Identification method for soft-hard interfaces at shield tunnel face in upper-soft and lower-hard composite strata

Shield tunneling technology has been widely applied in urban underground engineering. Nevertheless, when shield machines pass through upper-soft and lower-hard composite strata, insufficient geological information regarding the tunnel face frequently results in unreasonable tunneling parameter settings, severe cutter wear, and poor attitude control stability. To solve the above engineering problems, this paper systematically explores the vibration response characteristics of shield machines in such composite strata, and further proposes an innovative identification method for soft-hard strata interfaces. Impact load laboratory tests on disc cutters were carried out under static and rotating cutterhead working conditions to clarify the inherent vibration response laws of cutters. On this basis, field monitoring was implemented based on the Jinan Metro engineering project. The time-domain and time–frequency domain evolution laws of shield vibration signals were comprehensively analyzed under different hard-rock fractions of the tunnel face. The research results indicate that with the increase of hard-rock fraction on the tunnel face, vibration signals collected from multiple disc cutters produce superposition effects and form obvious characteristic signal clusters. Accordingly, this study deduces the time difference calculation formula for adjacent disc cutters penetrating hard-rock strata, and establishes the quantitative correlation between such time difference and hard-rock fraction. Meanwhile, a soft-hard strata interface identification method is proposed based on the peak time difference of vibration signals. Practical engineering applications verify that the proposed method can realize real-time accurate characterization of tunnel face strata distribution. This research can provide important technical references for the optimization of shield tunneling parameters, as well as improve the construction safety and efficiency of shield tunneling in complex composite strata.