An investigation into the effects of block size distribution function on the strength of bimrocks based on large-scale l

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ORIGINAL PAPER

An investigation into the effects of block size distribution function on the strength of bimrocks based on large-scale laboratory tests Saeed Mahdevari 1 & Parviz Maarefvand 1

Received: 2 January 2016 / Accepted: 25 April 2016 # Saudi Society for Geosciences 2016

Abstract Considering the geomechanical characteristics of rocks which are a mixture of rock blocks embedded in fine grain matrix can be challenging due to their complex structure and difficulty in preparing representative in situ or laboratory samples. In the literature, this group of rocks is called block in matrix Rocks (bimrocks). In this paper, the effects of volumetric block proportion (VBP), maximum block size, and distribution function on the compressive strength and failure patterns of these rocks were considered by executing the unconfined compressive test on large-scale synthetic samples. The compressive strength of bimrocks was found to have a direct relation with VBP and maximum block size and an indirect one with fractal dimension in accordance with multivariable regression analysis. The fracture patterns of remolded samples were changed from an inter-granular facture to a matrix and between-granular one by the reducing the VBP and increasing the fractal dimension. In addition, the failure mechanism was spalling for the low values of VBP and the high values of fractal dimension and it was changed to shear failure by increasing VBP and decreasing fractal dimension.

Keywords Bimrock . Fractal dimension . VBP . Unconfined compressive test

* Saeed Mahdevari [email protected]

1

Department of Mining and Metallurgical Engineering, Amir Kabir University of Technology, Hafez 424, Tehran 15875-4413, Iran

Introduction The block in matrix rocks (bimrocks) are a mixture of rock blocks embedded in fine matrixes which are at least two times weaker (Medley 1994, 2002; Medley and Lindquist 1995; Medley and Wakabayashi 2004). As results of their complex structure and difficulties in preparing laboratory or in situ samples, determining the geological parameters of these rocks is very challenging. So, there are a lot of engineering problems when underground and open structures are constructed within or on these rocks (Goodman and Ahlgren 2000; Glawe and Upreti 2004; Kahraman and Alber 2008; Shafiei and Dusseault 2008; Yasir and Tolgay 2010; Coli et al. 2011). The studies show that the most important parameters controlling the strength characteristics of bimrocks are the volumetric block proportion (VBP), matrix and block strength, block shape factors, block direction, and size distribution function (Hawkes and Mellor 1970; Kobayashi and Yoshinaka 1994; Lindquist and Goodman 1994; Andriani and Walsh 2002). In typical bimrocks, as the block strength is more than that of the matrix, there is an increase in the modulus of deformation and friction coefficient by increasing of the VBP up to 75 % and a decrease in the cohesion (Lindquist and Goodman 1994; Coli et al. 2011; Barbero et al. 2012). If the VBP is more than 75 %, by increasing the VB