Former Vallejo Manufactured Gas Plant Site

Significant Project Features

• MGP remediation requires both heavy civil and environmental remediation services.
• Demolition and ACM abatement of above grade site structures.
• Installation of 2 temporary fabric structures (TFSs) (136-ft x 238-ft). Each unit has air handling systems -2 – 48,000 scfm blowers and 1 – 20,000 scfm blower per building. The blowers are capable of 6.7 air changes per hour. The TFSs will house ISS, excavation, and soil processing operations.
• Excavation and offsite T+D of ~216,500 tons of MGP impacted soil.
• ISS of ~83,000 CY of MGP-impacted soils via bucket mixing (65% within TFS).
• Community relations is an important aspect of this project.
• Restoration includes storm sewer replacement; waterline installation; paving; and concrete gutters, sidewalks, and aprons replacement.

Project Gallery

History & Location Details

Project Description

Scope of Work

Mobilization and Site Set Up

Prior to mobilization, our project management team developed all required work plans for approval by the client. Sevenson mobilized company-owned equipment to the site including multiple excavators (Komatsu PC300, PC360 and PC490), an ALLU processing attachment, multiple dozers, cement silos, and off-road haulers. Crews performed topographic surveys, began clearing and grubbing, established staging/laydown areas, set up office trailers and personnel decontamination facilities, and established temporary power, water, and sanitary facilities. A 230 LF by 136 LF Temporary Fabric Structure (TFS) with an air handling system was installed to house some of the excavation and ISS operations. Four on-site concrete structures were demolished to enable excavation and ISS of soils underneath the structures.

Demolition of Existing Structures

Prior to initiating remediation operations Sevenson subcontracted the demolition and abatement of above grade structures. Demolition and removal required close communication between our project team and the subcontractor. Recyclable metals were sized and either stockpiled and covered with tarps onsite for final disposition or loaded into trucks for disposal and/or recycling at the designated facility. Asphalt stripped from paved surfaces was placed into tri-axel dump trucks and removed from the site for recycling. Brick, including brick which is coated with lead-based paint, was segregated, and stockpiled for disposal. Concrete was segregated and sized for recycling. Clean concrete was loaded for offsite recycling. Stained concrete was mixed with MGP impacted soils and loaded for disposal as MGP waste. Upon completion of the demolition phase, Sevenson restored the site to pre-existing conditions.

Dust and Odor Control

Since remediation is conducted in close proximity to public areas, Sevenson implements stringent dust and odor control measures. Water and odor suppressant foams are used in work areas to control dust and odors. Additionally, some ISS operations are performed under a TFS which effectively prevents dust and odors from escaping from the work area. Workers inside the TFS wear Level C PPE with respirators to work safely. Sevenson also applies Best Management Practices to work procedures that limit the spread of dust including proper loading procedures, sweeping truck tires, and covering and tarping loaded trucks. Sevenson’s  subcontractor provides street sweeping services to limit dust from escaping off-site. To date, Sevenson has used 12,226 gallons of odor and dust suppressant foam to contain odors and dust.

Excavation and In-Situ Soil Mixing

Sevenson is performing remediation of the Former Vallejo MGP site in a five phased approach. The phased approach allows Sevenson to remediate areas and expand stockpiling and support zones while also allowing public boat access to remain open to the public. For excavation, Komatsu PC360 and PC490 excavators are used to remove soils in 50-ft. by 50-ft. grids using conventional buckets to depths ranging from 1.5–8 ft. BGS. Excavation cells are delineated per project specifications and adjacent cells are overlapped to contiguously excavate material. Soils are removed and material is directly loaded into trucks for off-site disposal or is staged on-site for future disposal. Currently, Sevenson has completed excavation (including excavation within ISS cells) in 224 out of a total of 294 cells.

To date, Sevenson has completed excavation and ISS of >164,000 CY via the bucket mixing method (pictured above) to a depth of 20-ft. BGS within rectangular cells ranging from 15-ft. to 25-ft. long by 15-ft. to 25-ft. wide. To start, ISS cells are delineated, and overburden material is removed. Adjacent cells are overlapped to ensure complete ISS treatment and a continuous monolith. An excavator is then stationed at grade or on crane mats mixing at below grade at a depth of up to 6-ft. BGS. A grout slurry containing Portland cement (PC) and Sodium Persulfate (SPS) is added at a ratio of 10% PC for all cells and SPS at varied ratios between 1%, 1.5%, and 2% based on prior site characterization to achieve cleanup criteria. Crews utilize a Komatsu 490 excavator with a bucket attachment and a Komatsu 490 excavator with an ALLU attachment to homogeneously blend soils. A rotary mixing head attached to the ALLU is then used to polish off ISS cells for quality assurance (pictured right). The purpose of the second rotary mixing head is to increase break down of soil inclusions and help establish contact between the activated persulfate and the contamination in the soils.

Injection of the PC/SPS slurry begins at the center of each cell to the required design treatment depth. Grout is injected via the hydraulic excavators and continuously prepared in our on-site batch plant. As the slurry is added and mixed, the excavation area is gradually widened by taking small slices of soil from its sides. As the soil from the sides is removed, it is then mixed with additional PC/SPS grout. The soil from the initial cut along with additional slurry are incorporated into the cell and mixed until the cell reaches the desired width. Once the desired width is achieved, the entire ISS cell area is mixed and polished. Sevenson utilizes GPS to track each bucket movement and records the coordinates for each corner of the treatment cell and along the surface elevation (top of ISS elevation) of the treatment cell. To date, Sevenson has performed ISS bucket mixing of MGP-contaminated soils in 93 of 165 ISS cells. Crews have completed ISS operations in Phase 1 and 2 areas and are actively performing ISS of Phase 3, 4, and 5 areas.

Backfill

Sevenson backfills and compacts excavations daily. Clean imported fill material is placed using dedicated clean equipment to prevent potential cross-contamination. Sevenson stockpiles backfill material on-site in a designated clean storage area. A WA470 loader delivers material from the stockpile area to the active backfill areas.

Site Restoration

Once excavation and ISS operations are completed in a cell, Unconfined Compressive Strength (UCS) and permeability testing is conducted. Upon receipt of passing results (50 psi), the cells are backfilled using 3-in. minus aggregate. Final restoration requires asphalt, 3-in. minus aggregate, and topsoil for plantings. Backfill material is shipped in by truck, spread and compacted with dozers and compaction rollers. A public parking lot, public restroom, boat wash station, and plantings will be constructed at the end of the project. To date, crews have placed >100,000 CY of backfill material in remediated areas.

Quality Control/Health and Safety

Sevenson tests for UCS and Permeability to ensure the project specifications of 50 psi are met for each cell. To date, Sevenson has performed 912 UCS samples and 76 permeability samples with no failures. Sevenson’s Site Safety and Health Officer (SSHO) enforces Level D PPE primarily during work outside the TFS. For site work within the TFS, Level C PPE with respiratory protection is required. To date, Sevenson has completed >82,500 safe workhours without a lost-time incident. Sevenson won the 2022 Environmental Remediation Council’s Award for Safety Excellence from PG&E for our ISS work at the Former Vallejo MGP Site.