Mansfield, LA: Soil-Bentonite Slurry Wall, Oxbow Mine Dewatering Cell
The Oxbow Mine is a surface lignite coal mine just west of the town of Coushatta, LA. The Oxbow mine yields approximately 550,000 tons of lignite coal each year and, along with the neighboring Dolet Hills mine, has been the sole provider of fuel for the 650 mega-watt Dolet Hills Power Station since 1985.
Geo-Solutions Inc. was contracted by the American Electric Power Service Corporation (AEP) Dolet Hills Lignite Company to install the Cell #5 Slurry Wall to provide subsurface groundwater control and facilitate surface mining in Cell #5 of the Oxbow Mine.
The Cell #5 slurry wall included the installation of 437,000 vertical square feet (vsf) of soil-bentonite (SB) slurry wall. The wall was installed to a maximum depth of 72.5 feet below ground surface (fbgs), keying a minimum of 10-feet into the local Wilcox Group soil formation.
Excavation was accomplished under bentonite slurry using a Komatsu PC1250 excavator and Geo-Solutions' long-reach boom and stick attachment. Bentonite slurry for trenching was produced on site in Geo-Solutions' custom-made batch plant and pumped to the trench. The SB wall was 7,715 feet in length and tied into the existing Cell #3 slurry wall.
Trench spoils were blended with bentonite slurry as well as additional dry bentonite placed alongside the trench to create a controlled backfill mix. The target permeability for the SB wall was an average of 1 x 10-7. Target permeability was attained in all samples with an overall average permeability of 1.25 x 10-8.
The slurry wall was required to cross an existing 22-inch crude-petroleum pipeline, owned and operated by ExxonMobil. Geo-Solutions coordinated with ExxonMobil to protect the pipeline and install the slurry wall around it. This was successfully accomplished using pre-excavation and installation of flowable fill to protect the pipe during slurry wall operations.
Geo-Solutions served as the prime contractor for the project. The scope of work for the project also included:
- Site clearing/grubbing and disposal of debris
- Working platform preparation
- Stripping, relocation and stockpiling of existing topsoil
- Temporary erosion and sediment control
- Permanent surface-water runoff control including a levee and drainage ditch system
- Site Restoration/preparation for mining
The major earth work items were subcontracted to James Construction (Baton Rouge, Louisiana).
Geo-Solutions' commitment to safety was demonstrated on this project as it was completed with no incidents, accidents or near misses under active supervision by MSHA, OSHA and the Mine's safety department. Additionally, the Mine, including our project, was undergoing an active MSHA audit during completion. There were no MSHA findings associated with the slurry wall installation operation.
Because the surface mining operations depended on the completion of our work, schedule was a major focus. Local seasonal weather conditions made maintaining the project schedule challenging, but the project was ultimately completed ahead of schedule.