Tewksbury, MA: Soil Bentonite Slurry Wall, Vertical Barrier Installation
The Sutton Brook Disposal area consists primarily of two waste areas separated by the Sutton Brook. The disposal area was formerly known as Rocco's Landfill and began officially operating in 1957 as a "burning dump". It eventually became a sanitary landfill for the Town of Tewksbury in 1961, but continued to accept illegal commercial and industrial waste up until 1982, when it was officially closed. It is believed that it continued illegally accepting waste up until at least 1986, and possibly more recently. A Record of Decision (ROD) was completed in September 2007 to include the following:
- Excavation of contaminated soils and sediments
- Construction of a low permeability cap
- Construction of a vertical barrier to intercept groundwater from the southern lobe and prevent migration into Sutton Brook.
- Long-term groundwater, surface water, and sediment monitoring
Geo-Solutions Inc. was contracted by Charter Environmental to install the vertical barrier downstream of the southern lobe of the landfill.
The vertical barrier included the installation of 66,600 vertical square feet (vsf) of soil-bentonite (SB) slurry wall. The wall was installed to a maximum depth of 48 feet below ground surface (fbgs), keying into the underlying low-permeability till or bedrock.
Excavation was accomplished under bentonite slurry using a Komatsu PC800 excavator and Geo-Solutions' long-reach 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 1,733 feet in length.
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 with an overall average permeability of 2.82 x 10-8.
During excavation of the first 500 lineal feet of vertical barrier, an unexpected layer of very soft peat material was encountered, which caused instability of the trench and ultimately resulted in loss of backfill into the formation, and trench collapse. Geo-Solutions worked closely with the design engineer, Geosyntec, to develop some modified installation techniques to re-establish trenching operations and enable the completion of the vertical barrier as designed.