Solidification/Stabilization Treatment of
Dredged Material from the Port of San Diego
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Harbor
Tenth Avenue Marine Terminal
By Robert S. Austin, Port of San Diego, and Charles M.
Wilk, Portland Cement Association
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| S/S Blending Head |
Cement-based solidification/stabilization allowed the Port of San
Diego to economically and safely dispose or reuse material dredged
from the harbor. Dredging of channels and berths is a vital activity
to keep harbors open for business. Disposal of dredged material
is a significant cost to maintenance dredging. This is especially
true when contaminants in the dredged material prohibit ocean disposal
and require up-land disposal and increased costs. Cement-based solidification/stabilization
(S/S) is a treatment technology that can be used to manage dredged
material safely and effectively.
Project Description
This project involved dredging, treatment, and disposal of approximately
12,500 cubic meters (16,500 cubic yards) of sediment at the Tenth
Avenue Marine Terminal in San Diego, Calif. Sediment was dredged
to ensure safe ship maneuvering and docking conditions. The dredged
material was primarily storm drain runoff sediment discharged into
San Diego Bay out of the City of San Diego's Switzer Creek storm
drain outfall. Normal ocean disposal of the dredged sediment was
prohibited since the material was contaminated with below hazardous
levels of copper, zinc, lead, pesticides, and polychlorinated biphenyls
(PCBs).
Contaminated sediment is often disposed of in a confined disposal
facility (CDF). CDFs are dedicated landfills (or monofills) for
dredged sediments. Temporary CDFs can also be used to dewater or
dry sediment prior to off-site disposal. CDFs are often located
near the areas that are being dredged and can take up precious land
near port areas. This was the case at the Port of San Diego. A CDF
was not preferred and another sediment management solution was needed.
Cement-based S/S was selected to treat the dredged sediment allowing
for disposal in a local municipal solid waste landfill (MSWL). Cement-based
S/S involves mixing portland cement into the contaminated material
being treated. Cement reacts with water in the sediment to chemically
bind free water and dry the material.
Cement hydration reacts physically and chemically to immobilize
hazardous contaminants within the treated material. The dredged
sediment drying time for this process is a small fraction of that
required for air drying dredged material in a CDF. S/S treatment
reduces contaminant leachability allowing reuse or disposal in a
MSWL rather than a hazardous waste landfill. Reuse of treated dredge
material as engineered fill, as landfill cover material, or disposal
in a MSWL, is dramatically less expensive than disposal of the contaminated
dredged material in a hazardous waste landfill or CDF.
Dredging and Treatment
Sediment was dredged from the Bay using a clamshell dredge and
loaded into a barge tied alongside of the dredge vessel. A doublewalled
silt curtain (turbidity barrier) encircled the dredge area, including
the dredge vessel and barge. Free water captured with the sediment
during clamshell dredging was pumped back into the dredge area within
the silt curtain after it separated from the sediment in the barge
over several hours. The barge was then docked alongside the terminal
and an S/S blending head mounted on the end of a long reach extend-a-hoe
was used to mix a slurry of portland cement and water into the dredged
material. The mixing resulted in a 2% to 5% addition of portland
cement to the sediment.
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| Clam shell dredging |
S/S treatment using blending head |
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| Silt curtain encircles dredge area. |
Within a few hours the material was transferred by clamshell into
a k-rail bordered holding area on the terminal next to the mixing
barge, loaded into dump trucks lined with visqueen to keep the treated
sediment from sticking when it is dumped, and hauled to a local
sanitary landfill. The treated sediment was tested for pH and free
liquids. The MSWL criteria to accept the material for disposal was
a pH below 12 and above 2, and no free liquids per Paint Filter
Test (EPA SW846 Method 9095). In all, 12,600 cubic meters (16,500
cubic yards) of material were dredged and treated (approximately
27 barges). Actual dredging, free water removal, mixing, curing,
loading, hauling, and disposal operations required an average of
3 days per barge. By operating two barges simultaneously, the work
was accomplished in approximately 30 working days.
Benefits
Significant benefits resulting from the use of cement-based S/S
for dealing with the dredged sediment material include:
- Eliminates the need for a CDF.
- Binds soluble constituents and reduces chloride mobility
in the
resulting soil cement matrix.
- Generates soil cement with excellent engineering properties
for
use as landfill day cover, pre-conditioned (cement-treated)
roadway pavement base, slope or drainage channel surfacing
base, or structural fill material.
Credits
Plans and Specifications:
San Diego Unified Port District's Engineering Design Division,
San Diego, Calif.
Dredging and Treatment Contractor:
R.E. Staite Engineering, Inc., National City, Calif.
Solidification/Stabilization Mix Design:
Ninyo & Moore Geotechnical and Environmental Sciences
Consultants, San Diego, Calif.
Solidification/Stabilization Blending Head:
WBR Services, Inc., Newport Beach, Calif.
Photographs:
Port of San Diego
Robert S. Austin:
Project Engineer, Port of San Diego, Calif.
619.686.6251, rsaustin@portofsandiego.org
Charles M.Wilk:
Program Manager, Portland Cement Association
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