Project Summary/Scope:

This project extended an existing road, including construction of road embankment, a bridge over a railroad with MSE abutments, and the bridge approach.   Per the contract documents, the job required 280,000 cubic yards of borrow material for embankment (which locally was cohesive material); select material for MSE wall reinforced fill per a VDOT Special Provision; and separate high quality CBR 30 aggregate for bridge approach per VDOT Bridge Standards.

The Contractor examined various options and went with the use of No. 10 Tertiary Screenings (also called rock dust, quarry dust, rock screenings) where feasible as a value engineered initiative for all three of the classes of fill noted above.  No. 10 Screenings are an abundant quarry byproduct in most areas of the state, and VDOT recently published a Special Provision for their use after working with the Aggregate Industry in the state for whom re-use of this byproduct material was a significant initiative.  On this job, 176,000 tons of Screenings were used from a local quarry.

The use of Screenings on this job had several significant cost-saving and time-saving advantages:

1. It had been very wet in late spring, and had the Contractor stayed exclusively with the local cohesive borrow material for embankment, significant project delays would have likely resulted in order to allow the borrow to dry, or greater cost incurred to chemically dry it.
2. The Contractor found it very beneficial that rather than having to deal with 3 types of soil material and 3 different specifications (embankment, MSE fill, approach fill), that 1 type of material could satisfy all 3 uses.  This greatly simplified placement and delivery procedures.
3. The Screenings densified very well when compacted and were uniform in composition (with a maximum dry density of 126 pcf and an OMC of 8.5%).
4. Environmentally, any sediment contamination in site runoff was reduced over what cohesive soil would have contributed, as the screenings settled out of water rapidly during heavy rains.  Environmental monitoring of a nearby stream indicated no sedimentation.
5. Material cost savings.  Because Screenings are a byproduct material and were available locally in quantity, the Contractor realized a cost savings in using this material, even over locally available borrow.

Screenings do require confinement, including capping slope faces with cohesive material.  The Contractor did cap the slopes as construction progressed and used slope drains, diversion channels, and sediment basins and traps to help manage the material.  There were no dust complaints; no dust was created.  The Contractor did take steps to water the Screenings during placement and compaction, not only for compacting at optimum water content but also to prevent dust.

Photos:

Complementary Technologies Used:

Detail to construction activities, including employing timely erosion control measures.

Performance Monitoring:

Monitoring was typical VDOT procedure for Owner QC for compaction and moisture and environmental monitoring.

Cost Information:

Substantial cost savings were achieved on the material purchase itself but were offset due to longer haul distances.  However, the use of No. 10 Tertiary Screenings did aid constructability, as the excessively wet year would have otherwise prevented a successful completion of the project.

Case History Author/Submitter:

John Schuler, PE, VDOT Materials Division; john.schuler@vdot.virginia.gov; 804.328.3106

Date Case History Prepared:

September 23, 2019