Geotechnical core logging is the process of recording rock descriptions in drill logs, where the primary means of communicating rock properties is used in the design and construction of underground works, including foundations in and on rock, slopes of rock and tunnel supports, and rock excavations. The quality of rock descriptions can have far-reaching implications for the success of a project. An appropriate level of detail (as determined by the specific end-use objective of the project) must be collected and transmitted.
Important considerations for geotechnical core logging include:
1. Rock descriptions developed for each unit of rock material, defined as a discrete mass that exhibits a different set of engineering and geological properties than adjacent materials. Material unit based rock core logging minimizes the risk of overlooking critical rock mass conditions for design.
2. Material unit descriptions should be prepared using a standard descriptive code consisting of well-defined terminology arranged in a consistent format.
3. Standardization of the rock core log descriptive code (at least for individual projects) will result in more useful rock descriptions to facilitate geoengineering interpretation, spatial correlation of material units, and engineering analysis to develop recommendations design and construction considerations.
The following steps are suggested during the geotechnical core registration process:
1. Clean the core of drilling fluids or mud.
2. Mark major structures, proposed point load test locations, and depths (every 1-2 meters) in the intact core at the splits.
3. Photograph the core in the splits (if using the triple tube method) with a scale placed on the image and a whiteboard indicating how deep the core was obtained.
4. Complete the discontinuity and core description logs.
5. Transfer the core from the splits to a labeled core box.
6. Once a box of cores is full, take a single photo of the box of cores with a scale.
7. The steps are detailed in the following sections.
One of the most important things to do on the rig is to photograph the intact core in the splits. These photos can be used later to confirm the TV images and will be an invaluable resource on the rock mass and for reviewing design work.
Successful core photographs require the core to be cleaned prior to photographing. When the core is covered in drilling mud, structural information can be obscured, making it difficult to determine lithologies. Take the time to properly clean the core. The core should be wet if possible, as some structural features do not show up in the dry core, so be sure to moisten it with a spray bottle or brush.