The Geological Strength Index (GSI) was introduced by Hoek et. at. (1994) as an alternative method to classify rock mass due to difficulties in applying the Bieniawski’s Rock Mass Rating (RMR) to very poor rock masses. The GSI is an important tool to estimate parameters such as cohesion, friction angle and deformation modulus of rock masses. The author developed a new GSI table in 2000 to adapt the original GSI to metamorphic rock mass characterization in Venezuela. In 2007, the table was modified again to incorporate a non-foliated rocks structure column in order to include a broad range of metamorphic rock types. The experience in Venezuela in more than 2,000 classifications using the GSI table for metamorphic rocks, indicates an effective means to evaluate metamorphic rock mass in tunnels, slopes and foundations. In the last five years, the author has been working in the Blue Ridge and Piedmont Province areas of the Appalachian Mountains where metamorphic rocks such as schist, phyllite and gneiss are present. Similar characteristics between these rocks and metamorphic rocks in Venezuela allow the use of the modified GSI classification for metamorphic rock mass.
Since the beginning of important railroad tunneling projects in Venezuela by the end of the 1990’s and the beginning of the 2000’s, the methods selected for the evaluation of the quality of rock mass in proposed tunnels was the Geological Strength Index (GSI) of Hoek and the Rock Mass Rating proposed by Bieniawski (1989).
The original GSI table introduced by Hoek (1994) has had various modifications such as Hoek, Keiser and Bawden (1995), Hoek and Brown (1997), and the lastest version Hoek, Marinos and Benissi’s (1998) where the structure of “foliated/laminated” rock category was incorporated into the table. Table 1 shows that the very poor rock masses previously mentioned are located in the inferior part of the table, forcing one to adapt to the metamorphic rocks that exist in the Coastal Mountain Range of Venezuela. It means that most of the rock mass evaluations were always located in the inferior part of such a table. Further it implied that the classification of the excavation fronts in the tunnels was always sub-evaluated, and consequently with low values of GSI, the primary support to be calculated would be overdesigned. With the construction of the tunnels for the Railroad Project in Venezuela, geological surveys were performed in the excavation face of these tunnels in order to establish a statistical data base large enough to characterize the rock mass present in the area of the project.
After hundreds of geological surveys and having determined that the GSI table did not adapt well to metamorphic rocks observed in the tunnels, and that in most excavations, the rocks were represented by phyllites and schists frequently graphitic, and in less proportions marbles, cuarcites, and serpentines. For these reason, the author in 2000 developed a new table with minor modifications, that has an equivalence between the structures showed in the original GSI and the ones observed in practice.