Soils at Consolidated Edison’s former White Plains MGP facility had been identified as being contaminated with coal tar. Contaminants of concern in the tar included polycyclic aromatic hydrocarbons, benzene, toluene, ethyl benzene and xylene. In situ solidification using soil mixing was specified for the majority of the site. Jet grouting was to be performed only in areas of the treatment zone where the soil mixing rig was unable to reach. Ultimately, when the general contractor’s soil mixing operation encountered difficulties due to underground obstructions and an unmixable till layer, soil mixing was discontinued and the scope of Moretrench’s initial jet grouting work was expanded to encompass the untreated portion of the site.
The subsurface profile consisted of approximately 15 feet of granular surficial soils overlaying a variable sandy soil stratum. Beneath this, a 3- to 12-feet thick layer of till extended to top of rock, which sloped steeply in elevation across the site from 30 feet to 60 feet below working grade.
Moretrench installed pre-production test columns in two different areas of the site to evaluate the design parameters for production work. Retrieved core samples were inspected for continuity and quality, and wet grab samples were tested for unconfined compressive strength and permeability to confirm that the specified requirement of an in situ soil-grout product strength of 50 psi and a permeability of 1 x 10-6 cm/sec or less would be met during production work.
With health and safety of high priority, all site work was conducted under modified Level D protection, with engineering controls employed to ensure that the work zone could be maintained and the level of Hazmat protection did not have to be elevated. Large industrial blowers were used to maintain air quality during the drilling and jetting.
The revised scope of work at locations specified by the Engineer included re-treatment of areas where soil-mixing had not extended to full design depth or full overlap had not been achieved, as well mass treatment over the remainder of the site.
A special grout mix of cement and slag was used to create a very low-permeability finished product. Moretrench used double-fluid jet grouting to accomplish the in situ solidification, with grouting parameters designed to achieve 6-foot diameter columns. Rotary drilling was used to advance the jet grout monitor to top of rock. For mass treatment, jet grout locations were on a nominal 4.5-foot center-to-center spacing, realizing an overlap of a minimum of 9 inches. Grouting was accomplished in a primary/secondary sequence, with spoil generated from the jet grout operation pumped into trucks for off-site disposal.
Verification Monitoring and Testing
During production grouting, an automatic monitoring system recorded fluid pressures, flow rate, withdrawal rate and rotation speed. Verification testing measures included:
- Specific gravity testing at the batch plant to verify grout mix consistency
- Spoil sampling and testing for specific gravity and unconfined compressive strength (UCS)
- Retrieval of wet grab grout samples for 7- and 28-day UCS testing and permeability testing
Results of UCS testing showed an average compressive strength of 650 psi and confirmed that the permeability requirement of 1 x 10-6 cm/sec had been met.