High Frequency Sound Waves as Function of the Density and Water Content: Experimental Studies on Calcarenitic Stones of Southern Apulia
Abstract
In Apulia Region there is a dense historical-architectural and archaeological heritage built with soft calcarenitic stones. These materials present significant problems of conservation related to their low durability and therefore know the intrinsic characteristics is crucial to evaluate the performance properties and their susceptibility to degradation. To evaluate the physical-mechanical characteristics of the stone materials is very useful to use non-destructive and noninvasive technologies such as ultrasonic, applicable both in situ and in the laboratory. The factors influencing the propagation of elastic waves in the rocks are the structure, density, size and shape of the granules, porosity, anisotropy, the water content, stress and temperature. In this paper the effects of saturation on the elastic wave velocity and the relationship between density and elastic wave velocity were studied on some Apulia’s porous calcarenites such as Leccese stone, the Ostuni stone and the calcareous “Tufo delle Murge”. The ultrasonic P and S wave velocity measurements were performed on cubic samples, under natural condictions (e.g. without applying external pressure on the samples), using the transmission method. Variations of P and S wave velocity were related to density and percentages of water content. Furthermore in order to study the frequency influence on seismic velocities.
Keywords
Download Options
Introduction
Monumental structures represent our cultural and historical heritage. However, damage of historical buildings, monuments, works of art and other cultural properties is reported from all over the world. One of the greatest dangers for the historical monuments is weathering, caused by climatic changes and air pollution. Building stones are susceptible to various atmospheric factors causing their destruction, especially in Mediterranean basin, where the marine salts are a permanent cause of natural pollution, not only on the coast but also inland. Weathering effects on the physical and mechanical properties of natural stones of monuments. These properties can be studied using the microgeophysics methods that includes all the methodologies derived from geophysics with more or less miniaturized instrumentations. Particularly ultrasonic measurement is one of the non-destructive microgeophysical methods commonly used in order to provide data related to the elasticity, anisotropy and mechanical and weathering resistance of the stones, porosity, dry density, and water absorption. This method can be applied both in the laboratory. The study of P and S waves velocity has used in several area of application such as rock mass characterization (Boadu, 1997; Leucci and De Giorgi, 2006; Bery and Saad, 2012). There are several studies related to the application of ultrasonic method in order to study the damage of historical buildings and monuments (Zezza, 1993; Christaras et al., 1997; Christaras, 2003; Cosentino et al., 2009; Leucci et al., 2011; Leucci et al., 2012; Calia et al., 2013). Some authors have investigated on the relationship between seismic wave velocity and bulk density (Gardner et al., 1974; Miller and Stewart, 1991). Also the effect of water content on the ultrasonic velocities was studied by several authors (Wyllie et al., 2956; Wyllie et al., 1958; Thill and Bur, 1969; Nur and Simmons, 1969; Gregory, 1976; Carcione, 2001). Kahraman, (2007) performed P-wave velocity measurements were performed on 41 different rock types, 11 of which were igneous, 15 of which were sedimentary and 15 of which was metamorphic, he found a strong linear correlation between the dry- and wet-rock P-wave velocities.
Although several researchers have investigated both the effect of saturation and the variation of bulk density on elastic-wave velocity of different rocks, none of them has derived empirical equations between dry- and wet-rock P and S-wave velocities and between P and S wave velocities and bulk density. Furthermore these studies will be useful for the rock used in the construction of historical and monumental heritage in south Apulia region. In this paper, the predictability of both wet-and dry rock vs P and S-wave velocity and bulk density vs P and S-wave velocity was studied.
Conclusion
P and S wave velocity measurements were carried out on three different rock types used in the building construction in the southern Italy. The results were evaluated as function of bulk density and normalized saturation degree variations. The rock types were “pietra gentile”, “tufo delle murge” and “pietra leccese”. The following conclusions were obtained:
1) The Vp values increase as the normalized saturation degree increase in the three type of rock; P-wave velocity values rapidly increases follow a liner equation that varies in the three types of rock;
2) The Vs values increase as the normalized saturation degree increase in the “pietra gentile”, while decrease as the normalized saturation degree increase in both the “tufo delle murge” and “pietra leccese” rock types; this anomalous behavior may be due to the intrinsic characteristics of studied rock types. In fact, this phenomenon is to be related to the framework which determines the natural resistance of the rocks . The "Pietra Gentile" stone shows values of uniaxial compressive strength in dry conditions similar to the “pietra leccese”, while the Tufo delle Murge has poor resistance. In wet conditions the loss of strength of "Pietra Gentile" is relatively low (15.74% M; 3.5% B), while for the Leccese stone and "tufo delle Murge" has a fall of resistance even greater than 40% as seen from the values of uniaxial compressive strength shown in Table 1.
3) The low porosity values of “pietra gentile” makes it very compact makes it very compact and consequently the velocity Vs insensitive to changes of fluid present within the pores;
4) The Vp and Vs values increase as the density increase;
5) Regression analysis indicated that P and S-wave values were strongly correlated with the normalized saturation degree and density. When the regression analyses were repeated for the three types rock, it was seen that correlation coefficients were increased using a linear correlation.
6) The in situ measurements demonstrate the effectiveness of the relationships obtained from laboratory test.