Borehole Geophysics Applied To The Study Of Landfill Sites In Fractured Bedrock Terrains

Geotechnical and hydrogeologic studies for landfill sites in the fractured bedrock environment can<br>be a technically challenging prospect. Many of the problems involved in landfill construction in<br>fractured bedrock can be solved by detailed surface mapping of fracture density and orientation,<br>but ultimately data has to be collected from the subsurface via exploratory wellbores. Fracture<br>density and orientation can be obtained through acoustic televiewer logs or digital borehole<br>imaging logs, but identifying the fracture or fracture group that is water-bearing (hydraulically<br>conductive) is critical in characterizing the flow of water at the study site.<br>Hydrophysical logging and digital borehole imaging were used to log 13 wells to evaluate a<br>prospective canyon landfill site in weathered crystalline rocks. Hydrophysical cross-hole testing<br>was employed to evaluate hydraulic connections between wells and estimate medium scale<br>hydraulic conductivity. The results of this study found that: (1) For most wells, the more highly<br>transmissive intervals were within 30 feet of the piezometric level. This behavior is consistent<br>with the deeply weathered nature of the bedrock, which behaved in a manner hydraulically similar<br>to a silty sand. The hydraulically conductive intervals ranged in thickness between 2 and 8 feet,<br>and represented between 1% and 5% of the total length of the bedrock section, (2) In all but one<br>instance, the water bearing intervals were within 60 feet of the piezometric surface. In other<br>words, groundwater flow was largely concentrated in shallow fracture zones. (3) Based on the<br>hydrophysical logging results for ambient horizontal flow, a typical range of specific discharge<br>values for a discrete fractured interval between 0.3 and 0.02 A/day was observed at the study site.