Since 1999, Trenchless Construction Services, LLC has been performing horizontal directional drilling, micro-tunneling, pipe ramming and a number of trenchless pipe replacement processes such as pipe reaming, pipe bursting and sliplining. Recently, it served as general contractor for the $3.5 million Sedro-Wooley SR9 Township trenchless sanitary sewer improvement project - one of the first pilot tube projects in Washington State. The company was responsible for installing 14 manholes and 3,400 lineal ft. of 15- and 24-in.-diameter gravity sewer pipes at a .16% grade.
Trenchless methods were required because the pipe was to be placed under and adjacent to two state highways. "These methods were used because of a lack of detour options to maintain traffic flow," says John Gustafson, owner/manager of Trenchless Construction Services. "The soil conditions, as well as the accuracy of pilot tube micro-tunneling, made it the preferred method for the 15- and 24-in. portions of the project."
Pilot tube micro-tunneling, also known as guided auger boring, is similar to micro-tunneling systems with the addition of a guidance system, which consists of a camera-mounted theodolite that ensures high accuracy of the line and grade. "We chose the Bohrtec [BM 400LS] guided auger machine because we knew it would give us bullet-like accuracy," says Gustafson. With a jacking force of 150 tons and 75 tons of pullback force, the BM 400LS can perform guided drillings in one-, two- or three-phase procedures, including pilot drilling; reamer drilling with steel protective piping; alternative reamer drilling with pushing of product pipes at the same time; and drilling with welded steel pipes.
Prior to starting the project, Trenchless Construction Services sponsored a meeting with engineers, subcontractors and its employees in order to get everyone on the same page. ICON, the slide rail system manufacturer and Bohrtec pilot tube distributor, and Mission Clay, the No Dig clay pipe manufacturer, both made presentations on their products. "Our goal was to make sure all parties had a clear view of the project scope, as well as the products we were going to use," says Gustafson. "This was very beneficial as everyone was able to ask questions and get direct answers from the manufacturers."
The project required a total of six jacking pits, in which the pilot tube boring machine was placed to jack the sewer pipe between manholes. The slide rail system was used to shore the pits, which measured 11 ft. wide by 20 ft. long by 14 ft. deep. "ICON's slide rail system is very easy to install, which saved us a tremendous amount of time," says Gustafson. "The system reduces setup time and is substantial enough to withstand the forces of the BM 400LS." It also features temporary sheeting, which allowed small sheeting panels to be extracted rather than large shoring panels to create an opening for the pipe installation.
Trenchless Construction Services performed a three-phase guided drilling installation with the 24-in.-diameter clay pipe. The procedure consists of installing the pilot tube to secure an accurate line and grade. Once the pilot tube reached the receiving shaft, a 16-in. casing with an auger inside was connected to the last section of pilot tube in the jacking pit. The casing was jacked into place as the soil was extracted, following the pilot tubes. Once the casing reached the receiving pit, a reamer was attached to enlarge the bore hole. The 24-in. clay pipe was then jacked to the receiving pit, following the casing and reamer.
A two-phase procedure was used to install the 15-in.-diameter pipe. "We were able to jack the clay pipe after the initial pilot tube drilling with the casings because of the smaller pipe diameter," notes Gustafson.
The contractor purchased the 15-in. pipe auger and casings for the project and rented the larger auger and casings for the 24-in. pipe from ICON. "ICON was instrumental in securing the tooling and special equipment required for the specific pipe sizes on the project," Gustafson asserts.