Relevance OF Project
- Deep cutoff wall
- Use of Soil Bentonite Technique
- Excavation with a combination of extended reach backhoe and hydraulic clamshell
- Low permeability of the Backfill
In 1998, Inquip Associates Inc., as a subcontractor to Sverdrup Environmental, constructed a 1,500 linear feet soil bentonite cutoff wall, to maximum depth of 110 feet. The cutoff wall was constructed around a plume of chlorinated solvents. The trench was excavated, through very dense glacial till material, including cobbles and boulders in its lower 40 to 50 feet, with a Koehring 1266 equipped with an extended dipper stick and a Liebherr 853 crawler crane with a KS 3000 hydraulic clamshell attachment. The soil bentonite backfill was mixed adjacent to the trench with a low ground pressure dozer. The specified hydraulic conductivity for the backfill was k 1×10-7 cm/sec.
Principal Client Issues
- Depth of the soil bentonite cutoff
- Excavation through very dense till formations
- Cutoff was excavated to top of bedrock, which was sometimes difficult to identify because of the presence of large boulders.
- Concerns about the trench stability as the upper formations were soft.
Inquip hired a consulting engineer to perform a trench stability analysis prior to mobilization on site. This task included performing additional borings and soil testing to determine the characteristics of the soil formations. Once the criteria for trench stability were established, Inquip mobilized on site a powerful long reach excavator, (Koehring 1266) for the excavation of the upper 80 to 9 feet of trench. A specialized hydraulic clamshell KS 3000, attached to a 100 tons crane was used to deepen the trench to the top of bedrock. Inquip hired a geotechnical engineer to examine the cuttings and make the determination that the trench was well excavated to the top of bedrock.
The soil bentonite backfill, with a hydraulic conductivity of k ≤ 1×10-7 cm/sec, was mixed adjacent to the trench using a low ground pressure bulldozer.