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Pilot Tube Microtunneling Proves Effective Under Busy Houston Street
For the Westheimer project, Mann said an Akkerman 240A single stage jacking frame was used. Eleven drives were made, ranging in length from 57 to 301 feet. Most of the drives fell between 240 and 260 feet. Work shafts were at manhole locations. The guided boring machine (GBM) was placed in 8 by 12 foot trench boxes dug at manhole points. Subsurface soil conditions consisted of stiff, firm clay. The work area was in the center median to allow outside traffic lanes to remain open.
Lengths of clay pipe installed by pilot tube microtunneling were:
• 8 inch pipe, 260 feet;
• 10 inch pipe, 760 feet; and
• 12 inch pipe, 1,420 feet
Pipe joints were one meter in length which fit easily into the trench boxes.
The Akkerman 240A single stroke jacking frame can be accommodated in shafts 8 to 12 feet wide, and is capable of producing 100 tons of jacking force, 50 tons of pullback and 10,000 foot pounds of rotary torque. Its theodolite guidance system is designed for an accuracy of one quarter inch at 300 feet. Dual wall pilot tube design provides lubrication of pilot shaft as it passes through the ground, reducing the amounts of thrust and rotational force during installation. The unit's power package was positioned on the surface.
Vitrified clay pipe is used for more pilot tube installations than any other type of pipe, said Jeff Boschert, P.E., field engineer for the National Clay Pipe Institute.
"Clay pipe has high compressive strength for jacking, is chemically resistant to sewer gases and short, one meter lengths are perfectly suited to pilot tube technology," he added.
The Mission Clay No Dig jacking pipe used on the Westheimer project employs resilient gasket material and stainless steel collars at each low profile joint resulting in a leak free system. A chipboard compression ring distributes jacking forces from one section of pipe to the next.
For the tunneling industry, No Dig pipe is available in sizes ranging from 8 to 36 inches in diameter. Since the product was introduced in 1992, more than 300,000 linear feet of pipe have been installed on tunneling projects in the Unites States, said Billy Nelson, area manager for the No Dig Division of MCP Industries.
Magnum Tunneling completed its pilot tube segments over a two month period. Project owner is the city of Houston Department of Public Works and Engineering. Brown & Gay Engineers, Houston, was the consulting engineering firm on the project.