A feasibility study of the application of modal analysis to geotechnical engineering

Experimental modal analysis (EMA), developed in the field of vibration engineering, encompasses the most powerful aspects of both experimental and analytical techniques. It has shown great potential for assessing the characteristics of materials and engineering structures by their dynamic behaviors. This research was undertaken to investigate the feasibility of applying EMA as a nondestructive testing method in geotechnical engineering. The research involved two tasks. The first was demonstrating that the assumptions and fundamentals of EMA are valid in typical geotechnical situations. The second was showing structure reflections of defect or failure development. A positive conclusion was obtained.;A preliminary lab study was carried out to investigate the influence of defect development in a large rock specimen. Significant changes of modal behavior were found.;In the full scale field experiment, a rock block, part of a highway slope, was selected and modified several times to provide defect development. At each stage of modification, modal testing was conducted. First of all, a thorough statistical evaluation was performed. The data collected during the modal testing were proved to be random and transient with bell-shaped distribution. The system stability, time-invariance, and physical realizability were also demonstrated by analysis of the collected data. Slight nonlinearity was observed during the experiments and this was compensated for by limiting impact to a certain level. The applicability of EMA to the geotechnical structure was thereby justified. Secondly, modal behavior was examined after each stage of block modification to show the influence of defect development. Variations of mode shapes were found both visually and analytically after each block modification. Modal frequency down-shift was also manifested. No significant change could be determined for the damping coefficients (;Another technique used in vibration engineering, operational deflection shape (ODS), was also explored. The research showed ODS could reflect defect or failure development in a rock structure, especially the hammer-excited ODS.