Study on the field monitoring, assessment and influence factors of pipe friction resistance in rock

Accurate assessment and prediction of the pipe friction resistance are crucial for designing the jacking force, arranging intermediate jacking stations, and setting the sediment removal timing in long-distance rock pipe jacking (PJ) projects. However, the applicability of pipe friction resistance calculation models for PJ in stable and unstable rock strata was not verified in sufficient cases. Besides, research on the assessment of specific skin friction and the primary factors influencing pipe friction resistance in rock strata remains scarce. In this study, field monitoring was conducted to first assess specific skin friction in long-distance deep-buried rock PJ projects under different geological conditions. Then, the applicability of pipe friction resistance calculation models for PJ in stable and unstable rock strata was verified. Finally, the primary factors influencing the friction resistance of pipes in rock strata were analyzed, including formation stability, lubricating mud and sediment, groundwater, pipe misalignment, and tunnel burial depth. The main conclusions are as follows: (1) in unstable strata, specific skin friction (M) can be considered ’very poor’, with the jacking length owing to the primary jacking station (L₀) being just 0.21–0.48 times the value of M in stable strata. The value of M in stable strata can be considered at least ’very good’ in the initial jacking stage and ’good’ in the normal jacking stage. (2) In stable strata, the pipe-slurry contact model with a pipe-rock contact angle (2θ) of 0° reflects the measured value of M in the early jacking stage. The pipe friction resistance calculation model established by Deng et al. (2021) with 2θ = 75° can conservatively estimate the value of M during the normal jacking stage. In unstable strata, the pipe-rock full contact model (2θ = 360°) can predict the value of M with an error of just 8 %. (3) The dynamic change in the pipe-rock contact state is affected by the sediment at the pipe bottom and bentonite mud buoyancy. When the sediment causes 2θ to be smaller than 30°, the pipe floats; otherwise, the pipe sinks. (4) A highly confined groundwater inrush sharply increases the pipe friction resistance within a small jacking range and maintains constant friction resistance. The friction caused by pipe misalignment deviation increases instantaneously and quickly decreases to the normal level after rectification. (5) The influence of tunnel burial depth on the pipe friction resistance can be ignored for deeply buried rock PJ engineering in stable rock strata. The results of this study are expected to be useful for future rock PJ projects because such results are rarely reported.