Subsurface Exploration Using Bucket Auger Borings and Down-Hole Geologic Inspection

Abstract

The down-hole geologic inspection of 24 in. bucket auger borings has been a hands-on technique for collecting valuable geologic structural and lithologic detail in southern California investigations for over 35 yr. Although it has been used for all types of investigations for hillside urban development, it is of particular benefit in landslide investigations and evaluations. It has also been successfully used in fault investigations where trenching could not get deep enough to intercept the faults. The benefits of down-hole geologic inspection during detailed mapping of large landslide complexes with multiple slide planes are discussed in this paper. This paper is the result of numerous requests by city engineers, who have been faced with the monumental task of approving or disapproving urban development on ancient landslides within their jurisdictions. (Many of these areas have the best views.) Many of the geotechnical investigations of these massive landslide complexes have been very limited in their determinations of accurate landslide parameters and very deficient in proper engineering analysis while based upon this limited data. This has resulted in many cases where the geotechnical consultant erroneously concludes that ancient landslides don9t move and it is alright to build upon them, even though they have neither justified the landslide parameters, nor the slope stability or safety. In these cases, there was also no redundancy-in-design due to considering future landscape watering, future rainfall, and future earthquakes. These are a common problem throughout large portions of California and other mountainous states. Because this author and the many consultants contacted during the preparation of this paper were not aware of other publications regarding this method of collecting detailed geologic data, this author included the safety considerations, safety equipment, the cost and the Cal OSHA requirements for entering exploration shafts.