Kennedy Generating Station, in Jacksonville, Florida, is owned and operated by Jacksonville Electric Authority. When corrosion of abandoned water intake and discharge tunnels raised concerns about potential collapse, the owner retained consulting engineers Black & Veatch of Jacksonville to evaluate remediation options. Moretrench’s value-engineered alternative to the original proposal eliminated the need for underwater work and reduced the amount of excavation required.
The stacked, 5-ft square tunnels, approximately 290 and 230 ft in length, carried and returned water from the adjacent St. John’s River to a discontinued cooling system and were partially filled with water and sediment. The top of the upper tunnel was covered by 2 ft of ll and a 3-in. asphalt layer.
The initial remediation and restoration design included the installation of a bulkhead by divers to cut off the water source at the seawall and permit tunnel unwatering; compaction grouting of the lower tunnel; excavation of the roof of the top tunnel in segments; consolidation of the sediment; backfilling; and site restoration. However, in a value-engineered alternative proposed by Moretrench in cooperation with the owner and engineer, the final design included:
- Construction of the bulkhead by means of concrete-filled fabriform bags installed from the surface via access points cored through the crown of the upper tunnel, invert of the upper tunnel, and crown of the lower tunnel.
- Elimination of extensive excavation of the upper tunnel by the use of low-density cellular grout, capable of travelling significant distance, to fill the top tunnel. This would only require limited slot trench excavation along the tunnel alignment.
This proposal also took into account the need to minimize impact to plant operations and traffic flow, the limited work area, and the presence of buried utility lines.
Bulkhead design called for a double row of bags with a lean cement plug in the area between to ensure absolute water tightness. To construct the bulkhead, Moretrench excavated to the top of the upper tunnel and core-drilled two rows of 8-in. diameter access points at 1.75 ft on center and 10 ft apart in plan through to the base of the lower tunnel. The 3-ft diameter fabriform bags, wrapped around 2-in. steel tube à manchette grout pipe, were lowered in place and filled with cement grout such that each bag tightly abutted its neighbor. The intervening space was then filled via TAM pipes.
With all work being accomplished from the surface, the need for divers was eliminated, making bulkhead construction both a safer and less costly operation. Once the bulkhead was in place, a total of 27,800 gallons of water was pumped from the two tunnels, with total evacuation confirmed by camera.
Tunnel Void Fill Grouting
To access the tunnels, Moretrench core-drilled adjacent 8-in. diameter access points, one for each tunnel, at approximately 50-ft intervals along the tunnel alignment. The 6-in. diameter PVC grout riser pipes extended 4 ft above ground surface in order to generate the head pressure required to compress the layer of sediment in each tunnel.
The lower tunnel was completed first. After a survey of the existing conditions it was determined that the tunnel was essentially full of sediment. Compaction grouting was selected as the means for void fill/compaction of the sediment. Compaction grouting efforts commenced at the downstream end of the tunnel alignment and continued upstream until grout was observed exiting the stand pipe at the upstream end. Monitoring of the grout height in each subsequent standpipe verified that the lower tunnel was under sufficient pressure to consolidate any remaining sediment to within specified parameters. In addition, any void space in the lower tunnel prior to grouting was filled with water, so the volume of water in the upper tunnel was monitored prior to, during, and post compaction grouting and compared to the theoretical void in the lower tunnel prior to grouting. The resulting increase in water volume in the upper tunnel created by compaction grouting was essentially equivalent to the theoretical void space of the lower tunnel, further confirming all voids had been filled.
Once the lower tunnel was completed, the upper tunnel was filled with a Controlled Low Density Cellular Grout (CLDCG), gravity fed through the riser pipes. Visual monitoring of the rise of grout in subsequent stand pipes con rmed that the void had been completely filled. For the upper tunnel, cameras were installed at each riser pipe location to provide similar visual con rmation. Multichannel Analysis of Surface Waves (MASW) testing was conducted to verify both tunnels were completely filled. Approximately 400 cubic yards of grout was required for the project, which was completed on time and under budget.