Page 50 - Trenchless Technology Piping Installation and Inspection

New Pipeline Installations 23

a schematic of a microtunneling operation. The American Society of
Civil Engineers (ASCE) has also published Standard Construction Guide-
lines for Microtunneling (CI/ASCE 36-01, currently under revision) with
information on planning, design, pipe materials, and construction
aspects of microtunneling (ASCE, 2001). Figure 1.18 presents sequence
of microtunneling and pipe-jacking operation.

Pilot-Tube Microtunneling
Pilot-tube microtunneling (PTMT), also called guided auger boring,
guided boring, and auger drilling, originated in Europe for installa-
tion of 4- to 6-in. service lateral connections and was introduced in
1995 in the United States. Pilot-tube microtunneling is an alternate
and cost-effective method to conventional microtunneling. Pilot-tube
microtunneling combines the accuracy of microtunneling, the steer-
ing mechanism of a directional drill, and the spoil-removal system of
an auger-boring machine. PTMT employs auger and a guidance sys-
tem using a camera-mounted theodolite and a target with electric
light emitting diodes (LEDs) to secure high accuracy in line and
grade. When conditions are favorable (mainly soft soil conditions,
drive distances less than 300 ft, and pipe diameters less than 30 in.),
pilot-tube microtunneling can be a cost-effective tool for the installa-
tion of gravity pipes. This technique also can be used for house con-
nections direct from existing sanitary sewer manholes. Among the
most notable reasons for the popularity of this method are its low
initial cost, shallow installations, small workspace requirement, and
small jacking pits.

PTMT Operation The PTMT operation begins with excavation of driv-
ing and receiving shafts. These shafts are usually 6.5 to 8.0 ft in diam-
eter. The larger shafts are usually square or rectangular. The PTMT
machine is then lowered down into the drive shaft and is set to pre-
cise line, grade, and height from a control point established using
surveying techniques. The guidance system for PTMT consists of a
theodolite with a camera independent of jacking frame, and a moni-
tor screen. The accuracy of completed gravity sewer depends upon
setup of a theodolite, which is also adjusted to height, grade, and line.
Figures 1.19, 1.20, and 1.21 illustrate the stepwise PTMT installation
operation.
The first step involves the installation of a pilot tube precisely
over the center line of the prospective sewer or water line as shown in
Fig. 1.19. During this process the spoil is displaced by a slant-faced
steering head. Pilot tube is directed at precise line and grade during
advancement. The hollow stem of the pilot tube provides an optical
path for the camera to view the LED target in the steering head, dis-
playing the head position and steering orientation. This establishes
the center line for the installation of the new pipe.