Oelwein, IA: Slag Cement, Cement-Bentonite Slurry-Supported Excavation

Oelwein, IA: Slag Cement, Cement-Bentonite Slurry-Supported Excavation

Project Description

Slag Cement, Cement-Bentonite Slurry-Supported Excavation
Former Power House Area Excavation

Significant Project Features:

• Excavation of contaminated non-hazardous soil using cement-bentonite (CB) slurry
• Overburden excavation and demolition of various concrete structures located within the targeted excavation area
• LNAPL Collection System
• Disposal of impacted soil and debris
• Placement, compaction and testing of onsite and import materials above the CB monolith

Project Description:

As the prime contractor performing the remediation services, GSI excavated the impacted soils from within the Former Power House Area on a Union Pacific owned site located in Oelwein, Iowa using a cement-bentonite slurry method (“liquid shoring”) to maintain an open excavation and eliminate the need for dewatering or traditional shoring. The self-hardening slurry not only supported the excavation in an area of elevated groundwater and flowing sand conditions, but it formed a homogenous, low strength backfill to serve as soil replacement or backfill.

GSI completed the excavation to the horizontal and vertical limits provided by GHD, with GSI performing all confirmatory testing and surveying. GSI removed the overburden material until the work platform was located approximately 3 feet above the local groundwater table before performing the Piano Key excavation under CB slurry to depths reaching 16 feet below the platform (average depth: 10 ft.) The final volume of contaminated soil removed from the site was 6,880 cubic yards.

GSI provided the CB mix design to meet or exceed the project requirements of less than 1×10-5 cm/sec hydraulic conductivity and greater than 25 psi unconfined compressive strength (UCS). GSI utilized a custom continuous mix plant which combined water, bentonite, Ground Granulated Blast Furnace Slag cement and Portland cement. The excavation was performed using a Kobelco 350 equipped with stock boom and stick along with a 4 ft. wide bucket.


Prior to commencing the CB Piano Key excavation, GSI’s scope of work included the excavation and stockpiling of overburden soils to a depth of approximately 8 ft. below original grade, installation and monitoring of a LNAPL collection system, and demolition of various concrete structures and foundations found within the limits of excavation. In total, GSI disposed of 12,400 tons of impacted soil and debris using lined trailers and placed 8,875 cubic yards of fill in order to reestablish original grade across the site.

GSI provided a complete construction solution to UPRR, including engineering support, a comprehensive quality control program with daily reporting, an experienced site supervisor, a dedicated on-site project engineer and Health and Safety Officer, labor, and all equipment and materials. The work was completed safely, with no losses.

• Excavating “keys” or “cells” to depths reaching 16 ft. below the work platform in flowing sand conditions. Perimeter keys needed to be installed directly adjacent to sloped excavation walls.
• Due to the 7-hour turnaround to the designated landfill, sourcing an adequate number of trucks to accommodate GSI’s planned disposal schedule.