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Civil infrastructure

Lightweight approach fill and abutment backfill for bridges

A lighter approach fill adds less load to the soft ground beside the abutment and pushes less on the wall. Enviro-Rock is used for approach embankments and as free-draining backfill behind abutments and wing walls.[1]

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  1. 1Wall, abutment or foundation wall
  2. 2Foam glass aggregate backfill
  3. 3Lower lateral pressure on the wall
  4. 4Free-draining fill and drain
  5. 5Native soil

Foam glass aggregate behind the abutment reduces both the load on the ground and the pressure on the wall. Schematic, not to scale.

Bridge approach fill

The engineering problem

Long strip of foam glass aggregate fill with a roller and crew working along it

A bridge abutment usually sits on its own foundation, while the approach embankment rests on the ground beside it. Where that ground is soft, the approach fill keeps settling after the bridge opens, and a step forms where the road meets the deck.

The same fill pushes sideways on the abutment and wing walls. Heavy backfill means more lateral load for the abutment to carry, and more again if water collects behind it.

On a Virginia project reviewed for PennDOT, the estimated settlement at the deepest fill stations was 15 to 18 mm (0.6 to 0.7 in) with No. 57 stone, above the 13 mm (0.5 in) limit, and 10 mm (0.4 in) with foam glass aggregate, so the design used foam glass aggregate there.[2]

Benefits

What Enviro-Rock adds

01

Less load on the approach

At about 200 kg/m³ (12.5 lb/ft³) loose[3], Enviro-Rock adds much less load to the ground beside the abutment than conventional fill at 1,500 to 2,400 kg/m³.[4]

02

Less pressure on the abutment

FHWA states that the low weight and high friction angle of foam glass aggregate can reduce vertical and lateral forces by more than one-half.[4] Enviro-Rock's brochure friction angle is 39° at 65 kPa.[3]

03

Drainage behind the structure

Enviro-Rock is used as free-draining fill behind abutments and wing walls[1], and its open structure lets water move through the fill rather than collect against the wall.[5]

On site

How it goes in

  1. 1

    Set the lightweight zone

    The engineer sets the length of the approach fill and the extent behind the abutment; design guidance for foam glass aggregate carries the zone past the active wedge.[6]

  2. 2

    Prepare and separate

    Proof-roll the subgrade and lay non-woven geotextile below and around the fill.[7]

  3. 3

    Place and compact

    Place 300 to 600 mm loose lifts and compact 10 to 20% with a static roller, vibration off.[7] Keep wheeled machines off placed material.[8]

  4. 4

    Build the pavement

    Cover with geotextile, then the crushed gravel base and the pavement or concrete surface.[7] The pavement designer sizes that cover so traffic stress at the top of the fill stays within design limits; Illinois Tollway guidance uses about 96 kPa (2,000 psf).[6]

Specs that matter here

Key properties

Published Enviro-Rock values. Test reports are available on request.
PropertyValue
Dry bulk density200 kg/m³ loose (12.5 lb/ft³)[3]240 kg/m³ (15.0 lb/ft³) at 20% compaction. For design, use a moist, compacted unit weight: PennDOT practice and the Illinois Tollway design guide use 376 kg/m³ (23.5 lb/ft³) for foam glass aggregate[9], and the Kelowna design used about 2.5 kN/m³ compacted.[10] The engineer of record sets the project value.
Confined compressive resistance1.01 MPa (147 psi) at 20% deformation[3]0.56 MPa at 10% and 1.67 MPa at 30%.[3] Tested by modified EN 1097-11.[11] An index value for comparing aggregates, not an allowable bearing pressure.
Angle of friction39° at 65 kPa normal stress[3]The friction angle of foam glass aggregate falls as normal stress rises[9], so the geotechnical engineer selects the design value for the stress on the project from test data and design conditions.[12]
Intergranular void space50% loose, 30% at 20% compaction[3]The brochure notes these figures are based on worldwide averages. Open-graded, so water drains through the fill and capillary rise is interrupted.[5] No permeability value is published; ask for a test if the design needs one.
Placement and compaction300 to 600 mm loose lifts under a static roller, or 150 to 300 mm under a plate compactor; compact 10 to 20%[7][13]No vibration on rollers, to prevent particle breakdown.[7] With a plate compactor, use more passes rather than more vibration, and do not use a jumping jack.[13] Allow about 20% reduction from loose thickness.[8]

Industries

Who builds with it

Roles

Who specifies and installs it

Questions

Bridge approach fill: common questions

What is lightweight fill?

Fill much lighter than soil or crushed rock, used to cut the load on soft ground and the pressure on structures. FHWA lists foam glass aggregate at 160 to 400 kg/m³ (10 to 25 lb/ft³), against 1,500 to 2,400 kg/m³ (95 to 150 lb/ft³) for mineral soils and aggregates.[4]

What causes the bump at the end of a bridge?

A common cause is the approach fill settling on soft ground while the abutment, on its own foundation, does not. A lighter approach fill adds less load to that ground: FHWA states that foam glass aggregate can reduce vertical and lateral forces by more than one-half.[4]

Is Enviro-Rock accepted for BC highway bridge work?

Enviro-Rock is an accepted lightweight fill on the BC Ministry of Transportation and Transit Recognized Products List (product 1414), which means it may be used on Ministry projects as long as it meets the requirements of the Standard Specifications and/or the Work.[14] The bridge design still sets the project requirements.

Does foam glass aggregate creep under a bridge approach?

Illinois Tollway design guidance projects less than 0.1% creep over 50 years for foam glass aggregate under sustained stress up to about 96 kPa (2,000 psf).[6] The engineer checks the stress the approach puts on the fill against that limit.

What if the approach floods?

Foam glass aggregate is buoyant, so designs place it above the design flood level or ballast it with cover.[6] Maine DOT's provision does not allow the fill to be submerged until it is ballasted.[15]

Sources

  1. [1] Enviro-Rock applications: bridge approaches and abutments, Enviro-Corp Recycling
  2. [2] Lightweight and Sustainable Materials in Engineered Fills (FHWA-PA-2021-011), 2021, Pennsylvania Department of Transportation and Temple University
  3. [3] Enviro-Rock civil infrastructure brochure (metric), specification table, Enviro-Corp Recycling
  4. [4] Alternative Backfills for Highway Applications: State of the Practice (FHWA-HRT-23-110), 2024, Federal Highway Administration, Turner-Fairbank Highway Research Center
  5. [5] Enviro-Rock benefits: drainage and permeability, Enviro-Corp Recycling
  6. [6] Applications for Foamed Glass Lightweight Aggregate, final report with draft special provision and design guide, 2025, Illinois Tollway Research Program; Washington State University, Lafayette College and UW-Madison
  7. [7] Enviro-Rock installation guide, Enviro-Corp Recycling
  8. [8] Enviro-Rock installation, Enviro-Corp Recycling
  9. [9] Applications for Foamed Glass Lightweight Aggregate, final report, Illinois Tollway Research Program, Washington State University et al., 2025
  10. [10] Case study: 6-storey building, Kelowna, Enviro-Corp Recycling
  11. [11] Enviro-Rock technical data, Enviro-Corp Recycling
  12. [12] Enviro-Rock benefits: angle of friction, Enviro-Corp Recycling
  13. [13] Enviro-Rock compaction equipment sheet, Enviro-Corp Recycling
  14. [14] Recognized Products List, Lightweight Fill Material (October 1, 2026), BC Ministry of Transportation and Transit
  15. [15] Special Provision Section 203: Ultra-Lightweight Foamed Glass Aggregate, 2025, Maine Department of Transportation

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