Pipe Ramming Aids in Boston HDD River Crossing

Southeast Directional Drilling (SEDD), Casa Grande, Ariz., continues to prove why it is one of the premier drilling contractors in the world. On a recent project in Boston, SEDD overcame several challenges to complete three separate bores under the Chelsea River, adjacent to Boston’s Logan airport.

SEDD crews used a Conductor Barrel HDD-assist technique to ensure drilling went smoothly, as well as to combat contaminated soil issues. SEDD used a pneumatic pipe ramming system from TT Technologies, Aurora, Ill., to install three 48-in. diameter Conductor Barrel casings.

According to TT Technologies pipe ramming specialist Rick Melvin, this particular technique is effective when boring under rivers. “The concept behind the Conductor Barrel is creating a clear pathway through poor soil conditions so that drilling can begin in more preferable soil conditions,” he said. “The Conductor Barrel technique is often used for river or water crossings. The length of the Conductor Barrel is determined by soil conditions, the angle of the bore and the depth of the crossing below the waterway. In addition, the Conductor Barrel can prevent situations in unstable soils where drilling fluids under pressure force their way into waterways or wetlands, acting in a similar fashion to containment cells.”

Mel Olson, SEDD vice president of operations, municipal/industrial division, said in the case of the Chelsea River project, the installed Conductor Casings also help prevent the infiltration of contaminated groundwater, a priority for project owner, NSTAR, Massachusetts-based, investor-owned electric and gas utility. “There were a number of contaminates identified in the preliminary subsurface investigations, everything from creosote to benzene,” Olson said. “One of the key aspects of this job was dealing with that contaminated soil and preventing contaminated groundwater from infiltrating into the bore path and end up in NSTAR’s manholes and vaults. We took several measures to prevent that situation.”

SEDD crews are also well versed on HDD-assist techniques. Over the years, TT Technologies has introduced several pipe ramming techniques that help drill operators solve drilling problems and tackle tough conditions. On the Chelsea River project, the Conductor Barrel technique was used.

During the Conductor Barrel process, casings are rammed into the ground, at a pre-determined angle, until desirable soil conditions are met. The spoil is removed from the casing with an auger or core barrel. Drilling proceeds within the casing in the desirable soil conditions.

On the Job

The project for NSTAR included three separate bores under the Chelsea River. The project was ultimately completed to improve electrical back up and redundancy for Boston’s Logan Airport. SEDD was subcontracted to perform three 1,600-ft bores to place electrical conduit under the river.

In order to facilitate the bores, SEDD installed three 48-in. diameter Conductor Barrels on the north side of the crossing. Each conductor barrel was 120 ft long and installed at a 16-degree point on entry angle. “The entry angle was chosen in part due to space limitations. Obviously working in an urban environment we’re restricted with real estate on either side,” Olson said. “Plus, there was a brick-lined sewer main that was pile-supported, so we needed to go in at 16 degrees to achieve an adequate distance from our bore hole and the foundation of the brick sewer.”

To install the conductor barrel casings, the SEDD crew used a Grundoram Taurus and a single 1,600-cfm compressor. The SEDD crews used the drill rig bed angled to guide the Taurus during ramming operations and then fabricated a shoe that ran up and down the tracks of the drill rig to further guide the pipe rammer. Crews were able to weld two 20-ft casings together and ram 40-ft sections at a time. While ramming times were good, it took about a week to complete each installation with the removal of contaminated soil from within the casing accounting for much of the time. The three casings were installed approximately 20 ft apart. Once installed, SEDD crews removed the spoil from each casing with 42-in. auger segments connected through a box pin connection to the drill stem of the drill rig.

Contaminated Soil

According to Olson, the contaminated soil, and, ultimately, contaminated groundwater, posed a significant challenge and steps were taken to protect the site from contamination. “The pit area, where we installed the casings, was flow filled and sealed off to eliminate the potential of contaminated groundwater infiltration,” Olson said. “The entire area where the conductor barrel casings were installed was sheet piled off prior to installation and the exterior face of that sheeted pit was excavated down and sealed off with flow fill. The casing themselves were driven through cut outs in that steel sheeting and through the flow fill seal. Once the casings were installed to design depth and length, we provided an additional seal by pouring that flow fill material into and around the casings themselves.

“And the main reason for all of that seal was to contain contaminated groundwater that could migrate in and around the conductor barrel casings. What NSTAR has discovered as groundwater levels rise and fall naturally, is that a number of their manholes fill with contaminated groundwater. It becomes an issue then for them to access those vaults because of exposure to contaminate. So this was done as a preventative measure.” 

In addition to the measures taken to seal off the conductor casing, the entire conduit bundle was encapsulated with fill and sealed off so that groundwater would not following along the trench or HDD alignment and find its way into the manholes.

River Crossing

SEDD used a large directional drill with 500,000 lbs of pullback to complete the river crossings. Prior to the pilot bore, SEDD crews added a 16-in. centralizer pipe inside the 48-in. conductor casing in order to make sure the borehole would be aligned with the installed Conductor Barrel. Each pilot bore took approximately two days to complete using a 10 5/8-in. drill bit and was drilled out from the north side of the river, through the Conductor Barrels. SEDD crews then moved the drill rig to the south side of the river to complete reaming and final pullback.

“A single 42-in. back reamer was used on the first bore hole. For the next two, we used 24- and 38-in. reamers in tandem as we felt this would yield a better result,” Olson said. “Bentonite pumped continuously through each bore. And a swab pass was done to ‘prove the hole’ after reaming was complete. The soil itself was a mixture of sands, clays and silts.” 

Once reaming was complete, pullback was ready to begin. Each conduit bundle was comprised of seven 8-in. diameter HDPE conduits. The conduit bundle had a diameter of approximately 26 in. The bundle was not banded. A cradle assembly was used to bunch the bundle together as it entered the Conductor Barrel. Each pullback took just under 10 hours to complete.

Post installation, SEDD was required to do testing on each of the conduits to prove their integrity including running gauging plate pigs through, as well as hydro testing. “Once they were successfully tested and accepted by the owner, we turned the project over to the general contractor as they were ready to jump in and start the tie in process.  The entire project took approximately three months from start to finish,” Olson said.

According to Melvin, the SEDD crews have become extremely proficient at the pipe ramming HDD-assist techniques. “The Southeast Directional Drilling crews are exceptional,” he said. “The company owns two Grundoram Taurus pipe rammers and have them at the ready for any HDD situation they might encounter. They do a quality job and it really showed on the Chelsea River project.”  

Jim Schill is a technical writer in Mankato, Minn.
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