Basalts are the most common rocks of the Volcanic Complex of Lisbon. The interest of knowing the mechanical properties of this type of rock lays both in its interference with engineering projects (as tunnels and foundations) and its industrial exploitation (crushed stone for road paving, high resistance concrete and rockfill). Samples of basalt with various shapes (discs, cylinders and prisms) were prepared from quarry extracted blocks and submitted to uniaxial compressive strength, indirect tensile strength tests (point load, Brazilian and bending) and slake durability resistance. Results of that series of experiments are presented, compared and correlated in order to reveal insights about basalt properties and their mechanical characteristics.


Geomechanical properties of rocks are fundamental to predict rock mass behaviour for the purpose of adequately planning engineering projects. The present work was accomplished for studying basalts from the Volcanic Complex of Lisbon due to it industrial importance and reporting the obtained results from several different testing methods. Another objective was aimed to compare results gathered in the application of different methods in order to obtain a number of correlations that are valid for users who need simpler, faster and more economic ways of characterizing that rock type.


The Volcanic Complex of Lisbon (age 72 MA) is strongly represented by basalts, which cover an area of about 200 km2 of extension. This formation corresponds to one of the most important igneous activities that occurred during the formation of the Atlantic Ocean. The specimens collected for testing were obtained in the "Moita da Ladra" quarry, located in Via Longa (Alverca), 20 km North of Lisbon. The basaltic rock mass that is being extracted there shows low grade of weathering although frequent networks of irregular fractures with quite low spacing, as it is shown in Fig. 1. An homogeneous matrix with very fine grain characterize the hand samples of this type of rock. An attempt was made to collect blocks large enough to obtain all the test specimens needed. Whenever this purpose was not possible all blocks were chosen with the same orientation and visual characteristics. All samples were inspected for macroscopic defects, such as small fractures, joints and weathering, so that a common standard was imposed and all specimens were prepared in accordance with ISRM Suggested Methods. The subsequent operation of coring mainly used NX (54mm) diameter core bits with carefully prepared parallel bases in the specimens. All samples were tested at their natural water content and with an average of at least 10 measurements for each test (except for uniaxial compressive strength with only 5 results).


Almost all tests were performed according to ISRM suggested methods, except the bending test that followed the Portuguese Standard NP - 314 (three point bending).

3.1. Uniaxial compressive strength

Six basalt samples were tested for obtaining Young's modulus, Poisson's ratio and uniaxial compressive strength. The average value for Young's modulus was 79.2 GPa with a coefficient of variation of 10.28%.

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