Page 53 - Trenchless Technology Piping Installation and Inspection

26 Cha pte r O n e

The third step is to replace the auger casing with the product
pipe which has the same diameter as the auger casing. The product
pipes are pushed one by one as the auger casings are removed from
the opposite shaft as shown in Fig. 1.21. There is no spoil removal in
this step.
Depending on the application, the second and third steps of the
three-step PTMT can be combined, resulting in a two-step PTMT. In
this case, the reamer funnels the excavated material into the auger
casing which is coupled inside the product pipe. Once the product
pipes are installed, the auger casings can be removed by jacking
through either shaft.

Pipe-Ramming Method
Similar to horizontal auger-boring method, pipe-ramming method is
mainly used for installation of pipelines and utilities for road and
railroad crossings. Using an air compressor, this method hammers a
steel casing pipe inside the ground from a drive pit. The pipe might
be hammered closed end (for diameters less than 8 in.), or open end
(for diameters of 8 in. or more). When using large diameters, the spoil
is pushed out of the steel casing using air pressure, fluid pressure,
and mechanical means, such as a bobcat (for large diameters) or
combination of these methods.
The dynamic energy of a percussion hammer attached to the end
of the casing pipe is used to install the pipe from a drive pit to a recep-
tion pit. In this method, the ramming equipment does not create a
borehole; rather, it acts as a hammer to drive the pipe through the
soil. The product pipe can be used mainly for pressure applications
(such as water and gas); however, if the casing is large compared to
the product pipe, the product pipe may be installed using spacers on
grade for sewer applications. In this method, casing pipe provides a
continuous soil support and overexcavation is not required.

Method Description For pipes of up to 8 in. diameter, the pipe can be
driven either by having the leading end of the pipe in a wedge or cone
shape. For ramming operations with the pipe face closed (also called
impact moling or compaction method; see the following section), the
soil is compressed around the pipe as it is being rammed and there is
no spoil removal.
For pipes larger than 8 in., the leading end is usually left open
and a band is installed around the outside edge of the leading sec-
tion. The band serves a dual purpose: (1) it reinforces the leading
edge; and (2) it decreases the friction around the casing. Figure 1.22
illustrates a schematic of open-faced pipe-ramming operation. The
leading edge cuts a borehole equal to the diameter of the leading-
edge band. The spoil enters the pipe, is compacted, and is forced to
the rear of the pipe. For long lengths or in certain soil conditions (such
as stiff clays or sands), a steel pipe is installed on the top of the