Changing requirements at Blalock dam16 October 2000
Future water supply demands and hazard potential reclassification has led to refurbishment at Blalock dam in the US
Blalock dam was constructed in 1983 and is located on the Pacolet river in South Carolina near Spartanburg, US. The dam is owned and operated by the Spartanburg Water System (SWS) and the project consists of a 216.4m long, 21.3m high earthen dam; an 82.3m long ogee crested spillway; and low level outlet works. Discharge over the spillway is routed through a 40.5m long spillway chute to a stilling basin and turning pool.
At its current normal operating pool the H Taylor Blalock reservoir, also known as Lake Blalock, has approximately 56.3km of shoreline and a water surface area of 306ha. SWS needs to maximise the reservoir’s dependable yield to meet future water supply demands.
The original design of Lake Blalock was based on a normal operating pool at an elevation of 216.4m. At present, the existing spillway crest at el 210.9m is raised by using 2.4m high flashboards to create the current normal operating pool at 213m. The lower operating pool was the result of a phased construction approach used by SWS to defer the cost of installing permanent spillway gates or other means necessary to maintain the design operating pool at el 216.4m. Spillway modifications are now required to increase the reservoir’s dependable yield to the design value.
After the design and construction of Blalock dam, the Dams and Reservoir Safety Division of the South Carolina Department of Health and Environmental Control (DHEC) reclassified the dam’s hazard potential from ‘significant’ to ‘high hazard’. A significant classification requires that a spillway is designed to pass half of the probable maximum flood (PMF), which was the basis for the original design of Blalock dam. The dam must now be modified to pass the full PMF safely, and satisfy the current DHEC dam safety requirements and its reclassification to high hazard potential.
The existing low level outlet works for Blalock dam consist of a raw water intake structure and a 3.7m x 3.7m cast-in-place concrete tunnel below the dam. The intake also serves as an inlet to the tunnel for passing river flows downstream from the dam. A 274cm butterfly valve is located on the inlet of the tunnel, but it has not been operated since it was closed to fill the reservoir in 1983. Water is conveyed through the tunnel in a 152.4cm bypass piping system within the raw water intake structure. The capacity of the outlet works is limited by the size of the bypass piping, and the loud noise and vibrations that accompany releases of high flows are raising concern about the integrity of the piping.
During a high river flow event in 1995, a section of the spillway chute was damaged and others were displaced. Modification or replacement of the chute is required to rectify the damage and ensure long term reliable spillway operation.
A preliminary evaluation was conducted to select the most cost-effective and efficient means of upgrading Blalock dam. SWS’ engineering consultant, black-veatch Corporation, conducted a value-engineering workshop to facilitate the preliminary evaluation and to select alternatives that will be carried forward for further evaluation. Value-engineering is a systematic approach to identifying methods for reducing life cycle costs of projects or facilities without sacrificing their reliability or efficiency.
Known by several names, value-engineering involves a conceptual review of a project or project design. The conceptual design phase is one of the most productive times for value-engineering review. Changes or new concepts can be more readily incorporated before the detailed design.
For the Blalock project, value-engineering was applied early in the design process to evaluate alternative modifications. The process helped key individuals in both the owner’s and the engineering consultant’s organisations gain a better understanding of the relevant modification criteria and alternatives, enabling them to identify the most promising solutions.
During the value engineering workshop the areas of concern were addressed and the following overall project goals were established:
•Increase dependable reservoir yield. To meet future water supply needs the normal reservoir operating pool level must be raised from el 213.3m to el 216.4m.
•Satisfy DHEC dam safety requirements. DHEC has reclassified the hazard potential of Blalock dam from ‘significant’ to ‘high’. As a result, the dam must be improved so it can pass the full PMF, and the existing spillway section must pass at least the 500-year event before the dam is overtopped.
•Provide long term, reliable spillway chute operation. A slab panel of the existing spillway chute failed during a recent high discharge event. This failure and a preliminary review of the spillway slab design and construction indicate that the long term performance of the spillway chute is suspect.
•Provide positive outlet control. The existing low level outlet works of the dam can not be used dependably for lake drawdown. The outlet works must now be used for releases to facilitate the con-struction of modifications to the spillway and for future operation and maintenance.
•Low flow control. There are no provisions for accurate control and monitoring of low flow releases.
•Provide vehicular access to the dam section. A permanent bridge or other acceptable means of access for main-tenance and inspection is required from the west abutment across the existing spillway to the earthfilled section of the dam.
•Minimise construction costs. The value-engineering workshop produced several alternatives which needed to be evaluated in more detail. In order to raise the normal pool it was determined that the spillway section should be raised by placing a roller compacted concrete (RCC) fixed-crest overflow weir on top of the existing spillway. A gated system will be incorporated into the fixed-crest weir to provide SWS with a means of passing the 500-year event through the spillway. The dam portion will be armoured with RCC so it can be overtopped by floods greater than the 500-year event. The existing spillway chute will be replaced or overlaid with reinforced concrete or RCC. For the low level outlet works, a pressure pipe with a new downstream control valve will be installed in the existing tunnel.
The probable maximum precipitation (PMP) for the Blalock dam watershed was evaluated using National Oceanic and Atmospheric Administration’s Hydro-meteorological Reports 51 and 52. The US Army Corps of Engineers’ HEC-1 watershed programme was used to develop the PMF. The PMP was incor-porated into a HEC-1 model developed for the Blalock dam watershed. The resulting peak PMF inflow into Lake Blalock was found to be approximately 8666m3/sec.
The National Weather Service’s DAMBRK dam breach computer program was used to determine the reservoir levels for overtopping the dam during a PMF event. This program was also used to quantify the spillway modifications that are needed to pass the PMF and to determine whether an inflow design flood smaller than the PMF could be used for designing the spillway modifications, based on incremental damage effects downstream from the dam caused by dam failure.
The results of the PMF dam breach analyses indicate that the incremental damage downstream from a dam breach would affect property and could poten-tially cause loss of life. Therefore, the spillway modifications required to safely pass the PMF without overtopping the dam would not be feasible.
The results of the dam breach analyses for the various spillway alternatives developed in the value engineering workshop show that for a PMF event, the reservoir level will crest at approximately el 225m which is 4.6m above the dam crest with the peak outflow from the dam equal to approximately 7930m3/sec. The flow from the 500-year flood event was estimated to be approximately 864m3/sec, based on a frequency analysis of peak flow data from an upstream USGS gauging station, with adjustments for drainage area differences. This flow value represents the required discharge capacity through the existing spillway section prior to overtopping of the dam in order to satisfy DHEC requirements. It was also used to evaluate gate sizes and spillway modifications for the various alternatives.
It was recommended in the value-engineering workshop that to increase the reservoir’s dependable yield, the 82.3m long spillway should be modified by adding a new RCC fixed-crest weir in place of, or on top of, the existing ogee spillway. The modified spillway will include gates for low flow control, trash sluicing and additional flow capacity for passage of the 500-year event.
RCC is an economical material for large gravity structures. The RCC will be placed in 30.5cm maximum lifts. A reinforced concrete cap will be placed on the top and downstream sides of the RCC spillway to provide an efficient ogee crest and erosion protection during overtopping. The spillway will have a vertical upstream face that includes patented precast panels with impervious membrane facing, to prevent water seepage through the horizontal joints between RCC lifts. This design was used successfully on a similar RCC dam which was recently completed on the North Tyger river near Blalock dam.
The new RCC spillway will be placed on the existing spillway supported on bedrock, or directly on bedrock to ensure stability. The estimated quantity of RCC required for the new spillway is approximately 4282m3.
It was determined that hydraulically operated flap-type crest gates are the best way of providing low flow control and trash sluicing. The height of the gates was limited to 1.68m to eliminate the need for a new stilling basin in the turning pool as a result of the high unit discharges associated with deeper gates. Three 1.68m high and 10.7m long crest gates are needed so the new spillway can pass the 500-year event with the fixed-crest weir at el 216.6m.
A permanent access bridge will be installed over the top of the new fixed-crest spillway to provide access to the crest gates for maintenance and for raising and lowering the stoplogs located upstream from the gates. The bridge will also improve access to the east side of the project which is difficult to reach.
The portion of the bridge above the gates will be 37.2m long and 7.3m wide to accommodate a mobile crane used for gate maintenance and handling stoplogs. The remaining 45.1m of the bridge above the fixed-crest spillway will be 3.7m wide. It was decided that a precast bridge would be the most economical choice.
The spillway chute is used to route discharge from the existing spillway to the stilling basin. The spillway chute is 82.3m wide, about 40.5m long, and consists of 55 reinforced concrete panels of various dimensions and approximately 15.4cm thick. The chute has a gradient of 27% and is equipped with a vitrified clay pipe underdrain system.
Chute blocks are located at the downstream end of the spillway chute, adjacent to the stilling basin. The stilling basin is 7.6m long with a series of baffle blocks and an end sill.
During a high river flow event in the fall of 1995, several panels of the spillway chute were undermined and damaged. An 8.1m by 7.9m panel failed completely and several chute blocks were displaced. Hydraulic analysis revealed that the failed panel had been subjected to differential water pressures as a result of a hydraulic jump which occurred directly above the panel. Due to concerns about the ability of the spillway chute to withstand future high river flows, it was determined that the most economical way to repair the damage and ensure long-term, reliable spillway operation is to overlay the existing spillway chute with a reinforced concrete slab.
The new spillway overlay slab will be 38.1cm thick and anchored and bonded to the existing slab. The joints of the overlay slab will be placed over the joints in the existing slab to help resist uplift and horizontal forces. The overlay will be designed to withstand differential hydrostatic pressures from events up to the PMF. Uplift forces will be resisted by anchors grouted into sound rock. The damaged chute blocks at the bottom of the spillway chute will be repaired and all chute blocks integrated into the new overlay system.
The turning pool, located at the downstream end of the stilling basin, is about 48.8m long and varies in width from approximately 82.3m to 60.9m. A 2.4m high sloping wall at the downstream end controls the pool depth. The purpose of the turning pool is to maintain sufficient tailwater depth to force a hydraulic jump at the stilling basin and to dissipate the energy in the water before it enters the Pacolet river.
The foundation for the turning pool wall and the turning pool floor adjacent to the wall consists primarily of in situ soil. The wall and floor have been armoured with a 15.2cm concrete slab to prevent erosion. Some erosion has occurred on the downstream side. However, the pool wall slab does not extend down to bedrock, and there is concern that further erosion may cause undermining of the wall.
To avoid such damage and ensure stability, the turning pool wall could be replaced with a wall that does extend down to bedrock. Nearby boring logs indicate that the bedrock on the upstream side of the wall is anywhere from 1.5-6.1m below the pool floor elevation. Downstream of the wall, the bedrock elevations are considerably lower. Therefore, replacing the wall with a gravity structure that extends down to bedrock would be cost-prohibitive. It was decided that the most economical solution is to armour the turning pool wall with RCC. On the upstream side, RCC lifts will be placed over the wall and will extend down to bedrock at a 2:1 side slope. On the downstream side, the RCC will slope to 1.5m below the river bottom elevation. For added erosion protection, riprap will be extended downstream of the RCC armouring.
Low level outlet control
The existing low level outlet works for Blalock dam consist of a raw water intake structure for water supply and a low level outlet tunnel which passes flow downstream from the dam. A 274cm butterfly valve is located on the upstream end of the existing 3.7m x 3.7m cast-in-place concrete tunnel. The capacity of the tunnel is approximately 56.6m3/sec. The intake structure was apparently only designed for raw water supply and not for positive outlet control or flood control for the reservoir. The butterfly valve has not been operated since the reservoir was filled in 1983, and there is concern that if this valve is opened to discharge water through the tunnel it may not close properly. There is no other means to shut off the flow within the outlet works.
Flow can be diverted into the tunnel through a 152.4cm diameter bypass piping system located in a wetwell adjacent to the 274cm butterfly valve housing. The maximum flow rate through the wetwell bypass is approximately 8.5m3/sec. The flow rate to the tunnel is, therefore, limited by the capacity of the bypass, which is considerably less than the tunnel capacity. The bypass does not have sufficient capacity to reliably divert flows during construction of the spillway improvements. In addition, loud noise and vibrations that accompany releases of high flows are causing concerns about the integrity of the wetwell bypass.
In order to facilitate con-struction of the spillway modifications to maxi-mise the low level outlet works’ dis-charge capacity and provide re-liable reservoir operation and maintenance, modifications to the existing outlet works are required. During the value-engineering workshop it was determined that a liner pipe should be installed in the existing outlet tunnel, with a control valve located at the downstream end of the tunnel. A liner pipe was recommended because the existing outlet tunnel was not designed as a pressure conduit and may not withstand the pressures and surges associated with a downstream control, which would subject the dam to the risk of washout if the tunnel failed. A control valve on the downstream end will serve as a reliable means of controlling flow releases and lowering the reservoir. With the installation of a downstream control valve the 274cm butterfly valve can be opened, increasing the capacity above the current by 8.5m3/sec through the wetwell bypass.
In the final design it was decided that the liner pipe for the outlet tunnel will be a 3.1m diameter steel pipe encased in cellular concrete, which will provide additional support and minimise corrosion. The pipe will have a wye connection to the existing 152.4cm diameter wetwell bypass as a secondary source for inflow. The existing butterfly valve will be used as an inlet shutoff for inspec-tion and main-tenance of the pipeline and control valve.
A 198.3cm fixed-cone (Howell-Bunger) valve was select-ed as the down-stream control valve because of its high flow capacity, good flow control, ability to pass small objects in the flow, and energy dissipation. The maximum capacity of the fixed-cone valve with the reservoir level at el 216.4m will be approximately 39.7m3/sec.
A portion of the existing outlet structure will be replaced with a new structure to house the fixed-cone valve. The new structure will house the valve, a small blow-off line for dewatering the pipe, access for inspection and maintenance, and if desired, a wye pipe section for future connection to a hydro power facility. A concrete apron will be positioned downstream from the valve to direct flow and control erosion. The fixed-cone valve can be operated at the site or remotely from SWS’ Blalock water treatment plant.
Dam overtopping modifications
The earthen dam portion of Blalock dam is 216.4m long and 21.3m high, with its crest at el 220.4m. In the value-engineering workshop it was determined that the most feasible way to enable the dam to pass the full PMF was to armour the structure with RCC so it can be overtopped for floods higher than the 500-year event.
A hydrologic/hydraulic evaluation revealed that during the PMF event, with the new spillway and low level outlet modifications in place, the reservoir level would crest at about el 225m overtopping the dam by 4.6m.
The estimated quantity of RCC required for armouring on the dam is approximately 19,114m3. This will extend over the entire downstream slope, past the toe into a stilling basin at the bottom and a channel on the left dam abutment, across the crest of the dam, and some distance down the crest of the upstream slope. The armouring will extend along the length of the dam and tie into the east spillway abutment wall. The armouring will also abut the new low level outlet fixed-cone valve structure.
The RCC will have a nominal thickness of 0.9m perpendicular to the existing slopes and will be placed in a stair-step fashion, with a formed outside vertical face and 30.5cm step heights. Only minor reconfiguration of the existing dam embankment is required in preparation for placement of the RCC. The downstream slope will be stripped to a depth of about 30cm, and the upstream slope and the crest of the dam will be trimmed to accommodate the entire RCC armouring thickness.
Model tests of stepped RCC configurations by others have indicated that although this configuration will dissipate energy from overtopping flow, a considerable amount of energy may still remain at the toe of the dam. For flood conditions near or at the PMF, the tailwater depths are sufficient to dissipate the energy without the need for a stilling basin. However, for the 500-year or slightly higher flood events, the tailwater levels are too shallow for adequate dissipation. The bedrock levels at the downstream toe of the dam are too deep to economically place a cutoff wall to bedrock. Therefore, a stilling basin in the form of a trapezoidal RCC channel parallel to the contours of the embankment slope will be constructed at the toe of the embankment to help dissipate energy when the dam is initially overtopped and until the tailwater depth is sufficient to minimise scour.
The stilling basin will be approximately 73.2m wide by 12.2m long, with a 1.2m high end sill at the downstream side. Large riprap will be placed at the downstream toe of the stilling basin level with the existing grade for additional erosion protection during overtopping.
Erosion protection is also required where RCC armouring is in contact with the left dam abutment to prevent undermining during overtopping. Since the bedrock is very deep at the abutment contact, a cutoff wall to bedrock is not practical. An RCC armoured berm will be placed along the abutment to completely contain the overtopping flows and to channel the flow to the stilling basin.
To relieve the water pressures beneath the RCC armouring, drainage will be provided in the form of a 0.6m thick blanket of filter stone. Perforated longitudinal underdrain piping will be placed in the blanket at the toe and mid-height of the dam. The piping will drain into the stilling basin.
RCC mix design conditions
As there are no onsite major rock excavations, it will be necessary to obtain RCC aggregate from local quarries. More importantly, the aggregate quality for exposed RCC should essentially be the same as for conventional concrete which requires manufactured materials. Black & Veatch Corporation has prepared RCC mix designs on crusher-run aggregates from two local quarries for the North Tyger River dam which was constructed about three years ago. A third quarry was located which is closer to the project site. The Blalock dam project requires two separate mix designs:
•RCC with an unconfined compressive strength of 110.34MPa at 28 days for the spillway gravity section.
•RCC with a strength of 17.24MPa for slope armouring.
The two existing mix designs were prepared for the gravity mix only. Several other mix designs were prepared to determine the optimum proportions of cement and fly ash for the two types of RCC using aggregates from the three local quarries. Crusher-run aggregates were used which were produced without mixing and blending with other manufactured aggregates in order to establish the optimum cementitious material content. A significant difference in the required quantity of cementitious material was noted between the aggregates. The aggregates closest to the desirable gradation required significantly less cement and fly ash. The project specifications allow the contractor to use either the tested crusher-run aggregates that provides an economical mix based on the cost of the aggregate, consideration of haul distance, and cement/fly ash require-ments; or perform new mix designs on aggregates from the contractor’s own sources including blended aggregates.
The low level outlet control works must be modified before any other major construction activities can be undertaken because all other work depends on a reliable means of bypassing the spillway and lowering the lake. A contract for the work associated with the low level outlet works was awarded in autumn 1998. The 198.3cm fixed-cone valve was procured under a separate contract in spring 1998 to minimise SWS’ exposure to delay claims.
Construction to modify the low level outlet control works began in November 1998. Before work could begin, the contractor constructed a cofferdam/access road downstream of the new valve structure, to protect the construction area from the river and provide access to the site. The cofferdam was overtopped twice during the construction period by heavy rain which caused construction work to be delayed. In addition, the installation of the steel pipe in the existing tunnel was more difficult than the contractor anticipated due to alignment and welding problems. The contractor resorted to a 24-hour welding schedule to meet the July 1999 deadline for completing the tunnel modifications.
The 198.3cm fixed-cone valve was shipped to the job site on schedule and installed in early August 1999. However, when the valve was dry tested it failed to open and close properly. It was agreed by all parties to ship the valve to one of the valve manufacturer’s factories for testing. Unfortunately, South Carolina was experiencing a drought, and by the end of August 1999 SWS needed to use the low level outlet control works to meet minimum downstream flow requirements. Within three days a temporary flange with a concentric hole pattern was designed and installed in place of the fixed-cone valve to allow sufficient flow releases and back pressure to open the 152.4cm butterfly valve in the wetwell bypass. This temporary flange was used by SWS during the autumn of 1999 to meet their minimum flow requirements until the drought ended in late autumn. In January 2000, the 198.3cm fixed-cone valve was reinstalled and successfully dry-tested. Due to the continuing drought conditions, SWS has used the valve to release minimum flows from Lake Blalock during the spring and summer of 2000. The construction costs to modify the low level outlet control works, including the purchase of the valve, was approximately US$2.7M.
Bids for work associated with the spillway modifications, spillway chute modifications, turning pool improve-ments, dam overtopping modifications, and ancillary project improvements are scheduled for spring 2001, with construction expected to begin in July of 2001 and end in November 2002.
The ancillary project improvements include clearing and grubbing approximately 3.1m of the reservoir fringe which is approximately 64.4km of shoreline and improvements to existing access roads. The spillway improvements will require SWS to lower Blalock Lake level by 6.1m to el 207.2m for a duration of five months (October 2001 to April 2002). Contracts include the construction of cofferdams upstream from the spillway and downstream from the turning pool wall to prevent damage during flooding.
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