Development of a gamma logger to work in conjunction with the CPT for geotechnical and environmental applications

• Main Author: Singha, Sandeep
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/8404

The piezocone (CPTU) is a commonly used instrument to assess soil parameters and soil type in geotechnical investigations. However, it is sometimes difficult to distinguish between two soil types that are only subtly different such as fine sand and silty sand with the CPTU. In these cases an alternate method of determining soil type may be required. In order to assess soil type, we may measure the fines content of the soil. One method of assessing fines content within a soil is to measure the natural gamma radiation decay for the common clay mineral constituents potassium and thorium. A gamma logging module has been developed that fits the aforementioned cone penetrometer and records natural gamma data while the CPTU is performed. This test has been labelled the Gamma Cone Penetration Test(GCPT). Since emitted gamma radiation increases with increasing fines content, the instrument can be used to qualitatively distinguish soil type and fines content based on local correlations. Another application of the GCPT is for the measurement of insitu soil density. The back scatter from a module containing a small Cesium source mounted below the GCPT can be correlated to soil density. The GCPT also has environmental applications. It can be used locate radioactive contamination based on a gross gamma count log. Once the contamination is located, the GCPT can measure the gamma energy spectrum in order to identify the radioactive isotope. Since the GCPT is a penetration tool, there are no drill cuttings brought to the surface and worker exposure to potentially hazardous radiation is minimized.