Atti del 3° Congresso Nazionale AIGA - Centro di GeoTecnologie, Università degli Studi di Siena, San Giovanni Valdarno (AR), 25-27 Febbraio 2009, 17-18.
Atti del 3° Congresso Nazionale AIGA - Centro di GeoTecnologie, Università degli Studi di Siena, San Giovanni Valdarno (AR), 25-27 Febbraio 2009, 17-18
Some considerations on the application of the classical strength criteria for carbonate rock masses: a case study in a quarry in Apulia (Italy) ANDRIANI GIOACCHINO FRANCESCO (*), MICCOLI MARIA NILLA (*), PARISE MARIO (**) & WALSH NICOLA (*)
Pleistocene transgressive calcarenites (Calcarenite di Gravina Formation). The rock mass contains discontinuities such as bedding planes, joints, shear zones and faults. Karst features are recognizable by the presence of cavities, weathering surfaces and terra rossa.
RIASSUNTO
Alcune considerazioni sull’applicazione dei classici criteri di rottura agli ammassi rocciosi carbonatici: un caso di studio in una cava in Puglia (Italia) In questo lavoro viene presentato un esempio di applicazione dei “classici” criteri di rottura degli ammassi rocciosi su un fronte di cava per la produzione di inerti da roccia calcarea (cava “di Maso”, Bari). La successione ivi affiorante è costituita da calcari mesozoici su cui giace in trasgressione una sottile copertura di calcarenite pleistocenica. La roccia calcarea, intensamente fratturata e carsificata, è stata classificata dal punto di vista geomeccanico, con i sistemi RQD, Q, RMR, RMi, SMR e GSI. Le classificazioni utilizzate hanno indicato che l’ammasso roccioso calcareo è di qualità scadente o molto scadente, ad eccezione dell’RMi e del GSI per le quali lo stesso risulta, rispettivamente, buono e discreto. Successivamente, per la valutazione della resistenza di picco sono stati applicati e confrontati i criteri di Mohr-Coulomb e di Hoek-Brown, mentre per la resistenza residua quelli di Patton, di Barton e di Zhao.
KEY WORDS: Classification system, Failure, Rock mass, Strength criterior
Fig. 1 – Carbonate jointed rock mass of “Cava di Maso” (Bari, Apulia)
INTRODUCTION
After accurate geo-structural surveys (large-scale analysis), image analysis (small-scale analysis) and data processing, using the RQD (DEERE, 1964), Q (BARTON, 1974), RMR (BIENIAWSKI, 1976, 1989) and SMR (ROMANA, 1985) systems we have defined the rock mass quality as very poor or poor (RQD, Q, RMR, SMR); with the GSI (HOEK & BROWN, 1997) and RMi (PALMSTRÖM, 1995) systems it is described as discrete and good, respectively. This discrepancy is due to the different weight of the main parameters considered for evaluating the rock mass quality in the various classification systems (e.g. discontinuities, distribution, size, geometry, presence of residual materials). To complete the geomechanical analysis of the rock mass we have applied and compared the value of the peak shear strength obtained using the Mohr-Coulomb (BARMER, 1952) and the Generalized Hoek-Brown criteria (HOEK et alii, 2002) and that of the residual shear strength with the relations by PATTON (1966), BARTON (1976) and ZHAO (1997). These classic
The mechanical behavior of rock masses is strongly controlled by type and distribution of the discontinuities and the degree of weathering. A number of rock mass classification systems have been developed by numerous researchers over the past five decades aimed at evaluating the quality and expected behavior of rock masses. These classification systems have been progressively implemented with time, even though some of them has remained more appropriate than others to be used in specific geological settings. In this paper, we have applied the main classification systems (DEERE, 1964; BARTON, 1974; BIENIAWSKI, 1976, 1989; ROMANA, 1985; PALMSTRÖM, 1995; HOEK & BROWN, 1997) on the rock masses outcropping along the walls of an open pit quarry in carbonate rocks. The study area is located about 5-6 km south of Bari (Apulia, SE Italy). The rock outcrops (Fig. 1) are made up of limestones and dolostones (Calcare di Bari Formation), overlain by discontinuous and thin upper Pliocene to lower
(*) Dipartimento di Geologia e Geofisica – Università degli Studi di Bari “Aldo Moro” (**) Consiglio Nazionale delle Ricerche, IRPI, Bari. Lavoro eseguito nell’ambito del progetto “Analisi dei caratteri geologico-tecnici e idrogeologici per la tutela e la valorizzazione delle risorse naturali, ambientali e culturali” con il contributo finanziario dell’Università.degli Studi di Bari “Aldo Moro” (Fondi di Ateneo ex 60% - resp.: Prof. N. Walsh) 17
Fig. 2 – Envelopes according to the Mohr-Coulomb and Hoek-Brown strength criteria for the carbonate rock mass at “Cava di Maso” (Bari, Apulia).
strength criteria are relatively well established, but, at the same time, present some difficulties and uncertainties. Firstly, we have applied the Generalised Hoek-Brown failure criterion and determined cohesion and frictional angle that are necessary to reconstruct the equivalent Mohr-Coulomb curves. Successively, we have evaluated the Mohr-Coulomb envelopes and obtained from these the equivalent Hoek-Brown curves with the same values of cohesion and frictional angle. The Hoek-Brown Criterion defines higher values of shear strength than those from the Mohr-Coulomb. This difference increases with the increase in the tensional strength, in particular after that the normal stress reaches 1.5 MPa (Fig.2). As regards the residual strength, we have applied the Patton, Barton and Zhao relations and compared them. Observing the results, we have noticed the Patton line has higher values than the Barton and Zhao lines. Moreover, for low values of normal strength the last two show comparable results, while for higher values the differences become more pronounced.
BARTON N., (1976) – The shear strength of rock and rock joints. Matching Coefficient (JMC). Int. J. Rock. Mech. Min. ci. & Geomech. Abstr., 13, 1-24. BIENIAWSKI Z.T., (1976) – Rock mass classification in rock engineering. In: Exploration for rock engineering, Proc. of the symp., ed. Bieniawski Z.T., Cape Town: Balkema, 1, 97-106. BIENIAWSKI Z.T., (1989) – Engineering rock mass classifications. New York: Wiley. DEERE D.U., HENDRON A.J., PATTON F.D., CORDING E.J., (1967) – Design of surface and near-surface costruction. In: Failure and Breakage of Rock., Society of Mining Enginners of AIME, ed. Fairhurst C., New York, 15-99. HOEK E., BROWN E.T., (1997) – Practical Estimates of Rock Mass Strength. Int. J. Rock. Mech. Min. Sci., 34 (8), 11651186. HOEK E., CARRANZA-TORRES C., CORKUM B. (2002) – HoekBrown Failure Criterior – 2002 Edition. Proc. NARMSTAC Conference, Toronto 1, 267-273. PATTON F.D., (1966) – Multiple modes of shear failure in rock. Proc. 1st congr. Int. Soc. Rock Mech., Lisbon 1, 509-513. ROMANA M.R., (1985) – New adjustment Ratings for Application to Slopes. Int. Symp. On the Role of Rock Mechanics, Zacatecas, 49-53. ZHAO J. (1997b) – Joint surface matching and shear strength, Part B: JRC Shear Strength Criterior. Int. J. Rock. Mech. Min. Sci., 34 (2), 179-185.
REFERENCES BALMER G., (1952) – A general analytical solution for Mohr’s envelope. Am. Soc. Test. Mat., 52, 1260-1271. BARTON N., CHOUBEY V.D., (1974) – The shear strength of rock joints in Theory and Practice. Rock Mechanics, 1/2, 1-54.
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