## 5 Conclusions  
The support pressure at the tunnel face during mud-water-balanced pipe jacking is an important construction control parameter. If the support pressure is not set properly, it can cause stratum displacement or tunnel-face collapse, which causes high safety hazards in the pipe-jacking process. In this study, the support pressure on the tunnel face during pipe jacking under the conditions of a thin overburden, mud-water balance, and construction crossing a river channel was examined in depth, and the performed analyses allowed the formulation of general conclusions:  
Figure 12: Simple force analysis for jacking. 1. The damage mechanism of the tunnel face during pipe jacking under conditions of a thin overburden, mud-water equilibrium, and construction crossing a river was examined. Mathematical expressions for the lower and upper limit values of the tunnel-face support pressure, P\({}_{\rm min}\) and P\({}_{\rm max}\), were derived from analyses of the passive and active damage occurring at the tunnel face of the pipe tunnel, in conjunction with the limit equilibrium method. The jacking area was divided into a non-river impact area and a river impact area. Mathematical expressions for the optimal support pressure values, P\({}_{\rm o1}\) and P\({}_{\rm o2}\), for the two areas were also given separately. 2. By studying the Y25-Y26 pipe-jacking project crossing the Chu River in Hefei North City, and through numerical simulation, the displacement and ultimate support pressure of the tunnel face under different river overburdens and different support ratios were discussed. As the thickness of the overburden decreased, the soil monitoring point displacement caused by the active support pressure continued to increase, while the displacement caused by the passive support pressure continued to decrease. When large-diameter pipe jacking crosses a river channel, if the overburden thickness is small, the setting of the support pressure requires a reduced support ratio, while the support ratio needs to be increased if the overburden thickness is large. The optimal support force is in the interval of \(\lambda\) = 1.0-1.5 when H\({}_{\rm s}\) = 3 m, in the interval of \(\lambda\) = 0.9-1.0 when H\({}_{\rm s}\) = 2 m, and in the interval of \(\lambda\) = 0.8-0.9 when H\({}_{\rm s}\) = 1 m.
3. For pipe jacking crossing a thin-cover river channel, the theoretically calculated upper limit value P\({}_{\rm max}\), lower limit value P\({}_{\rm min}\), and optimal value P\({}_{\rm o}\) and the numerically simulated limit value of the support pressure were 2669.977 kN, 309.910 kN, 1044.870 kN, and 881.786 kN, respectively. The values measured during pipe jacking fluctuated between the upper limit value P\({}_{\rm max}\) and the lower limit value P\({}_{\rm min}\). They were more similar to the optimal value P\({}_{\rm o}\) in the non-river impact area and more similar to the upper limit value P\({}_{\rm max}\) in the river impact area. To overcome the complexity of geological conditions, the limitations of mechanical equipment, and other factors to ensure the stability of the tunnel face without damage, it is necessary to set the tunnel-face support pressure within a reasonable range between the upper limit value P\({}_{\rm max}\) and the lower limit value P\({}_{\rm min}\); we suggest adopting the optimal value P\({}_{\rm o}\). The analysis will make it possible in the future to develop longitudinally in pipe-jacking construction. The evolution characteristics of support pressure and damage alarm of a tunnel face under complex conditions should be more greatly explored and tested in practice in combination with artificial intelligence and algorithms. Conceptualization, Z.Z. and W.L. ; data curation, W.L., J.S. and M.Z. ; methodology, Z.Z. and W.L. ; formal analysis, Z.Z., W.L., J.S. and M.Z. ; investigation. W.L., J.S. and M.Z. ; writing--original draft, Z.Z. and W.L. ; writing--review and editing, Z.Z. and W.L. ; supervision, Z.Z. and B.L. ; project administration, Z.Z. ; resources, Z.Z. ; software, W.L., J.S. and M.Z.