## 2 Measurement of the Stress Conversion Coefficient
### Experiment Content
#### 2.2.2 Methods of Measuring the Stress Conversion Coefficient  
According to Equation (4), the stress conversion coefficient can be expressed as the ratio of operations carried out by internal tensile stress to the operations carried out by axial stress. In this paper, the uniaxial compression of large samples and its equivalent tensile test are used to calculate the measured stress conversion coefficient. (1) Uniaxial compression experiment  
In this study, the uniaxial compression experiment used an MTS universal material machine; for the corresponding large sample diameter size, the uniaxial compression instrument adopted a diameter of 20 cm and upper and lower pressure discs; moreover, the maximum control accuracy of the forward speed of the instrument beam is 0.1 mm/min, the range of the pressure sensor is \(-\)100 kN to 100 kN, and the precision of the displacement sensor is 0.01 mm. During the test, the system automatically collects experimental data and displays it on the computer in real time. (2) Tensile strain measuring system  
The strain measurement system consists of an S-type strain sensor and a data acquisition system. As shown in Figure 4, the S-type tension strain sensor consists of an S-type tension meter and a rubber ring. After production, the tension strain sensor is placed on the sample and connected to the data acquisition system. When the specimen is stretched in the process of compression, the radius increases, and the rubber ring expands; moreover, the S-type tension sensor tightens. Since rubber is an elastic material, after knowing the elastic modulus of the rubber ring, the perimeter of the section where the sensor is located can be converted by reading the tension of the S-type tension meter at this time, and the tensile strain on the section can be obtained using conversion. The S-type tension meter used in the test has a measurement range of 10 kN and an accuracy of 0.01 N. The elastic moduli of the three rubber rings are 0.0053 N/mm, 0.0041 N/mm, and 0.0059 N/mm. Because the k value of the rubber ring is not large, the lateral compression force caused by the rubber ring on the soil is not large. In the test, the pressure of the rubber ring is about 1/20 of the internal tensile stress at the peak stress of the soil; thus, the influence of the rubber ring on the soil can be ignored. In order to explore the influence of the measuring device on the sample, two large samples with the same sample production parameters were carried out at the same loading rate, and a control test was conducted on the soil without the tension strain sensor installed and after the sensor was installed. The experimental results are shown in Figure 5.  
In Figure 5, A-1, A-2, and A-3 are samples without tension strain sensors installed under different compaction loads, while B-1, B-2, and B-3 are samples equipped with tension strain sensors under different compaction loads. By observing the stress-strain curves of the two samples under the same compaction load, it can be observed that whether the tensile strain sensor is installed has little influence on the results of the samples. Figure 4: Tensile strain test system: (**a**) S-type tension meter; (**b**) strain measurement and analysis system. Figure 5: Influence test of the S-type tension sensor in the uniaxial experiment. (**A**) The figure shows the experiment and data without the S-type tension sensor; (**B**) the figure shows the experiment and data with the S-type tension sensor. (3) Equivalent tensile test  
The S-type tension sensor measured lateral deformations in all parts of the soil sample, and the relationship between the equivalent radius \(\overline{r}\) and time \(t\) was obtained. Then, the relationship between the equivalent tension strain (\(\epsilon_{t}=\frac{\overline{r}-\overline{r}_{0}}{\tau_{0}}\)) and time \(t\) was obtained, and lateral strain rate \(k_{2}\) was obtained (Figure 6). As observed in Figure 6, with the soil sample under compression, its transverse tensile strain presents a monotonically increasing trend.