Waco TX Labyrinth Weir Kleinfelder

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Refined elegance

February 2005 » Cover Story

The public will appreciate the labyrinth weir dam to be constructed on the Brazos River inWaco, Texas for the picturesque reservoir it creates in the heart of their city. Engineers willrespect it for its unique design.

By Scott M. Langerman, P.E., Victor M. Vasquez, P.E.

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. Thelabyrinth weir dam to be constructed on the Brazos River in Waco, Texas, is elegant from bothperspectives. The public will appreciate it for the picturesque reservoir it creates in theheart of theircity.

Engineers will respect it for its unique design.

The proposed new labyrinth weir will replace the existing gated dam, which is owned and operated bythecity 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 beautyof the downtown area, riverwalk, and trail system. There liability of the reservoir is, therefore, of greatimportanceto Waco.

The existing facility

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

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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 sectionof roller-compacted concrete (RCC) that was constructed in the late1980’s to reduce downstreamerosion. The second section is comprised of a 300-foot-long uncontrolled overflow, concrete-cappedearth embankment. The third section is the 400-foot-long, non-overflow earth embankment extendinginto the north (left) abutment.

A new solution

Engineers from the city of Waco and from the Freese and Nichols, Inc., offices in Austin and FortWorth, 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 ofWaco desired a low-maintenance and reliable structure that would be able to pass incremental floodevents without inundating the downtown area. A labyrinth weir was proposed to meet theserequirements.

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

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

Although the final configuration of the proposed labyrinth weir still is being evaluated, and modelstudies are being conducted currently at Utah State University to aid in sizing the weir, Figure 1 is agraphic illustration of the proposed structure that has been developed to date. The weir will have atotal 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 thechannel will be about 90 feet, which includes 15-foot, upstream and downstream approach/releaseslabs. Apron stepdowns will be constructed downstream of the release slabs, and will have lengths ofabout 20 feet parallel to the channel width. The V-shaped wall sections will be about 12 feet high inthe left half, and 17.5 feet high in the right half. The length of the weir walls will be approximately 60feet (parallel to the channel). A concrete retaining wall will be constructed as a containment structurefor the left upstream levee.

Reusing the RCC

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

Sliding stability analysis—Using the results of the laboratory and field investigations, as well as theproposed geometry of the new weir, a sliding stability analysis was conducted for the RCC. Thesliding 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 anda factor of safety defined as the ratio of the shear strength and the applied shear stress along thefailure plane. A minimum factor of safety of 2 is required for normal conditions, and 1.3 for extremeconditions.

The overall sliding stability analysis required the inclusion of headwater and tailwater levels for normalpool and 2-, 10-, and 100-year flood events, multiple failure plane elevations, varying shear strengthvalues, and three uplift conditions. Each of these variables was evaluated independently and thenincorporated 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 failureplanes were considered.

Also, engineers compared the data to other projects, and performed a sensitivity analysis todetermine 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

Cover Story

Preserving history while designing for the futu

A few years ago, administrators at The Rodeph Sholom Sc(RSS), an independent nursery through eighth grade schoofounded in 1958, were expecting a large second grade clasthe fall of 2009. RSS quickly realized that they needed morclassroom space, and fast! In addition, the school had agymnasium that was only half the size desirable. At that timthe campus was located on Manhattan’s Upper West Side consisted of four landmarked brownstones built in 1910 thahad been previously converted into a school. Comprised ofseveral staircases and narrow passages, the existing spacwere cramped and labyrinthine in layout, and RSS desiredmore usable space for hands-on activities such as readingteaching, and physical education. In 2008, the school had topportunity to acquire a fifth brownstone adjacent to theexisting school, measuring approximately 19 feet wide by 6feet long. The design team went to work immediately to cothis residential building into the spaces the school so badlyneeded and expanded their campus to include more classrspace, an expanded gymnasium, and a new library.

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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 berequired 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 thecritical condition to 20.5 for a 100-year flood event. A shear strength correlation recommended by theCorps of Engineers yields a factor of safety of 2.2 for normal pool conditions. As a result of theseanalyses, the engineering team determined that the right half of the new weir could be supported onthe existing RCC within an acceptable factor of safety of 2. However, the team also analyzed severalextreme 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 llyconnected to the existing gated dam section. The RCC pad was added in the late mid-1980s as adownstream erosion control measure, and a 3-foot-thick top slab was extended from the originalstilling basin over the RCC (See figure 2).

According to previous design drawings, as well as construction photographs reviewed by the designteam, a portion of the original top slab was broken out to expose the reinforcing steel. Reinforcingconsisted of #8 bars, top and bottom, spaced at 12 inches on-center each direction. The existingrebar were mechanically connected with couplers to the new rebar, and a new topping slab wasconstructed over the RCC. The stability analysis yielded a factor of safety of 2.1 for the connectedstructure, 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 thecalculations 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, wasdeemed 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 [email protected] . Victor M. Vasquez, P.E., is a design engineer with Freese and Nichols, Inc., and is the project engineer for the Lake Waco Dam

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