Frequently Asked Questions
Underground Flow Equalization System FAQs
The Underground Flow Equalization System (UFES) project consists of:
- An underground, concrete holding structure capable of temporarily storing up to 5.3 million gallons of wastewater. It is located in the east corner of the Event Center property. The facility is 200 feet by 150 feet, and 35 to 50 feet deep. Access hatches and manhole covers will be visible.
- Above ground electrical building
- Diversion System (diversion sewers, diversion structure, and force main) that routes wet weather flows to and from the existing Delaware sewer trunk line
- Odor Control System will continuously clean the air and vent the facility.
- During heavy rains, the Delaware sewer trunk line and other areas in the system reach capacity and cause sanitary sewer overflows (SSOs). The UFES will temporarily route diluted wastewater flows from the Delaware trunk line to the underground holding structure, where it can be stored for up to 24 hours.
- Once the downstream collection system and WWTP has available capacity, a pump station inside the holding structure will pump the wastewater back into the collection system.
- The facility can also be used to support operations and maintenance activities in other areas of the system, such as pipeline inspection and cleaning.
- After any use, self-cleaning mechanisms will flush and clean the facility. The interior is inspected after every event.
- Odor control systems will operate 24/7.
- The facility is expected to be used less than 20 times per year on average.
- During heavy rains, San Mateo’s sewer system capacity is overwhelmed and causes SSOs that impact the health of the community, creeks, beaches, lagoon, and San Francisco Bay.
- In 2009, the San Francisco Bay Regional Water Quality Control Board (RWQCB) issued a Cease & Desist Order that requires the City to eliminate SSOs.
- Many of these SSOs occur on Delaware Street and contaminate Borel Creek, which flows through the Fiesta Gardens neighborhood.
- In conjunction with the other Clean Water Program improvements, the UFES enables the City to prevent SSOs from occurring and comply with State and Federal regulations.
- Staff initially evaluated 55 sites throughout the City to potentially locate UFES. As part of the evaluation process, City staff and members of the Council visited several underground flow equalization facilities in Seattle, Washington. These facilities were located underneath parks and parking lots, and were adjacent to various residential, school, and commercial properties. Based on the information learned from the Seattle systems, parks at Bay Meadows and Fiesta Gardens were determined to be preferred locations for UFES. However, due to opposition to these locations from local residents, staff focused on further evaluating the City’s Corporation Yard and the County Event Center as potential sites.
- After further evaluation, the County Event Center Corner site was preferred over the Corporation Yard due to various factors (such as impacts to operations, proximity to residents, permitting requirements, and longer construction schedule). Because the Corporation Yard is the essential hub for the City’s Public Works and Parks and Recreation operations, staff would have to be relocated for the duration of the UFES project. However, the City was unable to find suitable temporary sites for the operations. Additionally, there were zoning restrictions on the Corporation Yard.
- The County Event Center site offered a shorter construction schedule and a less complex permitting process. Initially, the City identified the three corners (northwestern, southern, and eastern corners) of the County Event Center. Because the northwestern and southern corners presented challenging conditions for the operations of the Event Center, these were eliminated from further consideration. As a result, the Event Center staff determined that the eastern corner would be preferred and agreed to negotiate an easement for the location of the UFES project.
Construction of the 5.3 MG underground structure will be done in six phases: site preparation, shoring installation, excavation, micro-pile installation, concrete, and site finishing. Construction is anticipated to take approximately 25 months, with some of the phases overlapping. The diversion system will be constructed concurrently.
Phase 1: Site Preparation
- During this phase, the site will be cleared, construction trailers and equipment will be brought to the site, construction fencing will be installed, and dust control measures will be in place.
- At this time, equipment to monitor groundwater, vibration, and settlement will be installed.
- This phase is expected to take approximately 3 months.
Phase 2: Shoring Installation and Dewatering
- The shoring system will safely hold the area open during excavation. During this phase, shoring construction equipment will be brought to the site. The contractor will select one of three low vibration and noise systems for the installation of the shoring system. It is important to note that no sheet piles will be used on this project.
- Once the shoring equipment is installed, groundwater will be pumped out of the work area. The soil and groundwater have been tested to confirm there is no contamination. Additionally, dewatering tests were performed to observe groundwater level fluctuations in nearby wells. Water will be removed (dewatering) from inside the shoring to maintain stability. This dewatering method will not cause the groundwater levels to change from typical conditions beyond the property line.
- The City will survey and document nearby structures before, during, and after construction. During construction, the City will monitor groundwater levels, noise, and vibration. Enforceable thresholds will be set and enforced, where work can be stopped if work exceeds the specified limits.
- This work is expected to take approximately 4 months.
Phase 3: Excavation
- Once the shoring is complete, excavation will begin and the soil inside the shoring will be removed. During this phase, the City will monitor groundwater, vibration, and noise levels. Dust control measures will be in place.
- This work is expected to take approximately 3 months.
Phase 4: Foundation Micro-Pile Installation
- A network of foundation piles will be drilled to prevent uplift (floating) from the high groundwater levels, to support the structure and to keep it in place during seismic events. Seven different pile types and installation methods were evaluated. Micro-piles were selected because they meet the technical requirements, while being quieter and having less vibration than other methods. During this phase, approximately 270 micro-piles will be installed. The City will continue to monitor for groundwater levels, vibration and noise.
- This work is expected to take approximately 2 months.
Phase 5: Concrete
- In Phase 5, the concrete base, walls, column, beams, and roof elements of the UFES structure will be built. Concrete trucks and pumping equipment will be on site.
- The City will continue to monitor for groundwater, vibration, and noise throughout this phase.
- The duration of this work is expected to be 9 months.
Phase 6: Site Finishing
- Once the concrete is done, the new electrical building will be installed, in addition to new plantings and fencing along Saratoga Drive and new pavement. The permanent, active odor control facility will be installed (on the far west portion of the site, away from the park). The remainder of the site will be repaved and restored to Event Center usage.
- The construction crews will be demobilized, and the new facility will undergo startup and testing.
- The City will continue to monitor for groundwater, vibration, and noise throughout this phase.
- The duration of this work is expected to be 8 months.
- Work may take place from 7 am to 7 pm, which is allowable per City ordinance, with most construction activities only requiring 8-10 hours per day.
- Weekend and evening construction is not allowed. However, exceptions may be needed for certain activities, such as concrete pours. City approval and notice to the surrounding community would be required for this.
Yes. There will be a live response for citizen inquiries and a dedicated phone number.
The UFES project footprint is about 32,000 sf. The 400/450 Concar development has an approximate footprint of 134,000 sf. The SurveyMonkey building is approximately 87,000 sf. Both buildings had similarly deep excavations for their underground parking.
How will you prevent noise from negatively impacting the health and quality of life of the nearby residents? How will noise be monitored? Who will enforce this?
- The City will require construction noise to comply within the City’s noise ordinance.
- The City has eliminated impact pile driving and is now using a quiet pile alternative called “micropiling” that generates one-quarter the amount of noise.
- Piling will be installed 50 feet below ground surface and will require approximately 6 to 8 weeks to complete.
- The City is currently developing a monitoring plan which includes noise monitoring devices in the vicinity of the work. The final noise monitoring plan will be posted to the Program website once complete. The City will enforce noise violations through their independent construction manager who will be onsite during any work activity.
How will you prevent dust from negatively impacting the health and quality of life of the nearby residents? How will air quality be monitored? Who will enforce this?
- Dust control will meet or exceed regulatory requirements.
- Dust will be visually monitored per Bay Area Air Quality Management District permit requirements for construction dust particulate compliance needs.
- Measures will be taken by the contractor, such as wet sweeping on surrounding streets and around the construction site, covered trucks, site watering, fence screening and tire wash facilities.
- The City’s construction manager will enforce the dust prevention measures and compliance with regulatory requirements.
- A soil investigation was performed for the project and no contamination of the soil or groundwater was found.
How will settlement be prevented? How will you control groundwater levels? How will vibration be monitored? What type of monitoring systems will be put in place to prevent home damage? Who will enforce this?
- The UFES project will utilize a shoring system that acts as a cutoff wall during construction and that prevents groundwater from entering the excavation and isolates the excavation from the surrounding groundwater. This creates a closed “bathtub” effect, allowing work to proceed inside the structure without impacting the surrounding groundwater table and eliminating soil instability and settlement nearby.
- The City has conducted a geotechnical evaluation for the project to help develop prevention measures for settlement. The results showed that groundwater is 3’-5’ below ground, no contamination was found in the soil or groundwater and historical baselines of groundwater level fluctuations were obtained in wells installed around the site. Based on the subsurface conditions, the following measures will be taken to ensure settlement does not occur:
- Additional observation wells will be installed near the site before construction to monitor and establish baseline groundwater level measurements.
- Dewatering wells will be placed within the cutoff wall, and will be prohibited from being placed outside the excavation (this significantly reduces the potential for settlement).
- Dewatering drawdown will be minimized to 2’ below the excavation.
- The groundwater cutoff wall will extend at least 15’ below the excavation and watertight shoring will be required.
- Enforceable drawdown stop-action thresholds and corrective actions will be required.
- Groundwater levels will be measured during and after construction and compared to the baseline measurements.
- Existing structures will be surveyed and documented before, during and after. construction to differentiate between construction related impacts and pre-existing conditions (which houses in the area to survey is currently being evaluated).
- By implementing the actions above, the results will lead to a proven groundwater dewatering approach that will minimize the groundwater “cone of influence” to less than 10 ft. from the edge of the UFES basin. This will significantly reduce the potential issues that sometimes impact surrounding community structures.
- Vibration monitoring will occur during the use of heavy equipment. The Contract Documents will specify maximum vibration limits and enforceable actions if they are exceeded. The City is currently developing a monitoring plan which include vibration monitoring devices in the vicinity of the work. The details of this plan, including how it will be enforced and by whom, will be made available when completed. However, vibration is not anticipated to be a significant impact to the project with both driven sheet piles and driven foundation piles being excluded from consideration.
What is the plan to handle the increased construction traffic and construction road closures, especially in the context of all of the other construction projects in the area? What roads will be closed? What will the increase in traffic be? Can you consider alternate routes to access the site than Saratoga Dr.?
- The contractor will be required to submit an overall traffic management plan that must have City review and approval.
- The plan will attempt to minimize the traffic impacts to the surrounding community as much as possible.
- Construction from the project site is planned to be routed along Saratoga Dr. to the nearest designated and approved City truck haul routes: Delaware St. or Hillsdale Blvd.
- Lanes on Saratoga and Delaware St. will need to be closed for some of the work, but it is not anticipated that entire roads will be closed.
- This project is estimated to add an average of 120 vehicles per day over the entire project or about 10 additional vehicles per hour. For comparison, over 800 vehicles per hour use Saratoga Dr. (northbound and southbound) during the afternoon peak.
- The City will continue to evaluate alternate routes and traffic control options and will coordinate with other construction projects in the area.
Won’t this project create a safety risk to pedestrians and bicyclists? How will that risk be mitigated?
As part of the traffic management plan that will be required of the contractor and reviewed by the City, the plan will provide safe pathways for pedestrians and bicyclists attempting to navigate by the construction area.
Why aren’t you considering trenchless methods for the pipeline installation? Will trenching the pipeline have the potential to damage homes or nearby infrastructure?
- The force main along Saratoga Dr., from the 5.3 MG underground structure to Delaware St., will be constructed using trenchless methods for most of the alignment. Majority of the work will be pipe lining an existing 18” sewer pipe. A portion of the work to install a segment of new 18” sewer pipe will utilize a trenchless method called horizontal direction drilling.
- The 36” gravity diversion sewer will be installed by open cut methods. The design team previously looked at the possibility of microtunneling for this pipeline, but the tight curve of Saratoga Drive raised significant constructability, cost and schedule impacts.
- The City has been using open cut trenching throughout the City for many decades and is currently using open cut to numerous sewer projects throughout the City.
The draft EIR discusses specific environmental requirements for wildlife protection. For example, Mitigation Measure 5-2 requires biological surveys prior to construction to ensure the protection of any native bird nests and the establishment of protective buffers based on standards established by the California Department of Fish and Wildlife.
Yes. These details have yet to be worked out, but the City is interested in creating opportunities for the community to learn about and see the project during construction.
- The odor control system will continuously operate and will dry out the interior after usage
- Proven carbon scrubbing technology will be utilized, similar to the UFES projects the Council visited in the Seattle area in 2016.
- After each usage, the underground structure will be automatically pumped out and flushed clean with water, which will remove material that could cause odors.
- The City, as lead agency under the California Environmental Quality Act (CEQA), has prepared an Environmental Impact Report (EIR) for the UFES Project. The Draft EIR provides a project-level review of potential environmental impacts from the project. The draft EIR is no longer available for public review and comment. However, there will be additional opportunity for comment on the Final EIR when it is made available.
- For other inquiries, the public can call the hotline, 650-727-6870, or email info@CleanWaterProgramSanMateo.org.
- The Draft EIR public comment period closed on May 31. The Clean Water Program team has logged all submitted comments/questions and provided response with acknowledgements of receipt and indicated that all comments/questions would be addressed in the Final EIR.
- The project’s next community engagements will be at a Planning Commission Study Session on August 27.
- For the certification and adoption of the project’s Final EIR and approval of the special use permit, the project will return to the Planning Commission on September 24, then the City Council on October 7, 2019.
- Construction is anticipated to begin in Spring 2020 and last until Fall 2022.
The final design is required to meet seismic safety thresholds and standard building codes. The system will be designed to withstand the U.S. Geological Survey seismic acceleration of 1.9 g, which is specific to San Mateo. The design will also comply with governing codes, such as Reinforced Concrete ACI 350, California Building Code 2013, and Design Loads for Buildings and Other Structures (ASCE 7-10).
Will more taxes need to be paid to cover the cost of this project, or is there another funding source? How will the construction of this project be funded?
Taxes are not used to pay for this project or any of the City’s sewer projects. All City-supported wastewater related activities, including conveyance, treatment, maintenance, operation, repair, upgrades, and replacements are funded by sewer fees paid by property owners in San Mateo and through cost-sharing contributions from partner agencies, including the City of Foster City, Town of Hillsborough, County of San Mateo, and Crystal Springs County Sanitation District. The City has approved increases in sewer rates from Fiscal Year 2017/18 through 2022/23 to fund sewer operations and maintenance and clean water improvement activities. The City is also seeking funding options such as grants and low-interest rate loans to minimize the impact of this multi-year capital improvement program on sewer rates.
Geotechnical soil investigations have been completed as part of the design. Construction of the system will require soil excavation, and shoring will be utilized to maintain the integrity of the excavation.