## 3 Similarity Model Test of a Highway Subgrade
### Experimental Results and Analysis  
Exhausting the rubber balloon during the test resulted in differential settlement at each position of the subgrade model. The OFEB deformed with the soil at the different positions. Figure 11 shows the strain and displacement curves obtained under the working conditions 1-3, and all the test results are presented in Table 2.  
Table 2 and Figure 11 show that under working conditions 1 and 2, a very small subgrade settlement was measured by the drawstring displacement transducer and the OFEB. As the suction pipe was located on the left side of observation pole 1, the exhaust at observation pole 3 lagged behind that at observation poles 1 and 2. Thus, a \"pressure arch\" was created in the soil layer that supported the upper soil layer. After the rubber balloon was evacuated under Condition 3, the \"pressure arch\" in the soil layer could no longer support the overlying soil, and the upper layer suddenly collapsed. This sharp downward subsidence resulted in a large soil deformation. Table 2 presents the measurements obtained using the drawstring displacement transducer, indicating that the last subsidence was considerably larger at the settling plate A than at the settling plates B and C.  
Figure 10: Actual photographic images of the prototype. Figure 12a-c compares the measurements obtained using the optical fiber and drawstring displacement transducer under the three working conditions. The errors in the settlement measured by the optical fiber relative to the measurement by the drawstring displacement transducer were 2.9%, 2.8%, and 6.6% at Position C, 5.1%, 2.6%, and 4.8% at Position B, and 6.1%, 8.1%, and 37.4% at Position A. The errors at Positions B and C are acceptable and correspond to an OFEB measurement accuracy exceeding 93.4%. However, the OFEB measurement accuracy at Position A was considerably lower because settlement occurred at Position A before occurring at Positions B and C. Thus, uncoupling between the OFEB and the subgrade soil developed rapidly, especially when the soil foundation deformed to 125.3 mm, resulting in a large difference between the measurements obtained using the optical fiber and the drawstring displacement transducer. As the subgrade soil can slip on both sides of the OFEB, the deformation between the soil and OFEB was \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sensor**} & \multicolumn{3}{c}{**Test Settlement (mm)**} \\ \cline{2-4}  & **Condition 1** & **Condition 2** & **Condition 3** \\ \hline Observation Pole 1 & 134 & 489 & 489 \\ Drawstring A & 9.56 & 14.75 & 125.3 \\ Optic Fiber at Position A & 10.14 & 15.94 & 78.48 \\ \hline Observation Pole 2 & 138 & 491 & 491 \\ Drawstring B & 9.67 & 15.52 & 89.77 \\ Optic Fiber at Position B & 9.18 & 16.26 & 92.09 \\ \hline Observation Pole 3 & 44 & 254 & 490 \\ Drawstring C & 7.16 & 9.93 & 50.05 \\ Optic Fiber at Position C & 6.95 & 10.59 & 51.45 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of the test results. Figure 11: Distributions of the strain and displacement under conditions (**a**) 1, (**b**) 2, and (**c**) 3.  
completely uncoupled. This result is confirmed by the photo of the field test shown in Figure 12d: the subgrade soil at settling plate A completely fell into the cavity of the rubber balloon, showing that the OFEB was only coupled with the soil over a certain settlement deformation range. The OFEB should have a wide measurement range to accurately measure subgrade settlement. As a limited number of tests were performed, the accurate measurement range of the OFEB was determined in the experiment as 0-89.77 mm, which corresponds to the measurement range for the actual scaled-up highway subgrade of 0-897.7 mm. Thus, the proposed method can meet the requirements for measuring large deformations of subgrades. In conclusion, the calibration and model experiments show that the OFEB and the corresponding calculation method can measure subgrade settlement accurately within a certain measurement range.