Analysis and calculation of critical internal water pressure in fracture of surrounding rock of tunnel during hydraulic fracturing

Hydraulic fracturing is one of the important affecting factors in water inflow out of deep-buried tunnel construction. Its failure mechanism is a hot topic in geotechnical engineering fields. Based on stress states in rock fracture surface and fracture mechanics, fracture propagating models comprise tension-shear and compression-shear mixed fracturing. Using crack propagation criterion for mixed fracture model, calculation formulae of critical internal water pressure of two kinds of failure models are obtained respectively; and the affecting laws of fracture location and direction are studied. This study shows that fracture is most easily to propagate when it is parallel to the direction of maximal in-situ stress under tension-shear mixed model. For compression-shear mixed model, fracture is most easily to propagate when the angle between of fracture direction and maximal in-situ stress is within range of 30°and 75°.