Active Control Key Technology of NPR Cable in Soft Rock Tunnel

To solve the problem of large deformation of soft rock tunnel in Hengduan Mountains, Changning Tunnel of Changning-Baoshan Expressway in Yunnan Province is taken as the engineering background, and adopts the methods of theoretical analysis and field test to carry out the research on active control key technology of NPR cable in soft rock tunnel from the aspects of action mechanism, support scheme and construction technology. The results show that under passive support, the main causes of deformation and failure of surrounding rock in Changning tunnel are the development of joint fissures, high clay mineral content, low strength of surrounding rock, small dissolution pores in microstructure and loose structure. The active support technology of “NPR anchor cable + PR anchor cable + “ W” steel belt + high strength flexible net” is put forward, and the support parameters of NPR cable and PR cable are determined by field loose circle test. Aiming at the special environment of extremely broken surrounding rock and water-bearing soft rock, the optimization technology of NPR cable active support of “spray before support + re-spray” and “lower edge sealing of anchorage end + wire adding of steel strand end” is put forward. The field test results show that the NPR cable active control technology can effectively control the large deformation of soft rock tunnel, and the maximum deformation is controlled from 2125 mm to 300 mm, which ensures the safety of tunnel support structure.