The extension criterion of fracture mechanics is introduced to analytical solutions of hydraulic tunnel

and the macroscopic load-bearing analysis of hydraulic tunnel and mesoscopic crack propagation research are combined. After classifying cracks in surrounding rock nearby tunnel into shear-tensile and shear-compressive types

analytical solutions of critical inner water pressure of hydraulic tunnel are derived when hydraulic fracture happens in the surrounding rock

according to extension mechanism of different variety of cracks. Based on the proposed analytical solutions

the influence on critical inner water pressure of trend

length

depth of cracks is researched. The results show that both shear-tensile type and shear-compressive type cracks may occur because of high inner water pressure or release of geostress; but the latter one has more probability. Under high inner water pressure

the crack will extend much more difficult than other conditions when the trend of crack is perpendicular to the normal of tunnel. The longer of crack length

the smaller of crack buried depth

the bigger of contact area between pore water and rock masses

the more easily cracks will extend.