Pipe bursting is one of several trenchless methods for replacing sewer laterals.

 

Total project cost is not the sole factor in selecting the repair strategy. Others include service interruption time, surface disruptions and restoration costs, project site accessibility, terrain and the repaired line’s service life.

 

Pow-R Mole offers trenchless solutions that include the PD-6 thrust boring machine. It was used for a line replacement at Grampian Hills Apartments, a 198-unit community in Loyalsock Township near Williamsport, Pa.

 

The complex has an extensive collection system that eventually discharges to the township sewers. When faced with a mandate to replace several 8-inch terra cotta gravity mains and install two new manholes, the complex owners chose to replace the mains by pipe bursting. Pine Mountain Excavating of Avis, Pa., did the work using the PD-6 machine.

 

Grampian Hills head of maintenance Chuck Hutchins monitored the work, which also served as a demonstration of the bursting technology. The Pine Mountain crew included partner Mike Yarnell, backhoe operator Craig Yarnell, service technician Buck Dugan, and laborer Cory Brininger. Pow-R Mole sales representative Rob Jasinski was present to train the crew on their first use of the machine. The soil on the steep hillside site was bone dry and rock hard on the sunny autumn day when the first run was installed.

 

Walk-around

The PD-6 has three main components and several subsystems and task-specific options.

 

A heavy rectangular steel front gate or frame stabilizes the machine. When in use, the frame is oriented perpendicular to the direction of machine force. It is placed in direct contact with the excavated soil workface through which the pipe to be replaced protrudes. In the pulling mode, the frame transfers the load to the exposed soil surface.

A smaller frame supports a 3,000 psi hydraulic cylinder and operates in the same vertical plane as the protruding pipe section to be replaced. The piston has a 12-inch back and forth range of motion, and its centerline is about six inches below that of the driven rod. The piston frame’s flexible coupling to the front gate allows the piston to be angled to suit site conditions. A bidirectional grasping roller system binds the rod to the piston, transferring the piston’s motion to the rod. This also transfers 84,000 pounds of pulling force to the rod.

 

A segmented rod is assembled using 2-inch-diameter, 30-inch-long threaded rods, which are sequentially introduced to or removed from the cylinder’s grasping roller. For insertion, each rod is threaded into the exposed end of the string of rods previously inserted. Attached to the piston, the rods are either pushed into the pipe or drawn back into the working pit as the bursting head is pulled through the damaged pipe.

 

The bursting head is screwed onto the end of the first rod. Attached to the head is a pipe grip that makes the joint connecting the new HDPE pipe to the pulling rod. A variety of bursting heads and pipe grips are available to accommodate all pipe diameters.

 

Hydraulic power for the unit can be provided by a backhoe or similar machine, or by an auxiliary hydraulic pump available from the manufacturer. The auxiliary pump may be needed if the backhoe cannot deliver the necessary hydraulic pressure or flow rate.

 

The PD-6 unit measures 17 inches high, 54 inches long, and 21.5 inches wide and weighs 720 pounds. Hydraulic demand is 3,000 psi at 18 to 25 gpm. Delivered force is 84,822 pounds with a 12-inch stroke.

 

For this project, because of the working depth of the machine, the bursting unit was mounted in a 4-foot-deep by 6-foot-long by 3-foot-wide shoring push box. This box surrounded the machine and satisfied OSHA requirements.

 

Operation

The pipe replacement project began when the pits were excavated at two adjacent manholes. Because the bursting machine would be installed in one of the pits, the most accessible site was selected for the purpose.

 

The introduction pit’s bottom dimensions were about 60 by 72 inches, and the depth was governed by the depth of the pipe being replaced. The target depth was set about 15 inches below the pipe centerline, and the final depth was about 11 feet. The exit pit had final dimensions of about 3 by 8 feet and 12 feet deep. The two pits were about 140 feet apart.

 

With the existing pipe exposed in both pits, the manhole at the introduction pit was plugged to keep the work site clean and mostly dry. A trash pump was positioned near this manhole, and its discharge hose was run downslope to the manhole at the exit pit. The manhole was monitored for accumulated sewage and, as warranted, the pump was activated.

 

In both pits, the terra cotta pipes were cut, leaving a short stub of the original pipe protruding from each manhole. At the opposite ends of the pits, the pipe protruded into the pit from the enclosing soil.

 

Mounted in the shoring push box, the PD-6 unit was lowered into the introduction pit and positioned with the front gate straddling the old terra cotta pipe where it disappeared into the soil. Next, soil was backfilled around the working end of the unit to transfer thrust forces from the box to the excavation wall. This also minimized lateral box movement.

 

With hoses connecting the backhoe’s hydraulic system to the PD-6, the first rod section was fitted with a rounded end and inserted through the roller guide and into the old pipe. The rod was pushed into the pipe and another rod section was screwed in place. Because there was no significant resistance, muscle power was used to attach and advance additional rod sections. With about 30 feet of rod inserted into the pipe, it was no longer possible to push the rod by hand, so the hydraulic cylinder was put into use.

 

When the rod emerged in the exit pit, the bursting head was screwed to it, and the pipe grip was inserted into the end of the HDPE replacement pipe. As a threaded screw was turned into the gripper, the gripper’s body expanded within the pipe, tightly clasping the pipe between the gripper’s external collar and the expansion mechanism.

 

Because of the exit pit’s depth, the relative stiffness of the HDPE pipe and the need to make two sweeping 90-degree bends, the backhoe was used to position, guide and encourage the pipe into position at the point where it would enter the original pipe. Once in place, the gripper and bursting head were then joined by means of a clevis.

 

Work again focused at the introduction pit where the PD-6 was put into pulling mode. In 12-inch increments, the bursting head and trailing HDPE pipe were pulled toward the pit. As crew members took turns at the controls, each developed a rhythm that continued until the bursting head reached the area where the insertion was slowed by an unknown obstacle. Limited by the backhoe’s hydraulic capability, the auxiliary hydraulic power unit was put into service, and bursting continued.

 

When the head emerged in the introduction pit, the last rod section was removed from the bursting head. The backhoe was used to lift the shoring push box with the PD-6 out of the trench. The combined efforts of hand shoveling and backhoe excavation removed the previously placed stabilizing backfilled soil, and the head was exposed and removed.

 

The grip, still gripping the pipe, was connected to the backhoe bucket by a chain. With a motion resembling the opening of a fist, the action of the backhoe bucket drew the pipe farther into the pit. After the grip was removed, the pipe was trimmed and joined to the stub from the manhole. The process was repeated at the exit manhole. The holes were backfilled and the surface restored.

 

Observer comments

The PD-6 appeared to be an effective, uncomplicated machine. Worker safety within the shoring push box left little risk of injury. Workers’ hands and feet were naturally clear of the machine’s few moving parts during the pull. Operator training is minimal as the concepts are straightforward, and advanced skill levels are not necessary.

 

Manufacturer/user comments

“We selected this machine, and pipe bursting generally because together they offer the highest quality end product for our customers,” notes Mike Yarnell of Pine Mountain. “From a business owner’s perspective, this system is more productive than other technologies in linear feet completed per day and in dollars delivered to our bottom line. Every job site will bring challenges that cannot always be anticipated. At the end of the day, the repairs we set out to do are completed as planned.”

 

Jasinski of Pow-R Mole notes that the PD-6 machine has steerable horizontal thrust boring capabilities that enable installation of new pipes and cables in virgin areas.