Significant Project Features:

• Approximately 150LF of Permeable Reactive Barrier (PRB) installed to a max depth of 11 ft.

• Approximately 950LF of Soil-Bentonite Installed to a max depth of 15 ft.

• Transport and Disposal of approximately 1,200 tons of impacted soil.

• Excavating around/through various overhead and underground utilities.

• Restoration of concrete driveway, fence, and vegetated swale area.

Project Description:

GSI installed a PRB funnel and gate system at the Norris Sucker Rods Plant using Soil-Bentonite (SB) and In-Situ Chemical Reduction (ISCR) technologies. The scope of work as prime contractor also included demolition of existing fence and concrete, clearing and grubbing of vegetation, utility protection, earthwork, water management, transportation and disposal of impacted soils, and site restoration back to original conditions.

The PRB trench was installed with a 80:20 percent sand to zero-valent iron ratio. During excavation of the PRB trench, GSI determined that bio-polymer slurry support of the excavation would not be required due to stable ground conditions and shallow depths of excavation. The ZVI/sand backfill was mixed via mobile volumetric mix truck where it was then transported to the excavated trench. This allowed the backfill to progress with excavation concurrently, minimalizing any trench stabilty issues.

The southern SB wall alignment crossed multiple active utility lines. Field conditions required pivoting from a protect-and-dig approach via concrete encasement of the utilities, to a full soft excavation operation around each utility line. Soft excavation and utility location were completed using vacuum trucks and GPR.

The design included three types of cap in discrete areas along SB and PRB alignments: concrete, gravel, and topsoil, each underlain by a layer of clay composite separated by a nonwoven geotextile. GSI elected to add an additional layer of nonwoven geotextile or geogrid between the clay composite and SB backfill to achieve compaction requirements. The additional layer of geotextile improved failing results of 70-80% to a passing threshold of 95%. Additionally, the excavation was expanded 18” on either side of the alignment to embed the geotextile for further stability.

Project Challenges

• Achieving bedrock tie-in via soft excavation under utilities impacted budget and schedule.
• Pre-existing conditions in the concrete driveway required horizontal expansion of the proposed concrete cap as the existing concrete was not stable/intact for dowel tie ins.
• Evolving strategies for excavation around active utilities led to complex sequencing. At one time, the south wall alignment had three active excavations separated by “plugs” of undisturbed material. The plugs were removed in the final days of excavation, creating one continuous trench supported with bentonite slurry and SB backfill.