Project Abstract: While the effects of porosity on rock strength and elastic properties have been widely investigated both experimentally and analytically, the effects of macroporosity (large voids) on strength and elastic properties are poorly understood and are poorly addressed in the scientific literature. Besides the basic lack of scientific understanding of these materials, the results are of importance to a number of entities. One of the most important aspects of this study is its applicability to the analysis and design of the U.S. high level radioactive nuclear waste repository; portions of the repository will be constructed in tuff units that contain large cavities. The research will also be relevant to engineering applications involving weakly cemented aggregates, which are currently employed as backfill for mine openings and stabilization of sinkholes, and may possibly have military or homeland security uses as potential impact-resistant materials. The primary aim of this research is to investigate the engineering behavior of materials displaying macroporosity, by quantifying the variation in behavior as a function of macroporosity characteristics. A three stage approach will be employed, corresponding to master's thesis work of four graduate students. In the first and second stages of the research, the effects of cavity size and cavity shape, on the strength (unconfined compressive strength, friction angle, cohesion), deformability (Young's modulus), and failure behavior (stress-strain response and failure mode) will be quantified. The third stage will focus on studying the effects of multiple sizes of cavities, since real macroporous materials tend to contain a wide range of cavity sizes. All stages of the proposed project will be accomplished through a combination of laboratory testing and numerical modeling, utilizing Montana Tech's state-of-the-art rock triaxial testing apparatus and PFC3D distinct element numerical modeling software. Real rock (tuff) and cemented rockfill specimens as well as synthetic samples composed of plaster of Paris with Styrofoam inclusions will be tested. After performing validation using the experimental data, numerical models will be used to supplement the laboratory experimental data, facilitating analysis of a larger number of samples under a wider range of conditions. The proposed project team is both interdisciplinary and inter-institutional, involving faculty from three different departments (Civil, Geological, and Mining Engineering) on two campuses (Montana Tech of The University of Montana and the University of North Florida, both primarily undergraduate institutions). Funding for a 3-day short-course on Itasca's PFC software will allow project participants, as well as other interested members of the engineering community who will be invited to attend, to gain a more thorough understanding of this powerful numerical tool. The research results will be disseminated via journal publications and presentation at rock mechanics symposia, and will be incorporated into several courses at Montana Tech. The extensive opportunity for collaboration, which will allow sharing of tools, ideas, and expertise, is one of the most exciting and beneficial aspects of the project. The work will promote diversity within the engineering workforce by providing support for a female PI and several female graduate students. Undergraduate and high school students will be involved in various aspects of the work; interaction between these students and the graduate student researchers is expected to encourage the younger students to continue their education to the next level.
Some types of rock contain large holes called "voids" or "macropores" (think Swiss cheese) due to the way they are formed or environmental conditions they are subjected to after they are formed. While engineers agree that these voids make the rock weaker and more compressible (both of which are usually considered undesirable for engineering materials), the details of how much weaker and how much more compressible are not known with certainty. The purpose of this research project was to investigate the engineering behavior of rock and rock-like materials containing large voids, using a combination of laboratory testing and computer modeling. Real rock specimens containing large voids are difficult to get because they tend to fall apart during sampling. We were able to get some good specimens to test, but we also made hundreds of our own synthetic specimens using plaster of Paris to represent the rock material and Styrofoam inclusions to represent the voids. The use of synthetic specimens also allowed us to control the size, shape, number, and positions of the voids so we could investigate each of these aspects separately. The computer models were used to verify the laboratory test results and to expand the datasets so we could more thoroughly investigate different aspects of the macroporous materials. One way macroporous materials are often characterized is by their overall porosity value, which is defined as the volume of the void space divided by the total volume of the specimen. The results of our work suggest that the overall porosity is not really sufficient to determine the engineering behavior because the void parameters such as size, shape, number and positions of voids are not explicitly accounted for in the overall porosity. Our work has qualitatively shown that the void parameters have a significant influence on the strength and compressibility of the material; in fact, at low macroporosity values, the strength can range by as much as a factor of ten. With increasing macroporosity values the ranges of strength and compressibility decreases significantly. One exciting result showed the positions of voids within a specimen influenced the fracture patterns leading to failure of specimen. Incorporating upper and lower statistical bounds on the strength and compressibility was shown to be promising and at this stage in our research, is probably the best approach to dealing with these types of materials in engineering projects. Future work will focus on more sophisticated techniques of characterizing the voids in real specimens, as well as use of new imaging methods that will allow "looking" inside the rock specimens to determine the characteristics of the voids without destroying the specimens. The new imaging methods will be conducted on the specimens both before and after testing. This will allow the researchers to quantitatively assess the effect of void parameters on both strength and compressibility. A total of 6 graduate students (4 female) and 13 undergraduates (5 female) participated in this project during its five years of NSF support. The students were attracted to the combination of laboratory and computer work, the opportunity to contribute to their profession, and the excitement and camaraderie of working on a research team consisting of students of different levels and faculty at two primarily undergraduate universities. All of the students were encouraged to present their results at professional conferences, and almost all of them did so; the grant provided funds for the students to travel to conferences in Idaho, Utah, Colorado, Nevada, California, North Carolina, Vancouver, and Toronto.
