Geological Strength Index (GSI) Proxy

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:

• Recognition of the fact that it is pushing the limits of what is achievable from a core photograph; and

• It is not possible to reliably determine joint infill rating from a core photo.

Literature

This product is based on the following literature:

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.

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:

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:

• ProjectID_HoleID_GSI_3m.csv

• ProjectID_HoleID_GSI_by_user_intervals.csv

Both CSVs contain the following headers:

*Only included if project depths are in feet.

Product Limitations