To help meet future water needs in Georgia, US, the Douglasville-Douglas County Water and Sewer Authority has authorised the increase of storage at Dog river reservoir. Here Dan M McGill and Gary R Bailey describe the work involved in raising the project’s dam to increase the water level
THE Dog River dam was constructed in Douglas County Georgia in 1992. The dam provided a 70.8ha water supply for the Douglasville-Douglas County Water and Sewer Authority (the authority). Since 1993 the reservoir has provided the authority with a sustainable yield of about 64.6M litres of water per day. Accelerated population growth and projected future growth made it necessary for the authority to look for additional sources of water. The authority’s engineering consultant, R.J. Wood and Company, determined that one of the more feasible approaches to meeting the future water needs through the year 2018 would be to increase the storage in the Dog river reservoir.
Schnabel Engineering was retained by the Water and Sewer Authority to study the feasibility of raising the water level in the reservoir from elevation 750 to elevation 760 MSL. The study indicated that construction would be challenging due to constraints of maintaining the existing reservoir during construction, the limited access to the dam, the limited construction area and the original structural components of the existing dam.
The existing dam was constructed on the Dog river approximately 274.3m upstream of the river confluence with the Chattahoochee river in Douglas County, Georgia. The dam is a 16.7m high earthen dam with a length of approximately 198m. A bentonite slurry wall was constructed across about two-thirds of the foundation to provide a seepage barrier for a portion of the flood plain. The principle spillway consists of a 1.8m by 5.5m concrete water intake structure which outlets through a 1.8m x 1.8m box conduit. A small SAF outlet is used for energy dissipation for this spillway. The secondary spillway is a 73.2m wide, eight-cycle, labyrinth chute spillway constructed on the embankment. The labyrinth has a length to width ratio of four and provides an effective weir length of 292.6m. The existing labyrinth walls are 4.6m high and the existing sidewalls are 7.8m high. This spillway configuration will safely pass the estimated 1514.05m3/sec flow generated by the one-half PMP (43cm in six-hours) storm event.
The Water and Sewer Authority agreed to pursue the raising of the dam and reservoir. The design work for the modification of the dam was delayed several years while the Georgia Department of Transportation prepared plans for raising Georgia Highway 166 that passes through the reservoir. These plans included the construction of a new bridge. This work has to be completed before raising the reservoir. Obtaining a new 404 permit from the US Army Corps of Engineers (USACE) was also necessary for the project. In October 2004 the authority contracted with Schnabel Engineering to provide the engineering design and construction monitoring services to raise the Dog River dam and saddle dike located on a ridge east of the dam. Also involved in the renovation of the dam will be the clearing of the additional area around the reservoir that will be inundated by the new lake level. The design concept was simple; the constraints of design and construction made this a complicated puzzle.
Some of the constraints were obvious while others became known as several alternatives were studied. The primary constraints were as follows:
• The normal water level in this reservoir must be maintained during construction.
•The work is to be completed within the footprint of the dam
• Access constraints require that clearing of the new reservoir area be performed from barges
• Cofferdams needed for construction work on the dam or spillway must maintain the dams capability of passing storm events, while work is in progress without danger to the dam, spillway or reservoir capacity.
• Concrete structures (riser, box conduit labyrinth and chute intervals) were designed for existing loads with no thought of the reservoir being raised.
• Limited access to the work area.
The secondary constraints were:
• The sequence of the work that is critical to the success of the project.
• Seepage control under the concrete spillway.
During the feasibility study several alternatives were considered for raising the crest of the dam. These were: a raised earth embankment; a reinforced earth chimney section; a roller compacted concrete (RCC) chimney action; RCC overtopping protection with lowered crest; and a concrete parapet.
The raised earth embankment alternative required that the upstream and downstream slope of the dam be steepened and the top width of the dam reduced to achieve the additional 3m of dam height required. The change in the configuration of the embankment reduced the slope stability of the dam and the weight of the embankment caused an excessive surcharge on the chute spillway sidewalk. This added load along with the existing embankment load exceeded the design strength of the sidewalls. The added embankment also added a greater load on the 1.8 x 1.8m box conduit. Earthfill material for the construction of the added embankment would have to be hauled in from a considerable distance. This alternative was eliminated because of the effect on the sidewalls, limited work area and access to the site.
During the analysis of this alternative it was evident that the structural capacity of the chute spillway sidewalls would be critical.
The next several alternatives – including the reinforced earth chimney section, the RCC chimney section and the RCC overtopping protection with lowered crest – each caused excessive stresses in the chute spillway sidewall. Each of these alternatives would also cause construction sequence difficulties.
The concrete parapet alternative was considered the method with the least impact on the chute sidewalls and was selected for the final design. The parapet involves common construction practices and works well in the construction sequence.
The construction sequence is critical for the successful completion of this project. The dam must be kept safe from overtopping during all phases of construction. This means that the parapet must be in place before the height of the labyrinth walls can be increased. The chute sidewalls and embankment seepage control have to be in place before the parapet near the draft can be constructed. The following is the proposed sequence of construction for the renovations to this dam:
• Install embankment seepage gradient control through the
• Add gate by-pass structures on each sidewall section and reinforce chute sidewalls.
• Construct parapet on dam and dike to prevent overtopping of the dam during design storm events.
• Increase height of labyrinth walls sequentially by isolating each labyrinth cycle with cofferdams and install seepage gradient control under the inlet slab.
• Convert principal spillway water intake structure to reservoir drain structure.
• Close gate.
There are three existing structure elements that have to be modified on this project. They are the chute sidewall, the existing labyrinth wall and the water intake structure. Because of the complexity of the structures’ configurations and the varied loading, an expert in concrete structures was consulted to assist Schnabel Engineering on the project. Dr. Finley Charney provided finite element analysis and recommendations for construction of the chute sidewalls and labyrinth walls. The concrete and reinforcing steel were overstressed in the chute sidewall. It was recommended that the wall be tied to the new parapet to form a pinned connection at the top of the wall. A 20.3cm thick reinforced concrete wall, attached to the exposed face of the existing sidewall, should provide the additional strength necessary to safely carry the modified loads.
The labyrinth wall analysis was more complex. Two analyses were conducted. The first was to analyse the structure as if the new wall were constructed upstream of the existing labyrinth wall and the second analysis was as if the new wall were constructed downstream of the existing labyrinth wall. Constructing the wall on the upstream face of the existing wall was chosen. The existing labyrinth was overstressed in moment capacity. Thus a 40.6cm thick, reinforced concrete wall was added to the upstream face of the existing labyrinth as recommended. This additional wall provides the additional strength capacity for the labyrinth wall.
The results of the analysis also indicated that the existing slab was overstressed in shear and bending at the downstream apex of the labyrinth. The labyrinth wall was thickened on each side of the existing wall to better distribute the load from the wall to the slab and to reinforce the strength of the slab at the downstream apex of the labyrinth.
The final piece of the puzzle is the conversion of the principal spillway to a gate structure. A horizontal reinforced concrete slab will be constructed in the barrel of the riser structure. The normal water flow through the spillway will be diverted through the new gates through the labyrinth walls while the riser is converted. When the riser is converted, the construction work for raising the Dog River dam will be completed and the gates can be closed for the filling of the reservoir.
Dan M. McGill P.E. is a Senior Consultant and Gary R. Bailey P.E. is Senior Vice President, Schnabel Engineering South, LLC, 5975 Shiloh Road, Alpharetta, Georgia 30005, US. www.schnabel-eng.com.
This article was printed with kind permission of the Association of State Dam Safety Officials. For further information, please visit www.damsafety.org.