Tunnel and underground engineering composite lining contact surface relative displacement monitoring method

The invention belongs to the technical field of tunnel engineering detection and discloses a tunnel and underground engineering composite lining contact surface relative displacement monitoring method. The method includes the following steps that: one end of a displacement sensor is fixed to a lining which is constructed in advance, and the other end of the displacement sensor is fixed on a cast-in-place concrete lining reinforcing steel bar; the reading of the displacement sensor is recorded as L0 when concrete pouring is completed, and the reading of the displacement sensor at a certain subsequent time point is recoded as Lt, and the difference of the Lt and L0, which indicates by an equation that deltaL=Lt-L0, is a relative displacement amplitude; when deltaL is larger than 0, it is indicated that the displacement sensor is tensioned, and a lining ring contact surface where the displacement sensor is located is to be separated; and when deltaL is smaller than 0, it is indicated thatthe displacement sensor