Relevance of Project
- Deep cutoff wall
- Use of Soil Bentonite Technique
- Excavation with a combination of extended reach backhoe and hydraulic clamshells
- Insure trench stability
- Low permeability of the Backfill
- Need for excellent QC program
- Development and implementation of SWPP plan was critical due to the proximity of the Mississippi river as well as the risk of contamination
- Install a permanent crossing above the cutoff for heavy traffic
A chemical manufacturer was required by the USEPA to construct a barrier wall between a chemical landfill and the Mississippi River. Most of the alignment of the cutoff was within 60/80 feet from the Mississippi river. In 2003, Inquip was awarded the contract to construct 3,350 linear feet of soil bentonite cutoff wall to a depth varying between 135 and 155 feet. The trench was excavated with a combination of extended reach excavator and 2 hydraulic clamshells. A Koehring 1266, working one shift, was used to excavate the upper 80/85 feet and two Liebher 853 crawler cranes with KS3000 hydraulic clamshells, working two shifts, were used to excavate the remaining depth. The soil bentonite backfill was mixed adjacent to the trench using a low ground pressure bulldozer. Special considerations were given to the stability of this deep cutoff, in particular in an 800 feet long area where the excavation was through an old ash pond, i.e. very soft fine ash with high water content. The backfill was mixed with a low ground pressure bulldozer adjacent to the trench. The specified hydraulic conductivity for the backfill was k £ 1×10-7 cm/sec. An extensive SWPP program was implemented and developed on site. The specified hydraulic conductivity for the backfill was K£ 1×10-7 cm/sec.
Principal Client Issues
• Possible presence of unknown manmade obstructions along the trench alignment
• Trench stability in relation with the cutoff depth, in particular when trenching through old fly ash pond.
• Maintaining the characteristics of the bentonite slurry when excavating through contaminated soils and groundwater
• Excavation was rendered difficult as a result of the presence of limestone slabs above the bedrock formation. (Condition unknown at bid time).
• Cleaning the top of bedrock where the slurry trench was terminated prior to placement of backfill
• Work to proceed at some location within proximity of high voltage line.
• Risk of contaminated storm water flowing into the Mississippi river
Inquip first excavated a 10 feet deep pretrench to locate and remove all manmade obstructions along the cutoff alignment. Inquip hired a consulting engineer to perform the trench stability analysis. Special measures were developed to insure trench stability in the fly ash pond area including: Installation of wick drains to reduce the pore pressure, construction of a stable work platform and maintaining the trench slurry density between 75 and 85 pcf. Inquip performed some compatibility study to insure that the bentonite slurry as well as the SB backfill would not be attacked by the onsite contaminants. Specialized hydraulic clamshells, KS 3000, attached to a 100 tons crane deepened the trench to the top of bedrock, and a heavy chisel was used to break the limestone slabs. The soil bentonite backfill was mixed adjacent to the trench using a low ground pressure bulldozer. A very thorough QC plan was developed and implemented to insure trench stability and quality of the work. Inquip also worked closely with the electrical power company to develop and implement the measures to insure a safe environment when working next to the high power lines. We also designed a reinforced concrete cap for the permanent crossing.
A very thorough SWPP plan was prepared and implemented on site. Measures were taken to minimize contact between the storm water and the contaminated excavated soils. Potentially contaminated storm water was collected and pumped to an offsite treatment plant.