ADVANCING ROCK-TYPE SPECIFIC BRITTLE HOEK-BROWN PARAMETER “S”

Abstract

The work analyzes sandstone mechanics through laboratory tests consisting of UCS measurements using strain gauges, acoustic emission measurements, and microscopic observations. The first goal of this study was to classify sandstone types through petrographic analysis, followed by the measurement of their compressive stress responses in order to establish rock-type-specific parameters for the Hoek-Brown failure criterion. The study used ten sandstone samples: five fine-grained sandstones and five coarse-grained sandstones. The objective of the study is to determine the relationship between grain size, matrix type, and the brittle Hoek-Brown constant “s” (i.e., cohesion constant). Macroscopic studies combined with microscopic studies provide important results about the structure of the sample, but require more precise measurements between petrographic characteristics such as grain sizes and the Hook-Brown parameter. Petrographic observation showed thin-grained samples had higher matrix material that likely included clay-rich argillaceous material based on their compact, dull appearance. The fine-grained samples studied display arkosic wacke characteristics as a result of high matrix content and poor sorting and angular grain textures, which on thin sections resemble plagioclase feldspar. The thin-section analysis showed that the two sandstone types possessed poor sorting characteristics and included polymictic grain compositions of quartz and plagioclase feldspar, together with iron oxide cement. The fine-grained sandstones contained grains measuring between 60-150 microns, whereas the coarser sandstones held grains from 100 to 300 microns, accompanied by reduced matrix occurrence. The restricted transport patterns, along with quick deposition of angular to sub-angular grain shapes, probably took place in a setting of alluvial fans. Results from UCS tests showed noticeable differences between the strength of fine- and coarse-grained sandstones. The UCS values of coarse-grained sandstone (103.7-152.6 MPa) exceed the UCS values of fine-grained sandstone (73.9-124.1 MPa) because of different microstructural elements. The better grain interlocking and stronger grain-to-grain contacts within coarse-grained sandstones improve their load-bearing capability. The distribution of stress becomes less effective in fine-grained sandstones because their matrix material includes clay-rich components, which diminishes overall cohesion. The uniaxial compressive strength decreases because poorly sorted and less angular grains found in fine-grained sandstones lead to early stress-induced crack formation. The failure strain of fine-grained sandstones proved lower based on strain gauge measurements, thus indicating these rocks would fail in a more brittle manner than the coarse-grained rocks. The damage initiation stress values are lower in the fine-grained samples. Under stress conditions, the material experiences early microcrack formation since it contains higher amounts of matrix and grain bonding because of these factors. The research demonstrates that grain size and matrix content directly affect rock strengths as well as the processes through which they fail. The research studied how to improve the brittle Hoek-Brown failure criterion through proper adjustment of its 's' parameter that represents rock material cohesion attributes. Sample analyses indicate that fine-grained sandstone has s average parameter value of 0.142, while coarse-grained sandstone has s parameter average value of 0.34. The use of s = 0.11 as a generalized cohesion constant lead to forecasting inaccuracies due to significant deviations from the actual values measured during this study, which indicates the need to calibrate the parameter "s" for the type of rock. The study shows that sandstone exhibits varying mechanical responses according to grain size variation and matrix content. The strength values for fine-grained sandstones remain lower than those of coarse-grained sandstones due to their elevated matrix fraction, along with inferior grain bonding strength. This leads to earlier material deterioration. The advanced Hoek-Brown failure criterion gives geotechnical assessments better reliability, which leads to improved underground excavation planning efficiency and safety.