Bridges to Bypass in Alabama

It goes without saying, some bypass projects are more challenging than others. Sunbelt Rentals performed a recent bypass project in Montgomery, Alabama, which was no exception.

 

In the spring 2017, the Montgomery Water Work & Sanitary Sewer Board released a bid for sewer siphon rehab. The project consisted of rehabbing four separate sewer siphons using cured-in-place pipe (CIPP), with pipe diameters ranging from 14 to 64 in. CH2M designed this project while SAK Construction was the lowest bidder and awarded the project. In addition, Sunbelt Rentals was chosen as the bypass contractor to perform the flow diversion for SAK.

The CH2M bypass specifications relied heavily on details for success. These detailed specifications also helped guarantee that the bypass contractors were bidding on a level playing field. While several items required were a challenge, they were necessary for minimizing the owner’s risk of a sanitary sewer overflow (SSO).

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The first challenging requirement was to implement a higher design bypass flow rate for the rainy season (November–April). The next condition mandated the bypass to have a minimum of four primary pumps to perform the design flow with an additional 50 percent redundancy. The final and most challenging aspect of the project was that the bypass discharge piping could not be placed in the creeks where the siphons crossed.

Due to the rainy season, the sewers would see more inflow and infiltration, therefore the Montgomery Water Work & Sanitary Sewer Board and CH2M detailed different flows for the wet season vs. the dry season. The wet season flows were significantly higher than the dry season flows, which made more sense to perform the large diameter rehab in the dry months. The two largest siphons on the project were Siphon 4 and Siphon 6. These siphons had 35 and 40 million gallons per day (MGD) dry month flows, which increased to 65 and 70 MGD during wet month flows. Due to this, the larger siphon bypasses were planned to be performed first to stay within the dry months.

Redundancy was another important specification. To reduce the possibility of mechanical failure and achieve the desired flow, the larger diameter siphons required four main bypass pumps along with 50 percent redundancy. A minimum of six pumps were required to perform the larger bypasses. Siphon 4 and Siphon 6 included a total of six Sunbelt Rentals 18-in. Quiet Flow pumps with transducers to regulate the flow. This met the requirements for the design flow of the dry season with the adequate redundancy.

Project Challenge

Perhaps the greatest obstacle was preventing the fusible 24-in. high-density polyethylene pipe (HDPE) bypass discharge from entering the creeks where the siphons were located. This specification was incorporated to prevent the bypass pipe from being damaged during a high-water event, thereby causing a sewer spill. In the past, Sunbelt Rentals created temporary crossings of up to 90 ft for bypasses utilizing steel beams. However, the smallest crossing on this project was 90 ft and the longest crossing was close to 170 ft. Due to the longer distances, the beam idea was obsolete. At bid time, Sunbelt decided to place trench boxes turned on end in the bottom of the creek to support the HDPE for the crossings. Although, once evaluating the crossings post bid, it was determined this was not the safest approach. The creek bottoms were not level and even with the largest spreaders in the trench boxes, it still would not be level with the creek banks.

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Because Sunbelt did the math for the larger crossings using twin 24-in. HDPE discharge pipes, these pipes were close to 400 lbs per foot. Siphon 4 and Siphon 6 needed dual runs of 24-in. pipes to achieve the desired flow. With Siphon 4 being 90 ft of clear span and 36,000 lbs, and Siphon 6 being 170 ft and 70,550 lbs, Sunbelt needed to find something that would support the weight of a semi, or for Siphon 6, a couple of semis.

Luckily, after some searching, Sunbelt located the company that rented temporary bridges. After a few weeks of design, Sunbelt came up with a modular solution. The plan was to mobilize a single bridge to be utilized on all four phases of the siphon rehab. This bridge would be built on the first siphon, called Southlawn Siphon with a span of 150 ft. It only had a single 24-in. pipe with 200 lbs per foot (or 30,000 lbs) making this the ideal test to see if the bridge would work. To bypass this siphon, Sunbelt used four 12-in. Quiet Flow pumps and the single run of 24-in. HDPE pipe.

Sunbelt mobilized with the bridge in mid-June, and it took roughly a week to build the bridge, which came in multiple, heavy pieces. The bridge was assembled on cribbing with rollers to utilize the crane assist method during the instillation. This process included a 10k reach forklift strapped to the end of the bridge. The forklift pushed the bridge halfway over the span and a 200-ton crane picked up the end of the bridge. This synchronized action with the forklift pushing and the crane picking allowed the bridge to be placed on temporary abutments on the far side of the creek. The result was a resounding success as the bridge spanned the distance, without sagging, and our pipe was elevated off the creek bed.

Originally, due to the high flow requirement, Sunbelt was to perform the larger Siphon bypasses 4 and 6 first. Although, starting on the larger siphons didn’t go as planned, mostly due to accesses issues on the fiberglass suction manholes. Until a game plan could be established to gain access to those manholes on Siphon 4, Sunbelt chose to perform the smaller bypass on the Southlawn Siphon. The end result was that the five months turned into three months for Sunbelt to setup and relocate the two large siphon bypasses, and SAK to perform rehab before the higher flow requirement came into effect on Nov. 1.

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Once Southlawn was completed, the bridge was relocated to the Siphon 4 and then to Siphon 6. With each relocation Sunbelt became more efficient at relocating and setting the bridge. This made up for the lost time of performing the smaller siphon at the beginning of the project, and both Siphon 4 and Siphon 6 were completed before the Nov. 1 high flow period. Had trench boxes been utilized for the creek crossings the time frame may not have been met. All said, even though this bypass was a challenge its success was attributed to the detailed engineering bypass specifications, a thorough prime contractor, and a little post bid ingenuity to cross the creek spans.

Ladd Gould is national strategic account manager at Sunbelt Rentals.