City of Sunnyvale | Flood/Retaining Wall and Flood Gate Project Proposed Basis of Design Technical Memo 2365 Iron Point Rd. Suite 300 Folsom, CA 95630 T 916.817.4700 hdrinc.com CITY OF SUNNYVALE POLLUTION CONTROL PLANT PROPOSED BASIS OF DESIGN TECHNICAL MEMO - FLOOD/RETAINING WALL AND FLOOD GATE SLAB Draft November 2015 Prepared By: ______________________ Wesley Jacobs, P.E.* (LA, MS, TX, GA, VA, NY) Reviewed By: ______________________ Rob Natoli, P.E. Ronald Brown, P.E.* (WA, OR, AK, HI, OK) Reviewed By: ______________________ Jamel Demir, P.E. Reviewed By:______________________ James Wickstrom, P.E.
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City of Sunnyvale | Flood/Retaining Wall and Flood Gate Project Proposed Basis of Design Technical Memo
2365 Iron Point Rd. Suite 300 Folsom, CA 95630
T 916.817.4700 hdrinc.com
CITY OF SUNNYVALE
POLLUTION CONTROL PLANT
PROPOSED BASIS OF DESIGN TECHNICAL MEMO - FLOOD/RETAINING WALL AND FLOOD GATE SLAB
Draft November 2015
Prepared By: ______________________
Wesley Jacobs, P.E.* (LA, MS, TX, GA, VA, NY)
Reviewed By: ______________________
Rob Natoli, P.E. Ronald Brown, P.E.* (WA, OR, AK, HI, OK)
3.1.4 Flood Gate Base Slab .................................................................................................... 6
Appendices
Appendix A – Basis of Design Report
Appendix B – Draft Similitude Analysis Report
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1.0 Introduction 1.1 Purpose and Scope The purpose of this Technical Memorandum summary and Basis of Design Report (BOD) is to describe the flood control features proposed as part of the renovation of the existing City of Sunnyvale Water Pollution Control Plant (WPCP) so that the facility can continue to properly treat and dispose of municipal sewage for the next 75 years. To continually provide service for the next 75 years, the WPCP must secure a minimum level of flood protection against the 1-percent annual chance flood. A flood wall was designed (Task 1.1 of the scope of work) using both T-Wall and I-Wall sections design for soil retaining loads and flood loads per requirements of the Federal Emergency Management Agency (FEMA) and the United States Army Corps of Engineers (USACE) guidelines. Included in this design is the foundation for the floodgate that is to be designed in Sunnyvale WPCP Package 2. This BOD is a detailed look at the design criteria and parameters (attached as Appendix A) and will serve as the foundation for the work to be completed.
The scope of work includes these task items:
Hydraulic and Wave Analysis for Floodwall Design Loading
T-Wall Design of the Southern and Eastern Regions of the WPCP.
I-Wall Design of the Northern Region of the WPCP.
Floodgate Foundation Design at the south western terminus of the T-Wall.
1.2 Project Background and Description For the WPCP to provide uninterrupted services for the next 75 year, the WPCP must obtain a minimum 100-year level of flood protection to reduce some of the risk of flooding of their facilities. The WPCP is surrounded by both tidal flooding on the northern side and riverine flooding on the east and west sides. Currently, Santa Clara Valley Water District (SCVWD) is developing a design to improve capacity of the Sunnyvale East and West Channels that will contain the 100-year Still Water Elevation (SWE) event as well as consider sea level rise over the next 50 years.
2.0 Design 100-year Water Surface Elevation and Freeboard
Hydraulic and wave analysis is a key element to designing flood walls per USACE guidance; these are feature load cases that many times govern the design of the flood wall. Hydrostatic load on a portion of the wall is not sufficient to satisfy the criteria. The hydraulic and wave loading analysis summarized in this section were performed for the purpose of:
Providing a recommendation for the 100-year design water surface elevation (SWE) and required freeboard considering wave run-up and satisfying FEMA’s 44 CFR 65.10 regulations.
The 2014 BakerAECOM study titled, “A South San Francisco Bay Coastal Flood Hazard Study” indicated that the “South Bay study did not model swell waves because swell from the Pacific Ocean do not penetrate into the south San Francisco Bay” (BakerAECOM, March 28, 2014). The study did consider wind induced waves but found that for Transect 11 these were negligible, with wave heights less than one
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(1) ft. Nevertheless, for the purpose of evaluating potential wave loads on the proposed flood wall the values generated in the 2014 study were implemented using USACE guidance.
The 2014 study only considered wave runup for transects with wave heights greater than one foot. With a review of the terrain characteristics in the vicinity of the proposed flood wall, coupled with a design wave height of 0.6 feet, it was estimated that wave runup would be less than one foot. As a result, two feet of freeboard was applied to the SWE, per 44 CFR 65.10, to set the minimum top of flood wall elevation. Therefore, a SWE of 12 ft plus an additional 2.0 ft of freeboard sets the proposed top of floodwall at 14.0 feet.
3.0 Floodwall Design 3.1 Criteria Criteria used for the WPCP floodwalls is based on published federal technical guidance documents, including USACE Engineering Manuals, (EMs), Engineering Regulations, (ERs), and Engineering Technical Letters, (ETLs). Design criteria in the most recent publications supersede previous publications. This includes design criteria for flood wall stability and strength.
Based on recommendations and discussions with the City of Sunnyvale, two floodwall designs are proposed.
Design criteria established for the proposed floodwall types will utilize the following USACE criteria:
Cantilever Type Flood Walls (T-walls)
EM 1110-2-2502 Retaining and Flood Walls EM 1110-2-2100 Stability Analysis of Concrete Structures EM 1110-2-2104 Strength Design of Reinforced-Concrete Hydraulic Structures EM 1110-2-2906 Pile Design
Cantilever Sheet Pile Flood Walls (I-walls)
EM 1110-2-2504 Design of Sheet Pile Walls EC 1110-2-6066 Design of I-walls
3.1.1 Cantilever Type Flood Wall (T-Walls)
The T-walls were designed as retaining walls as well as flood walls as the eastern portion of the site is being raised as part of the plant process renovation. A majority of the flood wall designed will retain 8 ft. to 12 ft. of soil. There is a 40 foot section on the eastern alignment of flood wall that will require a pile foundation as a result of the loading conditions and estimated settlement (See Section 3.1.2). Since this perimeter risk reduction system is classified as a coastal flood wall system, seven load cases were examined for design per USACE guidance that included the 100 year SWE, wave loads, water to top of wall, earthquake, surcharge and construction loads. The walls were sized for stability, strength and serviceability to counteract these loading conditions. A sheet pile cut off wall is embedded into the base
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slab and is driven down into the subgrade to prevent seepage under the wall during a flood condition. This is continuous through all of the T-wall monoliths on the south and eastern alignments.
3.1.2 T-wall with Pile Foundation
The Sunnyvale WPCP floodwall alignment traverses an existing ditch between stations 9+00 and 10+00. The approximate height of soil retained by this 40 foot monolith is 20 feet. Due to this height of soil and the anticipated settlement of approximately 6 inches; a shallow founded T-Wall section will not be adequate per the USACE required criteria. Consequently deep foundations were required to mitigate this issue. Prestressed, precast concrete piles were selected as the foundation elements that are sloped or battered with a 1:3 ratio. Similar load cases were considered for this section as noted for the shallow founded flood wall sections.
3.1.3 Cantilever Sheet Pile Flood Wall (I-wall)
On the northern alignment, I-wall sections have been considered for design as the overall wall height or “stick up” is less than six feet (limited by new USACE guidance EC 1110-2-6066). This portion of the alignment would be subject to similar hydraulic loading conditions as noted above for T-walls. The sheet pile wall sections will be sized for length and strength in flexure based on the geotechnical stability analysis provided by the Fugro. The proposed I-wall section is a steel sheet pile section topped off with a reinforced concrete cap that is embedded approximately 1 – 2 ft below grade. Its appearance above grade will match the T-wall sections. During previous iterations, vinyl sheet pile was considered due to the potentially corrosive soils on site; however, per USACE guidance, vinyl is not allowed for mainline I-walls. Concrete sheet pile was also considered as an alternative for this reason but was dismissed due to potential difficulties in driving the sheets and the higher cost.
Based on the Floodwall Sheet Pile Similitude Corrosion Rate Analysis Technical Memorandum dated February 27, 2015 conducted for the site (See Appendix B), the use of steel sheet piles will require mitigation for corrosion protection. The recommendations in the tech memo were taken into consideration as well as the guidance provided in the USACE EM’s coupled with previous experience. Coating the steel sheet piles with a coal tar epoxy or bituminous coating system is an acceptable mitigation strategy for long term protection against corrosion. In addition to this, as a redundancy, a thicker steel sheet pile will be selected than what is required by design to counteract any additional potential corrosion that may be realized should the coating wear out or fail. This selection has been commonly used in USACE flood risk reduction projects all over the country with similar corrosive environments. The city of Sunnyvale, the program manager, and the designer discussed and concluded that a cantilever sheet pile I-wall would be the used for a portion of the floodwall.
3.1.4 Flood Gate Base Slab
At the entrance of the plant a passive bottom hinged floating flood gate system (by FloodBreak Inc.) is proposed. The base slab is being designed to withstand the axial, overturning and sliding forces that will be transferred from the flood gate superstructure. The geotechnical information used for the gate foundation design is taken from the Fugro Geotechnical Report, October, 2015. The foundation is designed for following load cases:
1: Flood gate open (down position) with HS-20 wheel loading 2: Flood gate closed (up position) with water to the top of the gate
The base slab was checked for overturning failure due to lateral hydrostatic load, bearing failure due to dead load of the slab and vertical hydrostatic load, and also for sliding due to lateral hydrostatic load. Due to the location on the southern portion of the alignment, hydrodynamic wave loadings are not applicable
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for the gate foundation. The design resulted in a rectangular reinforced concrete slab that was 20 feet x 16 feet x 4 feet thick. The American Association of State Highway and Transportation Officials - AASHTO LRFD Bridge Design Specifications, Customary U.S. Units, 7th Edition, with 2015 Interim Revisions was used in conjunction to the applicable USACE criteria.
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Appendix A –Basis of Design Report
Draft
City of Sunnyvale Water Pollution Control Plant
Proposed Basis of Design – Flood/Retaining Wall and Flood Gate Project
Version 1.0
Sunnyvale, California
October 12, 2015
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5.0 Flood Gate Base Slab ................................................................................................................. 16
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Proposed Basis of Design
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Tables Table 1 – Summary of hydrodynamic wave forces calculations .......................................................... 9 Table 2. Load Case Categories (Adopted from USACE EM 1110-2-2100) ....................................... 13 Table 3. Summary of Adopted Criteria .............................................................................................. 14
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Proposed Basis of Design
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1.0 Introduction 1.1 Purpose and Scope The purpose of this Proposed Basis of Design (BOD) is to renovate the existing City of Sunnyvale Water Pollution Control Plant (WPCP) so that the facility can continue to properly treat and dispose of municipal sewage for the next 75 years. To continually provide service for the next 75 years, the WPCP must secure a minimum level of flood protection against the 1-percent annual chance flood. A floodwall will be designed (Task 1.1 of the scope of work) using both T-Wall and I-Wall sections design for soil retaining loads and flood loads per requirements of the Federal Emergency Management Agency (FEMA) and the United States Army Corp of Engineers (USACE) guidelines. Included in this design is the foundation for the floodgate that is to be designed in Sunnyvale WPCP Package 2. This BOD is a detailed look at the design criteria and parameters(attached as Appendix A) and will serve as the foundation for the work to be completed.
The Scope of work includes these task items:
1. Hydraulic and Wave Analysis for Floodwall Design loading.
2. T-Wall Design of the Southern and Eastern Regions of the WPCP.
3. I-Wall Design of the Northern Region of the WPCP.
4. Floodgate Foundation Design at the South Western Edge of the T-Wall.
1.2 Project Objectives The objectives of the floodwall and floodgates include the following:
1. Design a flood protection system that minimizes long-term maintenance needs and associated costs.
2. Design a flood protection system based upon USACE EM’s Coastal (1110-2-1100), Stability of Concrete Structures (1110-2-2100), Concrete Design (1110-2-2104), Design of Sheet Pile Walls (1110-2-2504), and Retaining and Flood Walls (1110-2-2502).
3. Construct a flood protection system that protects against latest tidal and riverine study results (FEMA Preliminary FIRM and FIS reports, dated July 8, 2015).
1.3 Project Background and Description For the WPCP to provide uninterrupted services for the next 75 year, the WPCP must obtain a minimum 100-year level of flood protection to reduce some of the risk of flooding of their facilities. The WPCP is surrounded by both tidal flooding on the northern side and riverine flooding on the east and west sides. Currently, Santa Clara Valley Water District (SCVWD) is developing a design to improve capacity of the Sunnyvale East and West Channels that will contain the 100-year SWE event as well as consider sea level rise over the next 50 years. It is important to note, however, that bridge crossings over the Channels are not being upgraded as part of this project to contain sea level rise. Without these bridge improvements, flood waters will still be able to enter the WPCP
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Proposed Basis of Design
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from the south under existing conditions. It is anticipated that these bridges will be retrofitted to protect against sea level rise in the future as it becomes a problem and more funding becomes available. It is also anticipated that, dependent upon funding, the USACE will proceed forward with the design and construction of tidal levee/floodwall protection along the southern portion of Pond A4. Currently, the levees surrounding the WPCP are not certified and do not meet Code of Federal Regulations (44 CFR 65.10) and USACE standards. The City of Sunnyvale has selected to incorporate flood protection against the 100-year flood elevation into its planned retaining wall so that flood protection can be secured at the WPCP as soon as possible.
On July 8, 2015, FEMA released updated Preliminary Flood Insurance Rate Map (FIRM) and Flood Insurance Study (FIS) Report documenting updated Still Water Elevations (SWE) at and surrounding the WPCP. The proposed alignment of the floodwall crossed over a breakline between two SWE of 11 and 12 feet (NAVD 88). The higher of the two SWEs, 12 feet (NAVD88), was selected for the SWE.
This floodwall will be designed to secure additional fill material for future plant expansions, act as a security/privacy wall from the public, and provide flood protection against the 100-year SWE event. At the entrance to the WPCP, a floodgate foundation will be designed for incorporation of a floodgate in a future project phase.
1.4 Proposed Alignment HDR Inc. has identified the alignment for the new flood protection system that will be evaluated in Task 1.1 (Figure 1). The following considerations were taken to develop this alignment:
Existing facilities and topography
New site plan including grade elevation raise and new structures
Site constraints such as required wall foot prints and maximization of useable area
Adjacent construction improvements such as the Sunnyvale West Channel
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Proposed Basis of Design
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Figure 1: Floodwall Alignment
2.0 Existing Site Conditions 2.1 WPCP WPCP is located at 1444 Borregas Avenue, in Sunnyvale, California. Currently the WPCP is protected by uncertified levees along the northern side of the site, Pond A4, as well as the Sunnyvale West Channel flanking the west side of the site. The current ground elevation of the eastern portion of the existing site varies between 3 feet and 10 feet but will be designed to an elevation of 10 feet per North American Vertical Datum of 1988 (NAVD88). Even at this ground elevation, the site will be under the 100-year SWE of 12 feet based upon the FIRM and FIS report.
3.0 Hydraulic and Wave Loading Analyses 3.1 Introduction Hydraulic and wave analysis is a key element to designing flood walls per USACE guidance; these are feature load cases that many times govern the design of the flood wall. Hydrostatic load on a portion of the wall is not sufficient to satisfy the criteria. The hydraulic and wave loading analysis summarized in this section were performed for the purpose of:
i. Computing wave forces using USACE’s Costal Engineering Manual (EM) 1110-2-1100 guidance.
I-Wall
T-Wall
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Proposed Basis of Design
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ii. Providing a recommendation for the 100-year design water surface elevation (still water elevation) and required freeboard considering wave run-up and satisfying FEMA’s 44 CFR 65.10 regulations.
3.2 Data Collection and Previous Studies BakerAECOM performed a study of coastal flood hazards in the vicinity of the project site. The analysis and results from this study were summarized in 2014 stufy titled “A South San Francisco Bay Coastal Flood Hazard Study” (BakerAECOM, March 28, 2014). FEMA released Santa Clara County’s Preliminary Flood Insurance Study results in June 2015. The results of this study were used to compute wave forces on the proposed floodwall as well as setting the ultimate top of floodwall elevation for design. HDR did not perform any additional hydraulic modeling, but used the parameters and findings provided by FEMA and their mapping partners.
3.3 Hydrodynamic Wave Forces Analysis HDR reviewed the Preliminary Santa Clara County FIRM map (No. 06085C0042K) dated July 8, 2015 and determined that coastal Transect 11, located approximately 500 ft. east of the floodwall alignment, was representative of the project site conditions. The topography of the project site is complex due to man made berms that extend across the shoreline and around old salt ponds. These berms are not designed for flood control or accredited by FEMA; therefore, partial failure of these embankments was considered as part of the 2014 study titled, “A South San Francisco Bay Coastal Flood Hazard Study” (BakerAECOM, March 28, 2014)”. This is consistent with the conditions that the project floodwall is likely to encounter if engaged with coastal flooding.
The BakerAECOM 2014 study indicated that the “South Bay study did not model swell waves because swell from the Pacific Ocean do not penetrate into the south San Francisco Bay” (BakerAECOM, March 28, 2014). The study did consider wind waves but found that for Transect 11 these were negligible, with wave heights less than one (1) ft. Nevertheless, for the purpose of evaluating potential wave loads on the proposed floodwall the values generated in the 2014 were implemented using USACE guidance.
The USACE’s EM-1110-2-1100 was referenced for developing wave loads. As per the wave conditions described in the 2014 study only non-breaking waves were considered. Given the terrain in the vicinity of the floodwall alignment the following configuration was considered (Figure 2); a rubble (rock slope protection) layer was not included in the preliminary floodwall design but should be considered.
City of Sunnyvale | Flood/Retaining Wall and Flood Gate Project
The USACE’s EM-1110-2-1100 describes the following relationships (Goda, 1974) for Figure 1 parameters (Figure 3).
City of Sunnyvale | Flood/Retaining Wall and Flood Gate Project
Proposed Basis of Design
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Figure 3: Goda Formula for Irregular Waves (Goda 1974; Tanimoto et al. 1976 - USACE, 2011)
A Wave Height Analysis for Flood Insurance Study (WHAFIS) was performed as part of the 2014 study. The results of the WHAFIS in the vicinity of the recommended floodwall are illustrated in Figure 4.
City of Sunnyvale | Flood/Retaining Wall and Flood Gate Project
Proposed Basis of Design
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Figure 4: Wave Crest and 1-percent Stillwater Elevation at Project Site (BakerAECOM, March 28, 2014)
Given the relationships in Figures 2 & 3 and parameters derived from Figure 4 and the 2014 study; HDR found the following:
Table 1 – Summary of hydrodynamic wave forces calculations
Variable Description Value Units
h*
0.9 ft
p1
22.2 lbf/ft
2
p2
0.0 lbf/ft
2
p3
12.8 lbf/ft
2
pu 12.7
b Angle of incidence of Wave (angle between wave crest and structure)
0.0 degrees
Hdesign Design wave height defined as the highest wave in the design sea state at a location just in front of the breakwater.
0.6 ft.
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Proposed Basis of Design
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Variable Description Value Units
a*
0.0 NA
a1
0.6 NA
a2
0.0 NA
a3
0.6 NA
L Local Wave Length 0.6
Ft
hb Water depth at a distance of 5Hs seaward of the breakwater
frontwall
10.0 Ft
l1,l2,&l3 Modification factors depending on structure types 1.0
rw Mass density of water 2.0
slugs/ft3
g Gravitational Acceleration 32.2
ft/sec2
hc Freeboard 2.0
Ft
hs Still water depth 9.9
Ft
d Depth of wall submerged 4.2
Ft
hw Height of wall 6.2
Ft
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3.4 Design 100-year Water Surface Elevation and Freeboard FEMA’s 44 CFR 65.10 state that “for coastal levees, the freeboard must be established at one foot above the height of the one percent wave or the maximum wave runup (whichever is greater) associated with the 100-year Stillwater surge elevation at the site.” Additionally, 44 CFR 65.10 states, “under no circumstances, however, will a freeboard of less than two feet above the 100-year stillwater surge elevation be accepted.” The design of the floodwall shall comply with this regulation.
The 2014 study only considered wave runup for transects with wave heights greater than one foot. With a review of the terrain characteristics in the vicinity of the proposed floodwall, coupled with a design wave height of 0.6 feet, it was estimated that wave runup would be less than one foot. Since the wave runup was estimated as less than one foot, two feet of freeboard was applied to the SWE, per 44 CFR 65.10, to set the minimum top of floodwall elevation. Therefore, a SWE of 12 ft plus an additional 2.0 ft of freeboard sets the proposed top of floodwall at 14.0 feet.
4.0 Floodwall Design 4.1 Criteria Criteria used for the WPCP project are based on published federal technical guidance documents, including U.S. Army Corps of Engineer’s Engineering Manuals, (EMs), Engineering Regulations, (ERs), and Engineering Technical Letters, (ETLs). Design criteria in the most recent publications supersede previous publications. This includes design criteria for flood wall stability and strength.
Design criteria established for the following flood wall types will follow USACE criteria:
Cantilever T-Type Wall
EM 1110-2-2502 Retaining and Flood Walls
EM 1110-2-2100 Stability Analysis of Concrete Structures
EM 1110-2-2104 Strength Design of Reinforced-Concrete Hydraulic Structures
Sheet Pile Wall
EM 1110-2-2502 Retaining and Flood Walls
EM 1110-2-2504 Design of Sheet Pile Walls
EC 1110-2-6066 Design of I-walls
4.2 Top of Wall Elevation The top of wall elevation for the floodwall must be based on USACE, FEMA, and newly publish FIS report and FIRM. Key considerations should include the following:
Regulatory Requirements on freeboard and recurrence intervals to be used in design
Results of wind and wave evaluation
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Freeboard
Settlement
For this project, the top of wall design elevation is 14 feet.
4.3 Geotechnical Criteria The geotechnical investigation identified the type and distribution of foundation materials to determine material parameters for use in flood wall design (See Fugro Geotechnical Report, October, 2015). The geotechnical information will be used to select the foundation type and depth, design the flood wall / retaining wall system, estimate earth pressures, locate the ground-water level, estimate settlements, and identify potential construction problems.
For both the Cantilever T-Type and the Cantilever Sheet Pile Flood Wall, the results of the Geotechnical Investigation will be used to determine the relevant design parameters for each soil type encountered.
Floodwalls will be designed per EM 1110-2-2100 and EM 1110-2-2502. These floodwalls will be designed as retaining walls because a substantial part of the eastern portion of the WPCP site ground elevation is being raise to 10 ft. as part of the site renovation. A majority of the floodwall designed will retain 8 ft. to 12 ft. of soil. There is a 40 foot section of retaining wall that will have a deep foundation support of piles due to a need for approximately 20 foot of soil retained.
Four load cases will be examined for design of the flood wall. Cases C1 through C2c as specified in EM 1110-2-2502, Section 4-5, for coastal flood walls. These include; Case CI1 – Surge Still Water Elevation (~100 YR SWE), Case C2A – Nonbreaking Wave Load, Case C2b – Breaking Wave Load, Case C2c – Broken Wave Load, and note that coastal load cases C3 through C5 are checked within the retaining wall design. An additional case to be considered is when the wall has a WSE to the top of the wall.
Three load cases will be examined for design of the retaining wall. Cases R1 through R3 as specified in EM 1110-2-2502, Section 4-3, for retaining walls. These include; Case R1 – Usual Loading, Case R2 – Unusual Loading (Surcharge Load), Case R3 – Earthquake Loading.
These load cases will be assigned categories per EM 1110-2-2100 based on the expected return period, including; usual, unusual, and extreme. Table 1 was adopted from EM 1110-2-2100 and summarizes the load case categories. The EM provides guidance for increasing the allowable bearing capacity and decreasing the sliding factor of safety criteria, depending on the return period. These changes are also summarized in Table 2. An additional factor of safety of 2 for usual, and 1.5 for unusual load cases for soil bearing capacity was used due to the uncertainty of soil variation and floodwall placement conditions. For the design of the Floodwall, a hydraulic factor of 1.69 was used per ACI 318 and EM 1110-2-2104.
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Table 2. Load Case Categories (Adopted from USACE EM 1110-2-2100)
Load Case Category Return Period Notes Usual Less than or equal to 10 years No changes to criteria Unusual Greater than 10 years but less
than 300 years Increase qall by 15% Decrease Sliding FS to 1.3
Extreme Greater than 300 years Increase qall by 50% Decrease Sliding FS to 1.1
Case C1 – 100-Year SWE
This load case is classified as an unusual load case because it has a return period greater than 10 years. This results in a 15% increase to the qult and a decreased sliding factor of safety of 1.3 per EM 1110-2-2100. The SWE examined reached a height of 12 ft.
Case C2a – Unbroken Wave Load
This load case is classified as an unusual load case because the return period is approximately 100 years. As such, qult was increased by 15% and the minimum factor of safety for sliding was decreased to 1.3. The maximum unbroken wave force was approximately 11 lbs/ft^2.
Case C2b – Breaking Wave Load
This load case is classified as an unusual load case because the return period is approximately 100 years. As such, qult was increased by 15% and the minimum factor of safety for sliding was decreased to 1.3. This load case was not examined because for the 100-Year SWE, the breaking wave force was negligible.
Case C2c –Broken Wave Load
This load case is classified as an unusual load case because the return period is approximately 100 years. As such, qult was increased by 15% and the minimum factor of safety for sliding was decreased to 1.3. This load case was not examined because for the 100-year SWE, the broken wave force was negligible.
Case R1 – Normal Operations
This load case is classified as a usual load case because it has a return period less than 10 years. The sliding factor of safety for this case is 1.5. When subjected to overturning loads, the entire width of the wall base must remain in compression. The factor of safety for bearing capacity is 3.0 per EM 1110-2-2502.
Case R2 – Construction
This load case simulates loading on the wall during construction and is classified as an unusual load case resulting in a 15% increase to the qult and a decreased sliding factor of safety of 1.3. For this design, a surcharge of 300 lbs/ft^2 was used and applied based upon the Fugro Geotechnical Report. (See Fugro Geotechnical Report, Sept. 2015)
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CaseR3 – Earthquake
This load case is classified as an unusual load case because the return period is approximately 100 years. As such, qult would be increased by 15% and the minimum factor of safety for sliding was decreased to 1.3. The earthquake loading for the floodwall has a return period of approximately 480 years per Fugro Geotechnical Report. (See Fugro Geotechnical Report, October, 2015) This return period makes the earthquake load condition fall under the extreme load case.
Table 3. Summary of Adopted Criteria
Load Case Sliding FS Overturning RR Location Bearing FS
Case R1 1.50 0.333 < RR < 0.50 3.00 Case I1 1.30 0.333 < RR < 0.50 3.00 Case I2 1.10 0.25 < RR < 0.333 2.00 Case I3 1.30 RR within Bwall >1.00 Case I4 1.30 0.25 < RR < 0.333 2.00
Table 2 summarizes the criteria applied to the Cantilever T-Type flood wall. The values will be selected in accordance with EM 1110-2-2502 and EM 1110-2-2100. These values reflect the different factors of safety that are defined for the load case return periods.
The geometry of Cantilever T-Type Flood Walls will be determined by performing a stability analysis in accordance with EM 1110-2-2502 Retaining and Flood Walls, and EM 1110-2-2100 Stability Analysis of Concrete Structures, as applicable. The governing modes of failure are determined.
The modes of failure to be examined are as follows:
Sliding:
Sliding failure occurs when the calculated driving loads on the floodwall are greater than the available resistance of the floodwall/soil interface and the passive soil resistance. The sliding analysis is conducted along a shear plane along the foundation bottom, referred to as a “wedge.”
Overturning:
Overturning analysis calculates moments about the landside toe of the floodwall to ensure that the net loads on the wall will not cause it to overturn. EM 1110-2-2502 requires that the calculated resulting moment about the landside toe be located within a range of values along the base of the floodwall, which is dependent on the load case. This requirement is expressed as a ratio of the resultant location, XR, over the base width. This ratio is denoted as the resultant ratio, RR.
Bearing:
The bearing capacity of the soil is determined to ensure that the foundation soil possesses adequate strength when loading from the floodwall is applied. This analysis is
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completed along the wedge analysis failure plane. The total normal loads are compared against the ultimate bearing capacity of the soil, resulting in an FOS that is compared to the criteria.
4.4.2 Cantilever Floodwall with Pile Foundation
The Sunnyvale WPCP floodwall design includes a portion of terrain that is much deeper than the rest due to a pre-existing ditch between stations 9+00 and 10+00. The total amount of soil retained by this portion of wall is 20 feet for an approximate length of 40 feet. Due to this height of soil and the anticipated settlement of approximately 6 inches; a traditional Cantilever T-Wall section will not pass the USACE required factor of safety. The solution concluded from the given constraints is to design this portion of floodwall with a battered pile foundation. See the Cantilever T-Type Floodwall section for load cases and factors of safety per usual, unusual, and extreme loading. The design criteria used for the pile foundation is EM 1110-2-2906.
4.4.3 Cantilever Sheet Pile Flood Wall
Evaluations were performed for seepage, slope stability, and settlement for the two idealized sections. Full narratives of the geotechnical analysis are found in the Fugro Geotechnical Report, October, 2015. A summary of the analysis is presented below.
Originally, a vinyl I-wall was considered due to the potentially corrosive soils on site; however, per USACE (2010) EM 1110-2-6066, vinyl is not allowed for mainline I-walls. Concrete sheet pile was considered, but based on the stiff soil conditions and the potential for difficult drivability, it was concluded that a concrete sheet pile section will not be feasible.
Based on the Floodwall Sheet Pile Similitude Corrosion Rate Analysis Technical Memorandum dated February 27, 2015 conducted for the site, the use of steel sheet piles will require mitigation for corrosion protection. The recommendations in the tech memo were taken into consideration as well as the guidance provided in the USACE EM’s coupled with previous experience. Coating the steel sheet piles with a coal tar epoxy or bituminous coating system is an acceptable mitigation strategy for long term protection against corrosion. In addition to this, as a redundancy, a thicker steel sheet pile will be selected than what is required by design to counteract any additional potential corrosion that may be realized should the coating wear out or fail. This selection has been commonly used in USACE flood risk reduction projects all over the country with similar corrosive environments. The city of Sunnyvale, the program manager, and the designer discussed and concluded that a cantilever sheet pile I-wall would be the used for a portion of the floodwall.
Geotechnical - Design criteria and methodology was developed primarily from EM 1110-2-6066 provides interim guidance for I-wall design as indicated in ECB 2014-18. ETL 1110-2-575 is for evaluation of existing I-walls; however, aspects of that document were also used. Evaluations were performed using CWALSHT and GEO-SLOPE International (2013a, b, c), GeoStudio software Version 8.14.1.10087, SEEP/W, SLOPE/W, and SIGMA/W. Steady seepage evaluations were performed for the top of wall conditions. Design tip elevations, bending moments, shear forces were computed using CWALSHT for several design scenarios for top of wall water level and strength conditions, as summarized herein. The sheet pile section was sized to accommodate the bending moments presented in the geotechnical analysis. Rapid drawdown analyses were not performed as
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flood levels are attributed to diurnal tidal fluxes, which will not last long enough to result in sustained, long-term water level on the protected side of the floodwall.
5.0 Flood Gate Base Slab At the entrance of the plant a passive bottom hinged floating flood gate system (by FloodBreak Inc.) will be constructed. The foundation slab is being designed to withstand the axial, overturning and sliding forces that will be transferred from the flood gate superstructure. The geotechnical information used for the gate foundation design is taken from the Fugro Geotechnical Report, October, 2015. The foundation is designed for following load cases:
1: Flood gate open (down position) with HS-20 wheel loading
2: Flood gate closed (up positon) with water to the top of the gate
The flood elevation was provided by hydraulic analysis. The base slab was checked for overturning failure due to lateral hydrostatic load, bearing failure due to dead load of the slab and vertical hydrostatic load, and also for sliding due to lateral hydrostatic load. Due to the location on the southern portion of the alignment, hydrodynamic wave loadings are not applicable for the gate foundation.
The length of the slab will be determined by the gate opening. The width and depth of the slab will be determined by load cases and factor of safety consideration. The safety factor of overturning, sliding and bearing failure are based on EM 1110-2-2502: Retaining and Floodwall design. The live load criteria are based on AASHTO recommendations. A sheet pile cutoff wall will be designed at the bottom of the base slab to prevent seepage during flood events. The tip elevation of cutoff wall will be based on the geotechnical seepage analysis done for the adjacent T-wall sections.
Design criteria established for the following flood wall types will be using the following USACE criteria:
USACE, Strength Design for Reinforced-Concrete Hydraulic Structures, EM 1110-2-2104, June 30, 1992
American Association of State Highway and Transportation Officials - AASHTO LRFD Bridge Design Specifications, Customary U.S. Units, 7th Edition, with 2015 Interim Revisions
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Appendix B –Draft Similitude Analysis Report
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Date: Friday, February 27, 2015
Project: City of Sunnyvale Master Plan and Primary Treatment Design
Prepared For: Rob Natoli, PE, CDT HDR-Folsom
Prepared By: Mersedeh Akhoondan, PhD, and Brien Clark, PE, NACE CP4 HDR- Condition Assessment and Rehabilitation Group 431 W. Baseline Road, Claremont, CA 91711
Introduction The City of Sunnyvale Master Plan and Primary Treatment Design project includes the installation
of a driven sheet pile flood wall. HDR Engineering, Inc. (HDR) was asked to provide a corrosion
rate analysis on the proposed sheet piles.
The sheet piles will be steel per ASTM A328. They will extend approximately 8 to 10 feet vertically
above an existing roadway on one side and approximately 4 feet above the backfill materials on
the other side. The native soil beneath the roadway is exposed to fluctuating groundwater
approximately 5 to 10 feet deep. A culvert pipe will be installed in the backfill side with an invert
approximately 16 feet below grade. The sheet pile will extend several feet below the elevation of
the culvert, with the final depth to be determined by the contractor. The sheet pile system is to be
constructed by continuous interlocking unless splices are allowed by the Design Engineer. Per
current design plans, the exposed surfaces of sheet piles are to be covered with concrete or
shotcrete. A typical section detail is provided in Appendix A.
Laboratory soil corrosivity tests were completed on three samples by Cerco Analytical in
November 12, 2013, under Cerco project No. 1310250. The test results can be found in
Appendix B. Laboratory soil corrosivity tests were also completed on six samples by HDR on soil
samples selected from the boring logs provided. The purpose of HDR’s testing was to substantiate
Cerco Analytical’s test results and to determine if the soils might have deleterious effects on
underground materials, including the sheet pile wall. HDR assumes that the samples tested are
representative of the most corrosive soils at the site.
The scope of this study is to conduct a similitude analysis based on the available soil corrosivity
data, drawings, and specifications in order to forecast the corrosion rate of embedded surfaces of
the sheet piles and to provide recommendations on corrosion mitigation techniques for extending
the service life of the structure.
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Test Procedures The electrical resistivity of each soil sample tested by HDR was measured in a soil box per
ASTM G187 in its as-received condition and again after saturation with distilled water. Resistivities
are at about their lowest value when the soil is saturated. The pH of the saturated samples was
measured per CTM 643. A 5:1 water:soil extract from each sample was chemically analyzed for
the major soluble salts commonly found in soil per ASTM D4327, ASTM D6919, and Standard
Method 2320-B1. Sulfide and oxidation-reduction (redox) potential were determined per
ASTM G200 and AWWA C105 Appendix A. Laboratory analysis was performed under HDR
laboratory number 14-0592LAB and 14-0707LAB and the test results are shown in Appendix C.
Technical Background
CORROSION & CORROSION ALLOWANCE Corrosion is the primary durability limitation factor for attaining the designed service life of
major infrastructures such steel structures. Corrosion is an electrochemical reaction between a
material, usually a metal, and its environment that results in deterioration of the material and its
properties.
Sacrificial metal thickness or corrosion allowance refers to the thickness of metal, exceeding what
is structurally required, needed to compensate for the loss of metal that will occur as the structural
member corrodes. Service life of metallic structures is typically referred to the period from first
exposure to first perforation in case of pipelines or to the time that structural integrity is
compromised.
SOIL CORROSIVITY Soils contain chemical constituents that may react with buried construction materials, such as
concrete and ferrous metals. The chemical reaction taking place between soil and buried structures
may damage the structure and reduce its service life. The degree of aggressivity of soil toward
buried structures is referred to as corrosivity of soil. Soil corrosivity is typically a function of:
Porosity (aeration)
Electrical conductivity or resistivity
Dissolved salts
Moisture and site drainage
pH
To estimate the corrosion rates of buried structures a laboratory analysis of soil corrosivity is vital.
In such analysis, the major factor in determining soil corrosivity is electrical resistivity. The electrical
resistivity of a soil is a measure of its resistance to the flow of electrical current. Corrosion of buried
metal is an electrochemical process in which the amount of metal loss due to corrosion is directly
1 American Public Health Association (APHA). 2012. Standard Methods of Water and Wastewater. 22nd ed.
American Public Health Association, American Water Works Association, Water Environment Federation publication. APHA, Washington D.C.
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proportional to the flow of electrical current (DC) from the metal into the soil. Corrosion currents,
following Ohm's Law, are inversely proportional to soil resistivity. Lower electrical resistivity values
result from higher moisture and soluble salt contents and indicate corrosive soil.
Soil Resistivity
in ohm-centimeters Corrosivity Category
Greater than 10,000 Mildly Corrosive
2,001 to 10,000 Moderately Corrosive
1,001 to 2,000 Corrosive
0 to 1,000 Severely Corrosive
Other factors listed above also contribute to the aggressivity of soils.
Per Caltrans2 specifications, soil with chloride concentration of 500 ppm or greater and sulfate concentration of 2,000 ppm or greater is considered corrosive to buried structures. Florida Department of Transportation (FDOT) also specifies chloride concentrations of greater than 2,000 ppm and sulfate concentrations of greater than 5,000 ppm as highly corrosive3.
SIMILITUDE ANALYSIS FOR BURIED PORTIONS Data for corrosion of ferrous metals in a variety of soils was compiled by Melvin Romanoff of the
National Bureau of Standards in a Circular 579 entitled Underground Corrosion4. This database
along with the laboratory data was used to conduct a similitude analysis to assess the efficacy of
installing the proposed steel sheet piles exposed to site soil conditions and to calculate a corrosion
allowance for the piles. The basic methodology was to identify the representative soil
characteristics most likely to be encountered at the site and then use the data presented in Circular
579 to calculate the corrosion rates based upon the similitude between the soils documented and
the soils anticipated at the site. This analysis assumes that steel sheet piles are bare and will be
driven or vibrated into undisturbed soil. The results of this analysis will be discussed later in this
technical memorandum.
CORROSION CELL CONCERNS Microcell corrosion is the term given to the situation where active corrosion and the corresponding
cathodic half-cell reaction take place at adjacent parts of the same metal. In simple terms, when a
metal is exposed to two dissimilar environments such as soil and concrete (or soil with varying
properties), an accelerated corrosion (due to metal potential difference in two boundaries) may
take place. In case of sheet piles, the corrosion cell would exist between the steel embedded in
concrete/shotcrete on top and the steel portion embedded in the soil. The steel adjacent to the
boundary line between soil and concrete may suffer from accelerated corrosion. This issue can be
minimized by coating the part of the steel piles that will be embedded in concrete to prevent steel
from direct contact to concrete.
2 Corrosion Guidelines , November 2012, Version 2, http://www.dot.ca.gov/hq/esc/ttsb/corrosion/
3 State of Florida Department of Transportation, Drainage Handbook, Optional Pipe Material, 2012
4 Romanoff, Melvin. Underground Corrosion, NBS Circular 579. Reprinted by NACE. Houston, TX, 1989, p. 8.