New dam takes shape in the heart of Waco, Texas

Elegant is a word many might use to describe a beautiful and sophisticated piece of artwork. Engineers, on the other hand, apply the term to a particularly efficient mathematical calculation. The labyrinth weir dam to be constructed on the Brazos River in Waco, Texas, is elegant from both perspectives. The public will appreciate it for the picturesque reservoir it creates in theheart of their city.

Engineers will respect it for its unique design.

The proposed new labyrinth weir will replace the existing gated dam, which is owned and operated by thecity of Waco, approximately 2.5 miles east of downtown.

The upstream impoundment created by the existing dam is intended to be a recreational reservoir of constant water elevation in the downtown area. The Lake Brazos reservoir complements the beauty of the downtown area, riverwalk, and trail system. There liability of the reservoir is, therefore, of great importanceto Waco.

The existing facility

The existing dam was constructed in 1970 and substantially modified in 1985. It now is plagued by frequent operational problems and costly maintenanceassociated with its leaf gate systems. Maintenance costs for repairing hydraulic cylinders and removing silt in theleaf gatepits have climbed to as much as $300,000 per year. Additionally, problems with the gate system have forced thecity to lower thereservoir for as long as three months a year to perform necessary maintenance, detracting from the reservoir’s aesthetic benefits.

The existing dam is comprised of three typical sections. A 240-foot-long gated concrete spillway / stilling basin extends from thesouth (right) abutment. The stilling basin includes a 15-foot-thick section of roller-compacted concrete (RCC) that was constructed in the late1980’s to reduce downstream erosion. The second section is comprised of a 300-foot-long uncontrolled overflow, concrete-capped earth embankment. The third section is the 400-foot-long, non-overflow earth embankment extending into the north (left) abutment.

A new solution

Engineers from the city of Waco and from the Freese and Nichols, Inc., offices in Austin and Fort Worth, Texas, set out to design a new dam that would be elegant and efficient from a hydraulic, structural, and aesthetic standpoint. Because of the upkeep problems with the current dam, the city of Waco desired a low-maintenance and reliable structure that would be able to pass incremental flood events without inundating the downtown area. A labyrinth weir was proposed to meet these requirements.

A labyrinth weir was recommended over a straight, fixed weir because the former provides a larger discharge capacity with the same channel width, and reduces upstream flooding during moderate flood events.

Du ring later planning stages, the design team proposed that an offset weir be constructed in order to reuse an existing pad of RCC for the foundation for the right half of the weir. The left half of the weir will be supported with drilled shafts socketed into shale. Using the existing RCC provides a tremendous cost savings compared with construction of entire ly new foundations.

Although the final configuration of the proposed labyrinth weir still is being evaluated, and model studies are being conducted currently at Utah State University to aid in sizing the weir, Figure 1 is a graphic illustration of the proposed structure that has been developed to date. The weir will have a total width of 550 feet across the channel, and will be offset into left and right halves.

Overall, the weir is planned to have a total of 25, V-shaped cycles. The total width parallel to the channel will be about 90 feet, which includes 15-foot, upstream and downstream approach/release slabs. Apron stepdowns will be constructed downstream of the release slabs, and will have lengths of about 20 feet parallel to the channel width. The V-shaped wall sections will be about 12 feet high in the left half, and 17.5 feet high in the right half. The length of the weir walls will be approximately 60 feet (parallel to the channel). A concrete retaining wall will be constructed as a containment structure for the left upstream levee.

Reusing the RCC

In order to proceed with this elegant approach of reusing an existing foundation, the design team had to demonstrate that the existing RCC would meet the requirements of supporting the structural loads on the right half of the weir without sliding downstream. To validate this, Freese and Nichols, joined by engineers from Kleinfelder, Waco, Texas, performed a detailed geotechnical analysis based on exploratory borings and intensive laboratory testing.

Sliding stability analysis—Using the results of the laboratory and field investigations, as well as the proposed geometry of the new weir, a sliding stability analysis was conducted for the RCC. The sliding stability analysis was performed in accordance with Corps of Engineers procedures (EM-1110- 2-2200, ETL 1110-2-256, and Technical Report GL-83-13) based on the limit equilibrium method and a factor of safety defined as the ratio of the shear strength and the applied shear stress along the failure plane. A minimum factor of safety of 2 is required for normal conditions, and 1.3 for extreme conditions.

The overall sliding stability analysis required the inclusion of headwater and tailwater levels for normal pool and 2-, 10-, and 100-year flood events, multiple failure plane elevations, varying shear strength values, and three uplift conditions. Each of these variables was evaluated independently and then incorporated to determine the net effect of the interaction of the variables.

The complex geometry of the existing combined gated section and RCC dictated that multiple failure planes were considered.

Also, engineers compared the data to other projects, and performed a sensitivity analysis to determine failure envelopes for extreme conditions.

Overall, however, the factors of safety were most influenced by the selection of shear strength values. Although the team relied primarily on data from the exploratory borings, it also researched shear strength data from other projects in the Central Texas area, and other values presented in litera ture. In addition, it performed a calculation to determine the minimum shear strength values that would be required to have a factor of safety of at least 1 for the original dam strength.

Depending on the strength values selected, the factor of safety ranged from 2.1 at normal pool for the critical condition to 20.5 for a 100-year flood event. A shear strength correlation recommended by the Corps of Engineers yields a factor of safety of 2.2 for normal pool conditions. As a result of these analyses, the engineering team determined that the right half of the new weir could be supported on the existing RCC within an acceptable factor of safety of 2. However, the team also analyzed several extreme conditions to verify that the 1.3 factor of safety criteria was met.

Structural support—One critical element of the analysis was whether the RCC was structura lly connected to the existing gated dam section. The RCC pad was added in the late mid-1980s as a downstream erosion control measure, and a 3-foot-thick top slab was extended from the original stilling basin over the RCC (See figure 2).

According to previous design drawings, as well as construction photographs reviewed by the design team, a portion of the original top slab was broken out to expose the reinforcing steel. Reinforcing consisted of #8 bars, top and bottom, spaced at 12 inches on-center each direction. The existing rebar were mechanically connected with couplers to the new rebar, and a new topping slab was constructed over the RCC. The stability analysis yielded a factor of safety of 2.1 for the connected structure, and 1.6 if disconnected, which is acceptable for an extreme condition.

Conclusion

An independent peer review team led by Erik Kollgaard, P.E.; and John Focht, P.E., reviewed the calculations and results of the sliding stability analysis, as well as the overall structural assumptions. The peer reviewers brought their extensive experience in dam design and construction to the project. Based on their review, the overall project concept, including reuse of the existing RCC pad, was deemed appropriate. Construction is scheduled to begin during summer 2005.

Scott M. Langerman, P.E., is a senior geotechnical engineer and the Waco/Killeen area manager for Kleinfelder. He may be reached at slangerman@kleinfelder.com. Victor M. Vasquez, P.E., is a design engineer with Freese and Nichols, Inc., and is the project engineer for the Lake Waco Dam Replacement Project.