Acoustic Method Based on Integrity Coefficient for Testing the Loose Circle of Surrounding Rock

The thickness of the loose circle of surrounding rock is an important index in tunnel support parameter design and rock mass stability evaluation. Accurate and rapid determination of thickness is critical to ensure the safety of underground chamber construction. Field test is the most intuitive and accurate method for thickness determination. Acoustic method is also widely used because of its high precision, low cost, and simple operation. However, existing discriminative standards for analysis of test results are ambiguous and subjective; as such, a uniform standard that defines a loose circle has not been established yet. In this study, the complete degree of rock and loose rock mass is quantitatively analyzed by introducing the concept of and the calculation formula for the integrity coefficient of broken rock (R_v) and the integrity coefficient of loose circle (L_v). According to the formation principle of loose circle, the criteria and test method for surrounding rock loose circle are proposed on the basis of the integrity coefficient. The integrity coefficient of the loose circle (R_v) is assumed to be approximately equal to that of the broken rock (L_v) in chamber excavation. In the test of the surrounding rock loose circle in the Shimen tunnel at Baohan highway, the integrity coefficient (R_v) of the rock of the grade Ⅲ gneiss was 0.53 according to the combined indoor and field tests. The thickness of the type Ⅲ surrounding rock loose circle was 0.55-1.35 m, and the distribution along the circumference was not uniform. The arch shoulder was thick, and the wall was thin, consistent with the initial stress state of the surrounding rock. Comparison with the results of the original acoustic wave loosening test showed that the new discriminant method based on the integrity coefficient is more accurate and reliable. The proposed method could be used for the design and construction of tunnels.