Mineralogical controls on the engineering behavior of hydrothermally altered granites under uniaxial compression

• Reduction in strength of altered granite is directly related to changes in mineralogy.
• More intense alteration of plagioclase to secondary mica between Grades III and II
• Analysis of acoustic emission and strain gauge data used to assist characterization
• Further work is required to validate/optimize the crack threshold methodologies.

Effective engineering characterization of granites altered through processes of kaolinization is critical for safe extraction and design of slopes for the china clay industry in south-west England. By considering this important issue, representative samples taken from a range of decomposition grades of altered granite were used to assess the controlling influence of changes in mineralogy on uniaxial compressive strength, elastic modulus, dry density and ultrasonic velocity. Dramatic reduction in uniaxial compressive strength occurs between alteration Grades II and III. Analysis of acoustic emission data, constrained by evaluation of Scanning Electron Microscope (SEM) images and strain gauge deformation readings, has been used to identify crack propagation stages during uniaxial compression. Quantitative evaluation of mineralogy was determined from X-ray diffraction (XRD) analysis of representative samples in order to establish relationships between mineralogy, such as %kaolinite, and uniaxial compressive strength for the different alteration grade material. The results of the testing confirm that the degree of alteration or kaolinization, and associated changes in mineralogy of the granite, is directly related to reduction in uniaxial compressive strength and dry density.