## 4 Results and Discussion
### Change of Lateral Earth Pressure Coefficient  
[PERSON] [25; 26] proposed a value of 1 for K in the region where arching occurred. It was asserted that K varies between Ko and 1.5 depending on the vertical movement of the trapdoor and the soil height at which the frictional force appears. Researchers investigating arching mechanisms in various structures have reported K values for arching formations ranging from Ka (Rankine's active lateral earth pressure coefficient) to 1.6 [23; 30; 33; 34; 35; 36; 52; 53; 54; 55; 56]. [PERSON] et al. [39] explain arching in tunnels and claim that the lateral earth pressure coefficient in the central soil prism varies from Ko to Kp (coefficient of passive lateral earth pressure) depending on the burial depth, which is not constant. When comparing the stresses computed for different values of K in Equation (2) with the test and analysis outcomes, two distinct situations emerge. In the first four loading stages, where Pv is low and more homogeneously distributed, the soil behaves similarly to the Ko condition due to limited downward movement of the central soil prism. However, as surcharge stresses increase, stress concentration occurs predominantly in the crown, resulting in deflections that take on a heart-shaped form. The movement of particles towards the crown in the central prism is caused by intensified surcharge stresses, specific to shallow buried flexible pipes, negatively affecting the Pv by creating arching. Consequently, the lateral earth pressure coefficient is reduced, reaching a value between Ko and Ka (K = 0.3) (Figure 7). In other words, the greater the Pv in the crown of the pipe compared to other regions, the closer the earth pressure coefficient tends to be to Ka. The K value at SP-1 was computed for both S and P tests using Equation (5). Substituting \(\sigma_{\rm h1}\) and \(\sigma_{\rm v1}\) from Equations (3) and (4) into Equation (5) generates Equations (6) and (7). \[\rm K=\frac{\sigma_{\rm h1}}{\sigma_{\rm v1}} \tag{5}\]  
\[\rm K_{t}=\frac{22.22\times Ph_{t}}{26.67\times Pv_{t}}=c_{1}d_{r} \tag{6}\]  
\[\rm K_{a}=\frac{23.98\times Ph_{a}}{26.51\times Pv_{a}}=c_{1}d_{r} \tag{7}\]  
where, \(c_{1}\) represents the coefficient that characterises the correlation between pipe deflections and stresses in the adjacent soil prism, and the lateral earth pressure coefficient is defined for Ka analysis computations for Kt test measurements  
For SP-1, the variation of K was obtained from Equation (5) and the variation of K was computed as a function of \(d_{r}\) in Equations (6) and (7) for testing and analysis (Figure 13). Figure 13: Variation of the K with applied stress. In the S test and analysis, it was observed that the maximum horizontal stresses at the spring line were approximately 40 kPa, resulting in a horizontal displacement of 0.85 mm. This limited movement of the backfill towards the tank wall was influenced by the horizontal stresses. However, it did not have a significant effect on the backfill, which initially exhibited a Ko state. Considering the first four loading stages in both the P test and the analysis, the increment of horizontal stresses at low Ph values is greater than the increment of vertical stresses transferred to the adjacent soil prism by arching. This leads to higher K values in the first loading stages. However, after approximately 2-2.5 mm of horizontal pipe deflection, the increments for both horizontal and vertical stresses stabilize at SP-1, which means that K is constant.