Every genuine, experienced project manager and geotechnical engineer will vouch for the validity of undertaking trenchless projects with a good understanding of the prevailing geological conditions.
Many examples of failed projects stand as testimony to those who underestimated the need for a thorough geotechnical investigation. (Read Getting Technical: Information Required in a Geotechnical Investigation Report.)
Geotechnical investigation makes use of different methods to determine the physical properties of soil and rock below the surface of the earth. It also helps engineers to make the right decisions regarding the type of tools that will be required to dig through the site in the most economical manner.
One such method is the directional core drilling method that will be discussed in this section.
Directional Core Drilling Equipment
In the 1980s, a Norwegian firm Devico AS developed the first directional core barrel. Since then constant research and development have made it a popular tool for conducting a geotechnical investigation.
Directional core drilling equipment consists of a wireline operated core barrel to assist in directional control and has the same operational parameters as in a standard core barrel. This arrangement does not require any additional adjustments to the drill rig or drill string and can operate even in hard rock conditions.
The drilling trajectory and curvature are controlled by the tool-face angle and dogleg angle controls respectively. The toolface position is maintained during steering with the help of an inflatable packer at the rear of the core barrel.
Directional core drilling method with the Devico technology is able to control borehole deviation and accurately steer the hole towards the target while collecting core samples. In case another core from a different location is required, the first hole can be sidetracked and steered towards the second target by cutting straight through a curved section in the current hole. Branch holes like these are a great way to reduce drilling time and save money. The inner barrel of the core can be removed from the end of the borehole by lowering an overshot on the end of a wireline into the hole. The overshot attaches to the inner tube, and the wireline pulls back the inner tube which then disengages itself from the core barrel.
The Method
The borehole for the directional core driller can be begun at a suitable location near the planned trenchless project. The drill is steered along a predefined trajectory with the help of the directional Devico technology. The borehole for core sampling is usually planned in such a way that it will pass parallel to the path that the actual tunnel will take. As it passes, it can collect core samples over the entire length of the project such that fault zones, and fractures are also drilled through, giving the engineers a clear picture of what the real project will encounter.
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 the case of a deviation, the directional core barrel is reinserted and the hole steered to the desired direction. The standard tolerance for a borehole for core sampling from the planned trajectory is 1% of the borehole length, i.e. approx. 10 meters away from the tunnel for a 1000 meter long hole. The core samples can be studied in the laboratory using different methods, while geophysical and hydrological tests can be carried out in the borehole.
Why Use Directional Core Drilling
Directional core drilling has found its specific niche, not by no reason. The method can provide certain information that may not be easily available through other methods of geotechnical investigation. For one, directional core drilling can provide continuous core sample and geological information from the planned tunnel trajectory. In case a problematic rock formation is encountered, this method can provide data on the extent and orientation of such formation, allowing engineers to take special precautions when tunneling or boring through them. Other information can also be obtained easily using this method such as mechanical properties of the soil or rock formation, an estimate for the inflow of groundwater, an estimate for the amount of grouting and stabilization necessary, and time and cost of construction.
Benefits of Directional Core Drilling
Directional core drilling has many benefits when compared to other wireline drilling and directional drilling techniques.
Compared to standard wireline drilling:
- Reduced drilling length
- Less wear on drilling equipment
- Fewer drill sites
- Less impact on the environment
- Full control over natural deviation
- Improved borehole accuracy
Compared to other directional drilling techniques:
- Coring can be done in directional sections
- Low consumption of water
- High penetration rate in hard rock
- Easy adjustability of dogleg and use of higher dogleg due to smoother curve
- Drill string can also be rotated while steering
- Additional reaming not required
- Start position of borehole is not important as it can be guided to align with the planned tunnel
- Highly relevant data is obtained due to proximity to the actual borehole
Directional core drilling has shown many benefits since its inception in 1988 by Devico AS. Many international companies have used this method for constructing their tunnels and pipeline projects. Though in some cases long sections must be dug before reaching the desired location, making it expensive, the benefits that this method offers far outweigh the drawbacks.