Project Case Study: Getting to the point

San Diego sewer repair

Civil engineer
Psomas, San Diego

Product application
Point repairs of large-diameter, PVC-lined sewer tunnels prove more economical than pipe replacement or sliplining.

Point-repair sewer rehabilitation is more economical and less disruptive to the public.

With our aging national infrastructure, public agencies increasingly have to face the fact of sewer repairs. In many cities, the typical approach is to replace the pipes and structures and then abandon the old facilities—a costly capital improvement that results in significant public inconvenience over a lengthy period of time.

Click here to listen to a podcast of this article, Project Case Study: Getting to the point, by Curt Edwards.

Point repair is a more economical way to solve the problem of rehabilitating large-diameter pipes. And, just as important, this approach avoids the traffic nightmares, noise, and disruption caused by massive excavations for pipe replacement or relocation.

The city of San Diego is a leader in sewer point repairs, largely because portions of the sewer system can be shut down for limited, early morning periods. In 2004, the city’s Metropolitan Wastewater Department (MWWD) contracted with Psomas to prepare design documents for point repairs to the 84-inch South Metro Interceptor (SMI) sewer. This PVC-lined circular tunnel is constructed as deep as 90 feet beneath the streets and buildings of downtown San Diego.

The original PVC lining on the corroded area of the crown of this sewer tunnel was removed and hydroblasted in preparation for repair work.

After a video "walk through" of the 9,700-foot-long tunnel documented visual damage to the PVC liner and the underlying concrete tunnel wall, Psomas prepared repair plans for more than 96 small and large individual point repairs.

The construction period was limited, from 1:00 a.m. to 5:00 a.m., when Pump Station No. 1 was shut down during low-flow hours. Repair crews entered the pipes and, in the case of the small repairs, removed the liner, hydroblasted to remove the defective concrete, grouted the hole, and placed a new liner over the grout. The large repairs were typically deeper (6 to 12 inches), requiring crews to take additional steps prior to making repairs. These additional steps included preventing groundwater infiltration by injecting pressure grout to seal behind the cracks, and then adding a steel-reinforcing grid prior to placement of the new liner.

Finished PVC patches cover previously corroded areas on the crown of the sewer tunnel.

Most repairs proceeded as planned. However, during the construction process it was found that there were large voids behind some of the repair areas, apparently resulting from the original construction methods. These voids had allowed corrosive gasses to attack the tunnel lagging and support system. MWWD conducted an emergency effort to expedite the repairs and allow for full operation of the tunnel system. Psomas and the contractor, SANCON, quickly responded by arranging an early morning internal inspection of the voids and providing new structural repair recommendations within days that would meet the strict work schedule in this harsh, corrosive environment. Smaller point repairs could be completed during the brief shut-down period, but for these larger voids—from 10 to 15 feet long and as deep as 3 feet—it was important to design and construct a repair that would not be re-damaged when sewage flow was restored later in the morning. Psomas worked with SANCON to determine the maximum area that could be repaired during the brief shutdown period. The designs were then tailored to meet the schedule.

The $4.0 million project was completed in 2005. If MWWD had taken the approach of replacing the pipe, the project would have cost $5 million to $10 million more.

Point repairs during limited, sewer system shut downs were less expensive and caused less disruption to public services and other infrastructure in downtown San Diego.

A similar approach was used for repairs to the 87-inch discharge pipeline and outlet structure for MWWD’s Pump Station No. 2. The original, 19,000-foot-long reinforced concrete force main (Force main No. 1) was constructed in the early 1960s. The force main discharges to an outlet structure know as the East Portal, which also serves as the entrance to a 108-inch-diameter tunnel underneath the nearby Pt. Loma neighborhood in San Diego. The force main had a number of appurtenant fittings that were in various stages of corrosion and needed to be replaced to increase system reliability. The East Portal structure was PVC lined, but was corroding in a similar manner as the South Metro Interceptor sewer. Corrosive gasses and water somehow penetrated the liner, corroding the concrete and delaminating the liner.

To conduct investigations, sewage flows were diverted to Force main No. 2, which allowed Force main No. 1 to be drained and inspected. Concrete samples were taken and reinforcing steel was checked for corrosion. With the exception of the appurtenances, the force main was found to be in excellent condition.

After the design was completed, the force main was drained again and repair crews entered the pipe to remove corroded appurtenances, patch holes, and replace parts. The East Portal also was repaired during the early morning periods when Pump Stations 1 and 2 were shutdown.

Other alternative approaches, such as in-situ (or cured-in-place) lining, are not always practical for large-diameter pipes because of the large volume of hot water necessary to cure the resin and the need to have a completely dry pipe. Slip lining, since it involves inserting a rigid pipe, is not practical when the pipe is curved, deep, and has limited access points. Sending crews into a damaged pipe and performing a point repair is a solution far more economical than pipe replacement or relocation, and causes minimal disruption to public services.

Curt Edwards is a vice president with Psomas, a consulting engineering firm with offices in California, Utah, Arizona, Nevada, and Colorado. An expert in sewer repair, Edwards is based in the Psomas San Diego office.

Posted in Uncategorized | January 29th, 2014 by

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