Measurement and Prediction of In-Situ Weak Rock Mass Modulus: Case Studies from Nevada, Puerto Rico, and Iran

This dissertation aims to advance the state-of-the-art in measuring and predicting weak rock mass modulus in a variety of geotechnical conditions. This goal has been accomplished by collecting in-situ data from underground mines in Nevada and by reappraisal and critical review of existing geotechnical data from dam sites in Puerto Rico and Iran, which permitted the development of a multivariate correlation to estimate the rock mass modulus.;The underground mines of Nevada are often situated in highly-fractured and intensely-altered rock masses whose geotechnical properties occasionally resemble stiff soils, and existing mining excavations represented an excellent opportunity for performing in-situ tests. In order to efficiently and economically perform in-situ rock mass deformability tests in the underground mining environment, a portable plate loading device (PPLD) was designed and fabricated. The PPLD is transported on the surface as a highway-worthy trailer or underground by forklift where a plate loading test can be completed by two workers in about 10 to 15 hours. The PPLD was designed to be simple to operate and as such does not require any external power sources. Field and data reduction methods were developed to minimize potential sources of error.;Using the PPLD, in-situ rock mass deformability tests were performed at two of Nevada's producing underground gold mines yielding 14 high-confidence moduli values with associated detailed geotechnical characterizations. The tests were performed on a variety of rock types including fault breccias, argillized rhyolitic dykes, oxidized limestones, decalcified limestones, and argillized limestones. To augment the Nevada data and increase the general applicability of the analyses, plate loading test data was obtained for in-situ tests performed on highly-fractured rock masses at the Portuguese and Bakhtiary Dam sites in Puerto Rico and Iran, respectively. For plate loading tests at the dam sites, sub-surface rock mass deformations were recorded with multi-point borehole extensometers, and geotechnical logs were available for rock core recovered from below the center of each bearing plate. Moduli were calculated from force-displacement data for the Bakhtiary Dam site, and representative geotechnical properties for test sites were determined from core logs for both dam projects. Thirteen moduli from the dam sites were combined with 13 of the moduli measured in Nevada to create the first known published weak rock mass modulus database that includes associated detailed geotechnical parameters.;The predictive performance of published rock mass modulus models were evaluated against the weak rock mass database. Only two of the existing models were adequate for weak rock masses over limited ranges of alteration intensities, and none of the relationships provided good estimates of modulus over a range of geotechnical properties. In light of this shortcoming, a multivariate model was developed from the relatively small weak rock mass modulus dataset. The new correlation is exponential in form and has the following independent variables: 1) average block size or joint spacing, 2) field estimated rock strength, 3) discontinuity roughness, and 4) discontinuity infilling roughness. Prediction intervals were calculated for the proposed model to provide a ranges of uncertainty for predicted moduli, which has been largely absent in published models. The multivariate model provided better estimates of modulus for both hardblocky rock masses and intensely-altered rock masses. Although the multivariate model is somewhat rudimentary, it can be used to provide reasonably accurate estimates of weak rock mass modulus where only limited geotechnical data is available.