Perimeter Barrier Wall Installation to Facilitate Closure by Removal.pdf

Paul Schmall, Keller
Shawn Coughlin, Keller
Christopher Gee, Dominion

Description

Case Study: Perimeter Barrier Wall Installation to Facilitate Closure by Removal Authors Dr. Paul Schmall - United States - Keller Mr. Shawn Coughlin - United States - Keller Mr. Christopher Gee - United States - Dominion Abstract Hydrogeologic conditions of this project require a multi-faceted approach to facilitate ash dewatering and deep depressurization of underlying highly permeable soils within a 5,700 ft long perimeter cutoff wall, as deep as 80 feet. A perimeter cutoff wall was required because significant and sustained groundwater inflows from beneath the ponded ash were experienced with previous construction at the pond because the pond was built over the pre-existing riverbed in braided stream channel geology. A continuous trenching technique known as TRD (Trench Remixing Deep) was utilized to create the perimeter barrier wall. The technique utilized a vertical “chain saw” style trenching mechanism to build the barrier wall by mixing the in-situ soils with a cement/bentonite binder. The TRD technique was selected because it could be installed with a very stable, low-profile machine, it does not rely upon slurry for maintaining trench stability, does not require mixing of backfill alongside the trench, and provides very thorough mixing across a wide range of soil conditions. Several significant site constraints were addressed with the execution of the work. A sensitive perimeter embankment, immediately alongside the river, necessitated the insertion of steel beams through one section to increase the global stability of the embankment. In another area, micropiles were installed to support the TRD machine. Significant geotechnical investigation was performed to map the underlying permeable soils and to delineate the edges of the ash ponds so that the wall could be located outside of the limits of ash. This presentation will detail the necessary cutoff criteria, the relevant aspects of the installation, and the QA/QC methods. This presentation is intended to complement two other WOCA presentations focusing on the hydrogeological and practical dewatering aspects of the same project case study.

 
May 15th, 8:30 AM May 15th, 9:00 AM

Perimeter Barrier Wall Installation to Facilitate Closure by Removal.pdf

Grand Rapids, Michigan

Case Study: Perimeter Barrier Wall Installation to Facilitate Closure by Removal Authors Dr. Paul Schmall - United States - Keller Mr. Shawn Coughlin - United States - Keller Mr. Christopher Gee - United States - Dominion Abstract Hydrogeologic conditions of this project require a multi-faceted approach to facilitate ash dewatering and deep depressurization of underlying highly permeable soils within a 5,700 ft long perimeter cutoff wall, as deep as 80 feet. A perimeter cutoff wall was required because significant and sustained groundwater inflows from beneath the ponded ash were experienced with previous construction at the pond because the pond was built over the pre-existing riverbed in braided stream channel geology. A continuous trenching technique known as TRD (Trench Remixing Deep) was utilized to create the perimeter barrier wall. The technique utilized a vertical “chain saw” style trenching mechanism to build the barrier wall by mixing the in-situ soils with a cement/bentonite binder. The TRD technique was selected because it could be installed with a very stable, low-profile machine, it does not rely upon slurry for maintaining trench stability, does not require mixing of backfill alongside the trench, and provides very thorough mixing across a wide range of soil conditions. Several significant site constraints were addressed with the execution of the work. A sensitive perimeter embankment, immediately alongside the river, necessitated the insertion of steel beams through one section to increase the global stability of the embankment. In another area, micropiles were installed to support the TRD machine. Significant geotechnical investigation was performed to map the underlying permeable soils and to delineate the edges of the ash ponds so that the wall could be located outside of the limits of ash. This presentation will detail the necessary cutoff criteria, the relevant aspects of the installation, and the QA/QC methods. This presentation is intended to complement two other WOCA presentations focusing on the hydrogeological and practical dewatering aspects of the same project case study.