Crystalline rocks are made of interlocked minerals in a way that gives them high strength properties. These rocks are mostly igneous and interlocking patterns i.e., textures, varies among their major groups. In a novel attempt to connect these fundamental features with the macroscopic engineering and mechanical properties of these rocks, an extensive experimental study has been performed at the Indian Institute of Technology, Delhi. Blocks of different sizes and types made of dental plaster were prepared with split moulds and then assembled according to four different systematic patterns into 180mm × 180mm × 76mm large rectangular cubical specimens. These specimens were tested in biaxial loading condition after proper curing. The most notable aspects of the texture were discovered to be Lmax/Lmin or the ratio of maximum to minimum length of minerals, and the interlocking index number. The latter was proposed for the first time to be an index to the mechanical properties of crystalline rocks. A simple method was also proposed to calculate this parameter based on the thin section studies. The findings of the present study enhance the understanding of the rock strength that is a key factor in design of the rock engineering projects.


Crystalline rocks include most igneous and metamorphic rock types that expose a structure of interlocking crystals. The strength of these rocks is only due to their very complex and three-dimensional interlocking which, in case of igneous rocks, occurs as different crystals randomly put together as original magma solidifies. Role of the microstructure on the macroscopic rock properties has been discussed by several researchers (like Brace [1]), yet the details of this in terms of mechanism and important factors are not well understood. To explore the vague details of the matter, an extensive testing program has been carried out on rock-like heterogeneous specimens at Indian Institute of Technology Delhi. The results of the study are summarised in the following sections.

Materials and Method
Model Material

Since the majority of the crystalline rocks are igneous, they were studied in greater depth in the present work. The igneous rocks consist of five major minerals i.e. quartz, feldspar, mica, pyroxene and olivine; the percentage of other minerals is negligible [2]. Attempts were made to physically simulate these major minerals; their properties, i.e. σ c and E, were taken from literature or calculated through theoretical means (Table 1). A wide range of building and industrial materials were tried to get the same strength and modulus ratios as exist among these minerals. In the end, dental plaster was selected as the base model materials and mixing with different quantities of water it could approximately replicate the natural conditions. Model materials Q, F, B, and P were selected to roughly represent quartz, feldspar, biotite, and pyroxene respectively. To represent olivine, the same F material was selected since feldspar and olivine strength ratios are nearly same but olivine modulus ratio is too high to be achieved in the lab.

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