Pipe Ramming Through Glacial Wisconsin Conditions

Globe Contractors, Inc., of Pewaukee, Wisconsin, recently completed two successful casing installations for sanitary sewer systems that tested their planning savvy, adaptation ability and persistence under pressure. The major challenge of underground utility construction is integrating knowledge, experience and the best available technology to the task at hand to increase the probability of successful completion. Along the way, there are always challenges to overcome.

The local municipality of Beaver Dam, Wisconsin, wanted a new gravity sewer line installed under a state highway to service a middle school that was under construction. According to Bob Olson, Treasurer of Globe Contractors, Inc., changes to the original bid had to be made from the beginning of the project.

"We originally anticipated using an auger boring machine to install 20- or 24-inch steel casing to house the 8-inch sewer line over the 170-foot distance, but we quickly changed our method and the size of the casing when our excavator took its first bite of earth for the launch pit and dug up glacial boulders."

The excavator brought up basketball-size boulders that the glacial till of Wisconsin had buried. Glaciers that covered Wisconsin thousands of years ago plague underground contractors yet today. Fearing that the boulders would get caught in the flighting of the auger boring machine, Globe's crew quickly switched their plan to pipe ram 30-inch casing, with the help of 12- and 23-inch HammerHead Moles.

Olson explained why the casing was upsized. "Normally, we will use casing that is double the size of the carrier pipe. In this case, we upsized the casing even more because of the soil conditions. We tend to take a conservative approach. Our plans employ methods that increase the probability of success and reduce the risk of failure. In underground conditions with big rocks, there is always a risk of not making it.

So, despite the extra expense, we felt that the larger-size casing reduced the margin for error, especially given dealing with a gravity sewer project with tight tolerances."

Part of the crew prepared the launch pit for the HammerHead Mole and the welded 40-foot pipe section some 30 feet below the surface of the highway. Others were back at Globe's construction yard welding two 20-foot sections together for the first part of the ram, and welding onto the lead pipe a soil shoe, and lubrication lines.

Olson described why much of the preparation work was done off-site. "Basically, anything we can do to reduce on-site labor time improves the efficiency of the job.

Since we've learned that ramming time is such a small percentage of the overall time on a pipe-ramming job, and because on-site welding takes up so much time, we try to get as much welding done off-site prior to the start of a project."

Since the gravity sewer line had such a tight tolerance, the launch pit was carefully leveled. Heavy 6x6 pieces of lumber were set into the base of the launch pit and excavators lowered the welded 40-foot pipe section and the 12-inch HammerHead Mole into position.

Once in position, concrete was poured over the timbers and heaped aside the pipe to act as solid guides for this pipe section and for the ones that would follow.

"Even though we had to wait an extra day for the concrete to cure, we think it was worth it," stated Olson. "With a critical grade over this long push, we felt that the concrete supports were needed to maintain grade."

The actual start of the push occurred on a Monday afternoon with the 12-inch HammerHead Mole starting the ram. Once the lead pipe was started in the ground and the grade established, the 23-inch HammerHead Mole was lifted into place and locked into the collets. "We used this tandem team of rammers on purpose," Olson explained. "We thought that because of the heavy weight of the 23-inch tool relative to the weight of the first pipe section, the power of the 23-inch tool might overpower the lead pipe and lose the grade at the critical first section of the push.

The 12-inch tool served its purpose by allowing us to start the push slowly and get the grade of the pipe started exactly where we wanted it."

The entire 170 feet of the 30-inch casing were rammed in by Wednesday morning. Each 40-foot pipe section was rammed in under 30 minutes with the welding of the pipe sections taking up the rest of the work time. Total actual ramming time was under 2.5-hours. The spoil inside the pipe was pneumatically expelled. The casing was installed on grade less than .75" off its target grade.

Olson was extremely satisfied with the results. "The grade came out right on the mark; if I had known earlier that it would turn out this good, I would have downsized the pipe size to 24-inch, but these are things you never know for certain up front."

Tough ground conditions proved even more difficult on a second ramming job that Globe Contractors, Inc., accomplished with the 23-inch Vermeer Hammerhead Mole.

The project called for the placement of 130 feet of 48-inch diameter casing under a road south of Waukesha, Wisconsin, to house a 24-inch gravity sewer line. With this large-size casing the 23-inch HammerHead Mole was used at the outset.

"We had to pipe ram this casing," said Bob Olson. "Ground conditions made it impossible to consider auger boring. The ground was a wet, heavy gravel with very few fines in it. Even though the casing would be 25 feet under the road, if we had tried to auger bore this job, gravel would have come pouring down and risked the possibility of creating voids and settling the road. The only other way would be to hand tunnel it, but that method would take months to accomplish."

The 23-inch HammerHead Mole rammed in the first 80 feet of casing the first day. The remainder of the project was spent alternately augering out the heavy gravel spoil from inside the large-size casing to lessen the friction against the pipe and ram in more of the casing.

The heavy, wet gravel was under such strong ground-water pressure that despite the continuous cleaning of the casing, it created a binding effect. Even through the difficult soil conditions, the casing was successfully driven in and the carrier pipe installed.

"Compared to the alternatives I had, this tool did a good job," Olson stated. "It worked well in this situation. Hand tunneling and auger boring were simply not very good alternatives in these ground conditions. The larger HammerHead ramming tool provides the capability to drive larger-size casings longer distances. It increases our confidence in succeeding in tough ground conditions."

Written By: Richard Yach - Technical Writer Des Moines, Iowa
Provided By: Vermeer Manufacturing Company - Pella, Iowa