This document details Datarock’s product Geological Strength Index (GSI) Proxy.
The Geological Strength Index (GSI) was introduced in the early 1990s as a means to quantify and characterise the strength and deformability of rock masses for engineering and geotechnical applications. It was identified there was a need for a standardised index that could reliably assess the rock mass behaviour and provide valuable insights for construction and excavation projects. GSI takes into account various geological parameters such as rock structure, weathering, and joint conditions, assigning a numerical value to represent the overall strength and deformability of the rock mass.
Datarock has developed this product to extract as much information as possible from the core image, however it is named GSI Proxy for two key reasons:
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Recognition of the fact that it is pushing the limits of what is achievable from a core photograph; and
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It is not possible to reliably determine joint infill rating from a core photo.
Literature
This product is based on the following literature:
|
Item |
Title |
Author |
Year |
|---|---|---|---|
|
1 |
The weighted joint density method leads to improved characterization of jointing |
A Palmström et al |
1996 |
|
2 |
A discussion on the Hoek–Brown failure criterion and suggested modification to the criterion verified by slope stability case studies. |
Sonmez H, Ulusay R. |
2002 |
|
3 |
Quantification of the Geological Strength Index Chart |
Hoek, E., Carter, T.G., Diederichs, M.S. |
2013 |
Dependent Models
The outputs of the following computer vision models are used:
|
Model Name |
Model Type |
|
Fracture Detection and Classification |
Object Detection |
|
Drillers Break |
Object Detection |
|
Fracture Mask |
Instance Segmentation |
|
Geotechnical Weathering Intensity |
Classification |
Data Processing
Datarock has developed a detailed method for calculating a proxy of the Geological Strength Index (GSI) by integrating various Datarock products such as Weighted Joint Density, Geotech Weathering Intensity and Joint Roughness Coefficient.
The following steps provide a structured description of the method:
Calculating Structural Rating (SR)
Weighted Joint Density as basis for Volumetric Joint Count
As described by Palmström et al, 1996, wJd is practically the same as Volumetric Joint Count (Jv), therefore our calculated wJd has been used as the basis (see wJd product description for details).
-
Take the wJd for a given interval. This will be called
-
If residual soil exists within the interval (as detected by the Geotech Weathering Intensity model), wJd will be updated by assigning 10 fractures per 10cm of residual soil:
3. This value is then in the calculation of the Structural Rating.
Determining Structural Rating (SR)
The Structural Rating on the y-axis of the GSI chart is calculated using the wJd value. The relationship between SR and wJd is given by the equation as described by Sonmez H, Ulusay R., 2002:
As derived from empirical data, the Structural Rating quantifies the structural complexity of the rock mass.
Calculating Surface Condition Rating (SCR)
SCR includes considerations of infill rating, weathering, and joint roughness. However, it has been determined that it is not possible to reliably determine infill rating from a core photo, and therefore the SCR is determined based on weathering and JRC (Joint Roughness Coefficient) values. This is the primary reason we label this product as GSI proxy.
Roughness Rating (Rr)
The Roughness Rating is determined using Datarock’s Joint Roughness Coefficient as its basis (see JRC product description for details).
The JRC values are converted to a 0-6 scale using the following polynomial function (established by fitting a polyline to empirical data):
Weathering Rating (Rw)
The weathering score is determined by identifying the most dominant weathering type in the interval and assigning an associated Weathering Rating. The Weathering Score has been multiplied by 1.5 to take into account the absence of Infill Rating data.
|
Weathering Intensity |
Weathering Score |
Weathering Rating (Rw) |
|---|---|---|
|
Residual Soil |
0 |
0 |
|
Completely Weathered |
0 |
0 |
|
Highly Weathered |
1 |
1.5 |
|
Moderately Weathered |
3 |
4.5 |
|
Slightly Weathered |
5 |
7.5 |
|
Fresh / Unaltered |
6 |
9 |
Example Geotechnical Weathering Intensity predictions
GSI Calculation from the Quantified Chart
The GSI value is determined by digitising the GSI plot on the surfer and obtaining the surface equation (a quadratic-second order surface). With the calculated x (SCR) and y (SR) coordinates on the GSI chart, these values are plugged into the surface equation to yield the GSI value, providing a quantified measure of the rock mass quality.
Example Geological Strength Index data
This method provides a systematic approach to quantifying the GSI by integrating various rock mass characteristics, as detailed in the paper. The use of empirical relationships and polynomial functions allows for a more objective and quantifiable assessment of the GSI, contributing to a standardised evaluation of rock mass conditions.
Output Intervals
Default interval length: 3m
Customisable interval available: Yes
User Data
User data may be provided to the Datarock team via csv in the following format:
· HoleID_sampling_intervals_GSI.csv
CSV file to contain the following headers:
|
File Header |
Description |
|
depth_from |
Start of interval |
|
depth_to |
End of interval |
Data Output
Results from this product is delivered in a batch nature.
Integration of the required technologies into Datarock production is ongoing.
The available CSV files include the following:
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HoleID_gsi_by_3m_intervals.csv
-
HoleID_gsi_by_user_intervals.csv
Both CSVs contain the following headers:
|
File Header |
Description |
|
hole_id |
Customer’s Hole ID |
|
depth_m |
Depth of centre of interval (metres) |
|
depth_from_m |
Start of interval (metres) |
|
depth_to_m |
End of interval (metres) |
|
depth_from_ft* |
Start of interval (feet) |
|
depth_to_ft* |
End of interval (feet) |
|
wjd |
Weighted joint density |
|
wjd_measurable |
Weighted joint density using only “measurable” fractures |
|
rqd_wjd |
RQD - Weighted Joint Density measures rock solidity by assessing fracture frequency and size. |
|
rqd_wjd_measurable |
Assessment of rock quality by quantifying fracture impact on rock mass integrity. |
|
weathering_score |
Weathering Score evaluates the degree of rock degradation due to environmental exposure. |
|
SRC_JRC |
SRC_JRC is a measure that evaluates the shear strength and roughness of rock joints for stability analysis |
|
SR |
Structural Rating |
|
SRC |
Surface Roughness Coefficient |
|
gsi |
Geological Strength Index |
*Only included if project depths are in feet.
Product Limitations
|
Limitations |
Comments |
|---|---|
|
Lack of Infill Data |
The absence of infill rating in the Surface Condition Rating (SCR) calculation means that the model may not fully capture the influence of infill materials on the mechanical behavior of discontinuities, despite the scaling adjustment. |
|
Dependence on Visual Inspection |
GSI is traditionally estimated through visual inspection. The subjective nature of visual assessment can lead to variability in GSI values assigned by different assessors, potentially affecting the consistency and reliability of results. |
|
Version |
Date |
Author |
Rationale |
|---|---|---|---|
|
1 |
22 Jan 2024 |
L Yanez |
Initial release |
|
2 |
29 Jan 2024 |
S Johnson |
Updated to include depth in feet |