International, LLC and insulfoam.com)
2.0 STABILIZED SOILS BACKFILL
Soil stabilization is a useful technique in earthworks, aimed at enhancing the mechanical properties of soil to meet construction requirements. One significant advantage of soil stabilization is the improvement of soil strength, reducing the risk of settlement and increasing load-bearing capacity. This is a soil modification technique to use materials in the field that would otherwise need to be removed and replaced on the site. They are used to deal with expansive and weak soils. Some examples of the types of stabilized fills available are soil cement or lime-stabilized soil, no-fines concrete, lean concrete or roller-compacted concrete, and lightweight cellular foam concrete.
2.1 Soil Cement and Lime-Stabilized Fills
Soil cement is a widely used technique, offering a durable and cost-effective solution for stabilizing soil in construction projects. The process involves mixing soil, cement and water to create a hardened material (FIG. 7) with improved strength and higher frictional characteristics, improving the load-bearing capacity of the soil and stability when dealing with weak or unstable soils (REF. 4 and 5). The installation needs to be done by an experienced contractor, and extra quality control is needed during the mixing, placement and curing. Some special considerations in the design will also be needed for the reinforcement since the pH of the stabilized soil cement is higher than regular soil, and the durability reduction factor will need to be selected accordingly to the site conditions. Similarly, lime-stabilized soil improves the soil properties. When adding hydrated lime and water to the soil, a chemical reaction occurs that enhances the soil’s plasticity and load-bearing capacity (REF. 11). The dosage has to be carefully selected and the construction has to be done by an experienced crew following industry recommendations (REF. 7 and 5). The stabilized pH properties will be different from the soil, so the designer needs to be aware of how the reinforcement will be affected in the field and incorporate it in the design, if applicable. For both stabilization methods, remember to ensure proper drainage in/on the backfill to reduce the potential of efflorescence on the face of the wall. This is a concern because the stabilized backfill has lower permeability but has more salts that can be transported by water to the face of the wall.
FIGURE 7 — Cement Stabilization Example (Photo courtesy of American Cement Association (ACA), REF. 2)
2.2 No-Fines Concrete
No-fines concrete is a special weak concrete mix ordered from the ready-mix companies. The material is mostly coarse aggregates, cement and water. The material can be used when there is no space for reinforcement with SRW facing units or for repair jobs to fill voids caused by erosion (FIG. 8A). The material is porous (FIG. 8B), has a high strength (compared to soil) and the weight adds to the weight of the units to create taller walls. The design principles for this system follow the gravity wall principles, and the mix can be obtained from a local ready-mix company.
FIGURE 8 — A) Example of No-fines Concrete in an SRW B) Permeability Demonstration of the Material (Photo courtesy of CornerStone Retaining Walls)
2.3 Lean Concrete or Roller-Compacted Concrete
Lean concrete and roller-compacted concrete (RCC) are durable and versatile materials that are viable as backfill. One of the main advantages of using the lean concrete and RCC for backfill is its rapid construction capability. It is typically mixed with a lower water-to-cement ratio, resulting in a stiff, zero-slump mixture that can be placed and compacted using heavy rollers (REF. 13 and 14). Improves the frictional characteristics of very fine sand and also to reduce the required soil reinforcement length. The materials have a higher cost, are more difficult to construct and require specialized equipment and contractors (FIG. 9).
FIGURE 9 — Roller-Compacted Concrete (Photo courtesy of American Cement Association (ACA), REF. 23)
2.4 Lightweight Cellular Foam Concrete
Lightweight Cellular Concrete (LCC) contains cement, water and a preformed foam. The typical unit weight of LCC is about 32 lb/ft3 (512.6 kg/m3 ). This material can be used on poor foundation soils to reduce the weight of an MSE wall to about 25% of its weight with sand and gravel backfills (FIG. 10). It is more difficult and costly to install but not “undesirable” since its lightweight properties can make the total installed cost of the structure less expensive when compared to other alternatives involving deep foundation or soil improvement. (REF. 18)
FIGURE 10 — Lightweight Cellular Foam Concrete Used in a SRW Project (Photo courtesy of cell-crete.com, REF. 10)
3.0 RECYCLED BACKFILL MATERIALS
Recycled aggregates in earthworks present an environmentally conscious alternative to traditional construction materials. Including crushed concrete, asphalt or other reclaimed materials, the use of these materials diverts waste from landfills and reduces the demand for virgin resources. An evident advantage is the conservation of natural resources and energy, as the production of recycled aggregates typically requires less energy compared to extracting and processing virgin materials. However, potential disadvantages include variability in the material and the need for strict quality control to ensure the recycled material meets the specifications.
3.1 Recycled Concrete Aggregate (RCA)
Recycled concrete aggregates (RCA) offer a sustainable and resource-efficient solution for SRW construction projects. Created by crushing and processing waste concrete from demolition or construction sites, RCA provides a greener alternative to traditional aggregates (FIG. 11). RCA reduces the environmental impact of the project, as recycling concrete diverts material from landfills and diminishes the need for virgin aggregates, ultimately lowering carbon emissions associated with extraction and processing compared to new aggregate. Additionally, incorporating recycled concrete can lead to cost savings, making it an economically viable choice. Several challenges must be addressed when using RCA. These include variability in material quality and the resulting need for strict quality control to meet specifications. The material may crush during compaction. If the source concrete previously suffered a sulfate attack, it may cause expansion of the compacted material (REF. 11). Additionally, the high pH of RCA can affect the durability of geosynthetic reinforcement, requiring specialized design considerations. Finally, inadequate drainage can lead to efflorescence on the wall face. Proper assessment and testing are essential to address any concerns regarding strength, durability and potential contaminants. This material has typical unit weights, 𝛾, around 100 pcf (1601.8 kg/m3) (lighter than regular aggregate), high friction angles, the initial pH is generally 10-12. Still, it will dissipate over time (it needs to be incorporated in the design). Lower permeability may be a concern for certain applications (REF. 11). Higher properties could be specified, but their availability will need to be checked with the manufacturer.
FIGURE 11 — Recycled Concrete Aggregate (Ref. 22)
3.2 Recycled Asphalt Aggregate (RAA) — Used Mixed With Sand
Recycled asphalt aggregates (RAA) offer a sustainable and cost-effective solution for backfill applications in construction. Created by processing reclaimed asphalt pavement (RAP), RAA provides an eco-friendly alternative to traditional backfill materials (FIG. 12). These aggregates tend to be cost-effective, as they are readily available from road construction and maintenance projects, but they are less desirable than RCA. Potential disadvantages may include variations in the quality of the recycled material, the need for careful quality control to ensure that it meets project specifications, cannot hold water because of the bitumen on the particles, and the potential for large creep deformations on the aggregate. Proper testing is crucial to assess characteristics such as particle size distribution, strength, and the presence of contaminants. This material compacts well, is free-draining, has good friction angles, and has the potential for creep deformations that may lead to excessive deformation (REF. 11 and 9).
FIGURE 12 — Recycled Asphalt Pavement Aggregate (REF.24)
4.0 INDUSTRIAL BY-PRODUCTS
Industrial by-products are used extensively in construction and present an environmentally conscious alternative to traditional construction materials. Comprising of fly ash, bottom ash, steel slag, blast furnace, and mine waste/low-grade ore, the use of these materials diverts waste from landfills and reduces the demand for virgin resources. However, potential disadvantages include variability in the material, the need for strict quality control to ensure the material meets the specifications, the potential for degradation of the reinforcement, and the need toe valuate these materials on a case-by-case basis.
4.1 Fly Ash
Fly ash aggregates, derived from the combustion of pulverized coal in power plants, offer a sustainable option for backfill applications in construction. Fly ash aggregates are made by adding water to fly ash powder to create pellets. Fly ash aggregates are lightweight, making them easy to handle and transport, which can contribute to efficiency in construction projects (FIG. 13). Additionally, the pozzolanic properties of fly ash contribute to improving strength and durability when used in backfill applications. However, potential disadvantages include variations in the quality of fly ash depending on its source and composition. It is undesirable for use in reinforced segmental retaining walls due to poor frictional qualities, poor drainage, difficult to construct with, and potentially corrosive to the soil reinforcement (REF. 20).
4.2 Bottom Ash
Similar to fly ash, bottom ash is a by-product of coal burning but is much coarser than fly ash and it has better physical and electrochemical properties than slag (lower weights, good shear properties and gradations available) (FIG. 14). It has high corrosion potential and needs to be evaluated before use (REF. 20).
FIGURE 14 — Bottom Ash Aggregate (REF. 6)
4.3 Steel Slag
Steel slag aggregates, a by-product of the steel manufacturing process, are versatile materials for backfill in construction. One notable advantage is the sustainability aspect, as steel slag repurposes a waste material, diverting it from landfills. Steel slag exhibits good engineering properties, offering high-density and angular particle shapes that contribute to improved stability and load-bearing capacity in backfill applications. Steel slag can be used as gravel (FIG. 15), but some of the types of material are expansive and can exert high expansive pressures on retaining walls. Check the slag for expansion potential before using it. Steel slags can also have iron, which can oxidize and create rust staining (REF. 20).
FIGURE 15 — Steel Slag (Photo courtesy of carbicrete.com)
4.4 Blast Furnace Slag
Blast furnace slag aggregates are a lightweight by-product of iron and steel manufacturing and undergo a patented procedure to create air voids in the grains, resulting in its light weight. The physical properties of blast furnace slag, including its angular particle shapes and light weight, contribute to improved stability and load-bearing capacity in backfill scenarios (FIG. 16). The material is resistant to weathering and has excellent compaction characteristics, making it suitable fora range of construction projects. Concerns about the leaching of heavy metals or other contaminants must be addressed through testing before incorporation. The aggregate does not have the expansion problem that the steel-making slag has and has successfully been used in several MSE walls. It has a relatively high cost per cubic foot, compared to sand or gravel, but can be economical when combined with MSE walls on poor foundations, when compared to other retaining wall solutions. Like most lightweight backfills, the particles will crush under compaction forces, so the specification should limit the weight of the compactor, lift thickness and restrict the equipment to only use vibratory compactors (REF. 20).
FIGURE 16 — Blast Furnace Slag Aggregates (Photo courtesy of the National Slag Association)
4.5 Mine Waste/Low-Grade Ore
Mine waste aggregates or low-grade ore, consisting of materials excavated from mining activities, have gained attention as a potential resource for backfill in construction (FIG. 17). There are many different examples of mine waste or low-grade ore that mine owners are often keen to use as an alternative to sand and gravel. The material needs to be selected on a case-by-case basis; it may be contaminated and care should be taken so they do not damage the reinforcement or settle excessively. These materials have been successfully used in mining sites where importing gravel and sand is impractical (REF. 20).
FIGURE 17 — Mining Ores (Source: iStock)
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