College of Mining, NUST MISIS, Moscow, Russia:

V. V. Nabatov, Associate Professor, Candidate of Engineering Sciences, nv4@mail.ru

The article puts forward a new method to assess contact conditions behind lining. A bad contact is when a void is present behind lining, which can provoke accidents. Detection of such voids by nondestructive testing allows further grouting in the proper sections of tunnels and, thus, precluding disfunctions. The method is based on the impact excitation of lining with further recording and processing of seismic response. The processing means tracing of special signs (attributes) of a bad contact. The physical framework of the method is alteration in the intensity of resonance in the lining–soil system in the presence of a void. In this case, the lining acts as a membrane unforced by soil. The direct assessment of lining behavior by the acoustic Q-factor is often noisy. For this reason, aimed to improve quality of decision-making on the presence of a void, it is required to modify methods of seismic response processing. The newly proposed approach is based on using spectral attributes of Q-factor such as peak frequency and its inverse width. In this case, in the space of two-dimensional laws of distribution of these attributes, families of bad contacts (void) and good contacts (no void) appear linearly separable. Thereupon, the lining–soil contacts have been classified. In the tests in the Moscow metro tunnels in operation and under construction, the method proved its efficiency.
The study was supported by the Russian Foundation for Basic Research, Grant No. 17-05-00570 а.

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