Line 2 of the Guanjingkou Pipe Jacking Project in Chongqing encountered a pipe sticking problem, whose occurrence was\ninevitably attributed to the higher total frictional resistance of pipe strings rather than the maximum jacking force. Line 1, which is\nabout to start construction, has basically the same construction environment as Line 2 using the same microshield and pipe string\nsizes. To avoid repeating the pipe sticking problem of Line 2, the mutual friction characteristics between the surrounding rocks\nand jacked pipe strings are studied for Line 1 by adopting the same test method under seven complex contact conditions (the\npresence or various combinations of three substances, i.e., extrapipe string field debris, bentonite slurry, and sand-laden waste\nslurry, on the jacked pipe string-surrounding rock contact surface are mainly considered). The results show that sufficient\nbentonite slurry can effectively reduce the frictional resistance, and when the amount of bentonite is insufficient, the average\nfriction coefficient (AFC) of the later contact surface increases by 50%mm70%. The comparison of the monitored versus predicted\njacking forces indicates that the value predicted by the test is slightly higher than the monitored force and the variation trends of\nthe two match well, thus proving the correctness of the test results. It is possible to continue predicting the variation trends of the\njacking force and frictional resistance based on the contact situation outside the pipe string wall, which greatly lowers the\nprobability of re-encountering pipe sticking.The test results not only explain the important role of bentonite slurry in reducing the\npipe string wall frictional resistance but also suggest that an increase in the pipe string wall frictional resistance resulting from the\ncomplex contact inflow into the overexcavation gap is the root cause of pipe sticking; moreover, the number of jacked pipe strings\nmatching a single IJS is the second cause of pipe string sticking. The methodology of this study can provide a reference for other\nstudies concerning the jacking force of long-distance rock microshield tunnelling.
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