## 4 Results and Analysis  
The measurement is conducted in rainy weather. When the thickness of the water film on the surface of the model is maintained at 5-10 mm, the model is kept under the condition for 1 h. First connect the sensor with the laptop, and then open the software smart sensor 4.10 to verify that the connection is unblocked and set the initial parameters. The weight of the test vehicle is 1600 kg and the pore water pressure is recorded when the vehicle passes through the measurement point at the speed of 20 km/h, 40 km/h, 60 km/h and 80 km/h, respectively. In order to eliminate the influence of errors, parallel experiments were used, repeatedly collecting multiple sets of data, and using the coefficient of variation to eliminate unreasonable data to ensure that the errors in this study are within a reasonable range. The variation of pore water pressure with time at different speeds is shown in Figure 5.  
Figure 4: The photo of field measurement. Figure 5: The variation of pore water pressure with time at different speeds. Figure 3: (a) Section of the model, (b) Top view of the model. As can be seen from the Figure 5, the pore water pressure increases with the increase of speed, and the pore water pressure decays periodically over time, this phenomenon can be explained by Hydrodynamic water hammer theory [21]. As is shown in Figure 6, when the vehicle load acts on the pores, part of the pore water obtains the initial velocity \(v\), and the flow hits the bottom of the pore and the velocity of the flow at the bottom becomes zero instantaneously, the water pressure at the bottom of the pore increases from \(pgh\) to \(pgh+\Delta p\), and a compression wave is formed at the liquid interface between \(pgh\) and \(pgh+\Delta p\). This article assumes that the compression wave moves \(\Delta s\) within \(\Delta t\). After the compression wave reaches the bottom, the momentum equation of the water flow is described by the Equations (1) and (2). \[\rho\Delta sA(0-v)=[P-(P+\Delta P)]\Delta t \tag{1}\]  
\[\Delta P=\rho v\frac{\Delta s}{\Delta t}=\rho vc \tag{2}\]  
where \(c\) is the speed of the compression wave, \(v\) is the initial velocity of the flow under the vehicle load, \(\rho\) is the density of water, and A is the cross-sectional area of the pore. When the water pressure at the bottom of the pore increases from \(pgh\) to \(pgh+\Delta p\), the density of water and the cross-sectional area of the pores will change. The density of water increases by \(\Delta\rho\), and the cross-sectional area of the pores increase by \(\Delta A\). The mass of flow in the \(\Delta S\) section increases due to the effect of cross-section expansion and water compression, and the added mass is the mass of free water which is replenished from the upper part. According to the law of conservation of mass, Equation (3) can be acquired. \[\rho vA\Delta t=(\rho+\Delta\rho)(A+\Delta A)(c\Delta t)-\rho A(c\Delta t) \tag{3}\]  
Simplify and omit higher order infinitesimals to get the following Equation (4). \[\rho vA=c(\rho\Delta A+A\Delta\rho) \tag{4}\]  
Figure 6: Pore pressure propagation diagram. Then, Equation (5) can be acquired. \[v=c\left(\frac{\rho\Delta A+A\Delta\rho}{\rho A}\right)=d\left(\frac{\Delta A}{ \rho A}+\frac{\Delta\rho}{\rho}\right) \tag{5}\]  
Let \(\left(\frac{\Delta A}{\rho A}+\frac{\Delta\rho}{\rho}\right)\) be \(\Delta P\). The K bulk modulus of water is equal to the ratio of stress to strain. The Equation (6) can be acquired. Figure 4:0  
Then, Equation (7) can be acquired. Figure 4:1  
Since the diameter of the cylindrical pore is 2 \(r\), Equation (8) can be obtained. Figure 4:2  
Omit higher order infinitesimals, Equation (9) can be obtained. Figure 4:3  
The \(E\) elastic modulus of solids is equal to the ratio of stress to strain. Equation (10) can be acquired. Figure 4:4  
Then, Equation (11) can be obtained. Figure 4:5  
According to fluid mechanics, when the balance fluid acts on the pipe wall, the stress generated by the pipe wall is \(\Delta\sigma=\frac{\Delta Pr}{\xi}\).