Post-Tensioned Stabilization of Roy Inks Dam - Nicholson ...

POST-TENSIONED STABILIZATION OF ROY INKS DAMJohn S. Wolfhope, P.E.Project ManagerFreese and Nichols, Inc.jsw@freese.comJerome W. Scanlon, P.E.AssociateFreese and Nichols, Inc.jws@freese.comGregor ForbesProgram ManagerLower Colorado River Authoritygforbes@lcra.orgRoger A. BaldwinProject ExecutiveNicholson Constructionrbaldwin@nicholson-rodio.comABSTRACTRoy Inks Dam (Inks Dam) is a high hazard, concrete gravity dam located on theColorado River near Burnet, Texas. The dam, constructed from 1936 to 1938, isowned and operated by the Lower Colorado River Authority (LCRA). During theLCRA Dam Modernization Program, the extreme flood events were reevaluatedfor the Highland Lakes Dams. As a result, Inks Dam was found to be subject to amore severe loading condition than was considered in the original design. Thedam’s stability was analyzed under the new loading conditions and stabilizationimprovements were designed to protect the 880 feet long, uncontrolled, ogeeshapedspillway.Construction improvements to the dam included removal of the top lift of thespillway, construction of a reinforced concrete stressing beam, installation offorty-six 24-strand (1000 kip) post-tensioned anchors, and reconstruction of thecrest of the dam. The project also included an 18-inch pipeline penetrationthrough the dam to relocate an existing USFWS Fish Hatchery water supply line.This paper provides an overview of the challenges presented to the design teamdue to the unique nature of this project and the existing condition of the structure.It also presents a discussion of the construction and addresses how lessonslearned from previous post-tensioned spillway anchoring projects wereincorporated.

In the early 1990s the LCRA and Freese and Nichols, Inc. completed apreliminary study that determined four of the six Highland Lakes Dams neededimprovements to meet modern design standards. As a result of that study, theLCRA began a $50 million, 15-year program to perform detailed investigations,design modernization improvements, and complete construction of improvementsfor the four dams. Risk analysis indicated that the flood wave resulting from afailure of Inks Dam would endanger at least 46 lives and would cause damagesin excess of $16 million dollars. Inks Dam was the third of the four dams to bemodernized, with the modernization of Alvin Wirtz Dam completing in 1998,Buchanan Dam in 2000, and Inks Dam in 2001. The remaining dam, Tom MillerDam, is currently in the final design stages with construction scheduled for 2002through 2003.Figure 2 - Aerial Photograph of Roy Inks Dam (looking upstream)DESIGN OF MODERNIZATION IMPROVEMENTSFreese and Nichols designed the improvements and provided construction phaseservices for the modernization of Inks Dam. Other members of the Freese andNichols design team included Trinity Engineering/Kleinfelder, Hill CountryEnvironmental, and GeoSystems, Inc.The Team’s design evaluations included field investigations, installation ofmonitoring instrumentation, geotechnical drilling, laboratory testing, structuralsurveys, and visual inspections including detailed mapping of concrete cracks.Detailed design analyses were completed using conventional overturning andsliding stability analyses and included a physical hydraulic modeling programconducted at the Utah Water Research Laboratory (UWRL). From a macrostructuralperspective, the analyses indicated that the spillway and non-overflowsections of the dam were stable against sliding for all flood events. However,

overturning stability concerns were identified for the spillway section at floodevents greater than, or equal to, the 500-year flood. Preliminary designalternatives included stabilizing the dam using high capacity, multiple-strand,post-tensioned anchors, large numbers of lower capacity post-tensionedanchors, or mass gravity concrete overlays. High capacity, multiple-strand, posttensionanchors were determined to be the most cost-effective solution forstabilization the dam and were selected for final design.DESIGN CHALLENGESThe selected stabilization approach was successfully demonstrated for LCRAduring the stabilization of Alvin Wirtz and Buchanan Dams. Lessons learned onthe previous two dams were applied during the design and construction of theanchors for Inks Dam. These lessons learned are discussed later in this paper.However, Inks Dam presented several design challenges that made this projectunique. The configuration of the spillway and rock outcropping at the left bankresulted in a cross section that varies substantially and a discharge that includessignificant three-dimensional hydraulic effects. The challenge of making a goodestimate of the hydrodynamic forces on the spillway was accomplished throughthe UWRL hydraulic modeling program.Detailed crack mapping revealed the spillway was severely cracked in the upperlifts and the concrete was deteriorating. Visual observations of the drilling coresand laboratory test data indicated that the concrete was of poor quality with somehoneycombing and little or no bond between many of the lift joints. Furthermore,the top ten feet of the spillway had lower unit weights and compressive strengthsthan the rest of the structure. Finally, significant leakage was observed throughseveral of the lift joints across the entire length of the spillway, especially at thefirst lift joint below the crest.The design of the post-tension system required careful consideration fortransferring post-tension anchoring forces into the structure. The design teamwas faced with a challenging question: How do you take large point loads andtransfer them into a structure comprised of irregularly fractured, poor qualityconcrete blocks? The design team developed an innovative approach using apractical solution. The simplest approach to stabilizing the structure would be tojust clamp all the pieces together and hold the structure to the foundation below.Figure 3 shows the design teams simple stabilization concept. We knew whatwe wanted to do, but the challenge was how to do it.

Figure 3 – Stabilization ConceptIn a brain storming session, the design team developed the concept of a highlyreinforced concrete stressing beam that acts as the top of the clamp to bring thepieces of the dam together. Figure 4 shows the developed design approach.Additional crack mapping was conducted to characterize the concrete crackingand the existing construction documentation was researched. Once all theinformation was gathered from historic data, visual inspections, and laboratoryanalyses, the final design was completed for the reinforced concrete stressingbeam and the high capacity, multiple-strand, post-tensioned anchor system.Strict qualification requirements were developed for the bidders to help ensurethat a qualified contractor was selected.Figure 4 - Section of repair methodology

CONSTRUCTION CHALLENGESConstruction of the stabilization improvements also presented several uniquechallenges. The work was performed on a constant level lake with no freeboardin the midst of an environmentally sensitive area. The left banks of the lake arewithin a state park accessible to the public. The United States Fish and WildlifeService (USFWS) operates a fish hatchery immediately downstream of the damand depends on Inks Lake for its supply of fresh water within strict water qualitytolerances. Numerous high-end homes are present on top of the bluffsoverlooking the dam.Physical constraints for construction included difficult site access, the proximity ofhigh voltage power lines, and a small space-constrained working area. Themajority of the staging area for construction activities was the top fifteen feet ofthe spillway. The contractor, Nicholson Construction, only had access to thedam from one side of the spillway, and that access was only wide enough for onevehicle to pass at a time. Additionally, the side of the spillway that wasaccessible had a 24” siphon pipe crossing the crest of the spillway providingfresh, untreated water to the USFWS fish hatchery. Part of the project scopewas to replace the existing siphon with a gravity line to reduce maintenance andoperating costs for the USFWS and to remove this obstruction from the top of thedam. Due to the location of the pipeline and its obstruction to spillway access, ithad to be removed early in the construction schedule. In order to remove thesiphon, the USFWS required the contractor to provide the fish hatchery with acontinuous supply of fresh water up to 3,000 gallons per minute without anydown time greater than four hours. The task of installing the new USFWSwaterline included an 18-inch pipeline penetration through the dam. Thecontractor installed a specially designed skiff box on the upstream side of thedam to accommodate drilling a 22” diameter hole horizontally through thespillway. The skiff box allowed for the proper installation of a section of thepipeline and the intake. A specific grout mix design was developed by thecontractor to guarantee sealing the pipeline and the existing cracks in thesurrounding weathered concrete structure. As if these conditions were notchallenging enough, shortly after the start of construction, the hydroelectricgenerator, which is the only means of passing normal water flow through InksDam, needed unscheduled maintenance. This lead to a period of approximatelyfour months where all flows had to be passed over the spillway while constructionactivities were ongoing.To overcome the overall project access challenges, the LCRA obtained a permitfrom the Texas Department of Transportation for a temporary constructionentrance off the state highway and the contractor constructed about a one-halfmile long access road to the crest of the dam. Due to the environmentalconcerns, this road had to be carefully constructed and was completely removedat the completion of the project. To provide the fresh water requirements of the

USFWS, the contractor installed a temporary pipeline from the river to the watersupply line for the hatchery, installed two submersible pumps, and provided anemergency generator to provide uninterrupted flow to the fish hatchery.To provide personnel and vehicular access to the work area on top of thespillway, the contractor installed a steel cable safety rail on the upstream side ofthe dam and an eight feet wide steel platform was cantilevered off thedownstream face of the entire spillway. Figures 5 and 6 show the upstreamsafety rail and downstream work platform. Due to the access constraints,particular attention was dedicated to daily scheduling during anchor installation.This scheduling allowed the equipment to be setup efficiently on the spillway andto prevent interruption of daily activities.Figure 5 – Anchor Installation with Safety Rails and Platforms

Figure 6 – Downstream work platformThe design of the stressing beam was configured to accommodate the concretedemolition work and to provide a temporary work surface and access road for thecontractor on top of the spillway. Figure 7 shows the preparation of the spillwaycrest to allow for construction access. The contractor sequenced hisconstruction activities to accommodate the limited access. Demolition of thecrest started at the right end of the spillway (in the middle of the river) using atrencher, jack-hammers, and a lot of manpower. Material was removed from thework area along the crest of the dam and demolition proceeded towards the leftbank.Figure 7 - Construction access on the spillway crest

The reinforced concrete beam formwork included steel pipe imbedments forinstalling the post-tensioned, multiple-strand tendon anchors. These guidesleeves, which would become permanent after the beam concrete set up, had tobe carefully surveyed into place in order to maintain proper drilling alignment andto miss existing drain holes located at the base of the spillway. The constructionsequence for installation of the beam proceeded in the same manner as thedemolition, starting in the middle of the dam and working towards the left bank.A significant environmental concern for the LCRA and USFWS was preventingdebris or construction discharge from entering the river. To accommodate theLCRA’s strict environmental and water quality standards, the contractor utilized acustom made diverter-system on the drill rigs to discharge cuttings through steellines laid across the entire 880 ft of the dam. In some locations debris was blownover 1,000 ft into a temporary holding pond. As a secondary containment, thecontractor dewatered the flip bucket at the downstream toe of the dam and usedit as a catch basin for any falling construction debris. At normal tailwaterconditions, the flip bucket is covered with approximately 3 to 4 feet of water. Thecontractor used a series of sandbags and plastic sheeting to create a smallcofferdam along the edge of the flip bucket. The debris was removed and thesurface of the dam was cleaned on a daily basis.When the hydroelectric plant was taken off line, the LCRA designed,manufactured, and installed a flash board system along the upstream crest of thedam. The flash boards were designed to route normal flows and small floodevents around the active construction areas. LCRA’s manufacturing shopdeveloped an innovative design for the flash boards that allowed large sectionsof the system to be quickly raised or lowered as needed. A picture of the flashboards is provided in Figure 8.Figure 8 - Flash boards along crest of Inks Dam

One of the major construction challenges was replacing the concrete on the crestof the dam. The upstream and downstream edges of the repairs were unevendue to the partial demolition of the crest and an ogee shape that was constructedover 60 years ago. The hydraulic loading on the structure dictated that theconstruction documents specify tight construction tolerances. Adjustable metalforms were developed to match the existing concrete on both ends of the ogeeshaped crest.Based on the Wirtz Dam and Buchanan Dam anchoring projects, LCRA and FNIlearned that the most important aspects of the anchoring system include thequality of the materials, the cleanliness of the tendons in the anchoring zone, anextensive quality control and inspection program, and the uniform seating of thewedges. Considering each of these, the design team chose to install barestrands within corrugated sheathing, to use two stage grouting and doublecorrosion protection, to require adequate cleaning of strands, and required theuse of power seating for the gripping wedges. One of the biggest challengesfaced by the construction team was the installation of the 8-inch corrugatedsheathing for the corrosion protection of the anchors while maintaining a twostage grouted system. Installing this type of system requires a very delicatebalance of hydrostatic pressures in order not to collapse the corrugatedsheathing during installation, water testing and grouting. The contractordeveloped a specific procedure for the installation and grouting of the corrugatedpipes that permitted the successful performance of the anchors without impactingthe schedule. The contractor developed a custom power seating system toaccommodate longer wedge travel for the higher elongation tendons to avoidmultiple jack setups. The construction inspectors from FNI and LCRA closelymonitored the handling and cleaning of the materials along with an extensivequality control program established by the contractor.The final construction challenge, and risk to the contractor, was the constantthreat of flooding. As stated before, Inks Lake is a constant level lake with nomeans of passing flood waters other than the spillway on which the constructionwas occurring. Due to the threat of flooding, the construction contract wasdesigned such that the LCRA meteorological group would provide daily forecaststo the contractor and the contractor remained prepared at all times to removeequipment and personnel with as little as four hours notice. The contract alsoincluded a line item for standby time that would cover the cost of shutting downthe job for flooding events. The standby time provision in the contract documentsprovided a way for both LCRA and the contractor to quantify their risk duringconstruction. It also provided a fair manner for the contractor and LCRA to sharethe cost of such an event. Fortunately there were no major releases of waterover the spillway during construction. However, during the last month ofconstruction the drainage basin received significant rainfall that broke a droughtand filled Buchanan Reservoir (located about 2 miles upstream of Inks Dam) towithin one foot of the crest of the emergency spillway. Within a matter of twoweeks, Buchanan Reservoir rose over ten feet to capture nearly 200,000 acrefeetfeet of floodwater.

SUMMARYThe owner, design team, and construction team worked together closelythroughout the project to develop innovative, yet practical, approaches toovercome the engineering and construction challenges presented by thisstructure. Construction documents were developed to address theconstructability issues and were structured to clearly identify the responsibilitiesand risks of both the owner and the contractor. The owner, contractor, andengineer partnered together during construction to overcome unforeseenobstacles. As a result of the team effort during design, and the successfulpartnering during construction, all of the challenges were overcome and theproject was completed under budget and ahead of schedule.