## 6 Conclusions  
In order to improve the prediction accuracy, a new segmented friction calculation model was proposed that takes into account the effect of lubricants on the contact state, earth pressure, friction coefficient, and other factors during construction. Combined with the field-measured values and numerical simulation results, the effectiveness of the new method was verified. The conclusions of this study are as follows:  
(1) The measured jacking force data of two box jacking cases showed that there are differences in lubrication conditions during long-distance jacking, and the variation law of the friction force in each zone is obviously different. In the condition without lubrication, the full box-soil contact state occurs, the friction coefficient is large, and the friction force increases linearly with the jacking length. In the condition of gradual lubrication, the full box-soil contact gradually changes to three or two side contacts, the friction coefficient decreases gradually, and the friction force markedly decreases. In the condition of full lubrication, the friction coefficient decreases to the lowest values, and the friction increases linearly with the jacking length, but the increasing slope is obviously smaller than that without lubrication. (2) Compared with the existing models, the segmented friction calculation model proposed in this study has a better prediction accuracy for the changes in the lubricant conditions during the whole construction length and provides technical support for long-distance box jacking. Meanwhile, the existing models cannot reflect the changes in lubrication conditions, which generally misjudge the actual friction force. Especially for TSP, the friction coefficient was the same as the proposed model, but the calculated value was still far greater than the measured value. Meanwhile, the empirical value is used for TCB and TSR without considering the buried depth of the box, the earth pressure, or other factors, which limits its engineering applicability. (3) The numerical model could not simulate the actual injection and filtration of the lubricant, as there were some differences between the simulated results and the theoretical results for the earth pressure. However, by setting the interaction mode between the outer surface of the box and the soil, the numerical model can reflect the difference in lubrication conditions, and the simulation results prove the rationality of the theoretical hypothesis that this study considers the box-soil contact state, friction coefficient, and friction type. (4) From the perspective of the influence mechanism, it is suggested to select a calculation model to estimate the actual friction force combined with the real-time injection of the lubricant. The lubricant technology widely used at present can basically meet the construction requirements in terms of reducing the friction coefficient, maintaining soil stability, and preventing lubricant leakage. On this basis, adding injection holes at the top of the box and injecting a proper amount of high-viscosity bentonite slurry can further reduce the jacking force. It should be noted that due to the invisibility of underground engineering and the complexity of geological conditions, the transition points in each zone are still uncertain under actual conditions, and the changes in lubrication conditions need to be further studied via controllable and visual test work or more cases. Conceptualization, Y.Z. and P.Z. ; Data curation, Y.Z. and Y.X. ; Formal analysis, Y.Z. and P.Z. ; Funding acquisition, P.Z. ; Investigation, Y.Z. ; Methodology, Y.Z. and P.Z. ; Validation, TX. and K.L. ; Visualization, P.Z. and J.M. ; Writing--original draft, Y.Z., Y.X. and P.Z. ; Writing--review and editing, Y.Z. and K.L. All authors have read and agreed to the published version of the manuscript.