Determination of unconfined compressive strength and Young's modulus of porous materials by indentation tests

The formation of a compacted zone under the indenter seems to be the major factor controlling the indentation process in porous rocks, In the case of very porous materials, where the pore structure fails and deformation (by s tructural collapse) proceeds with almost no increase in the applied load an d with very limited damage to the surrounding material, no chipping is obse rved. The extent of the compacted zone is controlled by the porosity of the material and by the strength of its porous structure. This paper presents an interpretation model developed by the authors to obtain the uniaxial com pressive strength of porous materials from the results of indentation tests . It is based on the model proposed by Wilson et al. (Int. J. Mech. Sci., 1 7, 1975, 457) for the interpretation of indentation tests on compressible f oams and on an estimation by the authors of the extent of the compacted zon e under the indenter. The results of indentation tests can also be used to obtain the Young's modulus of the material with a model proposed by Gill et al. (Proceedings of the 13th Canadian Symposium on Rock Mechanics, 1980, 1 103), Uniaxial compression and indentation tests have been performed on art ificial porous materials showing porosities varying between 44 and 68%. The uniaxial compressive strength values obtained from both types of test show a very good agreement. For the Young's modulus, the values obtained from t he two types of test are different but the variation of the moduli with por osity is the same. Finally, a parameter called permanent penetration modulu s is proposed as a means of characterizing the uniaxial compressive strengt h of porous materials. (C) 2001 Elsevier Science B.V. All rights reserved.