## 4 FLC Feasibility Study
### Calculation of Additional Stresses and Settlements  
According to the design principle of FLC replacement, different degrees of settlement will be generated when FLC is poured on soft soil roadbeds. Based on the additional stress calculation Formula (3) for the foundation soil, the additional stress and settlement calculation was carried out in conjunction with the actual situation of the existing buried high pressure gas pipe section. The results show that the effective additional stress in the subgrade is 12 KPa, as shown in Formula (4). \[P_{0}=H_{\rm L}\times\gamma_{\rm L}+H_{\rm F}\times\gamma_{\rm F}-S\times \gamma_{\rm S}, \tag{3}\]  
\[P_{0}=0.83\times 26+5.63\times 5.5-3.086\times 18+15\approx 12\;\rm{KPa}, \tag{4}\]  
Choose the delamination total method for the settlement test. For the same soil layer each layer thickness \(h_{i}\) according to 0.4 base width, while taking into account the depth of groundwater. For this section \(h_{i}=6\;\rm{m}\), the water table elevation is \(-2.1\;\rm{m}\), so the stratification above the water table is \(2\;\rm{m}\), the following stratification needs to be no greater  
Figure 4: Calculation model of foam lightweight concrete replacement. than 6 m. Take the bottom of FLC as the depth Z = 0 m at which the soil self-weight stress \(\sigma_{cz}\) is calculated, as shown in Table 4.  
After dividing the foundation base into four equal parts a = 4.625 and b = 3.75, the corner-points method is applied to calculate the additional stress \(\sigma_{Z}\) in the foundation under the centre point of the foundation, as shown in Formula (5). \[\sigma_{Z}=4K_{C}P_{0}, \tag{5}\]  
where, \(K_{C}\) is the vertical additional stress coefficient at the corner point of the uniform rectangular load; \(P_{0}\) = 12 Kpa. Considering the soft soil roadbed with the depth of the foundation increases additional stress gradually decreases, often according to \(\sigma_{z}\leq 0.1\sigma_{cz}\) to determine the calculation depth [26], different depth additional stress calculation value as shown in Table 5, settlement calculation formula as shown in Formula (6). \[S_{C}=(\sigma_{Z}/E)\times h, \tag{6}\]  
where \(E\) is the compression modulus of the soil layer for settlement calculations and is taken as 4.74 MPa; \(h\) is the thickness of the soil layer. The settlement amount can be calculated to be 0.009 m, which meets the settlement specification requirements according to the \"Highway Roadbed Design Specification (JTG D30-2015)\" regarding the allowable post-work settlement specified value (\(\leq\)0.1 m) for roadbeds in soft soil areas [27]. However, in the process of soft foundation replacement, it is relatively difficult to protect existing gas pipelines, and the risk of pipeline deformation exceeds the standard is high [28]. There is no national standard for settlement control of buried pipelines in China, so this paper calculates the limit value for settlement control of pipelines through the allowable bending radius, and then judges the safety of buried pipelines under soft foundations [29]. \"Design Code for Gas Transmission Design Pipeline Engineering(GB 50251-2015)\" stipulates that the calculation of radius of curvature of the vertical surface elastic laying pipeline is shown in Formula (7) [30]. The geometric relationship of pipeline settlement curve is shown in Figure 5, which is deformed from ABCDE to arc AFGHE, and the radius of curvature of arcs AF, FG, GH and HE are all \(R\). Considering the pipe turning angle \(\alpha\leq 3^{\circ}\), the Formula (8) can be listed. \[R=\max\left\{3600\sqrt[3]{\frac{1-\cos\frac{\alpha}{2}}{\alpha^{4}}D^{2}},100 0D\right\}, \tag{7}\]  
\[\frac{L}{8}=R\times\tan\frac{\alpha}{2}, \tag{8}\] \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Z/m** & **0** & **2** & **4** \\ \hline \(\sigma_{cz}\)/(KN \(\cdot\) m\({}^{-2}\)) & 54 & 90 & 116 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Soil self-weight stress.