The presence of vesicles within basalt has a negative impact on both the unconfined compressive strength (UCS) and deformation modulus (axial stiffness) of basalt specimens. Vesicular basalt specimens (diameters of 44.45 mm) were cored from surface boulders obtained from southern Nevada. Each basalt boulder has its own unique pore sizes, pore spacing, and shape of pores. Sizes of these voids range from less than 1 mm to approximately 30 mm. To date, 26 specimens have been tested in unconfined compression to determine the engineering properties of peak strength and deformation modulus. The variation in these two parameters is assessed as a function of unit weight and total porosity. Results show the specimens fall within two distinct groups, one with relatively high unit weight (low total porosity) and one with relatively low unit weight (high total porosity). The high unit weight (low porosity) specimens had the greatest influence on developed relationships between peak strength or deformation modulus and unit weight or total porosity. Total porosity is a better predictor of peak strength and deformation modulus. Future work is presently underway to investigate potential size effects and characterize various macropore properties to augment the relationships developed through laboratory testing.


The presence of vesicles within basalt has a negative impact on the unconfined compressive strength (UCS) of the basalt. Vesicles are macropores within the basalt matrix. Vesicles are spherical or cylindrical cavities left by gas bubbles within solidifying magma [1]. Qualitatively, the strength of vesicular basalt is inversely proportional to the amount of pore space within the rock; as the macroporosity increases, the UCS decreases. Finding a reliable method to quantitatively estimate the strength of vesicular basalt without extensive laboratory testing would be of great benefit to the engineering community. Two studies have investigated the influence of macropores on engineering properties of vesicular basalt. One study investigated the properties of vesicular basalt from Saudi Arabia [2]. This study related various properties to porosity as determined through image analysis. The image analysis technique was not described in detail but it seems disks were cut from cored specimens, vesicles were filled with a white material, and macroporosity was calculated based on the ratio of area of macropores to matrix. The other recent study [3] investigated engineering properties of vesicular basalt from the western US and Mexico as a function of specific gravity. The study found an exponential relationship between unconfined compressive strength and specific gravity. Both of these studies are very interesting and provide valuable insight into the properties of macroporous rock. However the choice of independent variable is somewhat questionable. Porosity, whether macroporosity or total porosity, is not easy to determine but can be estimated using a number of different techniques for vesicular basalt. Specific gravity, which is often used when dealing with aggregate material in civil engineering applications, is a confusing property. There are a number of specific gravities, (oven dry, air dry, saturated surface dry, and wet [4]) which often leads to confusion when used without proper documentation.

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