Rock core logging is conceptually simple but realistically difficult.
One of the most challenging attributes of rock core descriptions is rock weathering. Cores give a glimpse into the rock mass and we are tasked with making standardized, reproducible, and accurate descriptions that apply to our project goals.
It's not easy and I suspect most of us who have described rock core can relate to these challenges.
Rock weathering rarely produces a uniform weathered rock mass where all rock is weathered to the same degree.
Complex variation of weathering throughout the rock mass is common due to variable factors such as orientation and spacing of discontinuities in the rock, groundwater flow paths and the removal of overlying weathered material by erosion.
The term "weathered" has introduced difficulty due to the subjective nature of observation and field interpretation. Instead, the degree of weathering can usually be qualified using three indicator parameters: 1) discoloration, 2) decomposition, and 3) disintegration.
We no longer advocate rock core descriptions that include the following terms:
- Slighty Weathered
- Moderately Weathered
- Highly Weathered
Instead, we suggesting using:
- Decomposition
- Disintegrated
- Discoloration
Decomposition is described in various grades or degrees of decomposition ranging from fresh (unweathered) rock to a true in-situ "Residual Soil". Some general parameters used for assigning a decomposition grade for a particular rock type include typical sequences of color changes, decomposition of certain minerals, and the results of other simple strength index tests.
Each are included on the
Disintegration is described according to the degree of mineralization or dissolution to either the rock core (matrix) or the cementation material. It should be noted that small-scale cracking and fracturing of rock could be caused by factors other than disintegration (i.e. mechanical decomposition). Once clay minerals start to form in the weathering process, cracks can be closed or "healed" as the original rock fabric begins to be destroyed, sometimes altering the hydraulic conductivity of the rock mass over time.
Discoloration is described using Munsell Colors, starting with the primary color first and adding up to two additional colors, when necessary. Similar to soil samples, rock cores should be described as soon as possible following coring.
Continual advances from field technologies to computer modeling make our work more challenging than ever, not to mention the effects of persistent contamination, the spectrum of remedial alternatives and site closure strategies.
Down hole viewers, flow meters, temperature sensors, and geophysical tools can lend priceless information for building the geologic framework.
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