Microtunneling is a process that uses a remotely controlled Microtunnel Boring Machine (MTBM) combined with the pipe jacking technique to directly install product pipelines underground in a single pass. This process avoids the need to have long stretches of open trench for pipe laying, which causes extreme disruption to the community. In the US, microtunneling has been used to install pipe from twelve inches to twelve feet in diameter. Therefore, the definition for microtunneling in the US does not necessarily include size. Microtunneling has evolved in the US to describe a tunneling process where the workforce does not routinely work in the tunnel. Microtunneling is currently the most accurate pipeline installation method. Line and grade tolerances of one inch are the microtunneling industry standard. This can be extremely important when trying to install a new pipeline in an area where a maze of underground utility lines already exists.
The microtunnel installation of large diameter pipelines in a marine environment presents a considerable challenge to the contractor. In south Florida, the water table averages about two feet below ground level. The soils profile consists of a top layer of sand, followed by layers of limestone and sandstone. The two rock units are porous with solution cavities and sand filled voids. This creates a mixed face tunnel condition unique to this marine location. Shaft construction can be difficult. The high water table and the nature of the soils generate extreme uplift forces on the shaft floor. Even tight sheeting of the shafts cannot prevent in flow of groundwater. Exiting a shaft or entering a shaft with a microtunnel machine is further complicated by these same conditions. Typical soil stabilization techniques such as chemical grouting and ground freezing have met with little success due to the permeability of the formations. Permeability also makes drawing down the water table impossible.
Huxted Tunneling completed the Miami River Microtunnel Crossing in May of 1998. This project required the installation of 680 feet of 52" diameter pipe from Claughton Island, crossing under Biscayne Bay, to the mainland in downtown Miami. This line was 50 feet below ground level in order to have sufficient clearance between the top of pipe and the bottom of the river. The alignment crosses the mouth of the Miami River, a navigable waterway used by commercial shipping for goods to and from the Caribbean, Central and South America. This traffic could not be interrupted. "All pipe installation methods were considered, including marine pipe laying using a spud barge. The only method that limited impact on shipping traffic and local vehicular traffic was micro- tunneling" said George Aguiar, Utility Design Engineer with Miami Dade Water and Sewer Department. Another unique feature of this project was the need for two horizontal curves in the pipeline so as to avoid existing seawall piles when leaving and entering the shafts.
Pictured Above: 52" Microtunneling Head entering the exit shaft after crossing the Miami River.
In 1997, Huxted Tunneling completed the installation of a 66" O.D. prestressed concrete cylinder pipe crossing of the Intracoastal Waterway from Miami Beach to the mainland at the Sunny Isles bascule bridge. This project also required shaft depths of fifty feet in order to provide sufficient distance between the top of pipe and the waterway. Exiting the shaft with a thirty-eight foot head of water is tricky and can be dangerous. The risk lies in the fact that the water pressure tries to push the tunnel machine and the pipeline back into the shaft. There is no danger of this happening while the machine is being jacked forward. The problem arises at the end of the jack stroke, when the jacks are retracted to add another pipe. If the machine and pipeline are allowed to come back more than an inch or two, the rubber gasket surrounding the pipe will flip back, allowing water and soil into the shaft. If not planned for prior to the tunnel machine launch, this could cause shaft flooding in a matter of seconds.
Microtunneling is economically competitive with direct burial when depths exceed twenty feet due to the costs of deep trench excavation and trench support. This method is cost effective when faced with unstable soil conditions and work below the groundwater level. These conditions increase the risk of surface settlement during a direct burial or conventional tunnel installation. These conditions also increase liability for all parties with regard to property damage and personal injury. Microtunneling is the safest tunneling option when faced with these conditions, because workers and the public are not directly exposed to hazardous conditions.
On the Oakwood Beach Interceptor Sewer project in New York City, costs were compared between conventional tunneling and microtunneling. Six contracts were let, two microtunneling, and the remaining four by conventional tunnel shield. The rate of progress on the microtunneling contracts was almost three times the rate achieved with conventional tunneling methods.
Minimal Disruption/Superior Pipe
Microtunneling is particularly suited for the urban environment where the disruption of business, traffic and other utilities is not acceptable for commercial, political and safety reasons. These disruption costs alone can equal the cost of construction in a large urban area. An added benefit to the utility owner is that the pipe used in the microtunneling process has a premium gasketed joint and a longer service life. The longer service life also comes from the increased pipe wall thickness required for jacking pipe. Manufacturing and quality control standards are higher for jacking pipe. All things considered, the microtunnel installation of a pipeline provides a superior asset for the owner.
In 1998 Huxted Tunneling recently completed the first North American microtunnel installation of FRPM (fiberglass reinforced polymer mortar) pressure pipe on the Bennett Road Forcemain project for the City of Orlando, Florida. This project called for the installation of 19,000 feet of 36" ID forcemain, including almost 5,000 feet of pipe by micro tunneling at eight different locations. These environmentally sensitive areas were at major intersections where the FDOT and the local business owners would not accept a direct burial operation due to the disruption. The pipeline will be operated in the forcemain mode but is also required to operate as a gravity line. This required gravity sewer installation tolerances of plus or minus one inch. After installation, all of the pipe on the Bennett Road project was required to pass a 100 PSI hydrostatic test with zero leakage over two hours.
Improved steering precision was realized when Huxted Tunneling became the first microtunneling contractor to purchase a theodolite based guidance system. The Dywidag Electronic Guidance System replaced the laser based guidance system that is provided by all microtunneling equipment manufacturers and used by all other microtunneling contractors. The motorized total station theodolite provides a degree of accuracy that is not possible with a laser based system. This system will provide an as-built location of the pipeline at every tenth of a foot if required. The system is also designed to do curved pipe jacking.
Microtunneling is one of many pipeline installation tools available to the utility contractor. The benefits of this technology include:
Huxted Tunneling achieved another first and a North American record on the Bennett Road project by installing 1,000 feet of this pipe in a single drive. This is a significant advance considering that one weakness of FRPM pipe is a lower jacking force capacity when compared to other pipe materials of the same internal diameter. Historically, larger diameter pipe and inter jack stations are used to achieve long drive lengths. Inter jack stations are jacks placed inside the tunnel to provide additional jacking capacity or to provide a more even distribution of the jacking capacity. Inter jack stations are not practical in pipe diameters less than 42" as they must be dismantled and removed from the pipeline when jacking is complete. Huxted Tunneling eliminated the need for inter jack stations and accomplished this drive length record by effectively managing jacking forces through strict pipe lubrication procedures and precise steering control.
- Reduced disruption of the community.
- Reduced liability for personal injury and property damage.
- Increased service life and asset value for the utility owner.
- Increased worker safety.
- Reduced restoration costs.
- Precise installation.
- Wet Conditions/Marine Crossings: often the only option.
- Faster rate of progress than conventional tunneling.
Pictured above: Cooling water intake MTBM wet recovery.
Pictured above: Intake ready for valve installation.
Pictured above: Diver at work preparing MTBM for wet recovery.
Pictured above: Float attached to MTBM.
Pictured above: Cooling water intake pipe installation, 36" Permalok Steel Pipe.