In 2018, SSR began working with a utility district in Middle Tennessee on the addition of a new force main at one of their largest pump stations for increased flexibility during wet weather events. The goal was to install a new 24” ductile iron force main approximately three miles in length along very difficult terrain from the pump station to an existing 48” interceptor located inside a 102” tunnel. Multiple routes were analyzed during the initial design phase and SSR was able to narrow the routes down to the final corridor after a series of brainstorming meetings with the utility district. Once the preliminary alignment was set, it was evident that this project would be extremely challenging and would require a strong collaborative relationship between all project team members, not limited to the owner, SSR, and contractor.

An Alternative, Team-First Approach

Given the scope and complexity of the project, the owner felt comfortable using an alternative project delivery method different than that of the typical traditional hard bid sequence. For this project, the Construction Manager At Risk (CMAR) method was chosen. SSR has experience with this delivery method and has worked with this owner on previous CMAR projects with great success.

The CMAR approach is an increasingly popular construction delivery method in which the project is advertised for construction at an approximate 30% design stage. Instead of a set bid price, the contractor submits a Guaranteed Maximum Price (GMP) for the work while also providing various Value Engineering (VE) suggestions that can be subtracted from the overall GMP price, if selected by the owner. From our experience, bringing the contractor on-board at an early stage has many benefits and this project confirmed that.

An initial benefit of CMAR is the contractor is able to work with the owner and engineer in a team first environment to meet the 60%, 90%, and 100% design milestones. The contractor can provide constant feedback, recommendations, and preferred construction methods to the design team to ensure the project is not only cost-effective, but constructible. Often, two contractors may prefer to complete the same task in different ways so tailoring the construction methods to the contractor on board often provides an expedited and cost savings approach to the work. An “open book” policy is utilized for project pricing and the entire project team works together to determine optimum pricing for individual project work items.

Another major benefit is the relationship building that this method brings involved parties throughout the project. The project team meets as often as every week during the design and permitting phase which brings all those involved into an almost family like unit by the time construction begins.

 

CMAR Design Phase

Once the 30% design set was completed by SSR, this project was advertised to a select group of highly qualified and proven contractors that both the owner and SSR were familiar with.  Cleary Construction, Inc. was the chosen contractor for this project due to their approach, low GMP price, and VE suggestions.

SSR then worked closely with Cleary Construction to finalize the design plans and submit them to the various regulatory agencies. The 24” force main started at the owner’s existing pump station which is located near the juncture of a major local river and one of its largest tributaries. From the pump station, the line quickly manifolds into the existing 18” force main for increased hydraulic flexibility before crossing the large tributary. The force main then continued across state owned property operated by a local sports league that fell in both the river floodplain and TVA Right of Way. At the end of the sports complex, the force main was connected to an existing 12” force main before it was bored 540 LF underneath a local road and Interstate to a termination point outside of State Right Of Way into the parking lot of a commercial business. The line was then bored 160 LF under a state highway before starting open cut installation in a local residential road located within State ROW. The 7,500 LF path along the residential roadway required two more blue line stream crossings, and a future connection to an existing force main. Cleary Construction was responsible for traffic control and full pavement restoration along the local residential road. The end of road was the start of a 760 LF bore through a large rock bluff hillside. Once on the other side of the bluff, the route continued approximately 3,700 LF crossing two  more blue line streams including another large tributary. The line terminated at the owner’s 48” interceptor sewer, a centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) gravity sewer located inside a 102” rock tunnel.

The project called for significant consideration to be given for restraint requirements to ensure no issues would be encountered from the constant, and sometimes inconsistent trust from the pumps used to transport the raw sewage. This required heavy coordination between the team and material supplier to ensure the project met those restraint requirements and the necessary appurtenances were placed in the appropriate locations.

 

Da Bores

This project contained three separate significant bores. The two shorter bores were achieved by traditional jack and bore method and were relatively straight forward to both SSR and Cleary Construction.

The longer 760 LF cross country bluff bore was one of the most difficult aspects of the entire project given its length and location. SSR leaned heavy on Cleary Construction’s boring expertise to devise a plan that could accurately install a 36” steel casing on grade throughout the entire 760 LF path to the other side of the bluff. After much discussion with the team, Cleary Construction finalized their plan and started establishing GPS coordinates and setting up the bore pit. Traditional boring equipment could not be used by itself for this work due to the length of the bore, so Cleary Construction elected to use a laser guided Axis System to establish an alignment for the jack and bore method. The Axis System would use the established survey points to bore a 13.5” pilot hole approximately 400’ into the bluff to predetermined point. The alignment was checked every 100’ to ensure they were on course to hit the desired point. To ensure its accuracy, three  separate surveyors were used throughout the bore process. Once the pilot hole was successfully complete, traditional boring equipment was used to ream the hole out for the 36” steel casing along the entire 400’. The surveyors then collected additional survey data of the bore, including the exact location of the casing pipe end which was used to formulate the next path the Axis System would drill.

All the boring equipment was then transferred to the other side or ‘end’ of the bore where a new boring pit was established in the steep hillside. Cleary Construction then used the Axis System to bore back into the hillside with the 13.5” pilot hole in hopes of hitting the center of their existing 36” casing pipe located 400’ inside the bluff. Cleary Construction was successful in their first attempt and was off-center by a margin of less than 1”.  To put this in perspective, it would be the equivalent to standing on each side of the pedestrian bridge in Nashville and throwing a football from each side that collide in the middle of the bridge. It was a great feat, especially considering the soil seams in the rock bluff. Once the pilot hole was complete, the hole was reamed out to accommodate the 36” steel casing.  Cleary then transitioned back to their original bore pit to load all 760’ of casing and carrier pipe from the same side.

 

Tunnel Interceptor Tie-In

The first of the two tie-ins occurred at the 102” tunnel housing the 48” CCFRPM Interceptor, the largest and most critical artery of the gravity system. Due to the sensitivity of the connection a by-pass pumping system capable of pumping the interceptor flow was on standby in case of an emergency. The excavation of this tie-in was completed in three distinct stages. First an excavator removed the top portion of the soil to expose rock. Then a grinder was used to excavate the rock without major disturbance to the foam grouted tunnel and carrier pipe. At approximately 2’ above the existing pipe, Cleary Construction crews removed the remaining foam grout material by hand to prevent damage to the existing pipe. The tie in consisted of the installation of a custom 48” x 24” doghouse wye fitting on top of the 48” CCFRPM pipe.  Use of this fitting allowed connection of the force main to the wye’s 24” sweep while the straight 48” portion extended up to near ground level where it became the base of a manhole structure. Once the CCFRPM pipe manufacturer finished the lamination process on the new wye fitting, the entire fitting and excavation area were encased in concrete to maintain long-term structural integrity of both the fitting and the tunnel. The SSR designed encasement included multiple concrete pours in separate lifts and rebar doweled into solid rock around the pipe. A custom fabricated transition FRP spool piece was then utilized to house a standard manhole frame and cover for permanent access to the interceptor. A concrete collar was poured around the top portion to distribute the load and provide a traffic rated structure.

 

Pump Station Tie-In

After completing the open cut installation, bores, and Tunnel Interceptor connection, the last step in finishing the 24” force main was to connect the new 24” force main at the pump station. This was done by connecting to the existing 18” force main with a cut-in tee fitting assembly and valves. Connecting the new force main to the existing force main would give the utility district increased capacity as well as redundancy moving forward. SSR coordinated with the owner to design a permanent bypass system for connection of the force mains as well as flexibility for future maintenance of both force mains. To accomplish the tie-in the SSR team provided design input, reviewed, and approved an elaborate temporary bypass pumping configuration designed for the connection to the existing 18” force main. After careful planning, the station pumps were shut off and the wetwell was pumped using two 1,270 gpm pumps connected to the new 24” force main bypass structure. The new line would be used as storage during this process, but the pumps were large enough to pump the contents the full length of the new force main to the 48” interceptor. The remaining contents in the existing 18” force main was also drained via pump from a tapping sleeve and valve installed at the connection point. The flow was pumped off the existing force main to the pump station’s closest inlet manhole where it flowed gravity to the pump station and was bypass pumped into the new 24” line. The tapping sleeve and valve were then removed for the permanent connection and valve configuration. This tie in was completed without issue after close coordination with the utility district’s operation staff and planning around the upcoming weather forecast.

 

A Successful Conclusion

In the end, this project posed about every challenge possible in a pipeline project and finished up under budget. In total, over 1.7 million pounds of steel and 16,000 tons of stone were put in the ground during this project. With a project of this magnitude and difficulty, clear and constant communication was necessary throughout the design and construction phases to achieve a successful project. We believe the addition of Cleary Construction to the team so early in the project played an important role during our design efforts. Their insight proved invaluable and directly translated to dollars saved by the owner during the construction phase. Throughout the duration of the project, Cleary had as many as three superintendents and three project managers directly involved which is a testament to their commitment to the project and team first approach. The CMAR approach has proven to be a great method to navigate difficult projects when the right players are involved and invested. We are excited for the next opportunity to implement similar alternative project delivery methods that bring value to our clients on other exciting and challenging projects.