Nyack, NY: In-Situ Solidification
A Manufactured Gas Plant (MGP) operated at this site from 1852 until 1965. The site covers a total land area of approximately 4 acres. The site is in an urban setting, with adjacent properties used for a mix of commercial and residential purposes. It is believed that gas was made from the coal carbonization process from 1852 until 1887. From 1887 until 1889 the plant used oil instead of coal, and from 1890 until 1938 the plant used both coal and oil as feedstock or the carbureted water gas (CWG) process. From 1938 until 1965, the site was used as an oil gas facility only during times of peak demand, a practice known as "peak shaving." Coal tars are present at this site in the form of a dense oily liquid, which does not readily dissolve in water. Materials such as this are typically found at MGP sites, and are referred to as non-aqueous phase liquids or NAPL.
Specific volatile organic compounds of concern are benzene, toluene, thylbenzene and xylenes. These are referred to collectively as BTEX.
NYSDEC selected excavation of upper terrace with In-Situ Solidification of the lower terrace for addressing the impacts in the soil for this site. ISS was completed using Geo-Con's custom-made Calweld Drill Platform mounted on a Manitowoc 3900 crawler crane. The Calweld Drill, which is capable of over 400,000 ft/lbs of torque, turned a ten-foot-diameter mixing auger up to 21 feet below ground surface (fbgs) to the bedrock surface. A mixture of water, Portland cement and Sodium Bentonite (Montmorillonite Clay) grout was manufactured in Geo-Con's custom batch plant and pumped through a hollow-stem Kelly bar to the mixing auger. Columns were mixed in an overlapping pattern to provide the specified 100% coverage of the treatment area. 11,400 Cubic Yards (CY) were completed using this method. Samples of the soil mix were collected every 500 Cubic yards and tested for Unconfined Compressive Strength and Flex-Wall Permeability to ensure the contaminants are immobilized within the solidified soil-mixed monolith. Soil-mix attained over 50psi and a permeability of approximately 1 x 10-6 cm/s.
Geo-Con also performed soil mixing by the High-Pressure Jet Grouting Method. This was completed to ensure the interface between the large diameter ISS columns and the site bedrock is sufficiently sealed. This method employed injecting the Cement/Bentonite Grout through ports located on a custom made sub-assembly on a conventional 4" drill string at ultra-high pressures (3500psi). Rotation and extraction of the drill string results in "columns" of soil-cement. These columns were installed along the outside perimeter of the treatment area and consisted of 755 lineal feet of Jet Grout column installation. Jet Grouting was also used to stabilize areas where obstructions prevented the completion of large diameter ISS columns. An additional 311 CY of soil were stabilized using the jet grout method.