5.5　Breakthrough survey

5.5.1　After caverns excavated from two opposite directions are broken through，breakthrough survey shall be carried out to determine the longitudinal，lateral and elevation breakthrough errors.

5.5.2　In the case of breakthrough survey，breakthrough points shall be set up in the breakthrough interface.

5.5.3　The longitudinal and lateral breakthrough errors of the caverns shall be projected onto the directtions of the tangent lines and the normal lines.

5.5.4　The allowable cavern breakthrough errors shall comply with the requirements in Table 5.5.4.