Electric resistance strain gauges have been widely used for the manufacturing the cone penetrometer. Measured values by strain gauges, however, are affected by various factors such as electromagnetic wave, temperature, self heating, and the length of the lead wire. In this paper, micro cone penetrometers with 3~7mm in diameter, are developed by using a FBG (fiber Bragg grating) sensor to improve the defect and the limitation of strain gage type cone penetrometer. Note FBG sensor, which is a kind of fiber optic sensor and has hair-size in diameter, is not affected by environmental conditions because the measured data is the wavelength shifting of the light instead of the intensity of the electric voltage. Stain gage type cone penetrometer shows that the output voltage changes and increases with an increase in the temperature. The FBG cone penetrometer, however, produces the steady state value because the temperature change is efficiently compensated. The temperature compensated FBG cone penetrometer shows excellent sensitivity and detects the interface of the layered soils with higher resolution. In addition, the temperature change near the cone tip is monitored during the cone penetration. This study demonstrates that the FBG sensor renders the possibility of the ultra small size cone and the new micro cone penetrometer may produce more reliable resistance.
To investigate the soil profile and reasonable soil properties, various field test methods are performed. Cone penetration testing (CPT) is a one of the most widely used in situ filed test because a fast and reliable data with minimum mechanical disturbance can be obtained. As the smaller cone produces the smaller disturbance, various types of micro cone penetrometers have been developed for the estimation of the soil strength and the detection of the non-homogeneity of soil layers (DeLima and Tumay 1991; Tumay et al. 1998, 2001; Tumay and Kurup 2001; Hird et al. 2003; Yun 2005; Liu et al. 2006; Lee et al. 2008).
