# Measurement of Unit Weight of Dry Sand Using Piezoelectric Sensor
## 3 Results of EMI Sensing
### Effect of Surcharge on EMI Sensing  
The effects of the surcharge application during EMI sensing experiments were investigated in terms of peak frequency and corresponding peak conductance. Figure 6 demonstrates the relationship between conductance and frequency for the three sand grain sizes. With an increase in surcharge levels on sand, the peak frequency was found to have gradually increased across all sand grain sizes. This result may be attributed to increased sand stiffness due to the increase in unit weight caused by the application of surcharge, as shown in Figure 8. Consequently, vibrations in the sand were reduced and the peak frequency increased. The peak conductance, on the other hand, demonstrated a slight decrease with an increase in surcharge levels. [PERSON] et al. (2015) [PERSON] et al. (2015) demonstrated a similar increase in peak frequency and decrease in peak conductance corresponding to an increase in hardness of mortar  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{3}{*}{**Cases**} & \multicolumn{3}{c}{**Small Sand**} & \multicolumn{3}{c}{**Medium Sand**} & \multicolumn{3}{c}{**Large Sand**} \\ \cline{2-10}  & **Unit** & **Peak** & **Peak** & **Coductance** & **Unit** & **Peak** & **Peak** & **Unit** & **Peak** & **Peak** \\  & **Weight** & **Frequency** & **Conductance** & **Weight** & **Frequency** & **Conductance** & **Weight** & **Frequency** & **Conductance** \\  & **(k/m\({}^{3}\)) & **(k/Hz)** & **(S)** & **(k/m\({}^{3}\))** & **(kHz)** & **(S)** & **(k/m\({}^{3}\))** & **(k/Hz)** & **(S)** \\ \hline Sand & 14.45 & 172.0 & 0.06 & 15.74 & 175.7 & 0.09 & 16.83 & 173.2 & 0.08 \\ \hline Sand + & & & & & & & & \\  6 kPa & 16.09 & 173.9 & 0.04 & 16.32 & 176.3 & 0.08 & 17.34 & 173.7 & 0.08 \\ \hline Sand + & & & & & & & & \\  11 kPa & 16.65 & 174.9 & 0.02 & 16.52 & 176.7 & 0.07 & 17.44 & 174.2 & 0.08 \\ \hline Sand + & & & & & & & & \\  17 kPa & 17.24 & 176.1 & 0.02 & 16.62 & 177.0 & 0.07 & 17.55 & 174.3 & 0.07 \\ \hline Sand + & & & & & & & & \\  23 kPa & 17.49 & 177.0 & 0.01 & 16.93 & 178.0 & 0.06 & 17.66 & 174.5 & 0.08 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Results of EMI sensing.  
Figure 7: Relationship between surcharge and unit weight for three sand types.  
Soils in the field comprise various grain sizes and, therefore, three different grain sizes of sand were used in the EMI sensing experiment. Figure 9 compares the values of the peak frequency and the peak conductance for the three sand grain sizes. The effect of an increase in the unit weight corresponding to an increase in surcharge levels was found to be most noticeable in the sand comprising small-sized sand grains. For medium and large-sized sand grains, the effect was not prominent. This result seems to be related to the contact area between sand grains and the PZT sensor. The relative size of sand particles compared to the PZT sensor could have an influence on the frequency and conductance. For example, the ratio of the diameter of the PZT sensor to mean grain size of the large and medium-sized sand grains (DPZT/D50) was 6 and 17, respectively. Compared to small-sized sand grains (DPZT/D50 = 47), such large grain sizes of sand are unlikely to effect a large change in the results obtained via EMI sensing.