November 24, 2015
While many pipe ramming projects have been celebrated for their sheer magnitude, large diameter pipe, difficult conditions or extreme distances, the pipe ramming method is often used in situations where precision is the key requirement to getting the project done.
Recently, Mocon Trenchless, Indio, Calif., completed a line- and grade-critical pipe ram in tight working conditions that highlights a side of this trenchless method that is sometimes overlooked.
The project took place in San Diego, as part of a multi-unit apartment complex development. The group overseeing the project — Garden Communities — is in the process of building several 16-story apartment complexes in the San Diego area. Mocon Trenchless was subcontracted to install three sewer lines running from the complex to the sanitary sewer main under highly traveled street. There were numerous underground utilities beneath the road and the general contractor did not want to open cut in that area, so a trenchless solution was preferable.
The first two sewer lines were successfully installed using a pilot tube system. During those installations, Mocon Trenchless crews encountered significant cobble, up to 10 in. in diameter. According to Mocon Trenchless president Rob Morrow, by the time they got to the third installation, bore logs indicated that ground conditions could potentially be worse in that location. In addition, the launch area for the third installation was inside a large excavation that was being developed as the underground parking area for the complex and about 25 ft below where the pipe would be installed. Building a suitable platform and backstop for a pilot tube installation was not possible in that location. Morrow elected to use pneumatic pipe ramming.
“We opted to ram the final casing because of the hardness of the ground and the presence of large cobble. We were hoping to either bust through the cobble or swallow it up in the casing,” Morrow said. “We were down in a large excavation where they were putting their underground parking area and we were probably about 20 to 25 ft below where the pipe needed to go. Because the pipe ramming method is self-contained, we didn’t have any significant engineering issues preventing us from using that application in that setting.”
For the project, Mocon Trenchless used a 10-in. diameter Grundoram Gigant from trenchless equipment manufacturer TT Technologies, Aurora, Ill.
Mocon Trenchless is a pipeline rehabilitation contractor serving the western United States. Offering a number of trenchless pipe rehabilitation services for wastewater, storm water, potable water, etc., Mocon Trenchless is recognized throughout the region as a leading trenchless contractor.
“Mocon originally started in Oregon over 30 years ago by my father,” Morrow said. “He was a second-generation contractor working on sewers, bridges and highways. So, I’m the third generation carrying on the Mocon name. Today, the company has really focused on the trenchless market and the more difficult specialty applications.”
Pipe Ramming Possibilities
Ramming tools essentially drive a pipe or casing through the ground with repeated percussive blows. During pipe ramming an entire length of pipe can be installed at once or, for longer runs, one section at a time can be installed.
“Basically a large range of pipe sizes and lengths can be installed through pneumatic pipe ramming, anything from 4 in. in diameter all the way up to 144 in. in diameter,” said TT Technologies pipe ramming specialist Scott Kneip. “The 10-in. diameter Gigant, which Mocon used, is somewhere in the middle of pipe rammers in terms of size and power. The Grundoram Apollo at 32 in. in diameter is the world’s most powerful ramming tool.”
Ramming can be used for horizontal, vertical and even angled applications. It is often used under roads and rail lines because it displaces the soil without creating voids or slumps. According to Kneip, another key benefit of the pipe ramming method is its ability to function in difficult rock, cobble and boulder filled soils. “Pipe ramming works well in difficult soil conditions,” he said.
“Boulders and rocks, as large as the casing itself, can be swallowed up as the casing moves through the soil and can be removed after the installation is complete. That’s one of the reasons that ramming was chosen for this project. ”
Pipe Ram Precision
For ramming in difficult conditions, a cutting shoe is often welded to the front of the lead casing to help reduce friction and cut through the soil. The cutting shoe fabricated by Mocon Trenchless included welded-on carbide bits to help break up the soil and any cobbles. “Because the ground was so hard in that area, we didn’t want the casing to literately bounce off the soil,” Morrow said. “The carbide bits would help break up the soil and keep the casing moving. The bits were welded onto the cutting shoe in a symmetrical pattern so that the casing didn’t veer off course.”
The new 12-in. steel casing needed to be installed approximately 85 ft with a 2 percent fall. The path took the casing over a large storm drain and just under a 30-in. water main. According to Morrow, there was about 5 in. of clearance under the main, which didn’t leave much room for error.
An I-beam was set up on line and grade to serve as a platform and guide for ramming operations. Once crews were satisfied with the ramming platform, the lead casing was put into place, and the connection was made between the casing and the ramming tool using a standard ram cone with soil port.
Crews began ramming the first 21-ft casing segment at partial power. During installation of the first segment, Mocon crews encountered resistance at the 15-ft mark, but were able to break through the obstacle after about 10 minutes. Once the lead casing was in place, the next 21-ft section of pipe was welded to the back end of the lead casing and ramming continued.
“Ramming times for the first casing were between one and two inches per minute at greatly reduced power,” Morrow said. “By the time we got about a casing and half in, we opened it up more and installation times improved to one to two feet per minute. It took about 45 minutes to complete the welds, and an hour and a half total installation time between casings. We pumped about 250 gallons of bentonite during the ram and ended up driving the casing to within 5 ft of the water main.
“Because of traffic restrictions, we needed to expose the water main, which was located in the median next to the street, at night. The street was particularly busy and the city only allowed a lane closure during nighttime hours. That was the reason for the trenchless approach to begin with, as the city did not want to close the street for an open cut project. We installed the pipe at a depth of 13 ft.”
With the casing installed, Mocon crews needed to remove the spoil from the casing. Several methods can be used to remove spoil including compressed air, water, an auguring system or other types of earthmoving equipment. Mocon crews used compressed air with a polyurethane pig. After inserting the pig, a seal plate was placed on the end of the casing and bentonite was pumped inside. Crews then applied compressed air to the casing, forcing the pig and the spoil out.
With the spoil cleared from the casing, crews inserted the new 8-in. HDPE sewer lateral and tied into the manhole. Mocon crews then grouted between the new pipe and the steel casing.
“This was a very well executed ramming project. Rob and his crews are very experienced in trenchless installation methods. They’re great to work with and great on the job,” Kneip said.
Jim Schill is a technical writer in Mankato, Minn.