Being a Good Neighbor During Construction

Trenchless methods—tunneling and drilling

Installing a new pipeline or repairing an existing pipeline can also be done using trenchless methods, such as tunneling or horizontal directional drilling. Trenchless methods are typically used to go under a busy roadway or stream—or to avoid a sensitive environmental area such as a wetland or steep slope. Trenchless methods are also considered when the sewer line is more than 25 feet deep, and cut-and-cover methods are high risk.

Trenchless methods can be more costly and can be used in only certain soil and ground conditions. For example, trenchless methods may not work well if the ground contains buried trees or large boulders.

Tunneling

Tunnels are built using specialized tunnel boring machines. King County has 344 miles of sewer pipelines, including 25 miles of tunnels. Building a tunnel often causes fewer impacts on a community because the tunneling takes place underground—usually more than 25 feet deep. Tunneling causes noticeable impacts at access shafts where the machine is inserted and dirt is removed. Also, because tunneling machines run in a straight line, other access shafts may be built so workers can redirect a machine to go a different direction.

Construction starts with site preparation—fencing, grading and clearing vegetation. Then workers build the access shaft. The shafts can be built using several shoring methods that support the surrounding soil. Those methods include sheet piles made of metal and walls or piles made of concrete.

After a tunnel access shaft is built, much of the noticeable activity centers on removing the excavated dirt, called spoils or muck. Trucks then transport spoils to a disposal site. Excavation and spoils removal will affect traffic as trucks come and go, and equipment will run on the site, sometimes continuously.

Microtunneling

King County has used microtunneling, which uses a small boring machine that’s remotely controlled from the surface, to install pipes up to 6 feet in diameter. Pipe is installed immediately behind the boring machine. In microtunneling, there are normally no workers in the tunnel, although they sometimes may enter to repair equipment. This method can be used below the water table and in most soil types.

Horizontal directional drilling

Other trenchless methods are available for different soil conditions. Horizontal directional drilling uses a drilling rig on the surface to install a drill pipe in a shallow underground arc. The drilling rig bores a pilot hole that’s filled with fluid and then uses a swiveling reamer to enlarge the hole to the final sewer pipe size. Sewer pipe is then pulled through the hole. Directional drilling often needs a large staging area so the pipe can be lined up before it’s pulled into the hole.

See also:

• Cut-and-cover or trenching
• Building or upgrading facilities
• Mitigation to minimize construction impacts

Updated: Dec. 6, 2005