New installation of pipe/cable/duct for gravity and/or pressure networks.
Applicable to gas, water, cable , sewer installations in diameters from 90 mm (3½ in ) to >1,000 mm ( 40 in) over lengths of up to 100 m depending on soil type. Generally used in softer soils but with the correct boring heads harder soils and soft rock may be bored.
Limitations may be that accuracy of this steered boring system is taken to be about ±25 mm over a 50 m length so any installation that requires higher accuracy may require a more accurate system such as microtunnelling.
As with other jacked pipe systems benefits include reduced impact on the local environment and economy with minimal disturbance to the local populous. The machines used are compact and no slurry is created. A variety of jacking pipes may be installed using the technique but usually clay or concrete provide the main options.
The process is quick and efficient and overall costs are usually less than those associated with more complex microtunnelling systems. The method also requires a much smaller footprint than the more complex options. As with microtunnelling the ground through which the bore passes is fully supported during the process.
Guided Auger Boring should not be confused with Auger Boring as it is a steered installation process which allows the bore to be maintained reasonably accurately on a design line and level thereby enabling gravity lines such as sewers to be installed.
The process begins by sinking a launch and reception shaft at either end of the desired bore route to the size required for the operation of the machine. The Guide Auger Boring machine which comprises a jacking frame and usually a thrust backing is located in the launch shaft aligned on the desired bored route.
An optical (laser) guidance system is installed and aligned on the desired line and level of the bore.
The bore is a multi-phase operation comprising first a pilot bore similar to the pilot bore used in HDD installations. The drilling head is angled to allow the steering of the head as it advances. Constant rotation provides a straight line advance whilst a non-rotational advance steers the boring head in a desired direction using friction on the slanted face against the ground to force the steering action. Any deviation from the desired line is indicated to the operator on a screen at the control point showing orientation of the head and which is then set for recovery steering to bring the head back on alignment.
This process continues until the pilot bore reaches the reception shaft.
Usually at the launch shaft the pilot drill rod is then replaced with a casing within which is an auger with a cutting head at the advancing head end. This is advanced through the ground, excavating the bore to the desired final diameter for the pipe being installed. The auger chain removes spoil from the bore to the launch shaft for collection and disposal.
As each section of the auger assembly is advanced into the ground so the next section is added and advanced again until the auger reaches the reception shaft/pit. The alignment of the bore is maintained by the pilot drill string that remains in the ground ahead of the auger assembly.
Once this phase is completed the product/casing pipe is then placed in the jacking frame and advanced into the bore created by the auger assembly. As the pipe is advanced the auger assembly, including the casing, is dismantled and collected at the reception shaft/pit for reuse.
Once the pipe string reaches the reception pit and all the auger and casing has been removed the installation is complete and the machine can be removed from the launch shaft.
If being used directly as a product pipe the installed pipe is then connected to the network it is to become part of. If the pipe is to be a casing pipe then the product pipe(s) or cables are installed as necessary.
Advances in Capability
As well as the standard pilot/auger/pipe installation technique, in recent years the machine manufacturers have devised options that enable bores to be completed in hard ground and at larger diameters. These generally take the form of a powered cutter head that is used instead of the auger during the second phase of the installation process.
The powered cutter head has a separate power feed that enables it to excavated larger diameters or harder ground than the standard systems. This results in its application range overlapping with microtunnelling in certain situations.