Tough Tennessee Sewer Casing Installation Requires Creative Underground Tactics

The City of Alcoa, Tennessee, located 5 miles south of Knoxville, was the site of a sewer casing installation in some of the toughest Tennessee clay any utility contractor could ever dream of. You know it was tough when once the job was successfully accomplished all anybody could say was "it was a heck of a learning experience."

The plan called for the placement of 530 feet of 16-inch steel casing under the Phillissippi Parkway. The casing was for an eight-inch ductile iron sewer pipe for a new development in Alcoa. Rick Potter, owner of Potter Pipeline, Inc. of Sweetwater, Tennessee, a road boring and tunneling contractor, took on the job, bringing 21 years of utility work experience to the project.

Potter was sought as a subcontractor for the job after a previous casing had been abandoned two months earlier. The casing had gone 20 feet too deep when contractors used conventional auger boring equipment from another subcontractor. Potter knew that the only way they could get the job done would be with directional boring equipment because the bore required a precise 2.5 percent grade over the length of the casing.

The main obstacle to a successful bore and pullback with the directional drilling equipment was the Tennessee firebrick clay. This clay has properties similar to gumbo, a fine silty soil that becomes sticky and non-porous when wet. When water came into contact with it, it would become gummy and stick to itself.

Meticulously, the directional bore path was laid out to achieve the desired grade for the 530-foot crossing. Drilling from the elevated north side of the four-lane, median-divided parkway to the south side, the bore path would be forty-eight feet deep.

The crew used a Vermeer D24x40 to drill the pilot bore, employing a combination of polymer drilling fluid and CON DET®, a detergent fluid used to prevent the sticky clay from adhering to the drill stems. The clay still stuck to the drill stems in spite of the use of the specialized fluid, and would not break down and flow out the bore hole.

According to Rick Potter, when he augur bored in this clay, the material would never slurry but came out through the auger flights in big clumps. "In the past, we had used a water line to soften up the clay and push steel casing in. With this horizontal directional drilling, we need it to break down into a slurry and flow out of the bore hole. It never did."

The job called for four pre-reamings, starting with a 12-inch reamer and increasing to a 21-incher. "For about thirty feet during the pre-reaming with the 12-inch reamer we got some flow, some slurry of the clay cuttings coming out of the bore, but then it refused to break down." said Potter. "We changed the drilling solution but the drilling fluid would not break down the soil. The soil just adhered to itself, and all we saw come out of the hole was the drilling fluid which had separated from the clay. Any continued pre-reaming was judged to be futile."

The crew knew they had to make this bore work since their "bore window" was very narrow and they had no other place to go. They built a "football" out of two six-inch gas reducers and shoved it back to the downhill side so they could attach a 20-inch reamer and the product pipe.

Five hundred feet of the 16-inch steel line was welded together and strung out, ready for installation. Even knowing that the cuttings were still in the bore, the crew had to attempt to pull the product pipe. They pulled the first 300 feet of the pipe and poured thousands of gallons of water and drilling fluid through the bore before they ultimately got stuck.

The Vermeer factory was alerted and they sent down a large D50x100 Navigator to pull the pipe the rest of the way. With 50,000 pounds of pullback strength and 10,000 foot pounds of torque, this directional drilling rig successfully pulled in the product pipe. It was assisted on the downhill side by a T-755 track trencher pulling with a couple of semi-trailer wreckers and a Link Belt trackhoe. But this pullback was not yet over.

When they cleaned out the exit pit, they could see that the power of the Navigator pullback had put so much pressure on the pipe in this tenacious clay that it had collapsed. The 16-inch casing had been crushed by the pressure and friction of the bore walls down to four inches in diameter, resembling a figure eight.

They tried to ram in the downhill side of the casing to push out the crushed pipe sections, but that was fruitless since the casing wall was too thin for this distance in this kind of soil. So the only option to them was to open up the crushed end.

Determined to know the extent of the damage to this end of the pipe, the crew sent a special downhole sewer television camera into the pipe which told them that 55 feet of the casing needed to be expanded out.

They brought in eight-inch and twelve-inch HammerHead Moles to punch in a reducer to open this section of casing back to its original size. To expand the walls of the crushed pipe, they built up 55 feet of steel reducers with a heavy-wall 14-inch casing at the rear to hold the collets for the HammerHead Mole pneumatic tools. The nose of this fabricated piece resembled that of a 22 caliber bullet.

Potter's experience with HammerHead Moles helped. "I've used the 5 and 3/4 inch mole to do straight bores under roads for 50 foot shots, so I knew they had power," explained Potter. "But this certainly would be an innovative use of the tool."

As the crew used the Hammerhead Mole to pound in the reducer, the casing would expand out to its edges. Periodically, they would get a wrecker, attach a chain to pull out reducer, and modify the nose to better fit the asymmetrical crushed casing. To save time and reduce their need for the cumbersome wrecker to pull out the reducer, they decided to weld a piece of 14-inch heavy wall pipe onto the top of the casing and use the HammerHead Mole to ram out the reducer when they needed to. They placed collets in this adapter and rammed the reducer in the opposite direction when they needed to remove it.

Eventually, the casing was opened up and the eight-inch ductile iron sewer pipe was installed. It took a combination of technologies, a lot of hard work, and solid dose of Tennessee tenacity, but the job was finally accomplished.

Potter described the positive feeling the entire crew had when the pipe was opened up. "It was definitely a good feeling when we made it. We could not have done this without the HammerHead Mole. This was the quickest and easiest way. In the future when I run into this clay again, and I know I will, I will have to consider using a combination of technologies from the start. This would include possibly increasing the pipe wall thickness, pulling with the directional rig and ramming the pipe with the pneumatic tool. I'd leave the pipe open and jet wash the casing clean. This project was a tough learning experience, but I know now that if I get in trouble I can always go to one of these pneumatic tools."

Written by: Richard Yach - Technical Writer Des Moines, Iowa
Provided by: Vermeer Manufacturing Co. - Pella, Iowa