By John A. Cima and Brian R. Reinicker
Roanoke Rapids Dam, located on the Roanoke River near Roanoke Rapids, N.C., is owned by Dominion Generation. The dam structures are a component of the 95-MW Roanoke Rapids Hydroelectric Project, a power generation facility regulated by the Federal Energy Regulatory Commission. The dam, completed in 1955, is classified as “high hazard potential” under FERC guidelines by virtue of potential loss of life and extensive property damage downstream should a failure occur.
When routine monitoring and inspection of the structure revealed that deformation was occurring over time, Dominion Generation and its partners found that concrete expansion caused by alkali- silica reactivity (ASR) was to blame.
Discovering the problem
The dam structure includes a gravity south non-overflow section (SNOS), service bay and powerhouse section, interior gravity non-overflow section, gated spillway and emergency spillway (ES) sections, and north non-overflow section.
Regular instrumentation monitoring and inspections of Roanoke Rapids Dam indicated the SNOS was experiencing some changes over time. Deformation monuments, first installed in 1985, had by 1995 indicated a pattern of downstream movement relative to other dam sections, prompting the installation of an inclinometer in 2000. Foundation piezometers showed a gradual increase, and seepage from the south abutment and entrance gallery showed a significant increase by the end of 2005.
The downstream deformation amounted to up to 1.5 inches at Monument No. 3, while seepage quantities increased from a few gallons per minute (gpm) to more than 100 gpm between 2005 and 2008. Meanwhile, foundation piezometers in the SNOS showed a gradual increase of a couple feet from 1995 to 2005.
|Longitudinal expansions caused by alkali-silica reactivity is to blame for increased leakage in the entrance gallery of Block S6 at Roanoke Rapids Dam.|
A two-phased investigation was performed by Gannett Fleming Inc. between 2006 and 2008 that included concrete coring and testing, down-hole imaging, underwater inspection, and installation of additional instrumentation – including additional piezometers, inclinometers and stress cells – to help identify and evaluate the observed progression of leakage, uplift pressure, concrete stresses and deformation. The results of the investigations led to the conclusion that the dam is undergoing concrete expansion due to ASR, most notable along the dam axis. Alkali in the cement reacts with silica in the concrete aggregate to form a gel that swells in the presence of water.
Downstream deformation and opening (as much as 2 inches) of a lift joint occurred in Blocks S3 through S8 of the SNOS as a result of the longitudinal expansion generated by ASR. Less severe cracking occurred along lift joints in Blocks 26 and 27 of the ES, adjacent to the north end of the gated spillway section. Analyses performed during the studies suggested that the ASR-induced expansion and associated deformations decreased the stability of the cracked SNOS and ES blocks at elevated reservoir levels.
Because these ASR-induced stresses were likely to continue to increase, Dominion retained Devine Tarbell & Associates (now HDR) in 2008 to design remedial measures to improve partial block stability of the SNOS and ES structures.
A 2008-2009 design evaluation included assessment of the condition of SNOS Blocks S3 through S9 and ES Blocks 26 and 27, as well as a reevaluation of the stability of these blocks in accordance with FERC’s Engineering Guidelines for the Evaluation of Hydropower Projects. It was concluded that the critical loading case controlling stability of the affected dam blocks is the probable maximum flood (PMF) condition and deep, multi-strand rock anchors were designed to stabilize these blocks to meet FERC factor of safety criteria.
Due to a concern that partial block movement could create additional damage during anchor installation and loading, grouting of the ASR cracking was the first step in the remediation design. A three-stage upstream seal was designed to be installed underwater along the upstream cracks that would allow a series of injection and relief holes to be drilled from the top of the dam to introduce and track travel of grout in each monolith, as well as to monitor grout pressures. The anchors were designed to be installed after crack grouting and extended from the crest of the dam into the bedrock foundation. At the SNOS (see Figure 1), the anchors had to be inclined and installed at a precise angle to miss the gallery and remain inside the base of the dam.
|Dowel anchors installed into Blocks S6 and S7 extend through the structure and into underlying bedrock.|
The remedial design featured several unique aspects to accommodate future ASR-induced loading and deformation over the life of the anchors:
– Greased and sheathed anchor tendons allow anchor stresses to adjust as additional ASR-induced concrete growth occurs;
– Anchors initially loaded to only 50% of guaranteed ultimate tensile strength (GUTS) to accommodate future expansion without overstressing the anchors;
– Instrumented tendons in five anchors monitor changes in anchor loads; and
– Re-stressable anchor heads with shims allow for future de-stressing, if necessary.
Crack grouting and anchor construction
A total of 33 30-strand post-tensioned rock anchors were installed by Brayman Construction Co. in Roanoke Rapids Dam in early 2010. They extended through the structure and into the underlying bedrock foundation. They were designed to improve the stability of the structure above the cracking and to accommodate future ASR growth.
During the crack grouting and anchor construction, the extent of cracking within the SNOS was found to be more extensive than had previously been understood. While grouting horizontal cracks in Block S5, grout was observed exiting the toe of the dam in Block S7. Additional probing of the downstream face revealed that the horizontal cracks joined and turned downward to follow the inclined face of the dam.
A reassessment of stability mechanisms was made and it was determined that the multi-block section of the SNOS where the inclined crack existed (Blocks S5 through S7) was globally stable without fear of failure. Temporary reinforcement using dowel anchors across the inclined cracked section allowed grouting and anchor construction to continue. The remediation construction was completed in May 2010.
Managing concrete growth
The crack grouting and anchor installation at Roanoke Rapids Dam improved the stability of the affected SNOS and ES blocks to comply with FERC requirements. However, ASR growth will continue to affect the structure and its stress and deformation status must be continually observed and evaluated going forward. The piezometric, deformation, stress, and anchor loading instrumentation, as well as monthly surveys, continue and the data is reviewed and analyzed to look for any changes in historical trends. Bi-annual and annual updates are presented to FERC to keep it informed on the current status of the dam.
|Anchors – designed for installation into the dam’s south non-overflow section (SNOS) – extend from the crest of the dam into the bedrock foundation.|
To more completely understand the historical behavior of the dam due to ASR and to provide a means to be able to predict the future response of the modified structure, a state-of-the-art finite element model (FEM) was developed by HDR, with assistance from Dr. Robin Charlwood, an internationally recognized expert in concrete dams and, in particular, those with ASR issues. The model development took place in three stages from 2010 through 2012:
– Stage I – Full dam global model developed and calibrated for the pre- anchored condition;
– Stage II – Detailed modeling and more refined calibration of the SNOS and ES, with anchors and cracking locations added; and
– Stage III – Update the model to account for observed changes in instrumentation after post-tensioned anchor installation.
Going forward, this model will be utilized to help Dominion determine if and when any additional remedial measures (typically slot cutting to relieve internal stresses) will be required for the dam.
The purpose of the Stage I model was to: calibrate the material behavior and geometric model to approximate visual observations and historical instrumentation data and to be consistent with the documented cracking in the SNOS; provide the framework and boundary conditions for the more detailed model of the SNOS to be developed in Stage II; and provide a general assessment of other potential ASR effects on the overall structure, such as deformation of the powerhouse and possible effects on generating equipment.
This 3D model was developed to include the major geometric features of the entire dam structure but did not include galleries or other powerhouse openings. The Stage I model also incorporated an advanced ASR expansion and material behavior component developed by HDR and Dr. Charlwood. The advanced FEM model included several important and key features:
– Transient thermal analysis;
– Temperature-dependent ASR expansion rates;
– Anisotropic stress-dependent ASR expansion rates;
– Visco-elastic creep behavior with creep function and effective modulus of elasticity; and
– Separation/opening along pre-existing cracks.
The model results were compared to key dam instrumentation and the calibrated FEM results appeared to be in general agreement with the measured data, including available survey monuments, inclinometers, and over-coring stress data. The model reasonably explained the observed past behavior of the various dam components. The combined thermal and structural model also reasonably predicted the typical seasonal oscillations observed in the measured data. This stage of the model was completed in October 2011.
The purpose of Stage II was to add detail in the area of the SNOS. In addition to adding the entrance gallery, refining the SNOS geometry and increasing the model mesh density, the crack fill grouting and post-tensioned anchors were added at their approximate dates of construction. This allowed the model to forecast conditions based on a continuation of the ASR-calibrated and time-dependent material model.
The Stage II model was used to perform further calibration based on the known structure behavior prior to anchor installation and refine that calibration based on instrumentation data collected after the anchors were completed; generate a short term (zero to three years) prediction of local behavior of the SNOS after anchor installation; develop an initial prediction of long-term (three to 10 years) behavior to assess the need for slot cutting or other remedial measures and to evaluate interactions with and side effects on the powerhouse and other parts of the structure; and recommend an initial time frame for further assessments and the need for and timing of additional remedial measures, including slot cutting, realigning generating units, and modifying gates should binding occur that would impede proper operation.
Based on the HDR evaluation of the Stage II FEM in 2012, the following conclusions were developed:
– The crack fill grout and post-tensioned anchors are performing as designed and provide adequate stabilization of the SNOS and ES. The current and forecast results indicate that the condition of the stabilized SNOS should continue to meet FERC stability criteria for normal and PMF loading conditions at least through 2021;
– The elevation 112 lift joint in the SNOS will remain closed and deformation and stress should remain within recommended threshold limits established for the SNOS for at least the next 10 years;
– ASR-induced stress and deformation forecasts for other sections of the dam remain consistent with historic rates and trends;
– Based on the FEM results, a decision on the need for design and construction of slot cuts or other remediation in the structures can be deferred at least five years in favor of continued observation and monitoring of instrumentation;
– A longer-term plan, looking more than five to 10 years ahead, should be developed in three to five years, once the model calibration and structure behavior have been confirmed based on continuing instrumentation data and evaluation; and
– A continuous program of visual inspection, monitoring and evaluation of the structure should be applied until the model and structure behavior have been confirmed.
The ongoing instrumentation data has been compared to model predictions and continues to indicate acceptable performance of the structures post- anchoring. The Stage II model has not had to be “retooled” or re-run based on data collected through the 3rd quarter 2015.
Ongoing activities and current status
Since the last model update in 2012, Dominion continues to monitor all dam instrumentation monthly and evaluate trends regularly. Bi-annual instrumentation updates are provided to FERC, HDR, and Dr. Charlwood. An annual update meeting is held with FERC to review the status of project instrumentation and how it compares with the most recent model predictions. It has been over five years since the crack grouting and anchor installation was completed, and the dam is performing well, with instrumentation results consistent with model predictions.
Monument No. 3 (see Figure 2 on page 32) shows the actual movement recorded at the top of Block S7 from 1995 through the first quarter of 2015. It is evident that since completion of the anchoring project, the trend of rapidly increasing downstream movement has been arrested, and smaller seasonal temperature variations are all that are being observed. This is because the previously cracked section was freer to move with season temperature variations prior to anchoring.
|This graph shows movement recorded at the top of SNOS Block S7 from 1995 through the first quarter of 2015.|
One of the key performance indicators for the structure is the state of internal stress as a result of ASR expansion forces. During the investigation phases in 2007 to 2008, a U.S. Bureau of Mines borehole deformation gauge, in conjunction with the over-coring drilling technique, was used to obtain in-situ stress determinations in multiple boreholes at discreet elevations. These test results were incorporated into the model and are the basis for continuing evaluation of stress conditions in the dam.
In August through December 2014, Agapito Associates performed additional in-situ stress determinations, both in areas of the dam previously tested to evaluate changes and in other sections of the structure, to provide a baseline stress profile to compare with future testing. In general terms, the stress profile in areas previously tested in 2007 to 2008 did not show appreciable changes. The 2014 test results were evaluated by HDR as part of its annual review of the instrumentation data and dam performance. There was an update meeting with FERC in summer 2015 to discuss current status of the project. The additional stress determinations and review by HDR concluded that the structure is continuing to perform in accordance with model predictions, the factor of safety of the anchored SNOS section continues to meet FERC criteria, and that additional remedial measures (slot cuts) are not necessary for the foreseeable future.
A review of historical instrumentation and the results of an extensive investigation in 2007 to 2008 confirmed that Roanoke Rapids Dam is undergoing concrete growth associated with ASR. Resulting cracking and downstream movement of the SNOS resulted in stability deficiencies of the cracked upper blocks. Post-tensioned anchors and crack grouting completed in 2010 restored the structure to an acceptable condition with regard to FERC stability criteria.
Dominion continues to regularly monitor and evaluate project instrumentation, and the results are regularly evaluated by HDR with respect to the model predictions.
The dam continues to perform in accordance with the model expectations and ASR concrete growth is being safely managed by this process. The dam meets FERC stability criteria, and potentially complex and expensive additional remedial measures such as slot cutting can be safely deferred in favor of continued observation and monitoring.
John Cima, formerly with Dominion Resources Services Inc., is now a senior consultant for Schnabel Engineering. Brian Reinicker is a senior geotechnical engineer for HDR.More HR Current Issue Articles
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