Pilot Tube Guided Auger Boring Takes on Horizontal Directional Drilling (HDD)
by ICON Tunnel Systems, Kleinfelder/S E A and Brierley Associates, LLC
Trenchless pipe installation such as Pilot Tube Guided Auger Boring is quickly becoming a popular way to install pipe as an open cut alternative. This method minimizes disruptions and costs, while proving to be an effective way to install pipe.
Pilot Tube Guided Auger Boring was developed in Germany by Dr. Peter Uffman of Bohrtec for the installation of trenchless house connection sewer pipes. The technology was later developed further to install mainline sanitary sewer pipes on line and grade over 300’ from manhole to manhole as an alternative to expensive tunnel boring machines.
For years, pilot tube machines have been installing steel, clay and other types of jacking pipes, but we are now seeing these systems expanding their role into High Density Polyethylene (HDPE) and other pullback pipe installations. Pilot tube machines can now install multiple types and sizes of pipe on one project while still achieving the pinpoint like accuracy for which they are known.
A recent project in Cambridge, MA highlights the pilot tube machines’ application versatility by installing HDPE and Hobas pipe as well as the benefits over Horizontal Directional Drilling (HDD) for this particular project.
The Cambridge Park Drive Drainage Improvement project is one aspect of the Massachusetts Water Resources Authority’s (MWRA) long term combined sewer overflow control plan for the Boston Harbor clean-up. This project was a key element of the Alewife Sewer Separation Project that will separate the combined sanitary wastewater and storm water infrastructure in the West Cambridge portion of the city.
Construction of this project will provide significant environmental benefits by eliminating the existing combined sewer outfall near the city’s public drinking water supply, minimize flooding in West Cambridge, protect Fresh Pond Reservoir from flood waters, and will reduce the discharge of pollutants to the Little River, which is tributary to the Alewife Brook, Mystic River, and Boston Harbor.
This project involves the construction of approximately 3,000’ of a new 8’ by 4’ box culvert that crosses several large business parcels and nine rail crossings, two of which are active high speed commuter rails. The box culvert will convey the newly separated storm water to a new storm water wetland basin where pollutants found in storm water will be treated by native vegetation and aquatic plants, detention, soil infiltration, and evaporation/transpiration before being discharged into the Little River.
Existing utilities of water, sewer, electric, gas and telecommunications all crossed the area of the new wetland basin and needed to be relocated as one of the first phases of work. The existing utilities cross the reservation and the Little River, both above and below-grade.
A soil investigation that included 17 test bores drilled to depths of 18’ to 37’ below ground surface found the following soil from ground surface down; fill, organic deposits, marine sand and marine clay. Engineers determined that the marine clay soil located approximately 8.3 feet below the river bed would be suitable for pipe installation.
Trenchless design engineers at SEA/Kleinfelder and Brierley Associates, LLC originally designed the Little River crossing as a horizontal directions drilling (HDD) installation. However, after the project was bid and awarded, the property owners on either side of the HDD crossing had concerns of construction disruption such as the considerable construction footprint needed for the HDD staging areas, and the extensive pipe laydown areas and drill path set up that would impact business operations and employee parking. As a result of prolonged and unsuccessful easement negotiations, a change of ownership for the property intended as the laydown area, as well as a new accelerated completion deadline by the property owner at the other end of the HDD, the project team realized that another construction method was needed.
The risk of frac-out was also a concern. HDD uses drilling fluids (drill mud) during the drilling and reaming procedure that are pumped at a high pressure through the drilling tools in order to transport/remove soil cuttings back to the receiving pit and to cool the drill bit, reamer and tracking system. Frac-out can be caused at times when the drill mud is being pumped at a high pressure while drilling in looser soil that cannot contain these pressures. When frac-out occurs, drilling fluids get injected into the surrounding soils/environment and possibly other surrounding utilities. To avoid this problem using HDD at the Cambridge project, operators would need to install the pipe at a depth sufficient to contain the drill fluid pressure, yet not so deep that the integrity of the HDPE pipe would be compromised by the ground pressure, also known as buckling. The minimum depth below the bottom of the river to avoid frac-out, as determined by the design engineers, was approximately 29ft.
HDD also required a 12.5’ offset between parallel bores to minimize cross-hole disturbance and drill fluid contamination.
These challenges lead the city, consultant team and the contractor to search for an alternative method for pipe installation. This portion of the project essentially became a design/build entity. The team would need to redesign the utilities crossing portion of the project in order to keep the project on budget and within the new constraints.
Utility Installation Solution:
Engineering consultants Kleinfelder/SEA Consultants of Cambridge, MWH Americas Inc. of Boston and Brierley Associates, LLC, of Manchester, NH evaluated the possibility of Pilot Tube Guided Auger Boring as an alternative utility installation method for the project. After meeting with ICON Tunnel Systems regarding the Pilot Tube system and capabilities, the engineers developed an HDD/Pilot Tube comparison spec table.
Pilot Tube Equipment:
The pilot tube machine required for the project was a Bohrtec BM600LS (265 tons jacking force and 22,500 ft-lbs torque) powered by a 75 Kilowatt Hydraulic Power Unit and Theodolite guidance system. Other equipment required included 5.5” O.D. double wall pilot tube rods, bentonite lubrication system for pilot tube rods, 16.5” O.D. casings and augers and a 38.3” O.D. hydraulic powered extension kit for the 36” I.D. Hobas jacking pipe. ICON supplied a 250psi bentonite mixing pump and 3,000psi-high pressure water pump station for jetting. For the HDPE pipe installations, a 4” pulling adapter designed and manufactured by ICON Tunnel Systems and an 8” pulling adapter/expander cone designed and manufactured by Bohrtec were also required. For the 8” HDPE line, NSTAR Gas, the local utilities provider, required that the forces acting on the pipe did not exceed a certain threshold. A special sensor was selected by the consulting team to monitor the forces being placed on the HDPE pipe during installation. The sensor system used was a Tensi Trak unit manufactured by Digital Controls Inc.
The utilities that would be installed consisted of three (3) 4” HDPE lines for AT&T Telecommunications, one (1) 8” HDPE line for NStar Gas and a 36” Hobas line for NStar Electric. Each drive would be 430 feet in length. The three (3) 4” HDPE lines were first to be installed followed by the 8” HDPE line and finally the 36” Hobas pipe. The invert of the largest pipe was approximately 23 feet below surface grades surrounding the river.
The installation process consisted of constructing two pits, a jacking pit and receiving pit. The jacking pit was 26’L x 20’W x 24’D in size where the Pilot Tube machine would be positioned and the receiving pit was 20’L x 20’W x 20’D in size.
4” and 8” HDPE Installations:
These installations were completed using a two phase method. Once the pilot tube machine was assembled in the jacking pit, the contractor, P. Gioioso & Sons, Inc. of Hyde Park, MA along with ICON Tunnel Systems would start the pilot bores. Once the pilot bores reached the receiving pit successfully on line and grade, the team would then remove the pilot head and replace it with the pulling adapter which was attached to the HDPE pipe. A Tensi Trak sensor unit was used when pulling back the 8” HDPE pipe. As the pipe was pulled into place the pilot tube rods were removed in the jacking pit. Both the 4” and 8” pipe used fusible pulling heads supplied by POLY-CAM, Inc. out of Anoka, MN.
The 8” pulling adapter/expander that was used has three water jets in front of the cone and a single bentonite port located in the back. The water jets assist in loosening the soil ahead of the expansion cone. This reduces the pulling forces acting on the pilots, adapter and the pipe. The bentonite is used to lubricate the outside of the pipe and bore path reducing skin friction.
Sensor Tracking Results:
Monitoring the forces placed on the HDPE pipe during installation was essential because if the pilot tube system exerts too much pulling force on the pipe it could elongate which reduces the wall thickness. If this should happen, would reject the pipe and the team would have to perform another installation, wasting valuable time and money.
The force threshold proved by for the HDPE pipe was 18,000lbs. During the pullback process, the forces maxed around 7,000 to 8,000 lbs, well below the threshold.
36” Hobas Pipe Installation:
Installation of the 36” Hobas pipe required the team to perform the traditional Pilot Tube Three Phase installation. They first established the pilot bore successfully on line and grade. They then attached the 16.5” auger head to the last pilot tube to expand the bore path diameter. The 16.5” steel casings with augers would follow the auger head until it reach the receiving pit. The team attached the 38.3” Hydraulic Extension kit to the last 16.5” steel casing to increase the bore diameter to its final size. The 36” I.D. Hobas product pipe would follow the 38.6” Hydraulic Extension kit on line and grade to the reception pit. The 36” Hobas pipe installation is one of the longest for this size and type of pipe to date using this process of installation.
What was Learned on the Cambridge Project:
• The project footprint was reduced by using Pilot Tube Guided Auger Boring.
• 5 pipes can be installed in close proximity out of one pit using Pilot Tube Guided Auger Boring. (3) 4” HDPE, (1) 8” HDPE and (1) 36” Hobas pipe.
• Installation of various types and sizes of pullback and jacking pipes (HDPE and Hobas) can be installed successfully on line and grade using one method and machine (Bohrtec BM600LS) instead of multiple methods and machines to accomplish this same task.
• Pilot Tube Guided Auger Boring reduced the risk of frac-out under the river versus HDD on this project.
• Given the success of the pullback installations on this job, we can safely say the Pilot Tube Guided Auger Boring process is a 100% field proven method for the successful installation of HDPE or other plastic pullback pipe for that matter.
Typically, projects that require HDPE pipe to be installed are designed with the HDD method of installation. By converting the project to the Pilot Tube Method, the team was able to meet their worksite constraints set by the property owners and achieve a successful installation of HDPE pipe with a bullet like line and grade accuracy without the fear of frac-out due to the specialized Pilot Tube Guided Auger Boring equipment used and supplied.
This project was undertaken by the Cambridge Department of Public Works (CDPW), in coordination with the Massachusetts Water Resource Authority and MA Department of Conservation and Recreation. Engineering consultants were John J. Struzziery, P.E.,Kleinfelder/S EA Consultants of Cambridge; MWH Americas Inc. of Boston; Nick Strater, P.G., Brierley Associates, LLC, of Manchester, NH; and the Bioengineering Group of Salem, MA. The general contractor for the project was P. Gioioso & Sons, Inc. of Hyde Park, MA and the Pilot Tube equipment and operators were provided by Daniel Paster and Andreu Kerry, ICON Tunnel Systems, East Brunswick, NJ. Ph: 800-836-5011 www.icontunnelsystems.com.