What Does Core Drilling Mean?
Core drilling is the process of drilling below the earth's surface to obtain a core of soil or rock sample in order to determine its properties. Core drilling is an important part of geotechnical site investigation that is carried out for the planning and construction of sub-surface structures such as tunnels and pipelines.
A core drill, which is cylindrical and hollow and made of metal with diamond or carbide coated drill tips is used for the purpose. The core drill cuts through the ground surface to extract the sample from the required depth. Core drilling retrieves usable samples for geotechnical investigation.
Core drilling can be accomplished using rotary drilling to obtain core samples from the bottom of the boreholes. A rotating core barrel fitted with an annular bit is used for this purpose. The core barrel retains the core, which is brought to the surface and recovered.
Another method is the directional core drilling method introduced by Devico technology, which is able to control borehole deviation and accurately steer the hole towards the target while collecting core samples. When the borehole reaches the area from where a sample is to be collected, a wireline core barrel is placed instead of the directional core barrel, allowing for a full-size core sample.
In case of a deviation, the directional core barrel is reinserted and the hole is steered to the desired direction. Directional core drilling can also provide continuous core sample and geological information from the planned tunnel trajectory, which is not possible with other core drilling techniques.
Trenchlesspedia Explains Core Drilling
Coring is primarily used to obtain intact rock samples that will represent the in-situ material. Usually, wireline core barrels are used for geotechnical investigation. Components of a rock core barrel include an outer and inner barrel and a bit. The bit turns along with the outer barrel to cut a core and the core is pushed into the inner barrel as the drilling progresses.
An overshot is used to retrieve the inner barrel along with the core sample. The drill bit is crucial to a successful coring operation and the type of bit is dictated by the rock type, and its hardness and integrity. Diamond chip and carbide coated drill tips are the main types of bits used for core drilling.
A rotary drill with a diamond drill bit attached is used for core drilling operations to create precisely measured holes. Diamond is the hardest known material and is capable of cutting through a wide range of materials. The holes created by diamond drills range between 8 mm to over 1500 mm and create clean and accurate holes. The method does not use percussion, hence the structural integrity of the material remains intact.
Devico Directional Core Drilling
Unlike other directional drilling technologies, Devico’s DeviDrill can collect continuous core samples as the hole is being steered. The DeviDrill uses a normal drill string and does not require additional water pumps, drill strings, or survey instruments. The survey instruments are a part of the tool and can measure while drilling (MWD).
The tool has a bushing offset from the centerline of the tool through which the drive shaft runs. While drilling on a curve, expanding pads operated by differential pressure keeps the drill in a fixed tool face. The inner assembly contains an inner tube that collects the core. As the system is operated by wireline, tripping drill rods in and out during directional coring is not necessary.