Projects
Swinging Bridge Dam (P-10482) Dam Safety Remediation Project
Project Rehabilitation Status - Previous Actions
Rehabilitation of the Swinging Bridge Dam is a complex process that was essentially completed Mid July 2007. The reservoir refilling is underway.
The complete remediation of Swinging Bridge that will restore the dam to all current dam safety standards has been divided into 2 phases of construction activities. In addition, several construction activities under Phase I were identified as emergency work to ensure the safety of the dam to handle the spring 2006 flood season. The Phase I/Emergency activities were completed on March 15, 2006. Phase II construction activities are the remaining repairs that will complete the necessary work.
Phase I
- Sinkhole: Repair of the sinkhole at the dam crest and of a number of depressions on the upstream slope was completed on January 13, 2006.
- Interim Repairs: Welding of cracked joints in the penstock steel liner and sealing of cracks in the concrete tunnel by chemical grouting was completed on January 14, 2006.
- Grouting of voids around the penstock and tunnel: Contact grouting of voids around the diversion tunnel was completed around March 10, 2006 under Phase I/Emergency. Contact grouting around the penstock was performed under Phase II, see below. Twenty one additional piezometers were installed from inside of the tunnel in the week of February 13, 2006. However, the pumps were stopped once the refilling was initiated on February 2, 2007.
- Dewatering Wells: Six radial jacked drains were completed from inside the tunnel by February 14, 2006. Drilling for twelve dewatering wells at the toe of the dam was completed March 15, 2006. The wells are all functional and pumping.
- Removal of Spillway Gates: The spillway gates were removed for the purpose of being able to pass more flow of water in the event of a major flood. The gates have been re-installed but remain open as the repairs to the dam are essentially completed but close instrumentation monitoring continue while the reservoir is re-filling at a controlled rate.
- Continuing Investigation: General subsurface investigation consisting of drilling 18 borings through the dam was completed in mid-November 2005. In addition, 13 Cone Penetration Tests (CPT) were started on February 20 and to date have been completed. Some CPTs were also performed from the tunnel floor.
Phase I was completed with the contact grouting from the penstock. As our knowledge of the dam and understanding of its behavior improved during the Phase I work, additional investigations and rehabilitation work will be performed as follows:
Phase II
- Cone Penetration Tests (CPT) from the tunnel were completed. Additional investigation by Low Mobility Grouting (LMG) tests were performed in October 2006. Installation of a filter around the conduit and along the toe, constructed to intercept any seepage going through the dam. Work on the filter began the first week of August 2006 and was completed Mid-November 2006. The work was done in a sheetpile enclosure on each side of the conduit. Placement of the filter material for the west side and removal of the sheetpiles was completed on October 31, 2006. Placement and completion of the East conduit filter material followed completion of the West conduit filter.
- As the crest CPT data do not support the need for grouting the core, the consultant recommended that LMG in the dam from the crest of the dam was not needed. This work was not done.
- Grouting of voids around the penstock. Contact grouting from the penstock started on July 17, 2006 and was practically completed on August 8, 2006.
- Conduit Filter - The East and West conduit filter, constructed to intercept any seepage going through the dam, has been completed. Work on the filter began the first week of August 2006 and was completed Mid-November 2006.
- Embankment Toe Filter and Blanket Filters - The east toe filter was completed in late December 2006 and work on the west toe filter was completed in late January 2007.
- Crest Seepage and Crest Wave Protection - Crest wave protection is being provided by adding a rock parapet on the upstream side of the crest road. This work was completed in late January, 2007. Crest seepage will be evaluated during the reservoir refilling operation.
- A viability assessment of repairing the penstock and tunnel, or filling one or both with concrete was made in late 2006. The former owner, Mirant, made the decision to fill the penstock and tunnel with volumetric fill similar to that of weak and easily excavated concrete. Final plans with specifications for this work item were submitted in February and March , 2007. As of April 12, 2007 authorization has been given to begin infilling the penstock/tunnel. This work has been completed by mid July 2007.
- Revised hydrology and hydraulics (H&H) analysis studies of all the projects on the Mongaup River. These studies were completed with the June 5, 2006 supplement.
In parallel, Mirant updated the flood studies and distributed the potential dam failure inundation maps and notification list under the Emergency Action Plan (EAP). An EAP Tabletop Exercise (TTE) was performed on July 8, 2005. A second TTE to solicit further comments from the downstream Emergency Management Agencies (EMA) occurred on February 22, 2006. An EAP Functional Exercise designed to simulate a real-time flooding scenario produced by a potential dam failure, and to simulate the necessary downstream evacuation measures, was conducted on March 23, 2006.
As a result of changes made to the EAP, an educational meeting and TTE was held on September 28, 2006.
Another functional exercise took place in November 2006.
Board of Consultants Meetings
The first Board of Consultant's (BOC) meeting took place on June 2, 2005. Mirant and its design engineer presented to the BOC and FERC staff a review of the dam safety incident and the initial plans for investigation and remediation.
The second BOC meeting was conducted on June 16 and focused on the investigation plan to assess the structural integrity of the dam and support the planning and design of the necessary remediation to the dam. Mirant presented to the BOC and FERC preliminary alternatives for the final comprehensive repairs to the Swinging Bridge Dam. An interim telephone conference took place on July 6, 2005 to update the BOC and FERC on the status of the field investigation and the design efforts.
The third BOC meeting occurred on July 29-30, 2005 and focused on the design and planning of a number of repairs and improvements to the current structural integrity of the dam necessary to restore a safe dam. Extensive discussions involved the rehabilitation plans that include a conduit filter, toe filter, conduit grouting, crest grouting, crest rehabilitation, and penstock rehabilitation.
The fourth BOC meeting took place on August 17-18, 2005. Issues such as permanently sealing up the penstock and access tunnel versus repaired were discussed. A limited braced excavation construction method for the conduit filter was agreed upon, rather then an open excavation. The conduit filter will be connected with a toe filter which will run east and west along the toe of the dam. A Potential Failure Mode Analysis (PFMA) was scheduled to identify and discuss any potential failure scenarios, to make sure they are all addressed by the rehabilitation process.
The fifth BOC meeting was conducted on September 9 and 10, 2005 and focused on remediation of the embankment crest at the existing sinkhole, the downstream conduit filter drain, and on the conduit grouting program.
The sixth BOC meeting was conducted on October 20 and 21, 2005 and focused on remediation of the embankment crest at the existing sinkhole, the downstream conduit filter drain, and on the conduit grouting program, and on the crest grouting program.
The seventh BOC meeting was conducted on November 17 and 18,2005 in Matamoras, PA and focused on remediation of the embankment crest at the existing sinkhole, the downstream conduit filter and drains, and on the conduit grouting program.
The eighth BOC meeting was conducted on December 15 and 16, 2005 in Matamoras, PA and focused on remediation of the embankment crest at the existing sinkhole, the downstream conduit filter drain, and on the conduit grouting program.
The ninth Board of Consultants (BOC) meeting was conducted on February 2, 2006 in Matamoras, PA and focused on the Emergency Response Action Program that includes a dewatering scheme, downstream conduit filter drains, and a conduit contact grouting program. The tenth BOC meeting is scheduled for March 3, 2006.
The tenth BOC meeting was conducted on March 3, 2006 at the project site and also at the Best Western in Matamoras, PA. This BOC meeting discussed the alternate remediation concept of adding a filter drainage system and berm to the entire downstream face of the dam. The eleventh BOC meeting is scheduled for April 10, 2006.
The eleventh BOC meeting was held on April 10, 2006 in Matamoras, PA. Findings of the investigations and tunnel contact grouting were discussed. Preliminary documents related to the alternate remediation concept (see tenth BOC meeting) were presented and discussed.
The twelfth BOC meeting was held on June 2, 2006 at the FERC Headquarters in Washington, DC. It was found that the filter berm solution is more expensive and takes longer to complete than the conduit filter and toe drain. Discussions covered the conduit filter, penstock remediation options, lake re-fill program, and project schedule.
The thirteenth BOC meeting was held on July 18, 2006 at the dam site and at the Best Western in Matamoras, PA. The assessment and methods of analysis of the penstock were discussed in details. Discussions also covered the conduit filter and toe drain schedule; the status of contact grouting from the penstock; and the results of the CPTs under the tunnel floor.
A short meeting was held by the BOC on August 26, 2006 at the dam site to discuss the status of the investigation of the Unit No. 1 penstock. This meeting has been designated as the fourteenth BOC meeting.
The fifteenth BOC meeting was held on September 6, 2006 at the dam site and at the Best Western in Matamoras, PA. The meeting discussed the status of the conduit filter, toe drain construction, contact grouting from the penstock and proposals of what would happen to penstock No. 1 upon completion of all other work.
The sixteenth BOC meeting was held on October 10, 2006 at the dam site and at the Best Western in Matamoras, PA. The meeting discussed the status of the conduit filter, toe drain construction, contact grouting from the penstock and proposals of what would happen to penstock No. 1 upon completion of all other work.
The seventeenth BOC meeting was held on November 20, 2006 at the FERC Headquarters office in Washington D.C. The meeting discussed the status of the conduit filter, toe drain construction, reservoir refill plan, intake tower bridge and Mirant's proposals of filling the penstock and conduit with volumetric fill material.
The eighteenth BOC meeting was held on January 9, 2007 at the dam site and at the Hampton Inn in Middletown, NY. The meeting discussed the completion of the conduit filter, completion of the east toe drain and the construction status of the west toe drain, reservoir refill plan and Mirant’s plan of filling the penstock and conduit with volumetric fill material.
The nineteenth BOC meeting was held on March 2, 2007 at the FERC Headquarters office in Washington D.C. The meeting discussed the plans and specs for filling the penstock and refilling the reservoir.
The twentieth BOC meeting was held on April 12, 2007 at the Best Western in Matamoras, PA. Final review of the infilling of the penstock/tunnel and of the instrumentation data available from the 1045 hold point. BOC approval and FERC authorization was given to begin the in-filling of the penstock/tunnel and to advance the reservoir filling to the 1050 hold point.
The twenty-first BOC meeting was held on May 8, 2007 at the Best Western Hotel in Matamores, PA following an early morning site visit. This meeting included the new owner, Alliance Energy Renewables, and how they would transition to take over the projects. Also, this meeting discussed the structural plug in the penstock and proposals by the new owner to speed up the remediation schedule.
With the work essentially completed and monitoring of the embankment during refilling underway, no BOC meetings are currently scheduled.
Investigation Program
A subsurface investigation test boring program was conducted for the purpose of evaluating the extent of the damage to the dam, and to provide information for the design of repairs. Piezometers used to measure the water pressures within the dam, and the foundations soils beneath the dam, have been installed in each of the completed bore holes.
The investigation commenced on the weekend of June 18, 2005. As of August 31, 2005 all ten original borings as well as five other shallow toe borings had been completed. The borings along the downstream toe of the dam ranged from 25 feet to 110 feet in depth. The borings on the mid and upper slope of the dam range from 150 feet to 170 feet in depth.
Additional borings were completed on the crest of the dam to determine the strength of the core soils along with other borings needed to obtain very specific information.
Since the crest repair work was completed, more detailed geotechnical investigations, such as cone penetrations tests (CPT's) have been performed in the immediate work area of the crest to evaluate the quality of construction and the condition of the upper part of the dam. As the crest CPT data did not support the need for grouting the core, the consultant recommended that this work was not needed. The CPTs performed through the tunnel floor have been completed and the results are under review for final design considerations.
Remediation and Repairs
During the Board of Consultants (BOC) meeting on July 29, 2005, Mirant's consulting engineering firm, DTA Engineers of New York, Inc. (DTA), discussed the plans and specifications submitted on July 12, 2005 for the proposed interim repairs to the penstock and sinkhole, and presented preliminary plans for the final remedial repairs to the dam.
The permanent repair measures that were proposed by the consultant included repairing the sinkholes and the cracks in the crest; installing a conduit filter and toe filter; a program of grouting from inside the tunnel and penstock, and from the crest; filling the tunnel with concrete; and filling or repairing the penstock. To date the crest repair has been completed.
The interim repair work on the penstock (the pipe that passes through the embankment dam) and the access tunnel which is below the penstock (the access tunnel was constructed to divert the stream flow during original construction of the dam) consisted of repairing leaks in the steel penstock and sealing cracks in the concrete access tunnel so that the penstock could be used in the event of a severe flood. The plans and specifications for the interim repairs to the penstock and access tunnel through the dam were approved and this work commenced September 19, 2005. This work was completed by March 15, 2006.
To accelerate the construction schedule the BOC recommended that DTA perform the reconstruction of the dam crest, in lieu of the proposed interim protective measures to the sinkhole, with the understanding that some additional crest repair work may be needed after the core remediation. This was implemented and the crest repairs have been completed. More detailed geotechnical investigations, such as cone penetrations tests (CPT's) have been performed in the immediate work area to evaluate the quality of construction.
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Project Rehabilitation Status Archives
January 2007
November 2006
June 2006
May 2006
March 2006
February 2006
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