Abstract. Two separate studies investigated the compressive strength of basic igneous rocks and its relation to index properties such as porosity, density and sound-wave velocity. The rocks tested were weathered dolerite from S. W. England and vesicular lavas from California. In both cases strength was affected considerably by porosity. In the dolerite, strength was also affected by fissures (microcracks) which were formed in the parent rock by shearing and later accentuated by weathering. While test results show that porosity, density and sound velocity are not reliable indices of rock strength, these properties are useful in interpretation of strength test results. Index testing can be used to classify samples according to similar behavior and to recognize the influence of fissures.


It is well known that the strength and deformability of so-called "intact rock" are influenced both by pores and by fissures. Fissures and microcracks are distinguished from spherical pores by their narrow, elongated shape and their high length-to-width ratios, typically of the order of 103 (Walsh & Brace, 1966). They are formed by stress, and result from tectonic shearing or from the expansion of minerals during weathering or alteration. In some cases, the crack or fissure pattern may be related to the macroscopic joint pattern in the rock mass, while in others it forms part of a separate shear fabric. Microcracks and fissures can range in length and spacing from the microscopic to a few cm, and can be distinguished according to the terminology proposed by Londe (1973): micro- cracks are features with a spacing of less than 1 cm, fissures have a spacing of less than 10 cm and joints have a spacing greater than 10 cm. In much of the literature, however, the adjective "fissured" refers to both microcracks and fissures.

As a consequence of their shape, microcracks and fissures are further distinguished from pores by their compressibility and preferred orientation. Their effects on the mechanical properties of "intact" rock, as reviewed by Jaeger (1972) and Goodman (1976), include the dependence of strength on specimen size, the change in slope of the stress-strain curve as cracks are closed with increasing stress, the stress-dependence of permeability, and hysteresis in load cycling. Cracks and fissures also influence static elastic modulus values, sound wave velocity, direct tensile strength, resistivity and conductivity. In all these cases, the effects of cracks and fissures are quite distinct from the effects of spherical pores, which are noncompressible and which do not have preferred orientation. This paper describes two separate studies of engineering properties of porous and fissured basic igneous rocks. The first study examined properties of weathered dolerite (diabase) from S. W. England and had the original objective of using simple index tests to aid in classification of the degree of weathering. The effects of weathering in these rocks are to create pores by leaching of relatively soluble constituents and to accentuate tectonic microcracks which occur in varying degrees in the unweathered rocks. The second study reported in the paper examined engineering properties of vesicular (i.e., porous) lavas from eastern California.

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