THE UNDERWATER CUT-OFF AT THE WALTER F. GEORGE DAM

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1 THE UNDERWATER CUT-OFF AT THE WALTER F. GEORGE DAM Gianfranco Di Cicco, Area Manage Special Project and Riccardo Pertocelli CEO, TREVllCOS Corporation, Boston, MA, USA Seepage problems were recorded for more then 40 years at the Walter F. George lock and dam. In 2001 the Mobile district of The U.S. Army Corps of Engineers turned to the installation of a 200 feet deep cut-off in a final attempt to remedy the situation. The advertised design-built contract requested the installation of the cutoff wall in front of the concrete structure of the dam starting from 90 feet of water, cutting through a concrete lock structure, a submerged retaining wall and extending on both sides on the earthen embankments. This paper summarize the history of the project and the installation of the first recorded cut-off wall constructed under deep water, upstream to an existing dam structures and under an existing concrete lock. The technology used combined with the project's results may be considered a milestone in the marine foundation and the marine cut-off wall construction technology. portion, the power units and two earthen wing dams extending to the Georgia and Alabama sides, respectively 5,810 feet and 6,130 feet long. As soon as the reservoir was impounded it became apparent that excessive seepage was occurring. In October 1961, during stripping operations on the final stage of construction of the Alabama dike, two sinkholes occurred. In 1962, still during construction, boils developed along the downstream toe of both Alabama and Georgia. HISTORY Project Location In order to control the seepage below the dam structure, relief wells were installed at the toe of the embankments and the foundation beneath the dikes was drilled and grouted. The Walter F. George lock and dam was constructed on the Chattahoochee River between 1955 and 1963 with the double function of improving navigation and generating power. Its 82 ft. by 450 ft. lock can accommodate large barge traffic and its four generating units with a plant capacity of 150 MW have an average energy output of 453,000 MWH. The dam consists of a 1,496 ft. long concrete structure housing the spillway, a non overflow Subsequently, after completion of the project, a large spring developed near the downstream lock guide wall and sinkhole activity increased on the Georgia side. In 1968 sinkhole activity increased and a new phase of remedial work began. Initially, the remedial work consisted of grouting a small area of the foundation under the Georgia

2 dike, filling the sinkholes with sand and grouting around each sinkhole periphery. In February 1969 a sinkhole was discovered in the reservoir near the Georgia dike and it was filled and grouted in a similar manner. A surface investigation downstream of the Georgia dike was carried out, a number of piezometers were installed and information concerning solution channels was obtained. After the evaluation of the new data, a sand filled filter trench was constructed downstream of the lock guide wall in order to intercept the water bearing solution channels which were discharging through a spring. The objective was to reduce the water velocity in order to prevent the movement of solid material. In 1970 a line of grout holes was performed downstream of the Georgia dike. During 1971, several small sinkholes were discovered at the Alabama dike toe and simultaneously pin boils were found in the relief well collector ditch in the same area. In 1978 a new design recommended the installation of a positive cut-off wall under the earth embankment to upgrade the dam to current standards. This recommendation was adopted, and a 24 inches thick concrete cut-off wall was installed to the bottom of the shell layer within the earthy limestone. In limited areas, in accordance with a previous soil investigation, the cut-off was installed deeper, generally to the bottom of the Shell Limestone. Phase I of the cut-off was installed under a portion of the Georgia dike in 1981 and Phase II was installed beneath an additional portion of the Georgia dike and the Alabama dike between 1983 and In 1982, between the two phases of construction, a boil was observed on the water surface immediately downstream of the Power House. The entrance point, called the "Hungry Hole". was plugged with a tremie pipe. The eroded channel under the Power House was filled with 175 c.y. of tremied concrete. Additional grouting was performed along the upstream face of the power house and part of the spillway. Figure 1: Aerial View of Cut-Off Wall Those efforts were only partially successful and resulted in a concentration of the seepage below the concrete structure. The seepage problems at the W. F. George dam continued to worsen. Discharges from piezometers SP-5 and from the power House drainage system increased. As the Shell Limestone material was piped from under the concrete structure, the passage ways through which the flow was occurring were being enlarged resulting in greater erosion. This situation compromised the generating potential of the dam as well as potentially jeopardized its stability. Finally, the decision was made to construct a deep cut-off primarily under the concrete portion of the dam to remedy the problem. (See Figures 1 and 2.) PROCUREMENT On July 5th, 2001, best and final offers were solicited by the Corps of Engineers (COE) for the construction of a deep cut-off to be built in front of the concrete portion of the dam from the bottom of the lake and tied into it. The cut-off had to cross an existing lock structure and an underwater retaining wall, as well as the remnants of a steel coffer cell left in place from the initial construction. The specifications required that the wall be built with a minimum thickness of 24" and that it extended from the bottom of the lake to elevation -5 into the Providence formation, an impervious layer.

3 While the land portions of the wall were on average 208 feet deep, the marine portion started from up to 90 feet of water and continued through different rock formations for another 100 feet. Contractors were invited to propose their own method of constructing the cut-off and offers were evaluated using several criteria, including technical, economic, and organizational and experience. Three proposals were received and on August 14, 2001 an award was made to the Joint Venture (JV) of Treviicos and Rodio. GEOLOGICAL CHALLENGES AND PROPOSED TECHNOLOGIES The geological challenges to overcome during the cutoff wall construction consisted in the existence of a very hard stratum (up to PSI) overlaying a loose sand layer, the presence of voids, fractures and pathways in the rock mass and the existence of sediments and various debris at the lake bottom. Nevertheless it was not requested; the proposal contemplated a preliminary drilling and grouting campaign in order to verify ground conditions and to fill any large voids which could disrupt the orderly progress of the work. Treviicos-Rodio proposed to do the marine portion of the work with secant piles installed using reverse circulation top-pile rigs and using water as drilling fluid, while the land portion would be a slurry wall done by Hydromill, using bentonite as a stabilizing fluid. of safety, quality, environmental, technical and economical conditions and requirements. Safety was considered in employing a technology able to reduce to the minimum the diving operation and the risks involved in performing underwater work at depths of almost 100 feet and in such operation. Quality was considered in employing the secant piles technology as the most reliable to do the marine portion of the work. Environmental concerns were considered in employing a reverse circulation methodology which allowed the use of water as circulation fluid and the elimination of any type of slurry from contaminating the lake environment. Technical concerns were considered in researching and detailing each different phase and challenge of the various areas of the project. Economical concerns were reduced in evaluating and implementing all the above considerations and insuring that the project team was formed by experienced personnel. A very detailed technical proposal containing documents, drawings and method of statements were submitted with the bid. However, to insure the client that the proposal and the project challenges were studied, detailed and covered by the design and build proposal, it was decided to include in the presentation a video CD showing and describing the various phases of the marine work. CONSTRUCTION METHODS The construction methods used are summarized in the following sequence of mainly marine operations: Drilling and Grouting Campaign Figure 2: Plan View of the Cut-Off Wall The chosen method for the marine work construction was the result of detailed analysis As explained, the specifications were not requiring the performing of any extra soil investigation. However, since the bid preparation it was decided to propose to drill a series of exploratory holes along the axis of the cut-off wall. (See Figure 3.) The results expected and obtained were to reduce the uncertainties over the extent of the Karstic phenomenon. The exploratory investigation,

4 which was the first of the marine operation performed, consisted in: Localized borings to ascertain: - Depth of sediments - Presence of fill materials placed upstream of the dam during construction - Thickness of hard sandy limestone layer - Characteristic of sand layer (apparently none cemented) within the sandy limestone Grout holes to fill major cavities, possibly with high water flows Installing Casing Templates Tied to the Concrete Dam Whenever possible, guide beams and templates were installed and secured to the concrete dam structures. The main beam was mounted on the props connected to the fenders of the dam. Its position and alignment was checked using reference points previously installed on the dam during the initial survey. (See Figure 4.) Figure 4: Standard Templates Figure 3: Drilling and Grouting Cleaning of the Lake Bottom and Constructing a Working Apron The construction underwater of an apron was required to insure the correct installation and positioning of the cutoff wall at the lake bed elevation. A cleaning operation in front of the dam structure was requested before the apron installation. Obstructions and debris were found in large quantities from the dam construction and from years of operation. After all the obstructions were removed the apron was installed excavating a six foot minimum trench was and then backfilled with engineering material along the cut-off wall alignment. When it was possible to attach the template to the dam, temporary piles were driven to support the original or a modified template. The templates were used to insure the correct piles positioning and to guide the temporary casing while installing and driving them into the apron. Drilling and Concreting Piles The cutoff wall designed using 50 inch secant piles, spaced 33 inches center to center. The piles were installed in a sequence of primary and secondary, in which the secondary piles were drilled between the two adjacent primary piles. (See Figures 5 and 6.) The piles were installed to form a continuous wall of the minimum thickness of 24 inches, from the lakebed to El - 5 MSL, into the Providence formation.

5 It has a rotary drill table mounted on the top of a frame, and a working platform. The rig is fixed on the top of the casing by means of clamps. A jib positioned on the top pile rig platform facilitates handling of rods. (See Figure 7.) Figure 5: Phase I Primary Piles Installation Figure 6: Phase II Secondary Piles Installation Typically, the execution of the piles entailed the following steps: Installation of the temporary casing supported and guided for the steel template Setting the reverse circulation rig on top of the temporary casing Drilling to design depth Removal of the drilling string Setting tremie pipes and pouring concrete Withdrawal of casing and setting in a new pile position Water pumped from the lake was used as drilling fluid. The main equipment for the installation of the secant piles was the following: Pontoons and spoil barges Manitowoc 4000 Crawler service cranes positioned on the pontoons Templates for the installations of casings Wirth PBA 612 reverse circulation rig ICE hammers to install the casing ICE 160 Hydraulic pile hammers to install the casing through hard strata IR 960 high pressure compressors Vertically control device The secant pile cut-off wall was drilled to full depth using the Wirth PBA 612 reverse circulation rig mounted on top of the 50 casing. Figure 7: Reverse Circulation Rig On the top of rotary table there is a two way swivel, discharging the cuttings into a 10 hose. The compressed air for drilling is supplied by means of 4pipes and hoses, connected to a swivel below the rotary table. Specially designed stabilizers were used in order to maintain the verticality of the drilling string during drilling operations. Flanged double wall drillings rods, 10 feet long with 6 " inner cuttings conduit and air conduit and the drilling bit completed the drilling equipment All the cuttings excavated by the reverse circulation rigs passed through a 10 discharge pipe and were delivered to the hopper barge. From this barge the cuttings were deposited to the bottom of the lake. Silt curtains, installed around the hopper barge, prevented the

6 contamination of the lake water by the spoil material. while secant piles did the rest of the crossing. (See Figure 9.) Constructing the Slurry Wall Slurry wall installation was the technology chosen to install the land portion of the cut-off wall, to cut the lock concrete monolith and the underwater retaining wall. (See Figure 8.) Figure 9: Construction of the Secant Pile Wall in the Water Figure 8: Construction of Slurry Wall on Land The cut-off wall installed using the slurry wall was built by primary and secondary panels using a Soletanche HI 2000 series Hydromill. The primary panel was formed by single or multiple bites with a total excavation length of 8 to 24 feet. The secondary panels overlapping the primaries were a single 8 foot bite. The de-sanding plant was a Sotres-450 consisting of a series of vibrating screens and cyclones capable of screening all the cutting sizes with a capacity of 500 cu. yd. per hour. Constructing the Cut-Off through the Lock Structure The crossing of the lock structure was accomplished by using the Hydromill on the Georgia side to cross the eastern retaining wall, With the Hydromill equipment, vertical excavation within required tolerance is assured when both cutter wheels mill similar material in terms of hardness and consistency. (See Figure 10.) On the Georgia side, the sloping contact between the soil embankment and the monolithic wall did not allow for the wall to be built in the correct sequence of primary and secondary panels, due to the fact that no panel could be excavated through the inclined concrete surface without a vertical guide The solution was to re-create in advance a uniform condition where: The inclined surface had been removed by a preliminary reaming of the wall The adjacent soil had been replaced by concrete Under this modified conditions, the standard sequence of overlapped primary and secondary panels was correctly carried out.

7 In detail the following steps were adopted: Construction of the slurry wall up to the last panel touching the monolithic wall foundation Milling the first adjacent bite passing through the wall for a certain depth Figure 11: Lock Structure Crossing Connection to Secant Pile Wall Figure 10: Hydromill and Guide Pipes The connection between slurry wall and secant piles was located in the footprint of the eastern monolith. Note that when the Hydromill reached the inclined hard surface, it reacted against and was guided by the previously concreted panel acting as a shoulder. This did not allow any lateral deviation After entering in the entire concrete mass for several feet, the excavation was stopped and the panel was concreted The same procedure was repeated in single bites in an uphill progression until the entire inclined surface was entirely reamed and the soil replaced by concrete. The last panel in this sequence was excavated and left open. After this operation, the slurry wall was built at full depth, re-excavating the newly placed concrete as well as the remaining portions of the lock wall with the proper sequence of primary and secondary panels. (See Figure 11.) Figure 12: Hydromill on Pontoon

8 The following steps were carried out: The soil between the walls of the Lock was excavated under water from El 108 to the bottom of the walls at El 81, for a width of 15 feet. The lakebed westwards of the western monolith was dredged as well to the bottom of the wall The excavation as filled with 3000 psi concrete to elevation 108 The Hydromill then continued to remove the concrete from the center of the Eastern wall, progressing westward until the concrete has been removed to El 108. Cutter wheels on the Hydromill for this operation were 63 inches in width in order to accommodate the casings for the secant piles. The progression was aided by installing a steel casing to act as a vertical guide Construction of the wall by the secant pile method was advanced eastward using the guiding template across the lock, until it encountered the lock wall. A secondary pile overlapping the last panel and the first primary pile was the joint element between secant pile wall and slurry wall. At the completion of the secant pile portion, the cut of the retaining wall was repaired. Constructing the Permanent Concrete Cap Tying the Cut-Off to the Dam The construction of the cap beam was done upon completion of the cut-off installation. Depending on the cut-off wall's position and the apron elevation, a portion, or all, of the engineered backfilling between the cut-off wall and the dam was removed in order to install the cap beam. The surfaces of the monoliths and of the cut-off wall were cleaned to insure a proper joint before pouring the 3000 psi. concrete of the cap beam. Figure 13: Cutting East Lock Monolith with Hydromill CONCLUSIONS The construction of a deep under water cut-off had never been attempted before and its successful completion will open new possibilities to Engineers and Owners on how to deal with difficult underwater seepage problems. The result obtained were the consequence of awarding the project to qualified companies and the assembling of a team of highly qualified experts working together from the bid preparation and through the completion of all the difficult phases of the project.