Spiral bolt의 지보효과에 관한 연구

Abstract

This study aims to evaluate the supporting effect of a spiral bolt that is superior to a rock bolt in terms of constructability, stability, environmental and economic aspects as a support system. This study thus analyzed the mechanical properties of a rock bolt which is widely used as a support and a spiral bolt. In addition, laboratory pull-out tests were conducted for the evaluation of properties of the supports such as displacement, pull-out load, confining pressure etc. Moreover, the differences between a rock bolt and a spiral bolt were drawn by comparing the two results of laboratory pull-out tests and in-situ pull-out tests. Then, the differences of the supporting effect of the two supports were analysed by comparing the results of the two pull-out tests with a numerical analysis using FLAC3D. The results of this study are summarized as follows; 1) In the mechanical properties of a rock bolt and a spiral bolt, the size of the cross-sectional area of a rock bolt was about 1.3 times larger than that of a spiral bolt. The rate of extension of a spiral bolt was about 1.2 times higher than that of a rock bolt, whereas the yield load and the pull-out load of a rock bolt were about similar and 1.2 times higher than those of a spiral bolt. 2) In the laboratory pull-out test of a rock bolt and a spiral bolt on a curing period, displacements of two supports were almost same or reduced at 7th and 28th day. It is considered that the force of adhesion between a support and a grout increases due to the fact that compressive strength increases according to a curing period. In the same pull-out load, the confining pressure of a spiral bolt was larger than a rock bolt. It implies that a spiral bolt is more stable than a rock bolt for securing the stability of a ground or rock mass when a support is installed on a ground or rock mass under the same condition. The maximum internal pressure and the maximum shear stress were higher at 7th day than 28th day and those of a spiral bolt were higher than a rock bolt. Therefore, it is suggested that the supporting effect of a support depends on a curing period and a form of the support. Furthermore, under the same condition of a ground or rock mass, a spiral bolt makes higher pull-out load and confining pressure than a rock bolt. In addition, considering the constructability and economic aspects as well as stability of a ground or rock mass before and after installing a support, it seems that a spiral bolt is more effective than a rock bolt. 3) In a in-situ pull-out test with a cement-mortar to a rock bolt and a spiral bolt, the pull-out load of a rock bolt was suddenly decreased and the displacement of a rock bolt was increased dramatically at the same time above the yield point. It is regarded that this is because the collapse would be occurred in an instant at the boundary between a rock bolt and a cement-mortar. On the other hand, the displacement of a spiral bolt tended to be increased above the yield point with the pull-out load. It is considered that this is because the collapse would be gradually occurred at the boundary between a spiral bolt and a cement-mortar. In the results of the pull-out load on curing period of resin, as the curing period of resin was longer, the pull-out loads of both supports were increased. The maximum pull-out loads of two supports were found in four hours, whereas the differences of others [in 8, 12, 24 hours] were almost same. In the three times of the re-pull-out test for checking variations of the maximum pull-out loads of two supports at each step, it was found that as the test repeated, the pull-out load of a rock bolt was decreased. It is thought that this is because the collapse would be occurred partly when the support reached the maximum pull-out load. However, the value of the pull-out load was not changed regardless of the number of tests in a spiral bolt. It is considered that this is because the collapse between the support and the filler would be rarely occurred although the support reached the maximum pull-out load. 4) The results of the numerical analysis of the relationship between the load and the displacement of a rock bolt and a spiral bolt according to a confining pressure in a tunnel are as below. There were positive relationship between the confining pressure and the shear yield strength. In both supports, as the confining pressure was increased, the shear yield strength was increased. Particularly, when the confining pressure was increased, the shear yield strength of a spiral bolt was higher than that of a rock bolt in a low displacement. Furthermore, the shear yield strength of a spiral bolt was about 4 to 25 percent higher than that of a rock bolt depending on a condition of confining pressure. In the low confining pressure, there was a big difference of the shear yield strength between a rock bolt and a spiral bolt, whereas there was a little difference of the shear yield strength between both supports in the high confining pressure. It implies that as a confining pressure is lower, the effectiveness of a spiral bolt is higher than a rock bolt. As a confining pressure was lower, the axial shear behavior of a spiral bolt was over maximum 25 percent higher than that of a rock bolt in its efficiency. It is considered that a condition of confining pressure can be a criterion of an application of a support. In conclusion, considering the fact that a fully-grouted form of a sub-support in a tunnel is finally collapsed by reducing the shearing strength at the boundary between a support and a grout according as the confining pressure is decreased by relaxation of rocks that is due to excavating a tunnel, it is reasonable to assume that the supporting effect of a spiral bolt is superior to that of a rock bolt.