Seattle City Light General Penstock Inspections: Concentration on Cedar Falls

Boundary Dam Completed in 1967 Pend Oreille River 1050 MW Total Capacity

(6) Individual Intakes Horizontal Penstocks Inspected every 10-years Vertical Penstocks Inspected every 15-years Vertical Intakes 51, 52, 54 Just Inspected

Ross Dam

Completed in 1949

Ross Lake Reservoir

460 MW

Two 24.5’ Concrete Lined Power Tunnels Bifurcates to steel-lined last 350’ Intakes, Penstocks Inspected Every 10 Years

Diablo Dam

Dam Completed in 1929

170 MW

19.5’ Diameter Single Penstock First 1800’ concrete lined, Last 190’ is steel lined. Inspected Every 10 Years

Gorge Dam Built in 1960 160 MW

Concrete lined power tunnel with an internal diameter of 20.5’, 11,000’ long 4 penstocks approx. 1600’ long Inspected every 10 years

South Fork Tolt 17 MW

Installed 1994 68” O.D. Welded Steel Pipe 25,000’ long Majority is buried 10-Year Inspections

Cedar Falls First Produced Power for Seattle in 1904

Cedar Falls Masonry Dam Completed in 1914 Cyclopean concrete 215’ x 1000’ 30 MW (Only) City Water Source (Hydro is not Primary)

One concrete-lined rock tunnel, 1500’ long that splits into two steel penstocks, each 7,500’ long and 78’ in diameter. Oldest sections are 92 YO. Newest sections are 54 YO.

Arch Bridge

Arch Bridge With (2) 78” Penstocks -5 & 6

Arch Bridge With (2) 78” Penstocks 5 & 6

Canyon Creek Trestle Bridge Carrying Penstock 5

One of Two Truss Crossings for Penstock 6

O&M Challenges Penstock •Oldest Sections 92 YO (1920) • New Sections 54 YO (1958)

Where/How/Why Do You Spend the Money??

Challenges Built in 1914 Carries Both Pnstks May not absorb MCE Cost to Repair/Replace: $20M-$25M

Challenges 1” Shotcrete Layer is Delaminating Some Exposed Rebar Vegetation

Challenges Cross Struts Appear Undersized for MCE Depth of Footings

Challenges • Built 1914 • Doesn’t Meet AASHTO Stand.

• Cost to Repair/Replace: $22M

Challenges • Bottom Chords of

Trusses Aren’t Properly Braced

• Abutment Tie Downs

Cost: $1M-3M

Challenges Failing Concrete Footings

Challenges Moss and Other Vegetation

Challenges Settling Concrete Footings

Around Surge Towers

Challenges Vegetation Around Surge Towers

Challenges Unstable Slopes

Challenges Erosion under Footings

Seattle City Light Contracted with Hatch Acres to Perform Risk Cost Analysis (RCA) to Evaluate Options To Address Seismic Vulnerability Along the Penstocks

1.Walked the Penstock 2.Identified PFMs 3.Site Geology and Seismic History 4.ID’d Frequent, Occasional or Rare

Events 5.Performed Stability Analysis

Identification of Frequent, Occasion and Rare Seismic Events The penstocks and supporting structures were evaluated for three earthquake events. The three earthquake events selected were: Frequent: Approximately 69 percent probability of exceedance in 50 years (1-in-43 year earthquake) as defined by Structural Engineers Association of California (SEAOC). Occasional (Operational Basis Event [OBE]): Approximately 40 percent probability of exceedance in 50 years (1-in-100 year earthquake) per City of Seattle residential slope analysis standards (City of Seattle Director’s Rule 33-2006). Rare (Maximum Design Event [MDE]): Approximately 7 percent probability of exceedance in 75 years (1-in-1033 year earthquake) per the AASHTO LRFD Bridge Design Specifications (AASHTO 2008).

A RCA was performed for each seismic event which modeled the risk cost using the probability of the seismic event, the economic properties of each failure mode, and the economic properties of the proposed interventions. Each RCA also modeled the risk reduction that each intervention would produce, and calculated a benefit-cost ratio by comparing this risk reduction to the cost of the intervention. These benefit-cost ratios allowed the economics interventions to be compared to optimize the rehabilitation plan.

A total of eight interventions were recommended based on risk cost analysis with an assumed 40 year project life and 4% discount rate. Each of the interventions had benefit cost ratios greater than 0.9.

ARCH BRIDGE 1. Isolate and Stiffen Penstocks Laterally 2. Remove soil behind right abutment to

elevation of strut 3. Install Rock Bolts to Strengthen Rock on Both

Abutments in Vicinity to Foundation

Steel Trestle 1.Strengthen Saddle to

Trestle Girder 2.Strengthen Trestle Girder

to Tower Connection

Penstocks Under pin foundations, R&R loose soils, repair saddle contact

Support at Erosion Points

Strengthen Lateral Stability of Surge Towers & Remove Vegetation