Interceptor Installed Using Microtunneling in Baltimore Park

InterceptorA popular Baltimore park was in need of a new interceptor, replacing an aging and deteriorating one being used well past its prime. Microtunneling was the trenchless method used to construct a significant portion of the 54-in. ID interceptor, forcing the contractor to work in some challenging soil and elevation conditions to successfully complete the project.

The City of Baltimore is carrying out the much-needed improvements to the Herring Run Interceptor in two phases. Phase 1 consists of the installation of 4,120 lf of sanitary sewer ranging between 8 and 54 in. Of the 4,120 lf, 2,300 lf of 54-in. ID reinforced concrete jacking pipe (RCJP) was installed using microtunneling.
Holmdel, N.J.-based Cruz Contractors LLC was awarded this challenging project; design firm was RK&K Engineering. The contractor had to navigate through some tough rock layers, as well as keep disruption to a minimum as the work took place during peak times of the year for the park, which remained open during construction.

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Herring Run Park is a great place to take a break from the stresses of everyday life. Park visitors can enjoy a plethora of activities such as its walking paths, softball and soccer fields and a paved loop for walking that leads into the wooded area. Phase 1of the project took place between September 2011 and March 2013; the microtunneling portion was done between May 2012 and December 2012.

The choice of microtunneling was a logical one for the Baltimore City Department of Public Works as the depth of the new interceptor and minimal disturbance to adjacent areas in the park and underground utilities were critical to the project.

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“For most of the park and even in our shallow areas, the pipe at the excavation point was from 30 ft and above,” says Cruz chief project manager Dominic Pillari. “We also crossed under an existing storm sewer culvert and that culvert alone from top to ground was about 64 ft deep and we had to go underneath that.”

40 ft Diameter ShaftProject Details
The sole purpose of the improvements to the Herring Run Interceptor Project Phase 1 was to replace approximately 2,500 lf of existing interceptor that was failing. The proposed alignment of the new 54-in. ID interceptor was designed to be outside of the existing Herring Run Stream as the existing pipe crossed the stream in many locations. Due to the age of the existing interceptor — which was more than 100 years old — there were several breeches that allowed infiltration and exfiltration between the sewer and the stream.
“[The design] basically re-routed the direction of flow outside the stream area of the park,” says Pillari. “The existing interceptor kept crossing the stream at various locations, causing sewage to pollute the stream. Now everything flows through the new pipe.”

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Cruz Contractors built sixshafts for the projectsIn order to install the tunnel, Cruz constructed six shafts to jack and receive the tunnel machine. The deepest shaft on the project was 60 ft, with 40 ft of it through solid rock, in which Cruz used blasting techniques to assist in the excavation. The standard shaft size used on the project was 25 ft in diameter and was comprised of a rings and vertical lagging design. However, the largest diameter shaft was 40 ft in diameter to incorporate an additional structure and connection to an existing 33-in. interceptor. In all, the project had more than 2,500 cubic yards of rock excavation required to install all the shafts, as well as some open-cut small diameter pipe work.

“The shafts took time to build,” says Cruz general superintendent Bob Rigle, who oversaw the Baltimore project. “Once we completed one shaft, we started the next one and so on. Once the microtunneling actually started, three of the shafts were complete.”

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To install the tunnel, Cruz used a Herrenknecht AVN 1500 tunnel machine with a 72-in. rock cutter head to excavate for the tunnel. Cruz also used a Derrick Separation System. The pipe installed was 54-in. ID and was made with an extra thick wall so the pipe OD was actually 69 in. The rest of the annular space between the pipe and the tunnel was pressure-grouted in place. The microtunneling runs ranged from 230 to 750 ft.

Project Challenges
Among the challenges Cruz faced was the density of the rock layers, which ranged from Gneiss and quartz, that went from soft to extremely hard and back to soft. “In some areas it was relatively soft [rock] and went from 8,000 to 10,000 psi to 26,000 to 28,000 psi,” says Rigle. “We had a consultant check the rock, take samples and tests before we started so we were prepared. We experimented with different cutters and in the end, we ended up using all Disk cutters because they were working better for us in this particular rock.”
Pillari says the rock excavation and transitions made for some steering challenges. “When microtunneling, if you have rock on the bottom half of the pipe and soil above, it can tend to make controlling the machine very difficult,” he explains. “That was a challenge our guys were able to handle. Bob [Rigle] made sure any problems or issues were resolved quickly. That is something we take a lot of pride in.”

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Some of the shafts were 60 ft deepThe soils and elevations made the construction of the shafts tricky, limiting site access, which resulted in some creative maneuvering and design to get equipment and crewmembers to the site. “What was prohibitive were the various elevation changes of the site. It was pretty much following the side of a mountain. Access in and out of the shafts from the actual roads was very limited,” says Pillari. “In one area, we constructed a temporary bridge across the [Herring Run] stream and all the equipment had to be carried in with excavators. There was not enough room to get to that shaft with any type of tractor-pull trailer. That was the shaft for our longest run. To fit through the easement area and make climbs up and down the hills, there was probably a 50-ft difference in elevation from the paved road to the point of access in the shaft.”

Prior to the microtunneling, Cruz installed 1,065 lf of 8- to 36-in. SDR 35 via open-cut, replacing some faulty, aging lines that entered the interceptor at various points. There was also a 650-ft open-cut run that Cruz used 54-in. HOBAS pipe.

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This was considered a very high profile project for the City of Baltimore, Pillari says, and due to the exfiltration into the Herring Run stream, the City tested the stream weekly. The new interceptor was fully tested and operational on schedule and on budget per the specifications required from the City of Baltimore.

Sharon M. Bueno is managing editor of Trenchless Technology.

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