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Measurement and Prediction of In-Situ Weak Rock Mass Modulus: Case Studies from Nevada, Puerto Rico, and Iran
AdvisorWatters, Robert J.
Geological Sciences and Engineering
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This dissertation aims to advance the state-of-the-art in measuring and predictingweak rock mass modulus in a variety of geotechnical conditions. This goal has beenaccomplished by collecting in-situ data from underground mines in Nevada and by reappraisaland critical review of existing geotechnical data from dam sites in Puerto Ricoand Iran, which permitted the development of a multivariate correlation to estimate therock mass modulus.The underground mines of Nevada are often situated in highly-fractured andintensely-altered rock masses whose geotechnical properties occasionally resemble stiffsoils, and existing mining excavations represented an excellent opportunity forperforming in-situ tests. In order to efficiently and economically perform in-situ rockmass deformability tests in the underground mining environment, a portable plate loadingiidevice (PPLD) was designed and fabricated. The PPLD is transported on the surface as ahighway-worthy trailer or underground by forklift where a plate loading test can becompleted by two workers in about 10 to 15 hours. The PPLD was designed to be simpleto operate and as such does not require any external power sources. Field and datareduction methods were developed to minimize potential sources of error.Using the PPLD, in-situ rock mass deformability tests were performed at two ofNevada's producing underground gold mines yielding 14 high-confidence moduli valueswith associated detailed geotechnical characterizations. The tests were performed on avariety of rock types including fault breccias, argillized rhyolitic dykes, oxidizedlimestones, decalcified limestones, and argillized limestones. To augment the Nevadadata and increase the general applicability of the analyses, plate loading test data wasobtained for in-situ tests performed on highly-fractured rock masses at the Portugues andBakhtiary Dam sites in Puerto Rico and Iran, respectively. For plate loading tests at thedam sites, sub-surface rock mass deformations were recorded with multi-point boreholeextensometers, and geotechnical logs were available for rock core recovered from belowthe center of each bearing plate. Moduli were calculated from force-displacement datafor the Bakhtiary Dam site, and representative geotechnical properties for test sites weredetermined from core logs for both dam projects. Thirteen moduli from the dam siteswere combined with 13 of the moduli measured in Nevada to create the first knownpublished weak rock mass modulus database that includes associated detailedgeotechnical parameters.The predictive performance of published rock mass modulus models wereevaluated against the weak rock mass database. Only two of the existing models wereiiiadequate for weak rock masses over limited ranges of alteration intensities, and none ofthe relationships provided good estimates of modulus over a range of geotechnicalproperties. In light of this shortcoming, a multivariate model was developed from therelatively small weak rock mass modulus dataset. The new correlation is exponential inform and has the following independent variables: 1) average block size or joint spacing,2) field estimated rock strength, 3) discontinuity roughness, and 4) discontinuity infillingroughness. Prediction intervals were calculated for the proposed model to provide aranges of uncertainty for predicted moduli, which has been largely absent in publishedmodels. The multivariate model provided better estimates of modulus for both hardblockyrock masses and intensely-altered rock masses. Although the multivariate modelis somewhat rudimentary, it can be used to provide reasonably accurate estimates of weakrock mass modulus where only limited geotechnical data is available.