In today’s world a major interstate highway and other modern transportation infrastructure crisscross the once picturesque ravine called the Trout Brook Valley located in Saint Paul, Minnesota. Some 300 years ago this area had an actual brook and was home to the Mdewakanton Dakota that depended on its fertile ground as both a source of food and a transportation corridor.
In the early 1800s as Europeans began settling the area for farming, they started filling in the brook, as well as using it as a power source for their mills.
Shortly after the 1880s and the arrival of the railroads, it was decided to bury the Trout Brook entirely. “It was becoming an area that was receiving a lot of raw sewage and rainwater runoff. It became an impediment to transportation but also a health hazard,” says Anna Eleria, a division manager of the Capitol Region Watershed District (CRWD), which owns and operates the tunnel.
The Trout Brook Sewer Interceptor was built by cut and cover in 1888 for USD 84,000 (about USD 2.1M today). “We actually have a copy of the contract document between the City of St Paul and the contractor,” she says.
The 10ft- (3m-) diameter, 2,500ft- (762m-) long storm sewer comprises limestone blocks and with granite and brick pavers in the invert. As the city grew during the 20th century the Trout Brook sewer grew too to six miles (9.6km) in length, constructed in phases with the last work dating to the 1950s.
Cover ranges between 3ft and 6ft (0.9m to 1.8m) along the alignment, which includes a stretch crossing beneath four active railroad tracks. In some places limestone blocks are being dislodged or displaced. Routine inspections carried out every five years also showed mortar degrading and leaving gaps.
A repair program this winter focusing on the original sewer section replaced the mortar and improved the structural integrity of the tunnel. The project team adapted its solution to accommodate the historic limestone, and retained much of the tunnel’s original aesthetic.
Inspection Results
As a CSO, the previous owner Metropolitan Council (a regional governmental agency and planning organization serving the seven-county metropolitan area) separated the Trout Brook Interceptor in the 1990s. It later transferred ownership to the CRWD in 2006.
“They had determined they no longer wanted to own stone sewer interceptors and were looking for an agency to take over its ownership,” Eleria says. Because the interceptor serves an area of 8,000 acres (32,400m2), from more than one community, she explains, it made sense from a more regional entity to assume ownership.
“It’s not typical for a watershed district or water management organisation to own infrastructure like this.”
CRWD is a special unit of government in charge of managing water resources within its designated boundaries. It relies on consultants for assistance in inspection, maintenance and repairing its tunnel system, and Barr Engineering has been in this role for about 10 years. The district inspects the tunnel on a five year cycle and has developed a five-year capital improvement plan to cover the cost of the repairs. Only the original 2,500ft (762m) stretch of tunnel is built with limestone block.
The rest of the interceptor is cast-in-place concrete and inspections show a majority of the tunnel is in fair condition. Joe Welna, an engineer with Barr Engineering, explains the work they’ve just completed is not in the worst rated section as far as the most recent (2014) inspection is concerned. Another section has a higher quantity of defects of higher severity. “We chose this section to start with because one of the defects in this limestone block tunnel was directly below a railroad track crossing with minimal cover.”
The implications of a failure at this location would have many impacts. He explains, “with this limestone block tunnel we’re counting on an arching effect of these blocks. During the inspection we found that several of the blocks in the keystone location had actually spalled out and split, and were in the invert of the tunnel.”
The main objective of the project was to address the railroad crossing but to take advantage of the contractor mobilization Barr recommended additional repairs. One of the major ones was mortar repair, Welna says. He attributes the missing and deteriorated mortar to hydrogen sulfinde from the tunnel’s former existence as a CSO.
“Typical defects had gaps in joints ranging from half and inch to 3 inches. In some sections the mortar was completely gone—it was overhead, on vertical surfaces.”
The work is labour intensive—cleaning out damaged mortar and installing new mortar—and in some sections with high concentrations of damage it wasn’t cost effective. Barr recommended a 2-inchthick unreinforced shotcrete overlay for a 300ft- (91.5m-) long section of highly-concentrated mortar damage, and a 12-inch-thick reinforced shotcrete overlay at the railroad crossing. CRWD awarded a USD 1.2M contract to low bidder Minnesota-based Engineering & Construction Innovations, Inc. (ECI) in late summer 2016, with work starting from December 2016 through February.
Tunnel Work
Conveniently there is a parallel tunnel owned by the City of Saint Paul with a crossover upstream from the section where ECI would be performing the repair work, which allowed for the construction of a bulkhead and diversion of water for three months during the project.
“The base flow is year round, even in the winter, and typically 15 cubic feet per second (0.42 cubic metres), and that’s basically just accumulation of infiltration from the entire system upstream, that’s when the lakes are not discharging,” says Barr Engineering’s Nathan Campeau.
Working through the winter months meant fewer chances of a rainstorm— though an unusually warm February did cause the contractor to pull equipment a couple times.
For the sensitive area below the railroad crossing Barr evaluated a number of options even including realigning the tunnel. The main parameters were cost, structural integrity, longevity, coordination, overall repair difficulty, and hydraulic impact.
“We found that a 12-inch-thick overlay within that limestone block tunnel was probably the best option,” Welna said. This new lining would be an approximately 200ft- (61m-) long section, and self supporting.
“Because a limestone block tunnel is somewhat hard to evaluate structurally—if you lose a block your structure is gone essentially—the 12-inch-thick overlay was designed as a stand alone structure so that the section of it is designed for the railroad loading. It doesn’t count on any of the structural support of the limestone block tunnel.”
The contractor prepped the surface with high pressure water to remove debris. The building process started with installing with the first of two mats of epoxy-coated rebar. Diameter in the tunnel can vary by ±6 inches on either dimension, Welna explains. He describes the overlay as a kind of composite structure with a cast in place 4ft- (1.2m- ) tall wall or “knee wall.” To seal the cold joint along the top, an expansive chemical grout water stop was injected, followed by the first shotcrete application along the arch.
“That’s the beauty of shotcrete is you can kind of conform to the existing shape.” Welna says. “If you were to try to use a form throughout this section you might be losing some hydraulic capacity.” After the first shotcrete lift was in place, ECI installed the second rebar mat and the second, final shotcrete application. The spacing between the rebar is 8in.
Welna says the main challenge with this section was mitigating water infiltration, which they did with a combination of chemical grout and drilling what he describes as relief ports. These would redirect the water and after shotcreting around the ports they were plugged.
In the section of the tunnel with the high concentration of mortar damage ECI sprayed the 2-inch-thick overlay directly on the blocks. “With it not being reinforced we were really counting on a high performance mix to bond to the tunnel,” Welna says. “It’s not being mechanically held up there.”
Barr Engineering chose a Sika prebagged mix with synthetic fibre reinforcement. Pull testing verified the application was getting a strong bond. “This was a pretty unique application because usually this material is used against concrete,” he says.
“This is a limestone block medium that we had to spray against.” The tunnel has encrustations on the limestone and part of the pull testing looked at whether the shotcrete could be applied directly or if it would need to be removed to get a good bond. The latter proved to be beneficial and the encrustations removed.
Mortar repair comprised roughly 5,700ft linear (1,737m)– it’s basically tuck pointing, explains Welna. “First it was going and painting all the sections we wanted addressed. Then it was removing any unsettled material, removing the dust, the soil, the latency from any of these joints.”
There was detail for mortar repair dry and mortar repair wet. “Some areas you actually had active infiltration water coming through. So we recommended that be sealed up prior to installation with chemical grouts.”
Preservation
Reflecting on the project Campeau highlights the district’s motivation to restore the original workmanship, and appreciation of the tunnel’s beauty and its historic nature—even though largely remains inaccessible to the public.
“Very early on CRWD decided it did not want to just automatically cover the entire tunnel and reline the whole thing. That was certainly an option, and certain areas definitely needed to be covered up because that was the safest thing to do structurally and the most cost effective thing to do.”
Eleria responds, “we were really proud of the fact that we’re taking responsibility for this storm sewer and highlight to the public the history of it—it’s built over a century ago, in fairly good condition, we just need some repairs to make it last another 50-100 years more.”
