Karst Features in Limestone Evaluated Utilizing an Acoustic Televiewer, I-70 Mississippi River Bridge, St. Louis, Missouri

摘要

Borehole televiewers are downhole inspection devices that can be utilized to view in-situ rock conditions and should be considered by geotechnical engineers and geologists as a supplemental means of investigating rock. The optical televiewer (OTV) produces a digital optical image and the acoustic televiewer (ATV) utilizes reflected acoustic waves to produce images of the borehole walls. Soft or fractured zones, solution features, open joints or other voids often unobservable during traditional rock coring techniques can be detected and measured utilizing this technology. The devices can be used to measure the strike and dip of joints, bedding planes, shear surfaces, or other structural features, eliminating the need for expensive oriented cores or downhole logging. An acoustic televiewer was utilized in the fall of 2008 during the investigation phase for the New 1-70 Mississippi River Bridge in St. Louis, Missouri. An ATV was utilized in each of ten river borings to produce images of the rock mass in the borehole and allow further investigation of voids identified during rock coring. Voids interpreted as karst features were identified in the ATV logs in multiple borings and at similar elevations, suggesting connectivity. Measurements of the void's width, location in the borehole and orientation allowed a more thorough understanding of the potential impact the karst features may have on the planned deep foundations. An extensive grouting program was eliminated based on the results of the ATV.