EFFECTS OF WATER ON STRENGTH AND FAILURE MODE OF COARSE-GRAINED GRANITES AT 300-DEGREES-C

Citation
E. Althaus et al., EFFECTS OF WATER ON STRENGTH AND FAILURE MODE OF COARSE-GRAINED GRANITES AT 300-DEGREES-C, Rock mechanics and rock engineering, 27(1), 1994, pp. 1-21
Citations number
36
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geosciences, Interdisciplinary","Engineering, Civil
ISSN journal
0723-2632
Volume
27
Issue
1
Year of publication
1994
Pages
1 - 21
Database
ISI
SICI code
0723-2632(1994)27:1<1:EOWOSA>2.0.ZU;2-V
Abstract
Deformation experiments have been performed in a triaxial compression cell at a temperature of 300 degrees C and confining pressures up to 6 5 MPa using samples of homogeneous, fresh two-mica-granite (RM) and mo nzogranite (CM). The cylindrical specimens (d = 70 mm, h = 140 mm, V = 540 cm(3)) were tested undrained under ''dry'' (105 degrees C), ''as received'', and ''water saturated'' conditions at deformation rates be tween epsilon = 12 x 10(-6) s(-1) and epsilon = 0.3 x 10(-6) s(-1). Th e mechanical behaviour of the two types of coarse-grained, crystallobl astic granites is critically influenced by mineralogical composition, porosity, and the amount of intergranular water present in the samples . The failure stress of the CM granite is at about 65 % of that of the RM granite; in both rocks strength decreases with increasing porosity and water content. The presence of interstitial water causes a failur e mode of non-localized, homogeneously distributed microcracking in th e central parts of the samples, whereas, in runs with dry granites, st rain localization along a single shear fracture was observed. When aqu eous fluids are present, the macroscopic style of deformation of grani tes appears to be ''ductile'' even at low P and T conditions. Strength and angle of internal friction are reduced to very low values. The st yle of deformation, as well as the reduction of strength of the water- saturated rock samples, is due to mechanical and chemical effects of i ntergranular water at elevated temperatures. The maximum differential stresses measured for these coarse-grained granites are much lower tha n the strength commonly reported for other granites, e.g. Westerly and Charcoal granites. Our data suggest that the strength of the granitic crust under differential stress is lower than currently deduced from laboratory experiments.