Indice geologico de resistencia

GSI: A GEOLOGICALLY FRIENDLY TOOL FOR ROCK MASS STRENGTH ESTIMATION
Paul Marinos1 and Evert Hoek2

ABSTRACT This paper presents a review of the estimation of rock mass strength properties through the use of GSI. The GSI classification system greatly respects the geological constraints that occur in nature and are reflected in the geological information. A discussion is given regarding theranges of the Geological Strength Index for typical rock masses with specific emphasis to heterogeneous rock masses. 1.0 INTRODUCTION

Reliable estimates of the strength and deformation characteristics of rock masses are required for almost any form of analysis used for the design of surface excavations. Hoek and Brown (1980a, 1980b) proposed a method for obtaining estimates of the strength ofjointed rock masses, based upon an assessment of the interlocking of rock blocks and the condition of the surfaces between these blocks. This method was modified over the years in order to meet the needs of users who applied it to problems that were not considered when the original criterion was developed (Hoek 1983, Hoek and Brown 1988). The application of the method to poor quality rock massesrequired further changes (Hoek, Wood and Shah, 1992) and, eventually, the development of a new classification called the Geological Strength Index (Hoek 1994, Hoek, Kaiser and Bawden 1995, Hoek and Brown 1997, Hoek, Marinos and Benissi, 1998), extended recently for heterogeneous rock masses (Marinos and Hoek, 2000). A review of the development of the criterion and the equations proposed at variousstages in this development is given in Hoek and Brown (1997). 2.0 ESTIMATE OF ROCK MASS PROPERTIES

The basic input consists of estimates or measurements of the uniaxial compressive strength (
ci) and a material constant (mi) that is related to the frictional properties of the rock. Ideally, these basic properties should determined by laboratory testing as described by Hoek and Brown (1997) but, inmany cases, the information is required before laboratory tests have been completed. To meet this need, tables that can be used to estimate values for these parameters are reproduced in Tables 1 and 2. Note that both tables are updated from earlier versions (Marinos and Hoek, 2000). The most important component of the Hoek – Brown system for rock masses is the process of reducing the materialconstants
ci and mi from their “laboratory” values to appropriate in situ values. This is accomplished through the Geological Strength Index GSI that is defined in Table 3. GSI has been developed over many years of discussions with engineering geologists with whom E. Hoek has worked around the world. Careful consideration has been given to the precise wording in each box and to the relative weightsassigned to each combination of structural and surface conditions, in order to respect the geological conditions existing in nature.

1 2

Professor of Eng. Geology, National Technical University of Athens, Athens, Greece, e-mail:marinos@central.ntua.gr Consulting Engineer, Vancouver, B.C., Canada, e-mail: ehoek@attglobal.net

Table 1: Field estimates of uniaxial compressive strength ofintact rock.3
Grade* R6 Term Extremely Strong Uniaxial Comp. Strength (MPa) > 250 Point Load Index (MPa) >10 Field estimate of strength Specimen can only be chipped with a geological hammer Examples Fresh basalt, chert, diabase, gneiss, granite, quartzite

R5

Very strong

100 - 250

4 - 10

Specimen requires many Amphibolite, sandstone, blows of a geological basalt, gabbro, gneiss, hammerto fracture it granodiorite, peridotite , rhyolite, tuff Specimen requires more than one blow of a geological hammer to fracture it Cannot be scraped or peeled with a pocket knife, specimen can be fractured with a single blow from a geological hammer Can be peeled with a pocket knife with difficulty, shallow indentation made by firm blow with point of a geological hammer Crumbles under firm blows...