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Microtunneling Begins Under Virginia’s Neabsco Creek

Thu July 31, 2008 - Southeast Edition
Angela B. Hurni

As work continues above ground on the contract to replace and widen the Neabsco Creek Bridge at Route 1 over the Neabsco Creek in Prince William County, E. Ann Jackson Inc. is working underground. The Petersburg, Va.-based company is microtunneling under Neabsco Creek in this Northern Virginia county so that water and sewer lines can be installed with minimal disturbance to the environment.

E. Ann Jackson is a subcontractor for Phillips Construction LLC, Henderson, Ky., who was awarded an $18 million contract by Virginia Department of Transportation (VDOT) for the replacement of the Neabsco Bridge. The project site extends from Neabsco Road (Va. 610) to Neabsco Mills Road (Va. 638). The bridge is being replaced and increased in height because it is 70 years old, and it is prone to flooding.

Work began last December on the contract for the new bridge, which will be 240 ft. (73 m) long and 9 ft. (2.7 m) higher than the existing bridge. The new bridge will have six lanes, three in each direction. Phillips’ contract also includes the installation of gutters, curbs, pedestrian walkways and a raised concrete median. In addition, improvements will be made to Neabsco Road, Cardinal Drive, Blackburn Road and Neabsco Mills Road. The bridge is scheduled to be completed in early summer 2010.

Jackson’s contract is worth $2.1 million and involves 277 ft. (84 m) of microtunneling under the Neabsco Creek Bridge to install three pipelines. A water line and a sewage line are being placed under the creek, and a sanitary line is being placed under U.S. 1. Once the tunnels are completed, the old bridge will be torn down.

Mike Jackson, site superintendent of E. Ann Jackson, said microtunneling is less disruptive to the environment. E. Ann Jackson was awarded the contract for this project because it has the experience required by VDOT to perform this type of work, and the company is known to be one of the best at what it does. Jackson describes his company’s experience with underground work as “years and years.”

Microtunneling is a trenchless method used for the installation of pipelines. An operator uses a control panel and remote control to operate a Microtunneling Boring Machine (MTBM) with a cutterhead attachment, which is a device that rotates and excavates at the face of a bore. The cutterhead has a cutting bit, which is attached to the front face of the boring machine and acts as the teeth and supporting structure of the machine.

In Jackson’s case, the cutterhead is laser-guided beneath the ground. The company owns microtunneling equipment made by Akkerman Inc. of Brownsdale, Minn. Jackson’s microtunneling equipment comes with a MT460 Jacking Frame, which houses the hydraulic cylinders used to propel the cutterhead and the pipeline. This particular jacking frame has a 400 ton (363 t) thrust capacity that will push up to a 60-in. (152 cm) pipe.

The microtunneling process begins with an entry pit; this job’s pit is 30 ft. (9 m) deep. The dirt that the cutterhead hacks through is sent through a slurry machine, which is a pressurized water filtration system. Once the dirt is filtered and separated from the water, the water can be reused. After that, Jackson said a clam shell bucket is used to remove the dirt.

The process involves equalized pressure with the slurry and the jacking device. Jackson explained the importance of having equalized pressure under the creek: “If you don’t have equalized pressure, the whole creek will dump into the tunnel.”

Initially, the company expected to achieve a 40 ft. (12 m) of progress per day average when installing the pipes. However, Jackson said the number fluctuates. “Our best day was 70 ft.,” he stated.

Once the pipelines are installed, the creek will be rechanneled to mimic natural stream conditions. Right now, the creek has several bends located on both sides of the bridge. These bends cause debris to build up, and water cannot flow due to the blockages. The rechanneling will improve these conditions.

Mike Jackson’s mother owns E. Ann Jackson, which has roots that go as far back as when the digging was done by hand instead of by machine. Jackson said the microtunneling that his company does takes the place of “sand hogging.” Jackson refers to this type of underground work as “911 holes” because the job is so dangerous.

In the late 1800s, men who worked underground while building bridges and tunnels, mainly in New York City, were called “sand hogs.”

They worked in compressed air far below the surface of land or water. To this day, sand hogs are working on one of New York’s largest ongoing projects — the City Water Tunnel No. 3, which began in 1969 and is expected to be finished by 2020.

As far as Jackson’s contract, the company should be finishing up by August. CEG

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