Lightweight Aggregate Solves Major Fill Challenge

By Jack Sinclair and Eric Nelson, Trinity Lightweight

Owner: Trinity River Authority of Texas

Location: Central Regional Wastewater System, Dallas, Texas

Engineer: CP&Y, Inc.

Contractor: Eagle Contracting

Engineering Solution: Expanded Shale Lightweight Aggregate

Expanded shale lightweight aggregate manufactured by Trinity Lightweight was used during phase 1 (Solids Management Improvements) at the Central Regional Wastewater System in Dallas, Texas. The physical properties (durable, lightweight, free draining, easily compacted and locally available) of the 3/8” expanded shale, which is commonly used in geotechnical fills, made it a perfect choice for this application.

The key issue at the Wastewater Plant was developable space. The most feasible way to make room for the needed improvements was to drain a portion of an existing pond, approximately 3/4 acre in size. Excavation to a depth of 23 feet would be required to construct a new gravity thickener, pump station, associated piping and site work. Soil borings indicated that the soft soils onsite could pose a problem for the new construction and network of piping. The engineers estimated if they used normal fill material they could expect settlement in the 8”-10” range. This was not an acceptable plan.

Expanded shale was introduced to two engineering firms during the preliminary design process. Both CP&Y and CDM Smith favored its lightweight properties because it reduced the overall loads by half compared to normal fills. The durable, angular particles would also contribute to quality compaction with far less settlement, critical to the network of piping and vertical construction. In all, some 13,000 cubic yards of expanded shale were used on the project.

Workability was a trait praised by job superintendent Luis Aguilar of Eagle Contracting. Weather never hindered the geotechnical fill operations due to the free draining characteristic of the material. The expanded shale lightweight aggregate was placed using a crane-and-bucket, spread with a skid steer loader, then compacted with a plate compactor in some areas and a skid steer loader in other locations. The project density requirements of 64-70 psf were easily achieved. Aguilar reported that “no settlement had occurred in the geotechnical fills despite being tested by heavy rain events during May and October.” The lightweight aggregate will be capped with two feet of soil before paving and landscaping.

The lightweight nature of expanded shale combined with a high angle of internal friction (over 45 degrees) is useful in other applications where weight is an issue including: fill behind retaining walls, bridge abutments spanning waterways, major pipe bedding, and coastal developments.

Photo Gallery

  • Photo shows extent of excavation, approx. 23 ft, where old pond was located.
  • Lightweight aggregate placed to depth of 23’ and compacted with vibratory plate to meet density requirements of 64-70 psf.
  • Geotechnical fill operations complete. Work in progress to complete above grade infrastructure.