Читать книгу Monument Future - Siegfried Siegesmund - Страница 186

The Hungarian tuffs generally have high effective porosity, from 17 % to 30 % approximately, with a well interconnected pore network, accounting for the almost exclusive presence of open pores in the rock volume. Capillary pores (> 0.1 µm) represent distinctively the most abundant size. The low bulk density, 1.5 g/cm³ on average, is directly related to the high porosity, as well as to the abundance of low-density pumice clasts and glass shards. Considering the mechanical properties, the studied tuffs are weak to moderately strong rocks with compressive strength of 7 and 28 MPa. Saturated conditions produce an extreme deterioration of the mechanical properties, with the strength that may decrease even by 90 % in the weakest tuff varieties (Table 1).

The Hilbersdorf and Weibern tuffs of Germany have high effective porosities of 26 % and 37 %, respectively. While Hilbersdorf tuff contains substantial amounts of micropores (43 %), Weibern tuff is characterized by huge portions (> 86 %) of capillary pores. In comparison to the other tuffs of this study, the Hilbersdorf tuff shows considerably high bulk (1.9 g/cm³) and matrix (2.6 g/cm³) densities, as well as moderate to high p-wave velocity (2.6 km/s), tensile (4 MPa) and compressive strength (32 MPa). The Weibern tuff, on the other hand, is characterized by lower densities and p-wave velocity and tensile strength (1.5 MPa) (Table 1). Both tuffs suffer a strong decrease (up to 40 %) in their mechanical properties when tested under saturated conditions (Table 1).

The Armenian tuffs show a broad range of petrophysical properties. The tuffs of this study have a high effective porosity of 21 % to 36 %. However, the Hoktemberyan tuff is characterized by a much lower ratio of micropores (9 %) and bulk density (1.6 g/cm³). Mechanical properties display the Armenian tuffs as moderately strong with maximum tensile strength values of 5 MPa. While Golden Armenia suffers from a distinct strength decrease under saturated conditions, the Hoktemberyan tuff does not seem to be affected by water in its mechanical properties.

The porosity of the Mexican tuffs ranges from 18 % to 36 % and the density values vary from 2.3 g/cm³ to 2.6 g/cm³, respectively. The pore-size distribution of the studied tuffs are unimodal and bimodal, in addition the average pore radius fluctuates from 0.15 µm to 4.02 µm, dominating the capillary pores. SLP volcanic rocks are also very soft to moderately hard, depending on the degree of welding and showing average values of tensile strength in dry conditions of around 8 MPa, falling extremely to values down to 1.33 MPa in water-saturated conditions. Well-welded ignimbritic tuff samples of SLP can have uniaxial compressive strength of 90 MPa (Wedekind et al. 2013).

Water saturation reduces the strength (both uniaxial compressive and tensile strength) of the tuffs with one exception (Hoktemberyan) (Table 1). The strength of the saturated samples can be as low as one-fourth, but in general, the value is half or two-thirds of the dry one. The porosity of the tuffs is in between 18 to 37 vol%, while matrix densities are relatively uniform with 2.3 to 2.6 g/ cm³. The P-wave velocity of the studied tuffs is in between 2.3 and 3.9 km/s. However, these values do not necessarily indicate differences in porosity (Table 1).