Coal

The Rehabilitation of an Arkansas Coal Tunnel

Issue 11 and Volume 116.

View of typical access shaft entrance in front of coal pile. All photos courtesy of Sauereisen, Inc.
View of typical access shaft entrance in front of coal pile. All photos courtesy of Sauereisen, Inc.

Lake H. Barrett, Sauereisen Sales Manager

There is an old adage that goes “out of sight, out of mind.” This mindset is particularly appropriate when it comes to problems with infrastructure that is located underground.

Plant engineers at a coal-fired generating station in Northeast Arkansas were forced to deal with such “out of sight” issues in the tunnel systems that support their plant’s coal conveyance system. Constructed in 1982, the plant’s below-grade conveyor mechanism sits inside a long, concrete, vault-like structure with numerous “escape tunnels” located at strategic points. These ancillary tunnels not only provide personnel in the conveyor areas with a means for emergency egress, but also serve as maintenance and service access points. Unfortunately, these access tunnels have suffered a number of problems stemming from water infiltration, corrosion and unstable soil conditions.

Unlike the concrete structure in which the conveyor mechanism resides, the access tunnels were constructed from heavy steel corrugated pipe, 84″ in diameter. This pipe was then protected internally with a coal tar epoxy lining. To provide a safe walking surface, approximately 3″ of concrete was poured over the bottom of the pipe to cover the corrugations and to provide a smooth walking surface.

Over the past three decades, many of the joints between the corrugated sections have failed, creating a large amount of flooding in the conveyor tunnels. Contractor Scott Fulton of Industrial Services & Solutions described the inflow from the access tunnels into the conveyor areas as “waterfalls,” especially following heavy rain. He also noted that such large amounts of water made areas impassable at times when sumps and sump pumps were overrun.

Fulton and his company were asked to develop a solution for the problems that would address the both the water inflow as well as the corrosion issues without disrupting operations or requiring earthwork from above.

Surveying the Scene

After surveying the access tunnels, Fulton noted that the problems originated from minor joint failures which led to a cascading chain of failures. As infiltrating water ran down the joints in the walls, it settled below the concrete walking surface. At that point, the water combined with the sulfur-rich coal dust on the walls and pipe bottom to form an acidic solution. Regrettably, the bitumastic coal tar lining installed during original construction was no match for this corrosive solution and it easily found paths to the steel substrate. This created holes in the pipe wall allowing additional water infiltration. Over time, the water infiltration created even more problems by bringing fines from the surrounding soil. As the fines were removed from the areas around the leak points, the soil shifted to fill the vacated areas, further dislodging the joint sections. Fulton reported separation in the joint sections of up to 3″ in some places due to either soil shifting or naturally occurring settling. In turn, these newly failed joint areas allowed additional levels of water to enter the tunnel. If allowed to continue, the cycle was set to progress to a point where the unstable soil conditions would pose structural problems for the tunnel, requiring extensive repairs and major costs.

As a solution, Industrial Services & Solutions prepared a three-pronged approached to the problem drawing from their experience with a number of restoration and corrosion resistant materials by Sauereisen, Inc. Their first task was to stop the active water inflow. They then needed to repair the joint areas and prevent future leaks in the face of potential ground movement. Finally, they had to provide a corrosion resistant floor in the tunnel offering a safe walking surface even in the presence of corrosive conditions.

Stopping the Active Water Inflow

Understanding that no repairs inside the tunnel could begin until water from the outside was stopped, Fulton chose to stop the inflow by way of grout injection. By utilizing a hydrophobic polyurethane grout material that would expand up to twenty times its original volume when exposed to water, holes were drilled into the corrugated steel so that grout could be injected behind the walls and into any adjacent voids in the soil. Access to the walls was fairly straight forward with the exception of some piping and conduit, but in order to access the bottom of the pipe the existing concrete floor had to be broken up and removed.

Joint being filled with Sauereisen F-88.
Joint being filled with Sauereisen F-88.

Once this was done Fulton’s crews had open access to the pipe’s full circumference and could locate corroded locations in the bottom, allowing them to “chase” their leaks a full 360 degrees around the joint locations. The choice of a hydroactive polyurethane grout for this application with adjustable catalyzation allowed Fulton the flexibility to “juice” the grout so that it would react immediately with water to stop aggressive leaks, or to back off of the catalyst levels permitting the grout to mix with the water and then follow it for a distance before reacting and expanding. Hydroactive polyurethane grouts come in a variety of grades and chemistries. Beyond the ability to simply expand and to stop water inflow, Fulton selected a rigid hydrophobic grade grout that would maintain its shape and size after expanding – even after water was removed. When using this type of material the grout serves to stabilize the soil surrounding the pipe by expanding to fill voids and caverns. It then maintains that volume through dry seasons, preventing shifting soil and sink holes.

Repairing the Failed Joints

View of tunnel wall showing deterioration of bitumastic lining.
View of tunnel wall showing deterioration of bitumastic lining.

Similar to selecting the ideal grout material for stopping active leaks from entering the pipe and stabilize the surrounding soil, the repair method chosen for joint areas also needed to serve multiple functions. Understanding that grouting can often be only a temporary fix, as natural ground movement will undoubtedly still occur, the choice of a rigid versus flexible joint repair was important, especially with regards to long-term performance and success. To provide the plant with the best option for both sealing the joints while allowing for future movement, the liquid-applied sealing material F-88 by Sauereisen was chosen. This modified urethane material was designed for stopping water infiltration in joints with high frequencies of movement. Incorporating glass fiber reinforcement, this highly flexible material has tenacious bonding capabilities and maintains its full array of physical properties, even when subjected to 125 percent elongation. To prepare the pipe for the new joint material, the joints were cleaned and mechanically prepared using hand tools to remove any residual rust, corroded materials and remaining coal tar linings. The F-88 material was then hand-packed into open joints and troweled at a minimum eighth-of-an-inch thickness over closed joints to prevent future infiltration should movement occur.

Replacing the Walking Surface

After the existing floor was demolished and removed to provide access to the pipe bottom, a new walking surface was needed for easy egress. In lieu of using the standard portland cement material, Industrial Services & Solutions selected an epoxy polymer concrete. A polymer concrete such as this offers many advantages over standard concrete in this type of application. Besides the natural corrosion resistance of an epoxy, a polymer concrete such as the one chosen for this project also creates an incredible bond to the prepared steel pipe, providing an effective seal against both groundwater coming in through holes in the pipe bottom as well as sealing the inside of the pipe bottom from water inside the tunnels. By sealing the bottom from water that would find its way in to the tunnels through other means, the method prevents acidic solutions from collecting under the walking surface where they can sit and promote further corrosion of the steel.

Conclusion

The three-pronged approach to these tunnel repairs eliminated or, at the very least, greatly reduced the previous water inflow and infiltration. The grouting has also stabilized the surrounding soil bed. The repairs installed on the once leaking joints have created a seal that will now tolerate a degree of movement without failure, and the new walking surface will now resist corrosion, both to its own infrastructure and to i the corrugated steel under it. These repairs have been implemented for nearly 22 months and both the procedure and the materials used have proven successful. To date, the plant has addressed a little over one-half mile of the access tunnels displaying the heaviest amounts of water infiltration. Plans are in place to continue this work through 2012, and into the future, until all of the leaking tunnels are rehabilitated.