A. Basu

Rock failure modes under uniaxial compression, Brazilian, and point load tests

Rock failure is a serious problem in rock engineering environments. Rock failure modes, however, are complex and difficult to quantify or predict. A comprehensive study on rock failure modes at laboratory scale is, therefore, potentially important as it helps recognize the adequacy of the support designed on the basis of the nature of an engineering work. With due need, this paper analyzes the failure modes of granite, schist, and sandstone under uniaxial compression, Brazilian, and point load tests in relation to corresponding strengths. The nature of the principal failure mode changes from axial splitting to shearing along a single plane to multiple fracturing in the case of both granite and sandstone specimens as uniaxial compressive strength (UCS) increases. In the case of schist, specimens failed at low UCS show failure along foliations whereas specimens which do not fail along foliations portray high strength. The relation between failure modes of all three rocks under uniaxial compression and corresponding UCS values was broadly explained in terms of damage evolution of the rocks. Granite and sandstone specimens failed mainly following central or central multiple type of fracturing whereas schist specimens principally failed by layer activation in combination with either central or non-central fractures over the entire range of determined Brazilian tensile strength. In the case of granite and sandstone, central multiple failure mode corresponds to high tensile strength. Descriptions of different failure modes under point loading were presented. It was found that granite and sandstone specimens generally fail through the rock materials in one or more extensional planes containing the line of loading. Failure patterns showing triple junctions correspond to high point load strength indices. In the case of schist, specimens failed along foliations show a low point load strength index whereas specimens failed through material with a single extensional plane result in high strength.

A method for estimating crack-initiation stress of rock materials by porosity

Crack-initiation stress of a rock under compression is the stress level that marks the initiation of the rock microfracturing process or in other words, the onset of new damage to the rock. This paper proposed a simple methodology with justifications to explore the feasibility of using total and effective porosities as estimators of crack-initiation stress of brittle crystalline rock materials under uniaxial compression. The validity/applicability of the proposed method was examined by an experimental study of granitic materials from Malanjkhand, Madhya Pradesh. It was found that effective porosity depicts better correlation with crack-initiation stress than with uniaxial compressive strength of the granitic materials. On the other hand, total porosity does not show any perceptible correlation with uniaxial compressive strength and crack-initiation stress. Plausible reasons for the nature of the obtained results were also explained in view of rock failure process under compression. It is concluded that following the proposed method, effective porosity can be used as a physical index to obtain a quick estimate of crack-initiation stress of the investigated rocks empirically.

Applicability of Weathering Classification to Quartzitic Materials and Relation Between Mechanical Properties and Assigned Weathering Grades: A Comparison with Investigations on Granitic Materials

The ongoing process of weathering in nature produces progressive but intricate changes in rock microstructure. Evaluating mechanical behaviors of rock materials with reference to weathering grades is, therefore, important for an engineering work encountering weathered rocks. The common 6-fold weathering classification for uniform materials is meant to capture gradational change of rock materials depending on degree of decomposition. Research in this regard has been limited within polymineralic rocks (e.g. granite etc.) where degree of alteration of constituent minerals helps recognize such gradation. Quartz being the most resistant mineral to weathering is the chief mineral constituent of quartzite and therefore, capturing intricate gradational change of quartzite in response to weathering or categorization of weathering grades of quartzitic materials is a challenging task. In line with the author’s involvement in three different research topics, this paper presents salient points in categorizing weathering grades of granitic rock materials from Hong Kong and southeastern Brazil that are subsequently compared with the issues in characterizing weathering grades of quartzitic rock materials from eastern India. An overall assessment of mechanical behaviors of these rocks with reference to assessed weathering grades is also outlined.

Quantification of microcrack anisotropy in quartzite — a comparison between experimentally undeformed and deformed samples

In this paper, microcrack patterns in a quartzite are quantified using fractal geometry based methods. Since the quartzite does not show a mesoscopic foliation, the fabric was recognized using anisotropy of magnetic susceptibility (AMS) analysis. Microcracks were investigated in thin sections prepared along the three principal planes of the AMS ellipsoid. Point load tests were performed on cores drilled parallel as well as perpendicular to the magnetic foliation. After experimental deformation, thin sections were prepared in two orientations — (a) parallel to the plane of failure (i.e., parallel to the direction of loading), (b) perpendicular to the plane of failure (i.e., perpendicular to the direction of loading), and microcrack patterns in these sections were investigated. The box-counting method of fractal analysis was first applied to microcracks traced from SEM images from each thin section of the experimentally undeformed as well as deformed samples to establish the fractal nature of the microcrack pattern. It was found that in thin sections perpendicular to the direction of loading, the box (fractal) dimension tends to marginally increase. This is inferred as a manifestation of the increase in complexity of the pattern. The software AMOCADO, which is based on the modified Cantor Dust method of fractal analysis, was applied to microcrack pattern from each thin section in order to quantify the pattern anisotropy. It is noted that the anisotropy significantly reduces in sections perpendicular to the loading direction. SEM data are presented to demonstrate that this reduction in anisotropy is on account of generation and/or growth of new cracks in random orientations. It is envisaged that the approach adopted in this investigation maybe useful in rock mechanics and mineral-resource applications in future.

Identification of paleochannels in and around ???Chandraketugarh???, Ganges Delta through remote sensing techniques using fuzzy inference system

A method for identification of most probable paleochannels in and around “Chandraketugarh” (an archeological site in the western part of Ganges Delta) was developed through remote sensing techniques using fuzzy inference system (FIS). The method was adopted considering the uniqueness of the geomorphology in terms of terrain homogeneity of the area. In order to categorize different geomorphological features, NDVI, NDWI, and elevation datasets were generated using multispectral images (LISS III) and CARTOSAT I datasets. Based on the unique spectral signatures of the different geomorphological units, the said datasets were critically examined to be utilized as the input parameters of the employed FIS-based image segmentation method. The result of subsequent analysis lead to development of a land use/land cover (LULC) map which portrayed inferred paleochannels. The developed method was validated with reference to a finer resolution image and ground truth datasets. It is concluded that the newly developed FIS-based method is quite efficient for identifying probable paleochannels in a terrain where slope variation is minimal and geomorphology does not seem to have been strongly affected by structural elements.