Geotechnical Core Logging

Geotechnial Core Logging is the process of recording rock descriptions on boring logs, where primary means of communicating rock properties are used in the design and construction of underground works including foundations in and on rock, rock slope and tunnel support, and excavations in rock. 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 project‐specific end use objective) must be collected and conveyed.

Important considerations for geotechnical core logging include:

1. Rock descriptions developed for each rock material unit, defined as discrete mass exhibiting a different set of geologic and engineering properties than adjacent materials. Material unit‐based rock core logging minimizes the risk of overlooking design‐critical rock mass conditions.
2. Material unit descriptions should be constructed using a standard descriptive code consisting of well‐defined terminology arranged in a consistent format.
3. Standardization of the rock core logging descriptive code (at least for individual projects) will result in more useful rock descriptions to facilitate geo‐engineering interpretation, spatial correlation of material units, and engineering analysis for developing design recommendations and construction considerations.

The following steps are suggested during the geotechnical core logging process:

1. Clean the core of drilling fluids or mud.
2. Mark major structures, proposed point load testing locations, and depths (every 1-2 metres) on undisturbed core in splits.
3. Photograph the core in the splits (if using triple tube method) with a scale placed in the picture and a whiteboard indicating what depth the core has been obtained from.
4. Complete the Discontinuity and core description logs.
5. Transfer the core from the splits to a labelled core box.
6. Once a core box is full, take a single photograph of the core box with a scale.
7. The steps are detailed in the following sections.

One of the most important things to do at the drill rig is photograph the undisturbed core in the splits. These photos may be used later to confirm televiewer images and will be an invaluable resource on the rock mass and for review of the design work.

Proper core photos require that the core be cleaned prior to photographing. When 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 on dry core so make sure to wet it down with a spray bottle or paint brush.

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