Abstract: The proper design of underground powerhouse support is the key engineering technique to guarantee the safe construction and operation of underground works. By regression analysis of surrounding rock bolting parameters of 20 underground powerhouses with a span of 19.2 m to 32.5 m, this paper puts forward the empirical formula of the relationship between the supporting intensity of the anchor bar, strength-stress ratio and workshop span, and the relationship between the supporting intensity of the prestressed anchor, strength-stress ratio and workshop span. The research results show that, when the strength-stress ratio is less than 3.0, the supporting intensity of the anchor bar of the surrounding rock increases significantly; and the supporting intensity is the function of strength-stress ratio with -2 singularity; when the strength-stress ratio is greater than 3.0 and less than 6.0, the supporting intensity of surrounding rock is in the nonlinear transition zone; when the strength-stress ratio is more than 6.0, the supporting intensity of surrounding rock tends to be constant.There exists a negative correlation (power exponent function) between the supporting intensity of prestressed anchor and the strength-stress ratio, and when the strength-stress ratio is less than 4.0, the supporting intensity of prestressed anchor increases significantly. The paper also puts forward the concept of the bolting index and the corresponding calculation formula, and finds that the bolting index of the anchor bar and the prestressed anchor of these 20 powerhouses are all distributed around 1.0. Therefore, the bolting index of practical engineering support design should reach 1.0 to ensure a reasonable support intensity.

Key words: underground powerhouse, support measures, empirical formula, support strength criteria, strength-stress ratio