PILOT TUBE MICROTUNNELING EXPLODES IN THE U.S. USING VITRIFIED CLAY JACKING PIPE

First introduced in the United States in 1995, Pilot Tube Microtunneling (PTMT) has been increasing in popularity year after year. This trenchless method of installing sewer pipe is essentially a hybrid of three trenchless boring techniques: 1- Having a slant faced steering head similar to that of a directional drill, 2- Utilizing the guided accuracy of a conventional microtunnel machine, 3- Using an auger type spoil removal system similar to a horizontal bore. Among the reasons for the popularity of this system are; low equipment costs, relatively small topside footprint, and small jacking pits. Each year more contractors are purchasing these inexpensive and easy to operate tunneling machines from three equipment manufacturers: Akkerman, BohrTec, and Wirth-Soltau. Initially, pipe sizes ranged from 4 inch to 12 inch with maximum drive lengths up to 250 feet. Currently, the largest diameter (in the U.S.) installed by the pilot tube microtunnel method is 27 inch I.D. /32 inch O.D. and maximum drive lengths are now just over 400 feet. Larger diameters and longer drive lengths are due to the development of better optics in the guidance system and more powerful hydraulics in the jacking frame. This paper will explain the method of how sewers are constructed using PTMT in detail (two pass and three pass methods), as well as briefly discuss in a case study manner, showing reasons the installation method was chosen, project statistics, and difficulties or challenges of numerous completed and current vitrified clay pipe pilot tube microtunnel projects. 1. ORIGINS The pilot tube method of microtunneling originated in Japan and Europe nearly two decades ago as a way to install 4 and 6 inch house connections using trenchless techniques. Today, this technology has grown to installations with pipe diameters up to 1200 mm (48 inches) (in Europe) and drive lengths in the 400 LF range. The primary reason for this growth is the achievement of the same accurate on-line and on-grade installation as conventional microtunneling, but with significantly reduced costs. Projects are often less costly than conventional open-cut methods and solve engineering problems such as utility obstacles, poor soils, deep installations and high ground water. Costly lift stations and maintenance costs