GEOPHYSICAL BOREHOLE LOGGING
DHLA has investigated a number of active landslides where the depth of failure, active movement and high groundwater conditions have precluded the usual Southern California practice of drilling large-diameter bucket-auger boreholes that are then entered by geologic personnel, who log the geologic structure and identify and sample the failure plane materials. Entering a borehole for the purposes of such “down-hole logging” is risky enough under perfect ground conditions. The complications inherent to an active landslide increase the risks of down-hole logging and usually decrease the success of obtaining useful samples for laboratory testing.
DHLA has developed an exploration procedure utilizing rock coring and a recently developed geophysical logging technique to successfully identify the orientation of the geologic structure and failure plane and to then obtain a relatively undisturbed sample of the failure plane for strength testing in the laboratory without having to remold samples.
The procedure involves first logging cores obtained from the landslide debris and underlying bedrock materials, followed by geophysical logging using an Acoustical TeleViewer (ATV). The ATV utilizes sound waves to produce an oriented graphic log of the borehole walls that can be correlated with the rock cores to provide an accurate lithologic log and the true orientation of the geologic structure. From this information the accurate depth and orientation of the failure plane can be determine and targeted for sampling in a companion borehole. The use of smaller-diameter rotary wash drilling rigs in this procedure also greatly facilitates the installation of slope inclinometer and pore pressure transducers for measurement of landslide movement and groundwater conditions.