Direct Push Electrical and CPT Logging: An Introduction

摘要

Direct push tools and methods for obtaining electrical and CPT logs have been developed to assist with investigation of unconsolidated formations. The direct push Wenner array probe for electrical logging is about 15-inches in length and 1.5-inches in diameter. The probe is advanced with a direct push machine equipped with a hydraulic hammer to depths exceeding 100 feet under favorable conditions. This portable system is operated from a lap top computer and provides real time on-screen logs. Logs may be printed and saved to diskette. Two people can complete a 60-foot log in about one hour. No previous borehole or well is required and no cuttings are generated while logging. The Wenner array is effective in defining lithology and locating aquifers and aquitards for geo-environmental investigations in alluvial and glacial deposits. The logs may be used to determine optimum locations for wells and screen intervals. In situ testing in geotechnical engineering has traditionally been carried out using ballasted CPT rigs with weights ranging from 15 to 30 tons. Smaller soil exploration units, such as Geoprobe soil probing units have most often been used in environmental site characterization. However, modifications in equipment design now allow these smaller units to be used for some geotechnical work. CPT probes have been successfully pushed by anchored soil probing units to depths exceeding 100 feet. The CPT is a reliable and cost-effective method for determining subsurface stratigraphy as well as obtaining many geotechnical parameters. CPT probes are pushed into the ground at a constant 2 centimeters per second (cm/s) rate, rather than with the use of a drop hammer to advance the tool string, as with the SPT. Unlike the SPT, CPT provides a continuous, detailed log of the soil characteristics and soil types.