LESSONS LEARNED AT THE TAUM SAUK REBUILD Paul C. Rizzo, Ph.D., P.E. 1 Carl Rizzo 2 John Bowen 3 ABSTRACT The Authors served in key roles for the design and rebuild of the Dam for the Taum Sauk Rebuild Project between 2007 and 2009. Taum Sauk is the largest RCC Dam in the United States and has a symmetrical cross-section with conventional concrete faces upstream and downstream. The curvilinear shape and the cross-section presented a number of placement issues. In addition, a large number of Lessons were learned because of the rapid construction schedule, highly variable temperatures, highly confined working space, numerous details related to waterstops, construction joints and crest-to-gallery drains, foundation preparation, lift maturity, bedding mixes, crack repairs, and the conventional concrete upstream face. The authors discuss these issues from the perspective of the Designer, Construction Manager, and Contractor. INTRODUCTION The Taum Sauk Pump Storage Project is located in southeast Missouri in the central part of the USA. It is on the East Fork of the Black River, approximately 90 miles southwest of St. Louis, Missouri. It is a reversible pumped storage project used to supplement the generation and transmission facilities of AmerenUE, with 450-MW of capacity (Figure 1). The Project has an Upper Reservoir (Reservoir), originally designed as a concrete faced rockfill dam (CFRD) and a Lower Reservoir impounded by a concrete gravity Dam across the Black River. The Upper Reservoir impounding Dam is 6,562-ft long and kidney-shaped (Figure 2). Figure 1. Overall View of Taum Sauk Plant 1 President/CEO, Paul C. Rizzo Associates, Inc., Pittsburgh, PA 15235, Paul.Rizzo@rizzoassoc.com 2 Vice President of Construction Management, Paul C. Rizzo Associates, Inc., Pittsburgh, PA 15235, Carl.Rizzo@rizzoassoc.com 3 President, ASI Constructors, Inc., Pueblo West, CO 81007, john@asiconstructors.com
The original impounding CFRD was constructed with dumped rockfill, uncompacted but sluiced with monitors in an attempt to remove the fines. The Reservoir floor is generally at El. 1505 ft and the 12-ft wide crest of the Dam is at El. 1589 ft. A 10-fthigh, 1-ft-thick reinforced concrete Parapet Wall on the crest of the CFRD extended the maximum pool to El. 1599 ft. The Upper Reservoir, which has no drainage area, was not designed with a spillway. Water fills the reservoir by pumping and direct rainfall. The powerhouse is located at the upstream end of the Lower Reservoir about 2 miles from the Upper Reservoir, in a narrow canyon through which a tailrace channel was excavated to connect to the Black River. A concrete and steel-lined shaft and tunnel connects the Powerhouse to the Upper Reservoir. The Taum Sauk Plant is a peaking and emergency reserve facility. A typical daily summer cycle involves generating power in the mid-morning by releasing water from the Upper Reservoir through the turbines to the Lower Reservoir, and pumping from the Lower Reservoir to the Upper Reservoir in the afternoon. In the fall, winter and spring, the plant typically generates once per day. The normal maximum level for the Upper Reservoir is El. 1596 ft. Typically, the Upper Reservoir is drawn down less than 30 feet. A 30-foot drawdown represents 6 hours of single unit generation or 3 hours of double unit generation. The Upper Reservoir can be drawn down to El. 1525 ft. without vortex problems at the morning glory outlet and to El. 1515 ft. if only one turbine/generator unit is operating. The Project operates under the direction of the load dispatcher in St. Louis. Both units can be put on full load in a few minutes. The December 14, 2005 Incident Figure 2. Upper Reservoir at Taum Sauk At approximately 5:00 AM on December 14, 2005, the Upper Reservoir CFRD overtopped due to overpumping and the northwest corner of the CFRD breached over a width of approximately 700 feet, causing an uncontrolled, rapid release of water down the west slope of Proffit Mountain and into the East Fork of the Black River. An overall aerial view of the flow path is shown in Figure 3. The uncontrolled release flooded nearby Route N while truck traffic was underway, and produced significant property and environmental damage to the Johnson s Shut-Ins Campground.
Figure 3. Overall View of the Breached Dike Forensic Investigation and Root Cause Analysis A forensic investigation and root cause analysis of the failure was conducted under the direction of the lead Author to identify the failure mechanism and develop design parameters for a rebuild of the entire Upper Reservoir. Economic and technical analyses indicated remediation of the breach section was not practical. The investigation concluded that the original design of the CFRD in the early 1960 s was flawed and poor construction practices exacerbated the poor design and compromised the long-term integrity of the Dam. The details of the Forensic Investigation are available in ICOLD Meeting Proceedings (Brasilia, May 2009). NEW UPPER RESERVOIR DESIGN CRITERIA The Dam impounding the Upper Reservoir was rebuilt between October 2007 and April 2010 as an RCC Dam. Rapid, fast-track scheduling was employed to restore the operability of the Taum Sauk Project, a vital power source in the region. An aerial view of the new RCC Dam is shown in Figure 4. Figure 4. Aerial Photo of the New RCC Dam The new RCC Dam was constructed along the same alignment as the original CFRD. As the Upper Reservoir is founded on top of Proffit Mountain, it has no watershed. The Probable Maximum Flood (PMF) for the new RCC Dam is based on the rainfall on the reservoir surface. Therefore, the hydrologic and hydraulic criteria were defined
by two major factors: the elevation of the crest of the new RCC Dam (including freeboard), and the capacity of the proposed Overflow Release Structure (spillway). The overall design basis was to re-build the Upper Reservoir such that it would have the same storage capacity for power generation as the original Upper Reservoir with a normal pool and overall gross head, as established in the FERC License for the Taum Sauk Project. The original design of the Upper Reservoir did not include an overflow release structure to deal with an accidental over-pumping during the pumping mode of operation. On the other hand, the re-built Upper Reservoir includes an Overflow Release Structure designed to operate if the water level in the Upper Reservoir rises more than 2 ft above the maximum normal operating level. The Overflow Release Structure is an integral part of the overall Dam design as redundancy to prevent overtopping. The new RCC Dam section shown in Figure 5 has a symmetrical cross-section to accommodate the relatively poor rock condition in the foundation a rhyolite intruded by granite dikes with considerable thermal alteration which resulted in clay seams and weathered dikes at depth. These conditions led to considerable concern with sliding stability, especially under seismic conditions. Figure 5. Critical Design Section Consequently, each 100 ft cross section of the Dam was carefully examined and the base was adjusted so that the maximum foundation slope was 10 degrees or flatter and the foundation rock was cleared and cleaned of all clay seams and weathered dikes within a 30 ft interval beneath the Dam. Based on two-dimensional and threedimensional stability analyses, poor rock was removed to a depth of at least 30 ft below the dam/rock interface. The rock surfaces sloping between 10 degrees and 25 degrees were excavated to achieve an average slope of 10 degrees or less. As shown in Figure 5, Station 55 + 00 was identified as the critical design section because it has the greatest height at the toe.
LESSONS LEARNED DURING CONSTRUCTION OF THE RCC DAM Because of the size of the new RCC Dam (largest in the USA at 2.8 million yd 3 ), the poor rock conditions and the rapid construction schedule, a number of lessons were learned (or relearned!) during construction at Taum Sauk. We summarize a few of these below: Foundation Preparation As mentioned previously, the foundation preparation work was intensive and comprehensive at Taum Sauk as illustrated in Figures 6 11. The lessons learned from these photos are summarized as follows: The time and cost of foundation preparation is difficult to estimate and varies with each project as the predictability of conditions is exceptionally difficult. The use of pressurized water guns mounted on small excavators and long reach vacuum truck hoses are major time savers, are much safer for craftsmen to operate, and are significantly less strenuous for the craftsmen. Control of water associated with the cleaning operation and rainfall is a key factor to be addressed in planning the work. At Taum Sauk, the excavations always sloped downstream around the perimeter of the Dam foundation, making this aspect relatively easy to plan and execute. Foundation preparation needed to be a multiple shift operation in order to maintain schedule with a single shift RCC placement operation. Figure 6. New RCC Dam Typical Cross-section
Figure 7. Typical Foundation Preparation Rock Surface Excavation Figure 8. Typical Foundation Preparation Exposed Rock Surface Figure 9. RCC Placement at Rock Surface
Figure 10. RCC Placement at the Rock Surface Fines and Debris in the Rockfill Figure 11. Foundation Preparation Activities One of the fundamental tenets of the design of the new RCC Dam was the use of the rockfill from the old CFRD. The rock in the old CFRD was excavated and hauled to a crushing operation setup inside the perimeter of the Upper Reservoir where it was crushed and processed into an aggregate suitable for the Project RCC mix. We specified an aggregate with a maximum particle size of 1-1/2 inches and not more than 8% fines passing the No. 200 sieve. Also, we specified two stock piles of processed aggregate: one for plus 3/4 and one for minus 3/4. The lessons learned in processing the rockfill were basically two-fold: The percent fines after processing was significantly higher than anticipated. But based on the photo of the failed cross-section shown in Figure 12, we should have planned for this eventuality more comprehensively. Clearly Figure 12
Figure 12. North Side of the Breach shows a very high percentage of fines for a rockfill dam, indicating that the sluicing operation used during the original construction was not effective. The excavated rockfill contained pieces of steel left in the rockfill during the CFRD construction including teeth from dozer blades, pieces of sluicing pipe and drainage pipe, steel bolts, and other unrecognizable pieces of steel. These pieces of debris caused havoc with the crushing equipment and steel magnets had to be added to the system an expensive lesson learned and to be applied in future applications. Effect of Leaving RCC Lift Surfaces Exposed for Long Periods As mentioned above, the new RCC Dam has a perimeter of about 6800 ft. For construction planning and sequences, we divided the Dam into nine (9) monoliths, each having a length of about 700 ft to 800 feet, and each having about 100 RCC lifts approximately 1 foot thick. Monolith No. 2 was constructed from the foundation level at El 1500 ft up to El 1572 ft, 28 lifts short of the crest and left that particular lift surface exposed to the weather and equipment tracking for about a year or slightly longer to allow for stockpiling of aggregate against the upstream face. When work was resumed in that area, the exposed RCC lift surface was freshly cut and prepared in accordance with normal practice but nothing special was done to the 3 ft thick facing concrete (CVC). The lessons learned from this experience are as follows: Even with our extensive RCC batching operation, elaborate combined conveyortruck transport system, and extended working shifts (12 hours), we learned that a lift surface having a length of 565 ft to 820 ft is about the maximum that can be used. Incidentally, it was incumbent upon the placement crew to place an RCC lift over the entire length of the monolith which meant occasional long shifts or short shifts as no intermediate transverse unplanned construction joints were allowed. We had designed the project with Construction Joints at 90 ft centers and therefore, in those instances where it was simply not possible to complete a lift over the entire length of the monolith, we simply stopped the
RCC placement at a planned Construction Joint (CJ), which included a waterstop and crest-to-gallery drain. The lift surface in Monolith 2 at El 1572 ft was a major source of leakage during the initial refill with seepage observed on the downstream face. This behavior indicated that water had penetrated the concrete facing (which had not be green cut or specially prepared) into the horizontal RCC lift joint and then traveled the entire width of the lift surface from the upstream face to the downstream face. During the first drawdown after refill, we went back into the Upper Reservoir and ran a seal of waterproofing material along the lift joint line at El 1572 ft for the entire length of the Monolith. This stopped the leakage entirely as shown below in Figure 13. The lesson learned here is that when one has a lift surface exposed to the elements for a long period of time, special treatment of both the RCC and the CVC on the upstream face is required, including conventional green cutting, extensive cleaning, bedding mix on the RCC and a bonding agent for the CVC on the face. Downstream Monolith 2 Crest-to-Gallery Drains Figure 13. Monolith Before and After Sealing At Taum Sauk, we drilled the Crest-to-Gallery Drains from the Crest of the Dam down to the Gallery using a downhole hammer rig as opposed to either drilling upward from the gallery or forming the drains at each lift with a telescoping pipe. We found that drilling from the Crest led to drains that are significantly offset from the alignment of the Construction Joint/Waterstop and that they did not line up with the drainage trench in the gallery. The lesson learned here is that one simply cannot control the alignment of the Drain hole with adequate accuracy from the Crest-to- Gallery Drain to function as intended. These drains should be formed with telescoping pipe on each lift and not be drilled down from the Crest or up from the Gallery. Intermediate CJ s for the Upstream CVC Face The new RCC Dam at Taum Sauk includes a conventional sloping concrete face (CVC) designed to (1) protect the underlying RCC from weathering effects (the downstream face also has a stepped CVC Facing) and (2) provide a measure of
watertightness by preventing the water in the Reservoir from coming in direct contact with the upstream edge of a lift joint in the RCC. The RCC was designed with fullwidth Construction Joints (CJ s), including a waterstop, Crest-to-Gallery Drain, and plastic sheathing across the joint running from the upstream face to the downstream face. The 90-foot spacing of the CJ s was determined from a comprehensive thermal analysis. This same analysis indicated that intermediate CJ s in the concrete facing were required at 45 foot spacing. These CVC CJ s have a crack inducer plate and water stop only in the CVC and not in the RCC. The lesson learned here is that 90 ft spacing is too large, an observation that was made in our Construction Test pad early in the Project, but not taken into account when we laid out the CJ s for the first Monolith. During construction of the first Monolith, we quickly learned from the observed crack spacing that intermediate CJ s in the CVC face were required. We installed these with chamfered V notches, crack inducers and waterstops as shown in Figure 14. GIN Method for the Grout Curtain Figure 14. Waterstops at Monolith Joints The original CFRD at Taum Sauk had a short grout curtain for approximately 300 feet of the 6800 feet of perimeter. Generally, this was a single line construction with what we believe was a cement-bentonite mix. Judging from flow measurements at a downstream toe ditch, the grout curtain was not effective. The new RCC Dam was designed with foundation drains to the Gallery and a complete, two line grout curtain- - a belt and suspenders approach to minimize uplift on the gravity dam section and minimize leakage thought the fractured and jointed bedrock. We selected and implemented the GIN Method (Grouting Intensity Number) for constructing the Grout Curtain using the GIN curves coupled with a real time, computerized monitoring and control system. Layne GeoConstruction performed this work as a subcontractor. The lesson learned with the grouting program (actually learned again) is that the GIN Method is a valid and technically attractive approach for grouting in a controlled manner to virtually eliminate the potential for hydraulic fracturing while maximizing grout penetration. Setting the GIN parameters must be done in a careful step-by-step
manner, including some trial and error testing, to guard against hydrofracturing. The experience at Taum Sauk was exceptionally positive on this aspect of the Project. Water Stop Configuration As mentioned above, the primary CJ s at 90-foot centers and the intermediate CJ s at 22.5 foot spacing included a waterstop. As seen on Figure 14, the a plastic crack inducer sheet was attached to each waterstop on both sides in the case of the main CJ s to induce a crack in the CVC facing and in the RCC, but on the upstream side only on the intermediate CJ s to induce a crack in the CVC facing. The plastic inducer sheets were about 6 mm thick and fastened to the waterstops in the pre-formed ears at the centre shown in Figure 15. Figure 15. Original & Revised Waterstop Designs The waterstops for the CJ s in Monoliths 1 and 2 were of the original design in Figure 15 recommended by the supplier. With this design, we found that ears were not large enough or stiff enough to hold the plastic crack inducer sheet in place and movement occurred during RCC and CVC placement. The Revised Design shown in Figure 15 with the deeper and stiffer ears did not allow the inducer sheet to move even under rough placement conditions. A wider waterstop may also have been considered for better performance. The lesson learned here is that particular care should be taken with the waterstop detail design, and more specifically, the supplier should be challenged to assure performance under actual RCC placement conditions. RCC conditions tend to be more challenging than CVC placement conditions where reinforcing steel is available to tie off the waterstop. RCC Placement under Winter Conditions During the intense construction planning phase of the Project, we concluded that placing RCC during the winter months at Taum Sauk would be necessary to meet the schedule for completion. Winter conditions were assumed to be temperatures at the lower range of acceptability for RCC placement, less than 28 o F. We invoked the
following practices for RCC placement during the winter months (December through February): Place RCC on the day shifts only. Night shifts were used only for stripping and placing forms. Start placement in the morning when the temperature was at least 28 o F and predicted to rise and continue until the temperature began to drop back toward freezing. Monitor the lift surface continually with hand-held infrared thermal readers. Although there was a continuing debate as to whether the inspector should point the reader at the surface or to scratch the surface and read about 1 inch below the surface, we concluded that a surface reading was adequate and sufficient. We set the triggering temperature at freezing, provided the temperature was on the rise for approximately the next 8 hours. If the temperature was not expected to rise, we set the triggering temperature higher on a case by case basis. Immediately after placement and compaction, the lift surface should be covered with tarps. When starting a placement directly on rock, use heated thermal tarps placed on the rock surface for a few days in advance to raise the temperature of the rock surface. We left the downstream and upstream forms on the Monolith as long as practical and worked to strip the forms only on those days when the temperature was expected to be above about 35 o F. When this practice was not followed, we experienced cracking in the CVC facing due to thermal shock between the warm RCC on the inside face and cold air on the outside. The lesson learned here is that RCC can be successfully placed at or near freezing conditions with proper control. Production during the winter months was severely restrained because of the single shift operation and the lost days when the temperature never went higher than 28 o F. SUMMARY The Taum Sauk Rebuild Project is in many respects a milestone in the construction of RCC Dams. It is the largest RCC dam in the USA and is believed to be the largest symmetrical low-strength RCC Dam in the world. Because of its size, location on top of a mountain, difficult foundation conditions, and rapid construction schedule, many lessons were learned; a few of the more obvious and tangible are reported herein. We hope that the dam building profession will find these lessons helpful on future RCC Dam construction to allow for highly attractive technical solutions at lower cost.