With aging infrastructure a growing concern and federal infrastructure funding on the horizon, municipalities are preparing to tackle pipeline rehabilitation projects just as communities are beginning to reopen after the pandemic. We have gathered tips and lessons learned from our work on a large diameter sewer rehabilitation and partial replacement project that allowed activities to carry on in bustling Silicon Valley.
The City of Santa Clara needed to rehabilitate one of its main trunk sewers, an aging, severely corroded 48-inch diameter pipe located in Trimble Road. A major conveyance pipeline, the trunk sewer carries approximately 12.5 million gallons a day from the City’s collection system in Santa Clara through a main traffic artery in the City of San Jose before connecting into 54- and 78-inch diameter sewer interceptors in San Jose that flow to the joint San Jose/Santa Clara treatment plant. Rehabilitation and replacement included a combination of pilot tube guided auger boring, cured-in-place pipe, manhole rehabilitation, and installation of a ten-foot diameter grit trap to capture gravel and debris upstream of inverted siphons under the Guadalupe River.
Investigate Subsurface Conditions
Because the project is located in a main thoroughfare near the San Francisco 49ers stadium, and in the area of many tech and innovation companies, the design team needed to identify a corridor for the new sections of the 48-inch trunk sewer that would minimize relocations of existing utilities and allow traffic to flow through the construction zone. Utility investigation, including potholing, was key to finding the right corridor. For complicated intersections, we created a staged potholing plan and allocated a robust budget for adequate subsurface investigation—over 20 potholes were completed at just one significant intersection.
In addition to locating underground utilities, we conducted a detailed geotechnical investigation—a best practice for trenchless applications. Together, the geotechnical information and the potholing data allowed the contractor to understand the existing conditions at the tunnel site prior to mobilization and facilitated troubleshooting any challenges encountered during construction.
Check Existing Information and Identify Gaps
Record drawings and other available documents provide useful information about the existing sewer system. Early in the design phase, these documents are reviewed to identify present conditions and missing information. After the review, the design team determines if additional investigation is needed to fill in the gaps.
For this project, record drawings for the 33-inch sewers beneath U.S. Highway 101 were unavailable so the design team used the City’s sewer atlas, which didn’t show that the pipeline diameter changed in the Highway 101 area from 33-inch to 42-inch. Closed-circuit television (CCTV) investigations conducted during construction revealed the variation in pipeline size. Fortunately, in this case the change in diameter did not adversely impact the cured-in-place pipe installation; however, discovery of a significantly different diameter could have delayed construction. To avoid such surprises, our designers use CCTV inspections during the design phase to confirm existing conditions where needed.
Obtain Permits during the Design Phase Where Possible
As a City of Santa Clara project occurring within the City of San Jose, the Trimble Road work required an inter-agency encroachment permit. Additionally, permits were required from Caltrans for work beneath U.S. Highway 101, from the Santa Clara Valley Water District for work along the Guadalupe River Trail, from the Santa Clara Valley Transportation Authority (VTA) for work adjacent to and beneath its light-rail tracks, and from the Federal Aviation Administration for manhole rehabilitation work around the San Jose International Airport. Our team invested time during design to obtain these permits so construction would not be slowed down by the application review times of these various agencies.
Knowing the requirements of each permit or agreement at the time of bidding also helped reduce claims for changed conditions since we were able to include appropriate requirements within the contract documents. For example, VTA’s network of light-rail tracks are critical for moving large groups of people, particularly when games or concerts are held at the San Francisco 49ers stadium. VTA mandated no track closures on event days, which weren’t determined at bid time, so our contract documents required the contractor to attend VTA’s Track Allocation Meetings when work was scheduled within 10-feet of the light-rail tracks. This requirement allowed the contractor to plan the close-proximity work on non-event days.
Due to high traffic volume, the encroachment permit required that no lanes of traffic could be closed during commute hours, which is very difficult for a tunneling operation. To meet this requirement, the tunneling contractor developed a jacking shaft that used a high strength structural beam along its center to support steel traffic plates during non-work hours and included folding handrails that could be quickly flipped up into position at the beginning of every work shift.
Incorporating trenchless technology into sewer rehabilitation and replacement projects already minimizes disruption to communities, but taking extra care to understand subsurface conditions, confirm data, and obtain permits during the design phase will further reduce risk and lessen impacts to residents, businesses, and commuters eager to re-engage in daily life.
