Design and Construction of an
11 MG Treated Water Reservoir
under Challenging Seismic
Conditions
Deborah C. Cohen, P.E. – Kennedy/Jenks ConsultantsCalvin Huey, P.E. – San Francisco Public Utilities CommissionSam Young, P.E. – San Francisco Public Utilities Commission
October 2, 2013
Presentation Outline
Project Overview
Harry Tracy Water Treatment Plant (HTWTP) Long Term Improvements Project (LTIP)
Design Elements
Seismic design criteria
Soil nail, MSE and soldier pile walls
11 MG Treated Water Reservoir
Construction
Construction sequencing
Lessons learned during construction
OVERVIEW OF HTWTP
PROJECT
SFPUC Hetch Hetchy Water System
HTWTP Facts
Built in 1972 and expanded in 1988 / 1992
Direct filtration plant
Rated capacity: 140 MGD
Sustainable capacity: 90 MGD
Average Flow: 20 - 40 MGD
Plant challenges
San Andreas Fault
Raw water quality
Site limitations
Project Objectives
Delivery Reliability
LOS Goal300 MGD with any source out of
service
Design Criteria: HTWTP
Reliably treat raw water of typical
water quality at a sustained flow
rate of 140 MGD for 60 days
Seismic Reliability
LOS Goal229 MGD within 24 hours and 300
MGD within 30 days of seismic
event
Design Criteria: HTWTP
Sustain limited damage following
Maximum Credible Earthquake on
San Andreas Fault and deliver 140
MGD within 24 hours after event
Project Overview
Region Peninsula
Project No. CUW 36701
Contract No. WD-2596
Contract Amount $174,197,000
Notice-to-Proceed March 16, 2011
Contract Time November 29, 2014 (1355 CD S.C.)
February 27, 2015 (1445 CD F.C.)
Project Map
DESIGN ELEMENTS
Seismic Design Criteria
Importance Factor: 1.5 for critical facilities
BSE-2 (Basic Safety Earthquake-2) per ASCE 41
2,475 year return period earthquake
2% occurrence in 50 years
Maximum Credible Earthquake of 7.9
Building Code regulates a return period of 475 years
Therefore higher acceleration (150% higher) required for design calculations
Concern for failure dictates replacement of existing 8 and 6.5 MG reservoirs with new 11 MG reservoir at new location
Earthwork Required to Support New
TWR
Cut and fill areas required to create flat surface for TWR
Soil nail and soldier pile walls
Designed to support cut face uphill of TWR
Mechanically stabilized earth (MSE) wall
Designed to support fill area downhill of TWR
Separates reservoir from main plant access road
Earthwork Required to Support New
TWR, cont.
Soil Nail Retaining Wall
Length: ~700 feet long
Height: 20 to 65 feet tall
Anchored by ~1,000 soil nails
Length of nails: 25 to 70 feet long
Diameter of nails: 1.375”
Required 6 to 8” diameter drilled holes
4” perforated horizontal drain pipes
10’ o.c. horizontal and 5’ o.c. vertical
Reduce pressure of non-drained condition
Design Drawing for Soil Nail Wall
Design Drawing for Soil Nail Wall
Mechanically Stabilized Earth (MSE)
Wall
Two tiers - upper and lower walls
Precast concrete facing panels
5’ by 5’ face area
Architectural detailing
Galvanized steel reinforcing strips
Anchor panels causing soil to behave monolithically
29.5’ long for bottom wall
19’ long for top wall
Spacing between strips determined by manufacturer
Design Drawing for MSE Wall
TWR - Process Flow
11 MG Treated Water Reservoir
Piles, Pile Caps, and Foundation
800+ Piles
HP-14 x 117 (14”x14” area; 117 weight per foot)
Depth: 12 to 61 feet
Depth based on the depth of bedrock
Bedrock varied 40’ throughout footprint of TWR
Pile-caps to connect piles to floor slab
Foundation
2’-3” thick slab
Design Drawing of TWR
TWR Inner Wall
To contain operational storage reservoir (8 MG)
Cast-in-place concrete wall
No prestressed reinforcement
Vertical post-tensioning tendons
Tapered wall
12” at top to 34” at bottom
Thickness at bottom required to withstand:
Hydrostatic pressure (water load 50’ of head)
Hydrodynamic (seismic) pressures
More cost effective to reduce mass of tank
TWR Outer Wall
To contain chlorine contact raceway (3 MG)
Strand-wound circular prestressed concrete tank
Cast-in-place concrete core
12” thick at top, tapered to 18” at bottom
Vertical post-tensioning tendons
Circular prestressed reinforcement
Design provided schedule of wraps for each layer
4 layers of strands wrapped around the tanks
1.5” thick shotcrete layer between wraps
Design Drawing of TWR
TWR Wall Connections
Connections to floor slab and to roof slab
Designed as unrestrained conditions
Allows tank to expand and contract when filling or draining the tank
Seismic cable connections
Transfer seismic load of the wall to the foundation
Prevents sliding of the tank off the foundation
Roof and Columns
Roof designed as two-way slab
Rebar resists bending stresses in each directions
Reduces required thickness of concrete in roof
More complicated design than one-way slab, but more cost effective
Columns
88 columns
30” diameter circular columns
CONSTRUCTION
Construction Sequencing
Completed to date:
Earthwork, including cut and fill areas
Construct soil nail, soldier pile and MSE walls
Installed piles and pile-caps
Placed buried pipelines
Installed leak detection system
Constructed grade beams and foundation slab in
quadrants
Poured inner and outer walls concurrently (in
progress)
11 MG Treated Water Reservoir
TWR AREA – PRE-CONSTRUCTION
11 MG Treated Water Reservoir
EXCAVATION AND SOIL NAIL WALL (SEPTEMBER 2011)
11 MG Treated Water Reservoir
EXCAVATION AND TEST PILES (MARCH 2012)
Soil Nail Wall
Soil Nail Wall
MSE Wall
Buried Pipelines
BURIED PIPELINE INSTALLATION (JUNE 2012)
Buried Pipelines
96” TW MANIFOLD PIPING SYSTEM
11 MG Treated Water Reservoir
PILE INSTALLATION (SEPTEMBER 2012)
11 MG Treated Water Reservoir
CONCRETE MUDSLAB (DECEMBER 2012)
11 MG Treated Water Reservoir
ENDURAFLEX COATING (JANUARY 2013)
11 MG Treated Water Reservoir
CLASS II PERMEABLE & LEAK DETECTION SYSTEM (FEBRUARY 2013)
11 MG Treated Water Reservoir
QUADRANT FORMWORK / REBAR / SEISMIC CABLES (MARCH 2013)
11 MG Treated Water Reservoir
2/4 QUADRANT CONCRETE POURS (APRIL 2013)
11 MG Treated Water Reservoir
COMPLETED QUADRANT CONCRETE POURS (MAY 2013)
11 MG Treated Water Reservoir
OUTER WALL CONCRETE & FORMWORK (JUNE 2013)
11 MG Treated Water Reservoir
OUTER WALL CONCRETE & FORMWORK (JULY 2013)
11 MG Treated Water Reservoir
TWR INNER AND OUTER WALLS (AUGUST 2013)
Construction Sequencing, cont.
To be completed:
Construct columns and contactor effluent box
Install roof
Prestress outer wall
Perform leak test
Place coating on inner and outer walls
Perform tracer study
Lessons Learned – Soil Nail Wall
Tall wall with high seismic loading resulted in extremely long soil nails with tight spacing.
Issue:
Potential for conflict between adjacent soil nails and drain lines.
Solution:
Require staking out/surveying of the soil nail alignment above and behind the wall to align drilling equipment.
Lessons Learned – Soil Nail Wall
Lessons Learned - TWR
Contractor developed two full-scale mock-ups of:
Seismic cables between foundation and walls
Reinforcement of outer wall
Purpose
Verify construction will follow design intent
Coordinate reinforcing in the wall and footing with seismic cables and rebar couplers
11 MG Treated Water Reservoir
MOCK UP OF FOUNDATION REBARS AND SEISMIC CABLES
11 MG Treated Water Reservoir
MOCK UP OF OUTER WALL
Questions?
Deborah C. Cohen, P.E. - Kennedy/Jenks Consultants
(415) 243-2528
Calvin Huey, P.E. – SFPUC
(415) 554-3189
Sam Young, P.E. – SFPUC
(415) 551-4651
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