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Lengthy microtunnel project beats schedule by 8 months: BRH-Garver uses 2 MTBM to install nearly 15,000 feet of force main.

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The city of Houston has an ongoing program to rehabilitate and maintain its wastewater collection system--no small task considering the system includes more than 6,700 miles of sewer pipelines ranging in diameter from 2 to 144 inches, and more than 120,000 manholes. These sewer lines are buried from 2 to 80 feet deep.

The structural component of the program involves rehabilitating more than 950,000 linear feet of sewer lines over a 10-year period. While construction can be disruptive to neighborhoods, the city's engineering and construction management groups are nationally recognized for the use of various trenchless construction methods to limit inconvenience to residents and businesses.

Those methods include making point repairs, pipe bursting and cured-in-place lining, plus removing and replacing pipes employing various methods. Progress is achieved one segment at a time.

BRH-Garver Construction LP recently completed the installation of 14,720 feet of 54-inch diameter centrifugally cast fiberglass reinforced polymer mortar (CCFRPM) pipe, replacing a smaller diameter sewer main. The project was in the Bellaire and Meyerland areas of Houston with the pipe route adjacent and parallel to existing and abandoned sanitary sewers and existing force mains.

In addition to installation of new pipe, the project included: one junction box to connect two existing 26-inch force mains; a cured-in-place manhole on an existing 72-inch MRC sewer for the downstream tie in; 25 manhole replacements; abandoning 21,066 linear feet of old, existing force main; traffic control; protection of trees; stormwater pollution protection; and site and pavement restoration. Work was completed eight months ahead of schedule.

The project manager, for the city of Houston Department of Public Works and Engineering was Brian Alcott, P.E.

No disruptions

"Trenchless construction was specified because the city didn't want to disrupt several major thoroughfares on the alignment," said Vanessa Moreno, BRH-Garver project manager. "Microtunneling was specified over other trenchless methods due to the accuracy needed to meet the extremely flat design grade."

Pipe installation via microtunneling is a single-step process; the microtunnel boring machine (MTBM) is advanced by pushing it with the pipe. Logistics of such a project are complex, and meticulous planning is essential.

"The project's traffic control plan," Moreno said, "was designed to have a total of seven segments to allow for installation of pipe as well as shaft installation, ahead of the microtunneling and manhole installation on completed segments.

"With the traffic control in place, we were able to schedule equipment to construct one jacking shaft and two receiving shafts per section. The jacking shaft was constructed first, then the two receiving pits."

Jacking shafts 40 feet long, 20 feet wide and 25 feet deep were installed as soldier piles by pre-drilling to drive steel beams (H-piles) to a depth of 10 feet below finish grade. The shaft was completed by driving steel plates and sheet piling between the beams as vertical excavation advanced.

Receiving shafts were typically 12 feet in diameter and constructed using a combination of drilling, machine excavation and hand excavation.

"Once we had a jacking shaft and two receiving shafts installed, we strategically placed the tunnel equipment around each jacking shaft to make enough room for our crane, loaders and dump trucks," Moreno said. 'We had multiple crews on site--two microtunnel crews; one or more shaft installation or backfill crews, depending on need; and restoration crews. The use of two microtunnel crews helped achieve the early completion.

'Tunneling was performed by the slurry microtunneling methodology employing the use of a closed-face MTBM," said Moreno. "Both machines were Soltau RVS models: a RVS 600 (600-ton jacking force) and a modified RVS 250 (400-ton jacking force)."

Face support is achieved by providing pressure at the excavation face by use of slurry pumps, pressure control valves and flow meters.

"The system," Moreno said, "includes a slurry bypass unit to allow direction of flow to be changed and isolated as necessary, and a separation process to provide adequate separation of spoil from slurry so that the slurry, with sediment content within the required limit for successful tunneling, can be returned to the cutting face for reuse."

Slurry composition

The system, Moreno continued, also used a separation process suited to the size of tunnel being constructed, soil type being excavated and the space available at each work area for the operating plant. That allows composition of the slurry to be monitored to maintain slurry weight and viscosity limits required for proper operation.

"The Soltau equipment has a remote control capability allowing operation without the need for personnel to enter the tunnel," Moreno said. "It has a display available to the operator, a remote operation console showing position of the shield in relation to design reference, together with other information such as face pressure, roll, pitch, steering attitude, valve positions, thrust force, cutter head torque, rate of advance and installed length of pipe."

The MTBMs active direction control maintains line and grade with a guidance system that relates actual position of the MTBM to the design reference by a laser beam. The laser is transmitted from the jacking shaft along the pipe string to a target mounted in a shield, providing active steering information and position information which is monitored by the operator at the control console.

Soils were primarily clay and sandy clay with sections of fine silty sand. Water table varied from 16 to 26 feet. Installation depth was between 22 and 26 feet.

Nineteen microtunnel runs were made of which the longest was 990 feet. Two runs were installed via hand mining for fear of conflicts with existing utilities.

The 540-ton CCFRPM jacking pipe chosen for the project was manufactured by Hobas Pipe USA. Pipe joints were 20 feet long. Because of the limited area of available space, pipe was delivered to the project as needed. The fact that the Hobas plant is in Houston plus the cooperation Hobas provided made such a schedule possible.

"The Soltau MTBMs are the two most powerful pieces of jacking equipment owned by BRH-Garver," Moreno said. "The slurry separation system used Brandt shakers and hydrocydones and a Derrick large-bowl centrifuge."

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Microtunneling work was completed in February 2016, with manhole installation, junction box construction and restoration finished in May 2016. The system tie-in was to be completed following resolution of a DS tie-in design issue.

"Overall, this project had few unexpected factors," said Moreno, "but those that did occur were addressed quickly. At the downstream line, we had to dewater at least four shafts due to underground conditions not listed in our geotech report."

"We were able to significantly beat the schedule due to the use of two microtunnel boring machines, the efforts of our project management staff and joh-site personnel," said David Ellett, BRH-Garver vice president. "David Kennedy and Vanessa Moreno were instrumental in coordinating the thousands of details needed to successfully perform a project of this scope."

FOR MORE INFORMATION:

BRH-Garver Construction L.P., (713) 921-2929, brhgarver.com

Hobas Pipe USA, (800) 856-7473, hobaspipe.com

Soltau Microtunneling Inc., (843) 2i6-1819

By Jeff Griffin, Senior Editor
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Author:Griffin, Jeff
Publication:Underground Construction
Date:Sep 1, 2016
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