Page 68 - Trenchless Technology Piping Installation and Inspection

38 Cha pte r O n e

1.8.4 Microtunneling

Advantages
The microtunneling method is capable of installing pipes to accurate
line and grade tolerances. It has the capability of performing in diffi-
cult ground conditions without expensive dewatering systems and/
or compressed air systems to pressurize the tunnel face. Pipe can be
installed at a great depth without a major cost increase. The depth
factor becomes increasingly important as underground congestion is
increased, or a high water table and difficult ground conditions are
encountered; where open-cut method becomes very costly. In this
method, safety is enhanced, as workers are not required to enter the
tunnel. The carrier (product) pipe, with sufficient axial load capacity,
can be jacked directly without the need of a separate casing pipe or
lining for soil support.

Limitations
The capital cost of equipment is high. However, on projects where
this method has been competitively bid against other tunneling meth-
ods, the unit price costs have been competitive. Some MTBM systems
have difficulty in soils with boulders more than one-third of the
machine diameter.

1.8.5 Pilot-Tube Microtunneling

Advantages
The change-over and setup times for this method are short. The
machine technology and operation is simple. The investment costs
for the equipment is reasonable. The space needed to set up the equip-
ment is small.

Limitations
The use of the soil displacement principle is only possible in soil that
can be displaced [approximately with a standard penetration test
(SPT) of less than 50]. Steerability becomes difficult with increasing
jacking distance. Because of the absence of position monitoring and
steering capabilities during the reaming process, obstacles or collaps-
ing soil layers may cause directional deviations. This may lead to
constraining forces at the coupling point of the pilot drill-rod string
with the reamer and, in extreme cases, fracture the joint.

1.8.6 Pipe Ramming

Advantages
The pipe-ramming method is an effective method for installing pipes
and utilities under roads and railroad tracks. The versatile pit sizes,