## 5 Conclusions  
Large deformation of soft rock is a prominent problem during the construction of underground engineering. This paper analyzed the specific cause and deformation law, discussed in detail the prediction and treatment of large deformation by physical and mechanical tests and a field-monitoring test. The following conclusions are drawn:  
(1) There are many flaky minerals in phyllite slate, and the size of flaky minerals is 2-5 microns, which has directionality and can cause anisotropic characteristics of the rock. When pyrite-bearing crystals are found during the sample grinding, attention should be paid to the corrosiveness of pyrite to reinforced concrete after weathering. According to the laboratory test, the free expansion rate of the rock sample is 20.0%, the average expansion stress of the rock sample is 11.0 kpa, and the expansibility of the rock is low. Unlike the common expansive large deformation, the large deformation in this paper mainly comes from the dilatancy effect with high geostress and relaxation deformation with weak support and is closely related to fragmentation and strength. (2) In order to avoid the low strength of surrounding rock, measures should be taken in time to deal with the three main factors (weathering, water and confining pressure) that affect the strength of surrounding rocks. For example, after excavation, shotcrete should be sprayed to expose surrounding rock to avoid continuous contact between surrounding rock and air; Early strength support and a second lining should be applied to provide confining pressure for the surrounding rock, to control the loss of confining pressure or to restore a certain confining pressure; and to try to control the water entering the surrounding rock. When there is water in the tunnel, the water should be drained out of the tunnel as soon as possible to avoid water immersion in surrounding rocks, etc. (3) Generally, the bolt must be penetrated into the elastic zone at least 1/3 of the length of the bolt to achieve better results. However, due to the small section of the tunnel, bolts need to be lengthened several times to form long bolts, and long boreholes are easy to collapse, which is very disadvantageous to the construction period. Short bolts are used to fix the steel arch on the surrounding rock horizontally and increase the number of bolts to effectively control the occurrence of large deformation in the soft rock mass. (4) The monitoring data show that the results of other monitoring items are in good agreement with the field observation, and can reflect the deformation of surrounding rock and the stress state of supporting structure more accurately, except for the unsatisfactory monitoring results of bolt stress. That is because of the poor grouting quality of constructed bolts. The monitoring of bolt stress is greatly affected by the construction quality. The quality of bolt construction is very important to the effect of the bolt, which should be guaranteed. Compared with other monitoring projects, the monitoring accuracy of bolt stress is susceptible to interference and is difficult to guarantee, so other monitoringprojects should be adopted as far as possible. In particular, clearance convergence has the advantages of intuitive results, easy quality assurance of instrument installation and high accuracy. It is suggested that clearance convergence should be the main monitoring work in construction. (5) For the support of soft rock, the pressure cannot be relieved excessively. Soft support first and hard support afterward are more reasonable. A second lining should be used to control the deformation in time after the initial support is stable. Otherwise, the rheological deformation of soft rock in the later stage will increase the stress of the initial support and cause instability and failure.