Directional Core Drilling for Tunneling and Geotechnical Investigation
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When planning the construction of tunnels, rock caverns and shafts, an extensive investigation of the rock formation in the area is required. The investigation is used to identify problematic ground and secure an efficient and low-risk construction.
Directional core drilling is an investigation method that allows core sampling along any defined trajectory, providing extremely accurate and relevant information about the rock formation near the planned underground construction.
WHAT DIRECTIONAL CORE DRILLING CAN PROVIDE:
- Continuous core sample and geological information from the planned tunnel trajectory
- Data on the extent and orientation of problematic rock formations
- Information about the mechanical properties of the rock formations
- Basis to estimate groundwater inflow
- Basis to estimate stabilization measures
- Basis to estimate construction costs and duration
Customers using Devico DeviDrill for their drilling solution
GEOTECHNICAL INVESTIGATION EXAMPLE
Where traditional core drilling (DHH-1 & 2) gives limited information, directional core drilling (DCD-1) penetrates the geological formations parallel to the planned tunnel.
By utilizing directional core drilling it is possible to steer the borehole along a predefined trajectory, no matter if it is curved or straight. With high accuracy the borehole will follow the specified path, for instance parallel to a planned tunnel, while at the same time collect a core sample over its full length. With this approach fault zones and fractures are drilled through in the same direction as the planned construction will hit them, giving relevant information about size, properties, material, leakage etc.
The borehole may be started at an available location near the planned underground construction and then steered as required to reach the direction and area of interest. When the borehole is in position the directional core barrel (DeviDrill) is pulled out of the hole and replaced with a standard wireline core barrel. This allows for a full size core sample to be collected through the most interesting zones. Should the hole deviate, or if there is a turn in the planned trajectory, the standard core barrel will again be replaced with the DeviDrill, and the hole steered back to the desired path. The planned trajectory can be anything from straight to curved with a radius of as little as 200 m.
Before starting the borehole a tolerance corridor is defined, specifying the maximum distance the borehole should be from the planned trajectory at any time. The tolerances depend on the project requirements, but are of often set to approximately 1% of the borehole length. This means, for a 1000 m long hole, the core sample is from a location never more than 10 m away from the planned tunnel.
The core sample, both from the directional and standard drilling process, can be tested in a lab to reveal the mechanical properties of the rock. Further geophysical and hydrological tests can be performed in the completed borehole. Combined these tests will significantly improve the geological mapping and reduce the risk of surprises during construction.
The benefits of directional core drilling is well documented, and since the start in 1988 Devico has participated in multiple international tunneling and underground construction projects. Several thousand meters have been drilled in countries like: Norway, Sweden, USA, Australia, Hong Kong SAR, South-Korea, Japan, and more.
Due to its frequent use at tunneling projects in Hong Kong the local Assosiation of Geotechnical and Geoenvironmental Specialists (AGS) prepared a guideline for the use of directional core drilling for ground investigation. The guideline and a presentation of it is available here:
AGS (HK) Technical Seminar
Introduction of New Ground Investigation Guideline
Ground Investigation Guideline
Directional core drilling (DCD) is done with the DeviDrill directional core barrel.
The principle behind the DeviDrill is a drive shaft running through a bushing offset from the center line of the tool. Expanding pads operated by differential pressure keep the DeviDrill in a fixed orientation while drilling in a curve.
The inner assembly carries an inner tube collecting the core, a muleshoe system, and an instrument barrel with the survey tool recording inclination and tool orientation.
Orientation data is stored inside the tool and downloaded to a PDA after each run.
Bit diameter 75.4 mm/2.97” (N-size)
Reamer diameter 75.6 mm/2.98”
Core diameter 31.5 mm/1.24”
Core length 3 m/10 ft