ABSTRACT

Bimrocks are structurally complex formations made up by a fine-grained matrix including heterometric hard-rock fragments which deeply influence their mechanical behavior. A new approach for characterizing the morphological and spatial variability of rock fragments in bimrocks is introduced, based on the geostatistical analysis of bimrock outcrop pictures through a single-scale variogram analysis of the rock fragment indicator variable IB (x, y). The analysis indicated the presence, all over the field, of a nested structure characterized by three or four common elementary variogram models, each taking into account the variability of a specific size range of the rock fragments. A first attempt was also made in order to investigate the possible correlations between the bimrock strength parameters, obtained through in-situ non-conventional shear tests (BimTests), and the geostatistical indexes, which quantify specific spatial and morphological properties of the fragments, by means of a Cross- Covariance study of non-isotopic regionalized variables (ReV).

1. INTRODUCTION

Bimrocks [1] are structurally complex formations characterized by a fine-grained soil, the “matrix”, which includes, in a typical block-in-matrix fabric, hard-rock fragments of variable dimensions. The presence of rock fragments above a critical threshold size, namely the block/matrix (B/M) threshold, deeply influences the mechanical behaviour of bimrocks [2, 3, 4, 5, 6, 7]. The B/M is not an absolute property of bimrocks but is related to a specific engineering scale of interest (i.e. several B/M can be identified depending on the working scale of the problem under investigation). Due to their complex structure, an exhaustive mechanical characterization of bimrocks requires some special investigations to be carried out. In particular a nonconventional in-situ large size test (namely BimTest) was developed by the Authors in order to properly take into account the presence of rock fragments and their interactions with the soil matrix [8]. The main advantage of BimTest is that that the shear plane is free to develop inside the specimen, thus allowing for an increase in tortuosity of the shear plane, leading to an increase in the bimrock shear strength compared to that of the only clayey matrix. However, the in-situ characterization through BimTests could result in a quite expensive and time-consuming task, especially when large volumes of the formation are involved. For this reason, the possibility to integrate the mechanical tests with an indirect method of characterization was investigated. The analysis aimed at correlating the strength parameters with 2D geostatistical indexes obtained through a single-scale variogram analysis of the rock fragment indicator variable IB (x, y), performed on digital pictures of outcrop exposures. The present study was carried out on a wide slope located in a dismissed open-pit mine in Tuscany, Italy, characterized by a wide exposure (about 345.000 m2) of an Oligo-Miocenic olistostrome of the Tuscan Nappe. The formation is constituted by a dark-grey clayey matrix containing rock fragments of micritic and arenitic limestone [9] (Fig. 1).

2. THE BIMTEST

Six BimTests were performed over the investigated area, on specimens of 0.3 m3 (80x80x50 cm) [8].

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