Texas-sized interceptor

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    LAN Project Manager Paul Banschbach and TRA Inspector James Williams observe placement of controlled low-strength material backfill on the 78-inch-diameter wastewater pipe. Photo: LAN

    The Trinity River Authority of Texas’ (TRA) Central Regional Wastewater System began operations in December 1959 as the first regional facility of its kind, serving Irving, Grand Prairie, and Farmers Branch, plus a portion of western Dallas. The system has since expanded to serve all or part of 21 contracting parties and approximately 1.2 million people in the Dallas/Fort Worth area.

    The Mountain Creek Interceptor System is one of five major interceptor systems that convey flow to the Central Regional Wastewater System treatment plant. The Mountain Creek Interceptor collects wastewater from portions of six contracting parties within the Central system including the City of Grand Prairie where the interceptor is located. The existing MC-7 and MC-8 segments consist of 17,000 linear feet of 48-inch to 60-inch unlined reinforced concrete pipe installed in the 1970s.

    In 2004, TRA initiated an assessment of the pipeline condition and capacity for a portion of the Mountain Creek Interceptor including segments MC-7 and MC-8. The assessment determined that the pipeline was in need of replacement due to extensive corrosion and additional capacity needs. Subsequently, TRA retained Lockwood, Andrews & Newnam, Inc. (LAN), a planning, engineering and program management firm, to design improvements for the interceptor and take the project from preliminary design to construction advertisement.

    Preliminary investigation and design

    Initial evaluation of the system included a condition assessment to determine the probable useful life, review of flow requirements — which, at the time, were for 2040 design conditions — sizing of the pipe for conveying future design flows, and recommendations for alternative alignments.

    Internal inspection of the system using standard closed-circuit television revealed severe pipe wall corrosion, including areas of exposed reinforced steel. Given the condition and age of the existing pipe, the remaining useful life was estimated to be 15 to 20 years using best case assumptions; however, with the rate of corrosion observed, substantial failures were projected to arise even earlier.

    Rehabilitation of the existing interceptor using cured-in-place pipe or fiberglass sliplining was first evaluated to determine the possibility of upgrading TRA’s current infrastructure; however, an additional parallel interceptor would also be required to meet projected design flows. As such, a new large-diameter parallel pipeline, ranging from 78 inches to 84 inches in diameter, was recommended to carry all future design flows. This approach also proved to be the most economical from a flow management standpoint, allowing the new interceptor to be constructed in the dry while the existing system remained in service to convey flows during construction.

    Three alignment options for the parallel interceptor were investigated. The evaluation included:

    comparison of the alignment’s impact to a forest preserve (Fish Creek Forest Preserve) owned by Dallas County;

    • crossing of Interstate 20 (IH-20);
    • impacts to two City of Grand Prairie parks areas;
    • impacts to multiple Fish Creek crossings, which included evaluation of at-grade or aerial crossings, siphon crossings, and lowered crossings;
    • permitting needs; and
    • construction costs.

    The selected alignment, approximately 16,800 linear feet in total, lowered the profile of the interceptor, eliminating the need for aerial creek crossings, which are historically a maintenance headache. It also included tunneled crossings of four municipal roadways and IH 20 to minimize traffic disruption.

    The view looking down the 15-foot-diameter, 45-foot-deep tunneling pit in the shopping center parking lot. Installation of the two-flange liner plate is being completed by hand tunneling. Photo: LAN

    Design challenges

    The size and length of this pipeline, in addition to being located in a very developed portion of the Dallas-Fort Worth Metroplex, presented the following design and coordination challenges:

    Easement acquisition — Due to the size and location of the proposed interceptor, easements had to be negotiated for 39 parcels, including negotiations with multiple corporations and international negotiations with a Canadian landowner. The process was quite lengthy due to specific needs and concerns of each property owner. Despite the challenges, the TRA land rights team’s proactive communication and coordination with the stakeholders resulted in successful acquisition of these easements.

    One specific challenge to land rights came after discovering old unpermitted septic systems on a group of properties located along the interceptor. Cooperation from the City of Grand Prairie was crucial in allowing the project to proceed without a major redesign. Rather than add several aerial crossings over Fish Creek, the City of Grand Prairie agreed to fund construction of a municipal collection line to serve the properties. The collector would then tie into TRA’s proposed interceptor, allowing the septic systems to be decommissioned.

    Tree preserve — Due to the location of the existing and proposed sanitary sewer easements within the Dallas County Fish Creek Preserve, the proposed pipeline posed a significant impact to the wooded park along Fish Creek. Therefore, an extensive tree survey was required prior to negotiations for land rights. The survey required a certified arborist to identify species and caliper for more than 500 trees in the preserve and mark each with an identification tag. To further complicate matters, the delay of schedule during property acquisition required re-surveying and re-tagging many of the trees due to additional growth.

    Permitting/coordination with stakeholders — The project required coordination with several critical stakeholders, including:

    • City of Grand Prairie: Being the largest stakeholder outside of TRA meant that the project team had to coordinate with multiple divisions within the city throughout design and into construction. These included the public works department for plan review and traffic control design as well as the parks department for construction requirements and access through Grand Prairie parks. Additionally, the city operates and maintains the park within the Dallas County Preserve. TRA modified the alignment and offset easements to best serve the public by maintaining access to a local playground, pavilion, and a portion of trails during construction.

    • Texas Department of Transportation (TxDOT): Several permits were required for work within and adjacent to TxDOT right-of-way, including a utility installation request for the proposed 78-inch interceptor crossing IH-20, an access permit for ingress/egress of TxDOT right-of-way, and a permit authorizing a temporary staging area along the north right-of-way.

    • U.S. Army Corps of Engineers (USACE): A permit was required from the USACE due to the multiple crossings along Fish Creek.

    • Floodplain permit: A floodplain development permit was required with the City of Grand Prairie because most of the proposed work is within the defined floodplain.

    • Texas Parks and Wildlife Department (TPWD)/Aquatic Resources: The proposed alignment crosses a small pond and, in order to open-cut construction across, a permit from TPWD was required as well as an aquatic relocation plan to relocate any wildlife prior to construction.

    • Shopping center: A large shopping center located on the corner of IH-20 and Carrier Parkway required special coordination. Not only were multiple landowners involved, but trenchless methods of construction were also required to minimize impacts to the adjacent businesses.

    A buried bridge is a buried structure supporting a roadway that derives its support from the interaction of the structure and surrounding soil with an unsupported span (bridge length) greater than 20 feet

    Tunneled crossings — To ensure the least amount of impact to the surrounding stakeholders, design included more than 3,200 linear feet of tunneled pipeline installation ranging from 94 inches to 100 inches in diameter. These tunnels included crossings of Carrier Parkway, Robinson Road, Bardin Road, and Lake Ridge Parkway in the City of Grand Prairie in addition to an 800-linear-foot tunnel under the parking lot of the large shopping center to minimize impacts to the businesses.

    Creek crossings — Although the proposed design removed the need for aerial creek crossings, it did not support tunneled crossings of the creek and required open-cut installation of seven creek crossings. This required protection of the pipe using concrete encasement and flowable fill/rip-rap installation from bank to bank.

    Construction staging area — Given the project location, a staging area for the contractor was not readily available. As such, the staging area was negotiated into the easement agreements where feasible and include several areas in City of Grand Prairie parks as well as in the parking lot of the shopping center.

    Crossings of existing interceptor — The proposed interceptor crosses the existing interceptor a total of three times. One of the locations had enough clearance to tunnel the crossing; the other two required a special gravity bypass design to keep the existing interceptor in service and negate the need for full-time bypass or large permanent junction structures.

    Impacts of delayed schedule

    The easement acquisition process took significantly more time than anticipated. This created a major impact to the overall project and required redesign through several key areas as the City of Grand Prairie continued to develop throughout the project limits. The most notable of these is the TxDOT IH-20 widening project that entered design in 2015 and scheduled major upgrades to the interstate and frontage roads right through the proposed pipeline alignment. This prompted TRA to break the overall project into two construction phases (Phase I and Phase II) and bid Phase I immediately. The resulting Phase I improvements were bid in July 2016, including 7,700 linear feet of 72-inch to 84-inch pipeline.

    The other major area of impact (located on Phase II) was construction of Lake Ridge Parkway in Grand Prairie. At the time of the initial route study, the Lake Ridge Parkway extension was still in conceptual design and the MC-7 and MC-8 segments were anticipated to be installed prior to construction of the parkway. However, with the delays experienced, Lake Ridge Parkway was not only designed, but also constructed, impacting more than 3,000 linear feet of the proposed interceptor. With this major change, LAN and TRA are investigating a reroute of the alignment to improve construction, construction access, and future maintenance, potentially saving TRA as much as $2.5 million in construction costs.

    Additionally, with the lengthy delay in schedule, revised flow projections became available for the 2060 flow conditions. This actually provided a benefit to TRA because the 2060 flow projections allowed for a reduction in proposed pipe size through 70 percent of the alignment, saving TRA as much as $500,000 in materials cost alone.

    Creative bidding strategies

    To move design of the proposed interceptor into long-awaited construction, several creative bidding strategies were utilized. Phase I, bid in July 2016, included two strict construction timelines required of the contractor. The first constraint involved coordination with the construction schedule for IH-20 improvements. TxDOT provided a deadline of Jan. 31, 2017 for completion of TRA construction activities in their right-of-way given their anticipated construction schedule for frontage road expansion; however, this deadline could be extended if the TxDOT bid schedule was delayed.

    The second timeline included limiting construction activities through the Dallas County Fish Creek Preserve to 365 calendar days once construction begins. This deadline was implemented during land rights negotiations and impacts the first 2,000 linear feet of the Phase I construction. This deadline is non-negotiable and construction activities must be completed prior to expiration.

    Most notably, however, a bid alternate was used on a middle 900-linear-foot section of pipeline between the Dallas County Fish Creek Preserve and the IH-20 tunneled crossing. This alternate was needed since land rights negotiations were not yet complete with the shopping center property owner at the time of bid. In order to meet the critical TxDOT construction schedule, the plans had to be bid with the assumption that the land rights would be completed in the following months. If land rights were not acquired, the bid alternate included a temporary flow connection that allowed the 900-linear-foot section of 78-inch interceptor to be built in a future TRA project. Fortunately, the easements were acquired in December 2016 and construction will be able to progress unimpeded as originally designed.

    Conclusion

    Notice to proceed on Phase I construction came in September 2016. More than 70 percent of the tunneled crossings of Carrier Parkway and the shopping center have been completed with the contractor actively tunneling IH-20 in preparation to receive the carrier pipe. The TxDOT bidding schedule for the IH-20 expansion project has been delayed to June 2017, allowing the contractor to continue to stage in the TxDOT right-of-way and minimizing impacts to the adjacent shopping center. Construction is expected to be at substantial completion in February 2018 and final completion in April 2018.

    Phase II is scheduled to complete final design in fall 2017 and, pending the land rights implications of the Lake Ridge Parkway reroute, could advertise for construction bids as early as spring 2018.


    Paul Banschbach, P.E., is an engineer at Lockwood, Andrews & Newnam, Inc. (LAN), a planning, engineering, and program management firm. He can be contacted at pmbanschbach@lan-inc.com. Kelly Davis, P.E., is the senior engineer and Pipeline Group leader for the Trinity River Authority of Texas’ Planning and Development Department. She can be contacted at davisk@trinityra.org.