# Measurement of Unit Weight of Dry Sand Using Piezoelectric Sensor
## 2 Piezoelectric Sensor and Sample Preparation  
A picture of the PZT sensor used in this study is shown in Figure 1a. Dimensions of the PZT sensor are indicated in the schematic shown in Figure 1b. When the PZT senor embeds in soil, it faces the risk of short circuiting due to moisture in the soil. In this study, the sensor was coated with a ceramic to prevent the event of a short circuit. Figure 2 shows the EMI signature of uncoated and ceramic coated PZT sensors immersed in 10% CaCl\({}_{2}\) solution. As seen in Figure 2, the EMI signature of the PZT senor showed the double resonance peaks in the frequency range of 50 to 350 kHz. In this study, the behavior of the EMI signature was analyzed using the peak corresponding to the frequency of 130 kHz under free vibration in air condition.  
The EMI signature of the immersed, uncoated PZT sensor was very different from that of the PZT sensor under the free vibration in air condition. In the case of the EMI signatures of the coated PZT sensor immersed in CaCl\({}_{2}\) solution compared with that in free vibration in air condition, the EMI peak magnitudes were slightly different but the EMI peak frequencies were the same. The difference in EMI  
Figure 1: Piezoelectric sensor (PZT) after coating and its dimensions (unit: mm): (**a**) PZT sensor used in this study; (**b**) dimensions of the PZT patch.  
Figure 2: Electro-mechanical impedance (EMI) signatures of PZT sensor under various conditions.  
peak magnitude was caused by the difference of the surrounding material of the PZT sensor. This means that a short circuit was avoided, and EMI sensing was possible even with moisture in the soil.  
Figure 3 shows the EMI signatures of the PZT sensor coated with ceramic under various temperature conditions. As shown in Figure 4, the EMI peak magnitudes were increased slightly with the increase of the temperature, but the EMI peak frequencies were not changed significantly. It can be seen that the increase of EMI peak magnitude with the increase of temperature is probably due to the thermal stress in the PZT sensor. Since the change of EMI signature by temperature was very small compared with the change of EMI signature in the soil experiment, the effect of temperature can be ignored in this study.  
The EMI measuring system employed in this study, as depicted in Figure 4, comprises an LCR meter, PZT sensor, GP-IB interface, and software. The PZT sensor, without being placed in the soil, demonstrated three modes of resonant frequencies (130, 468, and 662 kHz) below 1 MHz. It seems that the vibration occurs in multiple directions because of the circular shape of the PZT sensor. The frequency range of 50-350 kHz (500 Hz intervals) within the first mode of resonance was selected for use in this study to easily distinguish between different soil densities.  
The series of EMI sensing experiments for different levels of surcharge were performed on the Nakdong river sand. The sand was dried and sieved into three different grain sizes--small (0.075-0.85 mm), medium (0.85-2.0 mm), and large (2.0-4.75 mm)--as shown in Figure 5a. Grain size distribution curves and properties of the three sand types are compared in Figure 5b and Table 1. The specific gravity of each grain size was constant at 2.65. The sand was loosely poured into a compaction mold measuring 15 cm in diameter and 17 cm in height as shown in Figure 6. The PZT sensor was located at the center of the mold, and, subsequently, four levels of  
Figure 4: EMI measurement system using PZT sensor.  
Figure 3: The changes of EMI signatures with the increase of temperature: (**a**) Relationship between temperature and resonance peak magnitude; (**b**) relationship between temperature and EMI peak frequency at resonance peak.