The Quantification of the Geological Strength Index (GSI): A Review

The first Geological Strength Index (GSI) Chart was invented to classify rock mass properties. The GSI Chart developer assumes that qualified and skilled geologists or engineering geologists would evaluate and record the rock mass properties. Without a robust geological background and field experience, many researchers misuse the GSI charts. Due to the abnormal increase in GSI charts used in the recent decade, the update was necessary to eliminate visualization and assumption problems in GSI charts. The correlation between the quantified GSI charts was fair to reduce the uncertainty in estimating the rock strength properties. Many GSI charts were developed or updated to be quantified rather than visualized charts, which have become more specific and universal. The GSI Chart was modified by including additional parameters such as joint condition, rock quality designation RQD, volumetric joint count (Jv), and block volume (Vb). The modified GSI charts facilitate more practical use and reduce error. However, field observation and visualization are still essential for rock strength property estimation, particularly in the geomechanical classification of the rock mass. Some modifications add too much complexity to the original chart by adding a specific parameter or modifying it, making the decision-making based on the GSI Chart more difficult to find rock strength parameters. In some cases, a rock sample gets a significantly different GSI value for the same outcrop rock. Mixing 3D numerical modeling tools such as synthetic rock mass (SRM) or three-dimensional numerical modeling (3DEC) with GSI values is the most convenient method for estimating joints' strength and assisting engineering geologists in overcoming these obstacles.