ASSOCIATION OF STATE DAM SAFETY OFFICIALS VOLUME 14 ISSUE ISSN

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1 ASSOCIATION OF STATE DAM SAFETY OFFICIALS VOLUME 14 ISSUE ISSN IN THIS ISSUE: Depression at Nesbitt Dam Developing a Dam Emergency Intervention Toolbox Lake Delhi Dam Reconstruction Technology Solutions for Dam Safety Instrumentation Data Collection & Reporting

2 CARI R. BEENENGA JAMES C. MYERS ANTHONY M. NOKOVICH Completed in 1901, Nesbitt Dam is owned and operated by Pennsylvania American Water Company (PAWC). It is a 101- foot high, 538-foot long (including the spillway) composite earth embankment and stone masonry structure with a masonry core wall. The dam maintains 1.28 billion gallons of water at normal pool for serving 75,000 customers in 17 municipalities. Operational and regulatory issues identified at the dam led PAWC to complete a comprehensive rehabilitation of the structure. During the rehabilitation work, a depression formed on the upstream slope of the dam. PAWC, its design consultant Gannett Fleming, and specialty geotechnical contractor Moretrench worked closely together to respond to the situation and develop appropriate remediation measures to restore the stability of the dam. PLANNED REHABILITATION PROGRAM Pennsylvania American Water initiated remediation in April 2011 to address seepage, stability, and spillway capacity issues that were noted at the dam. Drainage systems, including relief wells, a toe drain, blanket drain, and several lateral drains, were planned to safely collect and convey seepage from the structure because the dam lacked these internal drainage features in the initial construction. The masonry section of the dam was being buttressed with RCC (Roller Compacted Concrete) and rock anchors were installed to improve the stability of the structure. Spillway capacity was being increased by armoring the earth embankment with RCC to prevent failure of the dam during extreme flood events that will result in embankment overtopping. GEOTECHNICAL INVESTIGATION Design of the remediation at Nesbitt Dam included collection of geotechnical data. Since subsurface data and functioning piezometers did not exist prior to the design engineer s work on this project, a thorough subsurface exploration program was completed. The subsurface exploration program included obtaining soil and rock samples to characterize the subsurface stratigraphy and installation of nested piezometers throughout the site to characterize pore pressures. During the subsurface exploration program, a concentrated seep just downstream of the non-overflow section and artesian conditions downstream of the dam in the vicinity of the spillway training wall extension were encountered. The presence of artesian conditions coupled with operational requirements [maximum 3 feet lowering of reservoir pool during rehabilitation] required the temporary dewatering of the dam embankment and foundation strata to permit installation of the drainage features. Four passive relief wells were detailed by the design engineer to provide adequate filtering of the artesian head. ISSN Association of State Dam Safety Officials THE JOURNAL OF DAM SAFETY VOLUME 14 ISSUE

3 CONSTRUCTION/DEPRESSION OBSERVED Construction was proceeding as planned until the morning of February 23, 2012, when a depression on the upstream slope of the dam was identified during the daily site walk. The depression was located above the pool elevation from Station 3+52 to 3+64 (Figure 1). The collapse of the depression was sudden, and although seepage had been noted throughout the history of the dam, evidence of sediment-laden seepage was not noted during past dam operation. Field measurements conducted by Gannett Fleming indicated the depression was 12 feet long, 8 feet wide, by 3 feet deep. Figure 2 shows a picture of the depression soon after it was discovered. Figure 1. Location of Depression 10 THE JOURNAL OF DAM SAFETY VOLUME 14 ISSUE ISSN Association of State Dam Safety Officials

4 Mobilization of a geophysical specialist to investigate seepage paths on the upstream and downstream embankment slopes. Test pit excavation in and near the depression. Backfill details and technique development. Subsurface exploration to determine the extent of embankment disturbance (Figure 3). Figure 2. Depression, February 24, 2012 The site staff immediately notified the owner and engineering design staff. This led to a series of rapid response measures which were executed in the following 48 hours: Lowering of the reservoir pool by the owner, Pennsylvania American Water, as the pool level had been only nominally lowered for construction activities. Site meeting with the owner, the project design engineer, the general contractor, and Pennsylvania Department of Environmental Protection (DEP). Internal drainage features were non-existent, and wet and saturated areas were clearly present downstream of the toe of the dam in its pre-rehabilitation state. Since seepage was evident, the rehabilitation included the installation of modern drains as part of the construction to properly collect and convey seepage from the structure. Figure 4 shows a cross section of the proposed drainage system for the dam. The subsurface exploration following depression identification yielded internal erosion as the likely cause. The engineer was confident that the proposed drainage system would address future failure modes associated with internal erosion and unfiltered seepage from the dam, which likely played a part in the development of the depression. Therefore, Gannett Fleming developed an initial remediation plan to mitigate further internal erosion during construction, and to locate and repair any defects. The scope of work included dewatering and grouting. Several pre-qualified companies were invited to bid on the work. Figure 3. Plan view of depression (black box) with location of postdepression borings (B-300 series). ISSN Association of State Dam Safety Officials THE JOURNAL OF DAM SAFETY VOLUME 14 ISSUE

5 Figure 4. Cross-section of the proposed drainage system for the dam. The initial work plan was geared towards locating and remediating the identified areas of concern and included a preliminary scope of work as well as expected materials and equipment to be used. It was made clear in the work plan that the scope of work could change as additional information was gathered and analyzed during implementation. During the development of the remediation plan, it was determined that there were three different probable flow paths that may have led to the depression: through the core wall, under the core wall, or around the core wall. flow was very closely monitored, and a penetration rate was used that permitted the cuttings to be easily lifted to the surface without collaring inside the casing. The drill spoils return was directed to a holding container by means of a discharge swivel so that an accurate accounting of water lost during drilling could be tracked. The drilling water was then recirculated through the drill tools and down the hole by pumping methods and fresh water was added as necessary. Figure 5 shows locations of dewatering wells and piezometer sensing zones. DEWATERING PROGRAM A series of dewatering wells (dd1-dd5 shown in Figure 3) was installed in the upstream embankment to control the gradient through the embankment dam. The upstream well submersible pumps were all located so as to provide a flat gradient through the earth embankment and core wall regardless of pool elevation. This was the first step in restoring dam stability, i.e., equalizing gradient upstream to downstream to prevent further migration of material. Installation of the dewatering wells was performed utilizing rotary duplex drilling with internal water flush. To minimize the potential for hydraulic fracturing, the bit was kept even with or inside of the casing at all times while drilling, the return Figure 6. Cumulative plot of drill water loss with depth (dewatering wells DW = dd from Figure 5) Figure 5. Section through dam, looking upstream. dd = dewatering well locations 12 THE JOURNAL OF DAM SAFETY VOLUME 14 ISSUE ISSN Association of State Dam Safety Officials

6 Figure 6 shows the tracking of cumulative drill water loss with depth of the borehole. Each well was properly developed prior to the installation of a submersible pump and discharge piping. The well heads were constructed significantly above normal pool elevation to prevent inundation of the wells during a high water event. Following the pump installation, each well was tested for flow rate and to verify that the designed filter pack prevented the migration of soil particles. The individual flow rates produced by each well varied from 0.1 to 3.0 gallons per minute. Throughout the process, a daily emergency monitoring program was enacted that consisted of the following: Measurement of water levels in upstream wells every morning and evening. Activation of upstream pumps when any well measured El feet or above. Monitoring of upstream well dd1 (which was not pumped) during pumping of the remaining wells, to confirm target upstream drawdown to El feet. Upstream and downstream piezometers were also used to confirm drawdown. Continuation of pumping until the source of gradient was removed or corrected. 24-hour site monitoring if upstream pumps were not effective at creating drawdown within four hours of activation. The monitoring actions discussed above were completed concurrent with commencement of work operations by Moretrench, the selected specialty contractor. Observations and data were continuously gathered during the treatment and remediation process. This required daily cooperation, coordination, and conference call discussions among project team members, i.e., owner, design consultant, regulator, and specialty contractor. The dewatering wells were effective and exhibited the capability to control gradients as illustrated by piezometric readings taken prior to and after upstream pumps were activated on May 16, 2012 (Figure 7). PRE-GROUTING PUMP TEST Prior to drilling any grout holes, the specialty contractor owner design consultant team decided to perform a pump test utilizing three previously installed dewatering wells on the downstream side of the cut stone and mortared core wall. While these three wells were pumped, the upstream devices would be monitored for water level change as a means to identify areas to target with the grouting program. Unfortunately, the pump test and associated data gathering had to be halted prior to reaching the intended length due to rain events consistently raising the pool elevation, leading the team to activate the upstream dewatering wells to limit the groundwater gradient across the core wall. CORE WALL DRILLING In order to keep the remediation program moving forward, drilling through the masonry core wall was initiated. In an effort to locate features within the core wall that could permit internal erosion, a series of six primary drilling locations (Figure 9) was planned with holes drilled at 30 degrees from vertical. Drilling for this work was accomplished with the use of a water-actuated down-the-hole hammer (Figure 8). Great care was taken to ensure the proper alignment of the borehole. The first step in the alignment process was to core drill an eight-inch diameter by three-foot-long pilot hole through a reinforced concrete cap that was added to the masonry core wall during the rehabilitation work. Once the coring operation was completed, a steel pipe sleeve was set, properly aligned with survey equipment, and grouted in place. When the drill rig was positioned over the borehole, survey equipment was utilized to ensure the proper alignment was achieved. Alignment of the drill rig was frequently checked throughout the drilling operation to verify the equipment was correctly positioned. Figure 7. Piezometer response to upstream dewatering well pumping ISSN Association of State Dam Safety Officials THE JOURNAL OF DAM SAFETY VOLUME 14 ISSUE

7 Figure 9. Section view showing the zone of highly disturbed material In addition to the care taken to ensure proper alignment of the borehole, the design of the water hammer aided greatly in producing straight boreholes. The water hammer has stabilizing ribs along the hammer case that provide a tight spacing between the ribs and borehole wall, maintaining the hammer in a centralized position within the borehole. Drilling was terminated within five feet of the bottom of the core wall at each location. Following the initial drilling, the boreholes were thoroughly washed and packer testing was completed. One of the initial observations from drilling through the core wall was that a noticeable loss of water return did not occur. This observation was proven with traditional packer testing methods for each borehole. The results showed that there were only minimal defects of the core wall in two of the boreholes and none of the defects seemed significant enough to permit piping paths that would mobilize soil. A borehole camera was on site to perform video inspection of the drilled holes but with minimal defects detected it was determined to forego performing the video inspection. After determining that the core wall was in acceptable condition and that the existence of significant flow paths through the wall was unlikely, the boreholes were advanced through the bottom of the core wall to investigate for flow paths that may have existed below the core wall. At this time, a zone of very soft, loose soil was discovered in the glacial till directly below the core wall, indicating a potential flow path under the wall. Two secondary drilling locations were Figure 8. Drilling of a primary angled grout hole location through the core wall Figure 10. Potential path of soil migration 14 THE JOURNAL OF DAM SAFETY VOLUME 14 ISSUE ISSN Association of State Dam Safety Officials

8 added to establish the limits of the disturbed material from Station 3+55 to Station 3+85 (Figure 9). The thickness of the disturbance was approximately 20 feet at a depth of approximately 70 to 90 feet below the existing ground surface. While drilling was underway in this disturbed location, immediate communication with the upstream dewatering wells was noted, as evidenced by discoloration of the discharge water. With the dewatering wells being located near the depression, this communication suggested that the depression and the soft, loose soils were connected (Figure 10). COMPACTION GROUTING The initial work plan did not anticipate locating a highly disturbed pocket of soil such as this and therefore had to be modified in real time by the parties involved. After careful review of the information and several discussions about the best way to proceed, a compaction grouting program was developed to treat the disturbed foundation soils, and core wall drilling and grouting was temporarily halted in order to focus on the foundation soils. It should be noted that an open dialogue and sharing of information by the project team had been taking place on a consistent basis. Therefore, when it became necessary to adjust the plan, the adjustments were able to be made smoothly and very rapidly. The specialty contractor owner design consultant team made the determination to locate the initial compaction grout holes on the upstream side of the core wall, offset sufficiently to avoid the core wall footing and to a target EL 1060 or five feet below the disturbed zone, whichever was greater. Compaction grouting equipment with upstream dewatering wells in the foreground is shown in Figure 11. After grouting of the primary compaction holes at Stations 3+62 and 3+80 was complete, two secondary compaction holes were drilled and grouted at Stations 3+72 and Two tertiary compaction holes were added at 3+66 and This completed the compaction grouting on the upstream side. On the downstream side, compaction grouting was performed at Stations 3+60, 3+68 and Including the approximately 0.8 cubic yards required to fill the borehole, compaction grout takes per hole varied from 4.5 to 60.3 cubic yards. The compaction grout placement is illustrated in Figure 12. Figure 11. View of compaction grouting equipment with upstream dewatering wells in the foreground ISSN Association of State Dam Safety Officials Figure 12. Compaction grout placement During the compaction grouting process, the existing instrumentation and other devices were constantly monitored for changes. Discharges from the dewatering wells and the downstream drains were monitored for turbidity and changes in flow rate. Piezometers close to the treatment area were monitored to evaluate a change in the water level and to identify pore pressure increases. After combining all of this information, it was possible to verify the selected parameters such as maximum grout pressure and pumping rate or to adjust the parameters in near real-time based on the responses observed. After the compaction grouting program was completed, the original primary grout holes drilled through the masonry core wall were washed and prepared for grouting with high mobility cement-based grouts. The grouting was performed in multiple stages using an inflatable packer within the core wall to isolate the borehole in stages from the bottom up. Six separate grout mixes were developed and tested to offer a variety of viscosities and penetrability. Marsh funnel testing of the different grout mixes produced flow times between 40 and 60 seconds. Components of the different mixes included the following materials: water, bentonite, Portland cement, micro-fine cement, whalen gum, and super plasticizer. Grouting of the boreholes with high mobility grouts was effective at treating any remaining highly disturbed soils at the base of the core wall without exceedingly high volumes being experienced, indicating that hydraulic fracturing had not occurred during the grouting program. Grout volumes and the number of stages utilized varied from 2 to 604 gallons and one to six stages respectively. The design team observed that a significant majority of the grout take occurred during grouting of the bottom stage of boreholes that fully penetrated the core wall. The high grout take was likely due to infilling of the small crevices at the base of THE JOURNAL OF DAM SAFETY VOLUME 14 ISSUE

9 the core wall that were not treated by the low mobility compaction grouting operation. Borehole P2 was the only location that had a considerable grout take within the core wall, which corresponded to the results from the packer testing. Neat grout takes in the boreholes ranged from 46 to 101 gallons. Following treatment of the disturbed foundation soils with low mobility compaction grout and grouting through the core wall with high mobility cement-based grout, final verification of the remediation program was executed. Vertical verification holes were installed from Stations 3+60 to 3+80 on 5-foot centers. These verification holes were advanced to an average depth of 115 feet, i.e., Figure 13. Permeation grout limits EL Upon reaching final depth and recording all pertinent drilling observations, a sleeve port pipe was installed within the borehole and a sodium silicate permeation grouting program was initiated. The intent of this was to treat any open-graded granular material that was present from the bottom of the core wall to below the disturbed depth. Minimal volumes of sodium silicate grout were able to be injected into the target zone and the remediation was considered complete. Limits of the permeation grout program are shown in Figure 13. The dam following remediation of the depression and construction rehabilitation to address operational and regulatory issues is shown in Figure 14. CONCLUSIONS The process for significant adjustments to the depression remediation work plan, and the unanticipated work in general, was executed exceptionally well for this project. Each of the five parties involved (owner, design engineer, contractor, specialty contractor, and regulator) was able to recognize where their contribution was the most needed and therefore most valuable. A highly technically competent and engaged owner was able to rapidly process the information presented and provide necessary decision making to keep the remediation work moving forward. The design consultant was able to gather and analyze information from prior to the first spotting of the depression through the entire remediation effort. This enabled the design engineer to provide key insight into the cause, the repair, and how both of these affected the overall structure. Multiple grouting techniques were instrumental in addressing the unanticipated loose soil condition revealed during the core wall drilling. The specialty geotechnical contractor understood the nature of the unanticipated work, was able to provide technical information when developing solutions, and could quickly adapt to changes in the work plan and mobilize the newly required equipment. The compaction grouting program to stabilize the pocket of loose soil, followed by permeation grouting to address any residual loose material, was constantly monitored and evaluated. Close cooperation and communication between project team members throughout the process was a critical factor in swift resolution of the problem. Lastly, the general contractor was keenly aware of its capabilities and provided the services in which it was proficient. This allowed the flow of information to take place in an efficient manner directly between owner, specialty contractor, and design consultant. Figure 14. Completed rehabilitation aerial view, October 2012 Regularly scheduled conference calls kept each of the parties apprised of the most current information, served as an opportunity to discuss and adjust the work plan as needed, and permitted each group to be aware of the next step and their associated responsibility. Despite the unanticipated nature of the work and the increased effort to analyze, interpret, and react on a nearly constant basis, the project team was able to execute the depression remediation as seamlessly as if it had been traditional specified work with design drawings. The success of the program was founded in open communication by all team members. Additionally, the rehabilitated Nesbitt Dam has been in continuous operation with full pool since depression remediation, with no evidence of internal erosion failure modes. 16 THE JOURNAL OF DAM SAFETY VOLUME 14 ISSUE ISSN Association of State Dam Safety Officials

10 Cari R. Beenenga, P.E. Gannett Fleming, Inc. P.O. Box Harrisburg, PA Anthony M. Nokovich, P.E. Pennsylvania American Water Company 852 Wesley Drive Mechanicsburg, PA Cari Beenenga is a principal geotechnical engineer with Gannett Fleming, Inc., leading the company s design and rehabilitation efforts to address geotechnical concerns for projects related to earth embankment dams, concrete and masonry gravity dams, roller compacted concrete (RCC) dams, levees, and other flood risk reduction features. Her responsibilities include the preparation of proposals; directing site reconnaissance and subsurface investigations; preparing geotechnical and foundation engineering reports; performing geotechnical analyses and design, geotechnical instrumentation design and monitoring; preparing construction contract provisions, plans and specifications; and providing construction inspection services and oversight. Ms. Beenenga is a graduate of Pennsylvania State University and a registered Professional Engineer in Pennsylvania, Virginia, and West Virginia. Anthony Nokovich is the engineering practice lead responsible for the dam safety program for Pennsylvania American Water Company (PAW), the largest investor-owned water utility in the state, with responsibility for 56 dams and reservoirs, 30 of which are classified as high hazard by virtue of size and proximity to downstream population centers. His responsibilities include review of design and construction procedures for PAW-owned dams and their appurtenances, preparing and updating emergency action plans, and coordinating dam safety related actions with state and federal agencies. Mr. Nokovich received a B.S. degree in civil engineering from the Pennsylvania State University in 1995, and is a registered Professional Engineer in Pennsylvania. James C. Myers, P.E. Moretrench 100 Stickle Avenue Rockaway, NJ jmyers@moretrench.com James Myers is a project manager with specialty geotechnical contractor Moretrench s Grouting and Groundwater Control Division. He has more than 10 years of experience in a range of specialty grouting techniques for challenging and complex subsurface conditions. These include low mobility (compaction) grouting, permeation grouting using chemical grouts and microfine cement grouts, jet grouting, fracture grouting, void fill grouting, and urethane grouting. His primary responsibilities include design, cost estimating and budget oversight, submittal preparation, project implementation, and overall management of the project team. Mr. Myers holds a Bachelor of Engineering degree from the Stevens Institute of Technology and is a registered Professional Engineer in New Jersey. Dam Monitoring and Safety Instrumentation RST Instruments Ltd. is a world leader in the design, manufacturing and sale of innovative dam monitoring and safety instrumentation. Since 1977, our customers have relied on our reliability & accuracy to help them make sound decisions to: Manage Risks Improve Safety Optimize Design Reduce Costs Increase Productivity CANADA: SALES + SERVICE + MANUFACTURING sales@rstinstruments.com TOLL FREE (USA & Canada) USA: SALES Etienne Constable econstable@rstinstruments.com Jeff Keller jkeller@rstinstruments.com EUROPE / MIDDLE EAST / AFRICA: SALES Grant Taylor gtaylor@rstinstruments.com MIG0235C ISSN Association of State Dam Safety Officials THE JOURNAL OF DAM SAFETY VOLUME 14 ISSUE