sec8 recommendations

Toledo Bend Project – Part 12D Section 8 Safety Inspection Report Recommendations

Rev. 0a Page 8-1 03/31/2005

SECTION 8

RECOMMENDATIONS

8.1 PROJECT DESCRIPTION

No recommendations related to Project Description.


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8.2 POTENTIAL FAILURE MODE ANALYSIS REPORT

Based on the results of the PFMA, the project was found to be in generally good condition, with good maintenance and operating procedures. Twelve potential failure modes (PFMs) were identified in the potential failure modes analysis of the Toledo Bend Dam Project. Of these, two Category I PFMs, five Category II PFMs, three Category III PFMs, and two Category IV PFMs were identified.

The following PFMs were identified as Category I:

• Stability of Spillway due to Failure of Low-flow Discharge Bypass Pipes

• Piping Under Spillway The following PFMs were identified as Category II:

• Spillway Stability – General

• Gate Failure in an Open Position

• Stability of Embankment Slope

• Piping through Embankment

• Stability of Powerhouse The following PFMs were identified as Category III:

• Failure of Spillway Chute Slab

• Stability of Spillway due to Tailwater

• Overtopping of Embankment The following PFMs were identified as Category IV:

• Gate Failure, Forced Open from a Closed Position

• Piping at Powerhouse

8.2.1 Category I PFMs As listed above, the PFMA found two critical items (Category I) indicating an alternative method of passing the low flow through the spillway structure is needed to reduce the hazard for spillway stability problems from possible rupture of the low-flow bypass pipes in the spillway gallery. These pipes have had to be replaced in the past because of cavitation problems that were found before the pipes ruptured. The other critical item is the discovery of soil in the key drain discharge pipes that are pending classification of soil type. If the soil turns out to be clay, that would indicate a problem with the drainage system under the spillway structure.


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8.2.2 Category II PFMs Additionally, the PFMA found the following items that should be performed or completed to improve safety and provide for monitoring or evaluation regarding the Category II PFMs of the project:

• Inspect the spillway drain pipes, test the drainage system and monitor run-time on the relief well pumps to evaluate the condition and effectiveness of the spillway drainage system.

• Confirm or provide a backup power supply for the relief well pumps at the powerhouse and provide a system to monitor pump run-time.

• Evaluate the trunnion anchor capacities with one or two rod failures.

• For the embankment drainage and monitoring system, install barriers around the piezometer stand pipes to prevent damage, install a measuring device and monitor flow from the drainage system, and inspect the drain pipe with video equipment.

8.2.3 Category III PFMs The PFMA found the following items requiring calculations or equipment needed to determine the appropriate category for those PFM’s placed in Category III:

• Calculate the negative and positive pressures produced by high-velocity flows on the spillway slab during discharge through the spillway gates.

• Calculate tailwater levels and velocities inside and outside the spillway stilling basin where erosion could jeopardize the dam or spillway during passage of large floods.

• Calculate the reservoir levels for leaving some gates closed during passage of the PMF (to help evaluate the required operation during the PMF).

• Provide a backup motor for gate operation and store it at the site for use during an emergency (failure of gate motor during a flood).


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8.3 PERFORMANCE MONITORING AND VISUAL ASSESSMENT WITH RESPECT TO POTENTIAL FAILURE MODES

8.3.1 Review of Historical Instrumentation Readings.

The fluctuation in piezometer readings, observed over time, appear to be primarily a result of changing headwater and tailrace water levels. However, wet weather and dry weather seasonal effects have had some influence on the recorded piezometer levels. Also, embankment piezometers located near the spillway appear to be more effected by tailrace water levels than piezometers located further away.

Note: Several spillway piezometers have not been working since July 1983. However, observations for these piezometers are presently reflected by levels measured in adjacent relief wells. These piezometers are identified in Appendix D.

One area of interest that needs to be addresses is the area of the piezometers in the vicinity of the north end of the main embankment just before it joins the southern end of the concrete spillway. For instance, piezometer F-21 shows 5-year high elevation reading higher than the maximum allowable phreatic surface. Unfortunately the tip elevation is not known. Likewise, piezometer D-17 shows the five year high elevation reading to be above the maximum allowable phreatic surface. Because the area of the embankment in the vicinity of these high piezometers reading has exhibited seeps both in this inspection and past inspections, these high reading should not be ignored. All piezometers with 5-year high elevation readings exceeding the maximum phreatic surface should be flushed and the reading taken frequently to verify if the piezometer is plugged or otherwise malfunctioning. If high elevation readings persist in these piezometers, then it is recommended that additional new piezometers with their tip elevations at the bottom of the select pervious drain layer be set in the vicinity of the piezometers with high readings. These new piezometers would be used to confirm whether or not the maximum allowable phreatic surface is in fact being exceeded. If the maximum phreatic surface is not being exceeded, these new piezometers should continued to be monitored and read on the same basis as the remainder of the piezometer instruments. If the phreatic surface proves to be high as established by these new piezometer readings, then a program to reduce the phreatic surface to acceptable levels should be developed.

8.3.2 Underdrain System Observations

Underdrain Discharge Pipes: There are two key drain discharge pipes that release water from the heel drain under the spillway foundation. Soil


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particles were discovered in the underdrain discharge pipes exiting onto the spillway from the heel drain. Because of the significance of this issue, it is of utmost importance that further monitoring and investigations be conducted to determine what is really occurring. The discharge pipes should be monitored at least weekly for further material discharge. The recommendation includes the collection of another soil sample of sufficient quantity, perhaps by extending some sort of tool up in the pipe and scraping/pulling any material forward out the end. If there is a determination that the material being discharged is clay, the discharge pipes may require investigation by the use of a closed circuit television camera.

8.3.3 Seepage from Downside Slope of Embankment

As indicated in paragraph 4.1.2, seepage continues to exit on the downside slope of the embankment. The amount of seepage observed does not appear to be solely due to rainfall. The seepage is attributed to one or both of the following possibilities: seepage flowing through the cross section of the embankment at an elevation above the vertical portion of the sand drain, or seepage flowing up from the foundation through the horizontal sand layer.

a. To reduce the amount of seepage collecting along the bottom of the down slope of the embankment the TBPJO has installed perforated drainage tile at several locations. This system appears to be working adequately and the slope has dried considerably since the system was installed. Other areas where the system has not been installed continue to show seepage. It is recommended that drainage tile be installed in these areas as well.

b. Further field inspection should be performed to determine the source of the seepage. If the source is found to be from the embankment foundation, a problem may exist with the horizontal portion of the sand drain. Should this be the case, measures to repair the sand drain system may be justifiable. If the seepage is determined to be traveling through the embankment at an elevation higher than the top of the vertical sand drain, this may indicate a problem with the impervious core of the embankment. Should this be the case, repairs may be warranted; however, this scenario is thought to be unlikely.


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8.3.4 Damage to Soil-Cement Slope Protection

a. Visual inspection of the embankment has revealed moderate damage to the soil cement slope protection system on the upside slope. Sustained high winds have resulted in high waves that break on the soil cement.

b. To maintain the profile of the cement soil layer the TBPJO has replaced sections of damaged soil cement with cast-in-place concrete. This approach to maintaining the adequacy of the slope protection system is suitable and should be continued.

8.3.5 Spillway Approach Slab

a. The approach slab immediately upstream of the spillway will require visual inspection to verify that the slab is providing a waterproof barrier that prevents/minimizes the passage of seepage underneath the spillway section of the dam.

b. To adequately inspect the approach slab, divers will need to be used. This type of inspection should be scheduled on a periodic basis by the TBPJO.

8.3.6 Spillway Gate Motor Drawing Excessive Amperage

a. During inspection activities, one of the spillway gate motors was observed to be drawing more amperage than normal during opening of the gate. At the time of the inspection, the brake on the lifting mechanism was adjusted to relieve the load on the motor.

b. Further inspection is underway to determine the cause of the high amperage draw by the motor.

8.3.7 Spalling of Spillway Chute

a. The concrete chute at the spillway is experiencing localized spalling at various places along the joints in the slab. In the past, areas that have spalled were repaired by chipping away concrete around the steel reinforcement in the affected area; the steel reinforcement was then cleaned and coated with an epoxy compound before replacing the concrete.

b. The repair and maintenance procedures are outlined in Section 5, Field Inspection, paragraph 5.1.3. This approach has been used for several years, is suitable and should continue to be used for repairing new areas experiencing spalling.


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8.4 FIELD INSPECTION

8.4.1 Embankments

The main embankment, Dike 1, 2 and 3 are all well maintained and appear to be in good condition. Minor seepage is occurring in some isolated areas. The soil-cement upstream embankment protection was exhibiting continued wear. However it was readily apparent that repairs had been made since the last Part 12D inspection. For the age of the structure, the soil cement is in fair condition, but maintenance efforts should continue as explained in the main body of the report.

8.4.2 Powerhouse

There were no unusual conditions observed at the powerhouse. The emergency action plan was current and readily available. The key to maintaining stability at the powerhouse is the continued operation of the pumped relief wells.

8.4.3 Spillway

The spillway continued to display a good amount of concrete spalling at the construction joints throughout the chute slab. Repairs should be scheduled and budgeted in the near term since the situation will only worsen every time a significant release is made.

8.4.4 Gate Motor Amperage Overload

The gate motor amperage overload issue needs to be resolved. It is currently under investigation by other parties. The gate trunnion arms and gate bracing need to continue to be cleared of water and debris on a regular basis to avoid continued corrosion. The rolled J-bulb at Gate 5 needs to be replaced when possible. Minor leakage at the other gate seals continues but is not a significant issue.

8.4.5 Low Flow Bypass

In the gallery, the 20-inch pipes that provide low flow by-pass need to be watched for signs of corrosion and cavitation damage, since there have been problems with the elbows in these pipe sections before. To date, they appear to be in good condition. Also, the continued build up of calcification on the gallery walls especially in conjunction with the minor spalling that occurred on the downstream side of the gallery wall should be monitored. This problem is not yet significant but should not be ignored.


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8.4.6 Spillway Underdrain System

The spillway heel underdrain system should be fully investigated as soon as possible in light of the clay like sediment found in the key drain discharge pipes up on the chute slab. The underdrain pipe is a 6-inch perforated cast iron pipe that is surrounded by a filter of gravel, followed by a layer of Type I sand, and finally a filter of Type II sand. The spillway base material is a bentonite clay material. The underdrain pipe is subject to corrosion and is already 35 years old. The pipe may have failed in one or more areas. If clay like material is exiting the discharge pipes, that would lead one to believe the sand material has already been lost in one or more areas and foundation material is now being lost through the discharge pipes. At this point we do not know what is happening, but the consequences could be very serious, thus the need for monitoring and investigation. The Independent Consultant has recommended continued monitoring for sediment in the discharge pipes. Attempts should be made to collect additional sample material for testing. If the problem persists, a closed circuit television exploration of the pipes may be required.


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8.5 OPERATIONS AND MAINTENANCE PROGRAMS RELATIVE TO POTENTIAL FAILURE MODES

8.5.1 O&M Overview

a. TBPJO personnel seem to take their responsibilities very seriously, however they need to continue their vigilant upkeep of the facility. The communication and response procedures are adequate. There exists a certain level of redundancy with power provided via two separate circuits. A backup generator is also available for gate operations.

b. The report recommends a back-up motor for tainter gate operations be procured and stored nearby for emergency installation. This item is critical because there is some indication that the motors may be overloaded during gate operations. This is still under investigation, but acquiring a back-up motor is a simple precaution to take.

c. The human factors are addressed adequately. Changes to the O&M procedures based on the PFMA are listed in the following paragraph.

8.5.2 Corrective Actions for O&M

Corrective action should be considered for the identified PFMA operation and maintenance related items indicated in this section. The following recommendations should be considered for immediate implementation upon approval of this report. These items include, but are not limited to the following:

a. Improvements to the internal drain system of the dam embankment. b. Stability improvements related to liquefaction as recommended in

the 1983 report by Rone Engineering. c. Change data collection forms to allow field personnel to compare

newly collected piezometer data with previous measurements as new data is collected in the field to ensure immediate warning of potential problems or identification of inaccurate data measurements.

d. Construct barriers around piezometers to protect exposed portion of riser from mowers.

e. Calculate water velocities at various high tailwater levels in the stilling basin beyond the spillway training wall and downstream of the stilling basin.


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f. Calculate downstream tailwater level for the PMF outflow and determine the effect on stability of the stilling basin.

g. Identify potential adverse effects due to the suction effect during high flows over the ogee crest.

h. Procure backup gate operator motor to be stored on site to minimize gate operation downtime in the event a gate motor fails.

i. Confirm flow rate and nappe depth over crest of spillway gates. j. Calculate PMF elevation for scenario where one or two spillway

gates are closed and inoperable. k. Contact transportation departments of both states regarding issue

with highway speed limit for spillway bridge. l. Develop a way to restrict flow from the spillway gallery into the

underdrain system. m. Install check valves in well pump discharge piping to prevent

backflow through the pumps and back into the wells. n. Design and implement modifications for minimizing flow into the

spillway gallery to eliminate potential for flooding of the gallery (i.e., develop alternative methods of passing low flow through the spillway structure to reduce the hazard for spillway stability problems from possible rupture of bypass pipes in the spillway gallery).


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8.6 ASSESSMENT OF SUPPORTING TECHNICAL INFORMATION DOCUMENT

(No recommendations; STI is adequate)

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