Slag Cement, Cement-Bentonite (SCCB) Slurry Wall
Hunter’s Point Naval Shipyard Parcel E-2 – Upland Slurry Wall
San Francisco, California
Project Features:
- Installation of Slag Cement, Cement-Bentonite (SCCB) wall
- Target Strength of 30 PSI
- Target Permeability of 1×10-6
- Long-term laboratory mix design process for chemical compatibility
- 570 Linear feet of cutoff wall
- Average depth of 20 feet, Maximum depth of 26 feet
- 2 feet key into Bay Mud (Soft Clay)
- 6 foot wide Final Structural “Key” Cap
Background:
Hunter’s Point Naval Shipyard was a commercial dry dock facility until the Navy purchased the property in 1939. It was used as a naval submarine and ship repair facility until 1974 when the Navy ceased shipyard operations there. During that time, the Naval Radiological Defense Laboratory (NRDL) used the facility for radiological decontamination of ships exposed to atomic weapons and research on the effects of radiation on materials and on living tissue. In 1989 the site was placed on the EPA’s National Priority List (NPL) of Superfund sites to be remediated. There are soils contaminated with petroleum fuels, pesticides, heavy metals, polychlorinated biphenyls (PCBs) and volatile organic compounds (VOCs) as well as radiological contamination. Parcel E-2 specifically is having contaminated soils excavated and screened for contaminants and will eventually have a liner and soil cap installed over the entire parcel for revegetation. As part of this phase of the remediation, a slurry wall was required along the Upland alignment along the west side of the historic landfill.
Description of Work:
GSI was contracted by CB&I Federal (which became APTIM Federal during the course of the project) to install the SCCB cutoff wall along the Parcel E-2 Upland alignment. The SCCB seepage cutoff wall was installed with a Komatsu PC-490 equipped with GSI’s custom long stick. The slurry wall was keyed 2 feet into the underlying Bay Mud soft clay layer. The “key” trench, or final structural cap was installed in sections following the installation of the slurry wall.
The excavation was supported by the SCCB slurry which was produced on site using a frac tank for water, frac tank for bentonite slurry, and a continuous mixing plant for producing the SCCB grout. Water was sourced from a local hydrant and was brought to the site by HDPE fusion welded pipe. The permeability target was far surpassed by all of the samples, with an average permeability of 3×10-7. The UCS target was met by all of the samples tested.
Prior to the field work, GSI conducted a long-term laboratory mix design process to ensure compatibility with the groundwater chemistry. GSI made samples of numerous candidate mixes in the laboratory, using 3 different types of clay, and variable proportions of slag cement and Portland cement. The mix design was done in 2 phases; the initial mixes were tested for UCS and permeability in order narrow down the options. Then, 3 mixes were selected for long-term compatibility testing. The samples were permeated for 3 months with groundwater from the site to ensure they were compatible.
Project Challenges:
This primary challenge on the project was a large sub-surface obstruction that was encountered along the alignment. GSI worked with APTIM and the Nave representatives to change the depth of the wall in areas to work around the obstruction.
Due to the nature of the potential radioactive contamination, and the fact that the wall alignment was within the Exclusion Zone, radiation level monitors had to be worn by all personnel working in the Exclusion Zone. All personnel and equipment had to be manually screened prior to demobilization out of the zone and off the site.
