A variety of hybrid methods incorporate the use of a pilot tube to establish an accurate alignment between two shafts or the two sides of a crossing. The pilot tube typically is a small steel tube of around 100 mm diameter. In its basic use, it is thrust (with or without rotation) in a straight line through displaceable ground using a slant head to maintain line and grade in a similar fashion to the steering technique in HDD installations. Monitoring of line and grade of the pilot installation is carried out using a digital theodolite sighting down the pilot tube to an illuminated LED target at the front of the tube. The LED target displays the head position and steering orientation, which is viewed by the operator on a monitor screen. In a typical pilot tube application, once the pilot tube has been installed, the excavation and installation process continues by enlarging the hole through soil augering using temporary casings as necessary. In the final installation step, the last temporary casing is replaced by the final product pipe. Although the process typically has more installation steps than a full-face microtunneling installation, the installation equipment and the required size of shafts is smaller and the product pipe does not need to resist the same level of jacking forces as in a full-face microtunneling installation. In non-displaceable ground (hard ground or rock), there are variations of the technique using steerable drilling techniques to create the pilot hole. The pilot tube approach can also be used in conjunction with other trenchless installation techniques such as pipe ramming to establish an accurate line and grade for the subsequent installation to follow. In the trenchless technology literature, the pilot tube method may be found under the terms “guided boring” or “pilot-tube microtunneling”. The pilot tube method was initially used to install small diameter pipes such as service laterals. In this type of installation, the pilot tube is launched from a manhole or pit. Once the pilot tube string has reached the recovery pit, a service lateral is pulled in as the pilot rods are retracted.
Pilot Tube
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