PIANC Dredging 2012 Conference

Castro Cove Sediment Remediation Project, Richmond,

CaliforniaIan Austin PhD PE - URS Corporation

Paul Ryan – Chevron Environmental Management CompanyMark Sutton – Dixon Marine Services

Why of Interest?Technically challenging mudflat environmentSpecialty equipment designed to place “no-net-fill” 20-

acre capConsolidation of hydraulically placed beneficial re-use

mud layerShear stress modeling prior to sheet pile removal

Castro Cove Sediment Remediation Project Site Location

• Embayment of San Pablo Bay in north San Francisco Bay

• Adjacent to active refinery• Mudflats at +2 to +3 feet MLLW• Shear strengths; 50, 150 psf at 1, 2 foot depths

Castro Cove Sediment Remediation Project AOC

• Effluent discharged into Cove from early 1900s until 1987

• ERA: 20-acre AOC based on PAHs, mercury, benthic risk

• 2007: Dredged 97,000 cy sediment, placed upland • Approximately 6-inch thick layer of residual

contamination remained in AOC after dredging

Castro Cove Sediment Remediation ProjectEngineered Cap Design

• Cap designed using lab tests and chemical flux modeling, expert peer review (Danny Reible)

• Sequential Batch Leach, Thin Column Leach, and cap-loading tests used to develop partition coefficients and sorption capacities

• Design includes; • “No-net-fill” requirement, < 97,000 cy• 1.5 acres of Reactive Core Mats

(RCM), • 6-inch+ sand layer for chemical

isolation and containment, • 18-inch+ clean Bay mud layer to

restore biological viability for benthic communities

Castro Cove Sediment Remediation Project

Sand Placement Challenge• Performed “in-the-wet” with 3 to 5 feet

of water inside 2,700 foot sheet pile enclosure

• Adjacent to an operating refinery• Place a nominal 6-inch layer of sand over

20-acres (45,000 cy maximum) • Could not disturb or re-suspension

residual contamination• RWQCB Order only allowed discharge of

water inside sheet-pile after sand placement and WQ confirmation testing: closed loop system

• Required finesse, rather than bulk production

Castro Cove Sediment Remediation ProjectSand Placement

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Specialized Equipment Design: • Worked with DMS on hydrodynamic design of

spreader• Hydraulic placement of sand pumped 5,000 feet,

needed to reduce velocities from 14 fps to 2 –> 3 fps• Used computer model (HEC-RAS) to design spreader

plate, flare width and slope• Modular construction based on two 10-ft by 40-ft

pontoons

Sand Placement

Automated sand slurry system used to produce accurate slurry concentrations: spreader position only variable

Castro Cove Sediment Remediation Project

Sand Layer Confirmation: Sand barge delivery volumes

Bi-weekly bathymetric surveys using purpose-built, shallow-water, 5-beam

“sweep” systemSand core collection in clear PVC tubes

Castro Cove Sediment Remediation Project

Mud Layer Placement Challenge: Two weeks prior to start-up, required to change from hydraulic to mechanical dredging due to post-permit (DF&G)

change in position regarding impacts to long-fin smelt

Beneficial reuse of 37,500 cy of dredge sediment from Richmond Rod & Gun Club

Yacht Harbor dredgingBeneficial reuse of 13,000 cy from Point

San Pablo Yacht Harbor; 8,000 feet of pipe

Castro Cove Sediment Remediation Project Mechanical Hydraulic Dredge

Castro Cove Sediment Remediation Project AOC Confirmation Cross

Sections

Total sand and sediment volume: 95,500 cy

Castro Cove Sediment Remediation Project

Clay Layer Consolidation Challenge: Dewatered AOC to promote consolidation of mud

slurryExtended consolidation time from one month to

four months based on shear strengthsMeasured in-situ shear strength in surrounding

mudflatsMeasured strength of mud in AOC along diagonal

transect – field shear vane measurements

Castro Cove Sediment Remediation ProjectShear Vane Transect

Castro Cove Sediment Remediation ProjectMIKE 21 Shear stress modeling of sheet pile removal

Modeled tide-specific sequencing options for reintroducing the tideFirst: 300-ft pushed down to create 1-foot high weir -> minimize initial tidal prismSecond: sheet removal with two barge cranes along north face

Castro Cove Sediment Remediation Project Conclusions

• Cooperation between regulatory agency, owner, engineer, contractor needed to complete the project (from 1997 to 2012)

• Integration of science, engineering, construction needed to meet technical challenges (material delivery, mud consolidation, sheet pile removal)