Different Hungarian volcanic tuffs (andesite, basalt and rhyolite) were investigated with the goal to determine the influence of the water on their strength. The following petrophysical constants were measured for all the samples both in dry and saturated condition: bulk density, ultrasonic wave velocity, unconfirmed compressive strength (UCS) and Young's modulus. The destruction work (strain energy) was calculated from the measured stress-strain curves, as well. The influence of the water for the UCS, impedance (scalar of the density and the ultrasonic wave velocity) and the destruction work is shown. In these cases linear relationship can be written between the dry and the saturated constants. Both linear and power equation can be used for the effect of the water on Young's modulus. Finally the ucs is written as function of the density and the impedance.


Recently several investigations were carried out for determining the strength of different volcanic tuffs in Hungary: i.e. the most of the vine-cellars in North Hungary were mined in tuffs (e.g. the famous Tokaj wine cellars) or some of the castles and towers were built on the top of this type of rock (as important monument as for example Visegrad, the capital of Hungary during the Medieval ages). The purpose of this paper to analyse the results of 12 rhyolite tuffs, 8 andesite tuffs and 10 basalt tuffs which were from different parts of Hungary. Certainly, every tuff is from different formation thus had different mineral composition. Although the tuffs have different mineral contents, grain-size, porosity, etc. the results show same general characteristics for this type of rocks. The results of tests are summarised in Table 1, which values are the average of 5 tests. Specimen preparation and testing were performed in the Rock Mechanics Laboratory at the Technical University of Budapest. Each tests were carried out in two petrophysical states: dry and water saturated and measured the bulk density (p) and the ultrasonic wave velocity (v) in which the impedance was calculated. Right circular cylinders were prepared, following the ISRM suggested methods (ISRM 1978), with a diameter of 54 mm and the height: diameter ratio is 2: 1 or slightly above. In addition to the standard values of unconfined compressive strength (UCS) and modulus of elasticity (Young's modulus, E), the complete stress-strain curve was measured. From the stress-strain curve the destruction work (or strain energy - W) was also calculated in both petrophysical states.


Firstly, the influence of the water for the UCS was examined. Figure 1 shows the plotted results, using linear regression determining the relationship between the dry and saturated compressive strength. Using the squared fit method for writing the relationship between the dry and saturated Young's modulus we found that the squared regressions coefficients for linear and exponential laws were not significantly different. The theoretical UCS of these tuffs without porosity could be determined with these equation and it is around 122 MPa.

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