Why weight matters
Ordinary soils and aggregates weigh 1,520 to 2,400 kg/m³ (95 to 150 lb/ft³).[1] On soft clay or organic soil that weight drives settlement, behind a wall it drives lateral pressure, and over a pipe or culvert it drives the load the structure must carry.
A lighter fill reduces the load itself. For foam glass aggregate, the low weight and high friction can reduce vertical and lateral forces by more than one-half.[1]
Types of lightweight fill
Unit weights published by US transportation agencies, lightest first.
| Material | Unit weight | Notes |
|---|---|---|
| EPS geofoam | 11 to 48 kg/m³ (0.7 to 3 lb/ft³)[1] | Lightest; flammable and dissolved by fuel spills, so it needs a protective membrane[2] |
| Foam glass aggregate | 160 to 400 kg/m³ (10 to 25 lb/ft³)[1] | Granular, free-draining, non-combustible; buoyant[3] |
| Lightweight cellular concrete | 320 to 960 kg/m³ (20 to 60 lb/ft³)[1] | Placed by a specialty contractor in lifts that must cure[2] |
| Expanded shale and clay | 600 to 1,040 kg/m³ (37.5 to 65 lb/ft³)[1] | Granular, heavier than foam glass |
| Natural volcanic material (includes pumice) | 800 to 1,200 kg/m³ (50 to 75 lb/ft³)[4] | Natural volcanic aggregate |
| Conventional soil and aggregate | 1,520 to 2,400 kg/m³ (95 to 150 lb/ft³)[1] | The baseline |
Choosing a lightweight fill
Geofoam is lighter, so it reduces load the most where the very lowest weight governs. Its block shape limits it in narrow or irregular areas, where a granular fill such as foam glass aggregate places like ordinary aggregate.[5]
A load of lightweight fill is usually limited by the truck's volume, not its weight, so haul distance affects the delivered cost, and supply close to the project keeps it down.[6]
Enviro-Rock as lightweight fill
Common questions
What is lightweight fill?
Any fill much lighter than ordinary soil, used to cut the load on soft ground, walls and buried structures. Types include EPS geofoam, foam glass aggregate, cellular concrete, and expanded shale and clay[1], plus natural volcanic material such as pumice.[4]
How much lighter is foam glass aggregate than gravel fill?
Foam glass aggregate weighs 160 to 400 kg/m³ (10 to 25 lb/ft³), against 1,520 to 2,400 kg/m³ (95 to 150 lb/ft³) for ordinary soils and aggregates.[1] Enviro-Rock is about 90% lighter than conventional crushed rock.[9]
Where is lightweight fill used?
Road embankments and widening over soft soil, bridge approaches, wall and abutment backfill, and fill over pipes and culverts.[1]
Sources
- [1] Alternative Backfills for Highway Applications: State of the Practice (FHWA-HRT-23-110), Federal Highway Administration, 2024
- [2] How Low Can You Go: Materials Used by MDOT to Reduce Lateral Earth Loading Pressures, Michigan DOT, 2025
- [3] Applications for Foamed Glass Lightweight Aggregate, final report, Illinois Tollway Research Program, Washington State University et al., 2025
- [4] Geotechnical Manual: Ground Modification, Caltrans, 2026
- [5] Lightweight and Sustainable Materials in Engineered Fills (FHWA-PA-2021-011), PennDOT and Temple University, 2021
- [6] Guidelines for Using Lightweight Fills in Transportation Infrastructure Applications, Texas DOT and University of Texas at Austin, 2025
- [7] Enviro-Rock civil infrastructure brochure (metric), specification table, Enviro-Corp Recycling
- [8] Recognized Products List, Lightweight Fill Material (October 1, 2026), BC Ministry of Transportation and Transit
- [9] Enviro-Rock benefits: lightweight, Enviro-Corp Recycling