This stage consists in drilling a pilot hole along a predesigned customer approved drill path. The pilot hole can be drilled from surface to surface, between trenches or pits.
There are various methods and consequently different tools to carry out the drilling of a pilot hole; the selection of the tools that make up the drill string which will be used depends fundamentally on the composition of the predominant strata of the soil to be drilled.
The predominant technology for the construction of the pilot hole developed over the past years has been jetting. This entails drilling with a bit attached to a hollow pipe drill string through which drilling fluids are exhausted at high pressures. The fluid exits from small jet nozzles placed in the drilling head at the end of the string causing a “hydro-jet” effect that bores through the soil, whilst as a result of the thrust created by the rig, the assembly advances occupying the penetrated space producing a slow but constant advance.
When drilling in hard soils or rocks, down hole mud motors coupled to tricone bits similar to the ones used in the petroleum industry are used.
Drilling begins by entering the formation with a penetration angle determined by the ground’s geology and the behavior of the tool.
The pressure of the mud injected through the drill pipes and exhausted from specially designed jet nozzles, which are selected according to the intended impact flow and force, are used for cutting.
The fluids superintendent will combination the mud components according to the prevailing circumstances in order to enhance bit penetration rate , suspend cuttings, produce a stabilizing mud cake on the tunnel walls preventing loss of circulation and controlling formation pressures. Prevention of friction, soils welling, tool overheating together with pipe support and lubrication are also functions of a proper combination of components.
The drilling tool must be guided along its underground path and to that end two different tracking methods are used:
The end of the pipe string is fitted with a hydrokinetic force nozzle equipped jet drill bit. Immediately behind the head, inside a hollow section, there is a probe which emits radio frequency signals that are received by a portable unit, handheld by an operator experienced in surface tracking. The receiver has a display unit by means of which the operator can read the drilling tool’s inclination, dial position and depth. The proficient operator receives the data and compares it with his layout plan so as to be able to issue orders to the drill rig operator regarding corrections required to ensure that the bore hole coincides with the projected drill path. This system is used for most HDD network installations where walking on surface above the drill path line is possible and when the maximum depth is not greater than 12 meters.
For this system the probe emits signals corresponding to its inclination, dial position and magnetic azimuth. These signals are transmitted by means of electronic impulses carried out by a cable threaded through the drill string to the drillers’ cabin where they are decoded within an interface, which feeds a computer that through an adequate program shows the location of the probe. As the signal is conducted through a cable and not the ground, it becomes unnecessary to be located on surface to receive the information. This is the most suitable system to be used on those installations where access to the location surface may not be possible such as in river or lake crossings, buildings, high circulation highways or runways and contaminated, environmentally sensitive, or geologically unstable areas.
Magnetic grid or solenoid, verification systems
So as to achieve greater control of the drill path, electromagnetic guidance systems are linked with the layout of an electrified grid on ground surface. This grid creates an independent magnetic field which allows for a better control of the drill path depth and advance by means of triangulation.
Tracking with solenoid
A gyrocompass is a kind of nonmagnetic compass which bases its operation on a rapidly rotating disc and on the rotation of the Earth to locate its geographic direction. Constructed to be able to use the effect of the gyrocompass precision, in the guidance of more complex directional drilling, it travels in the probe inside the nonmagnetic housing, and, as do the electromagnetic systems, feeds and connects with computers in the control cabin through cables threaded through the drill pipe string .Even though used less frequently because of its greater operational complexity, this system, in combination with accelerometers, provides an extremely accurate tracking tool possessing three significant advantages over the magnetometers: