Pilot Tube Guided Auger Boring Takes on Horizontal Directional Drilling

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Trenchless pipe installation such as pilot tube guided auger boring is 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.

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 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, Mass., 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 (CSO) control plan for the Boston Harbor cleanup. 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 ft of a new 8-in. by 4-in. 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 ft 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 ft below the river bed would be suitable for pipe installation

Project Challenges

Trenchless design engineers at SEA/Kleinfelder and Brierley Associates LLC originally designed the Little River crossing as an 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. As a result of prolonged and unsuccessful easement negotiations, a change of ownership for the property intended as the pipe 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 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. 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 29 ft.

HDD also required a 12.5-ft offset between parallel bores to minimize cross-hole disturbance and drill fluid contamination.
These challenges led 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

Kleinfelder/SEA Consultants, Cambridge, MWH Americas Inc. of Boston and Brierley Associates LLC, of Manchester, N.H., 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.

The pilot tube system afforded a smaller jobsite and layout than HDD, avoided impacting adjacent properties. In addition, the bore path using pilot tube guided auger boring would not have to be as deep as HDD to avoid frac-out and the horizontal offset between the utilities would only be 3 ft opposed to 12.5 ft with HDD.
Because of the findings, pilot tube guided auger boring became the approved method of trenchless pipe installation for the project.

The pilot tube machine required for the project was a Bohrtec BM600LS powered by a 75-kw hydraulic power unit and theodolite guidance system. Other equipment required included 5.5-in. OD, double wall pilot tube rods, bentonite lubrication system for pilot tube rods, 16.5-in. OD casings and augers and a 38.3-in. OD hydraulic powered extension kit for the 36-in. ID HOBAS jacking pipe. ICON supplied a 250-psi bentonite mixing pump and 3,000- psi, high-pressure water pump station for jetting. For the HDPE pipe installations, a 4-in. pulling adapter designed and manufactured by ICON Tunnel Systems and an 8-in. pulling adapter/expander cone designed and manufactured by Bohrtec were also required. For the 8-in. 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 4-in. HDPE lines for AT&T Telecommunications, one 8-in. HDPE line for NStar Gas and a 36-in. line for NStar Electric. Each drive would be 430 ft in length. The three 4-in. HDPE lines were first to be installed followed by the 8-in. HDPE line and finally the 36-in. pipe. The invert of the largest pipe was approximately 23 ft below surface grades surrounding the river.

These installations were completed using a two-phase method. Once the pilot tube machine was assembled in the jacking pit, contractor P. Gioioso & Sons Inc., Hyde Park, Mass., 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-in. HDPE pipe. As the pipe was pulled into place the pilot tube rods were removed in the jacking pit. Both the 4-in. and 8-in. pipe used fusible pulling heads supplied by POLY-CAM Inc., Anoka, Minn.

The 8-in. 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.

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 18,000 lbs. During the pullback process, the forces maxed around 7,000 to 8,000 lbs, well below the threshold.

Installation of the 36-in. 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-in. auger head to the last pilot tube to expand the bore path diameter. The 16.5-in. steel casings with augers would follow the auger head until it reach the receiving pit. The team attached the 38.3-in. hydraulic extension kit to the last 16.5-in. steel casing to increase the bore diameter to its final size. The 36-in. ID product pipe would follow the 38.6-in. hydraulic extension kit on line and grade to the reception pit.  The 36-in. HOBAS pipe installation is one of the longest for this size and type of pipe to date using this process of installation.

TT edited this article submitted by ICON Tunnel Systems, Kleinfelder/S E A and Brierley Associates LLC.

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