The drilling machine must be mounted on some type of carrier in order to drill and move about the site and even from location-to-location. The type of carrier has an effect on the versatility of the machine and the efficiency of the overall operation. Drill rigs may be mounted on trucks, crawlers, excavators, cranes, or may even be designed to operate directly as a top drive unit mounted onto a casing. The following sections provide a brief description of drill rigs by methods of mounting the machine.
Truck Mounted Drilling Rigs
Mobility is the greatest advantage of truck-mounted drilling rigs, which can range widely in size and drilling capabilities. Truck-mounted rigs can range from small, extremely mobile rigs most suited to small holes to large, heavy rigs capable of drilling rock. If the site is accessible to rubber-tired vehicles and conditions are favourable for drilling with truck-mounted rigs, construction of drilled shafts can be accomplished very efficiently with these machines. With the mast or derrick stored in a horizontal position, lighter units can move readily along a roadway. The truck can move to location, erect the derrick, activate hydraulic rams to level the rotary table, and begin drilling within a few minutes of reaching the borehole location.
Truck-mounted rigs are normally mechanically-driven with a fixed rotary, and therefore may have limited capability to reach over tall casing or to handle tall drilling tools. The space below the rotary table can be increased by placing the rig on a ramped platform, but this procedure is obviously slow and expensive and would be used only in unusual circumstances. However, some truck-mounts are now supplied with a hydraulic sliding rotary which can overcome many of the limitations of older truck-mounted rigs.
While the truck-mounted unit has a secondary line with some lifting capacity, that capability is necessarily small because of the limited size of the derrick. The drilling tools can be lifted for attachment to and detachment from the kelly, but, if a rebar cage, tremie or casing must be handled, a service crane is usually necessary. Some truck rigs can handle light rebar cages and tremies of limited length.
Crane Mounted Drilling Rigs
A power unit, rotary table, and kelly can be mounted separately on a crane of the contractor's choice. Crane mounted drill rigs can have substantial capabilities and versatility on a bridge project, especially over water. The crane-mounted machine is obviously less mobile than a truck unit. Mobilization to the jobsite generally requires “rigging” or assembly of the equipment with significant cost and effort.
Power units of various sizes can be utilized to supply large torque at slow rotational speeds to the drilling tool. Usually, the downward force on the tool is due to the dead weight of the drill string, but the dead weight can be increased by use of heavy drill pipe (drill collars), “doughnuts,” or a heavy cylinder. Special rigging is available for crane machines that will apply a crowd for drilling in hard rock. The cross-sectional area of the kelly can be increased to accommodate high crowds.
The framework or “bridge” that is used to support the power unit and rotary table can vary widely. The rotary table may be positioned 75 ft or more from the base of the boom of a crane by using an extended mount. The ability to reach to access the hole from a distance makes crane mounted machines very attractive in marine construction when working from a barge or work trestle. The bridge for the drilling unit can also be constructed in such a way that a tool of almost any height can fit beneath the rotary table. Therefore, crane-mounted units with high bridges can be used to work casing into the ground while drilling or for accommodating tall drilling tools.
A service crane or the drilling crane itself is used on the construction site for handling rebar cages, tremies, concrete buckets and casings. The secondary lift line on the drilling crane can be used for common lifting by tilting the derrick forward and away from the rotary table, thus making the crane- mounted drilling unit a highly versatile tool.
Crawler Mounted Drilling Rigs
A good “in-between” option for truck-mounted and crane-mounted options is the crawler mounted rigs. Crawler mounted drilling rigs may be less mobile than truck mounted equipment for accessible sites, but can provide excellent mobility on the jobsite. Compared to a crane mounted rig, the drilling equipment is usually a permanent fixture on the crawler with a fixed mast serving as the lead for the rotary or kelly guide system. The crawler mount is the most common system used for hydraulic powered rigs, although it is also a popular system for conventional mechanical rigs.
Lightweight crawler mounted drilling rigs can be extremely versatile for work on difficult to access sites for applications such as slope stabilization, sound wall foundations, and foundations for signs, towers, or transmission lines.