Abstract
Investigation of a hard rock site for the development of engineered structures mainly depends on the delineation of weathered and unweathered rock, and the fractures/faults. Traditionally, borehole tests are used in such investigations. However, such approaches are expensive and time-consuming, require more equipment, cannot be conducted in steep topographic areas, and provide low coverage of the area with point measurements only. Conversely, geophysical methods are non-invasive, economical, and provide large coverage of an area through both vertical and lateral imaging of the subsurface. The geophysical method, electrical resistivity tomography (ERT), can reduce a significant number of expensive drilling tests in geotechnical investigations. However, a geophysical method alone may provide ambiguity in the interpretation of the subsurface, such as electrical resistivity cannot differentiate between water and clay content. Such uncertainty can be improved by the integration of ERT with induced polarization (IP). Similarly, self-potential (SP) can be integrated with other geophysical methods to delineate the groundwater flow. In this contribution, we integrated three geophysical methods (ERT, IP and SP) to delineate the weathered and unweathered rock including the weathered/unweathered transition zone, to detect the fractures/faults, and to map the groundwater flow. Based on ERT, IP and SP results, we develop a geophysical conceptual site model which can be used by site engineers to interpret/implement the findings for build-out. Our approach fills the gaps between the well data and geological model and suggests the most suitable places for the development of engineered structures in the hard rock terrains.
Highlights
Given natural heterogeneity, the evaluation of hard rock sites for the construction of engineered structures is a challenging task for planners and engineers
The subsurface was interpreted for the delineation of distinct layers, fractures/faults, clay, and groundwater flow path. 2D electrical resistivity tomography (ERT) models were integrated with the borehole lithological logs to obtain a subsurface model of four distinct layers/zones with specific resistivity values range
The main fractures/faults were delineated by the integration of all geophysical models using low values of ERT, induced polarization (IP) and SP
Summary
The evaluation of hard rock sites for the construction of engineered structures is a challenging task for planners and engineers. In the hard rock terrains, the subsurface geological knowledge about the weathered and unweathered rock and the fractures/faults needs to be known for the success of the engineering construction [6]. ERT coupled with IP can detect the saturated weathered/fractured zones more efficiently since low values of electrical resistivity and chargeability suggest water content [3]. We performed an integrated geophysical survey of ERT, IP and SP at a highly heterogeneous hard-rock terrain in order to evaluate the subsurface for site suitability prior to engineering construction. Compared with conventional geotechnical studies, our contribution reduces the uncertainty caused by the spatially limited data, provides a more accurate geological model via lateral and vertical (2D) imaging of the large area, and suggests the most suitable places for the development of engineering structures in highly heterogeneous hard-rock terrain
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