Rock is a complex structure which is naturally produced by one or more minerals under geological conditions. In the rock engineer, most rocks belong to pressure-shear condition. Therefore, it is necessary and urgent for the study of unstable rock in pressure-shear condition. A lot of scholars have devoted themselves to the study of unstable rock. For example, Nara Y. etc.  had found that fissure water would accelerate the speed of the subcritical crack growth of granite and affect the structural strength of the granite; With rock surrounding the Skolis Mountain and the Acrocorinthos area as the research object, Zygouri V. etc.  showed that shallow earthquakes could cause a wide range of rock collapse; Chen H. K. etc.  put forward the failure criterion of unstable rock under the excitation effect and established the evaluation method for its safe; Johari A. etc.  used the method of joint distribution of random variables to evaluate the stability of rock in the critical state; Li Y. etc.  simulated the crack development of rock mass under the action of water pressure by using FLAC3D software, and the studies showed that the strength and stability of the jointed rock mass was obviously decreased; Liang L. etc.  sampled the shale in the Long maxi area, and the research results showed that the aqueous liquid of had significant positive impact in Crack growth of shale. However, So far, many scholars have studied the rock crack by the method of experiment and numerical analysis, and the theoretical analysis of the rock crack is slightly deficient. This text used the method of fracture mechanics to deduce the unstable rock, and the results have certain theoretical guiding significance and economic value for prevention of disaster and engineering safety assessment.
2. Coordinate Transformation of Stress Components
Figure 1 is a rock model of establishment. Where is center of gravity of unstable rock; u is Fissure water pressure of control fissure; PL is horizontal seismic force per unit length; PV is vertical seismic force per unit length; W is Gravity of rock mass per unit length.
In the straight line of the unit which parallel to the axis, the included angle between the outer normal and the coordinate axes is and respectively. We may know, , and. The known parameters are brought into the boundary condition equation
Figure 1. The unstable rock model.
Figure 2. Crack stress at the tip of the element.
It will find and. In the straight line of the unit which parallel to the axis, it can know and in the same way. As we all know:
A new and old coordinate system is established in Figure 3. Where is Stress tensor of the old coordinate system; is Stress tensor of the new coordinate system. The projection of axis of the new coordinates system in the old coordinate system is and respectively; The projection of axis of the new coordinates system in the old coordinate system is and respectively.
It can know
Combining formula (2), formula (3), formula (4) and formula (5) can be obtained
By formula (6) can be known
3. Deduction of Type I and Type II Stress Intensity Factor
The article assumes and. Westergaard proposed the stress function 
Figure 3. The old and new coordinate system.
Because of and, it can be obtained
We can carry out partial differential to Westergaard stress function
From formula (13), formula (14) and formula (15), we can get
As shown in Figure 4, there is a center crack with a length of in the infinite space. And its infinite distance is affected by bidirectional uniform stress. Its boundary conditions are: when and, it is; When and, it is; When, it is.
The stress function is obtained
The original O point translates to the new coordinate O' point. We assume complex coordinates of any new coordinates of a bit is.
Combining formula (16) and formula (17)
Figure 4. Type I crack under bidirectional uniform stress.
When, we can get
Because of, formula (19) is a constant. So we can assume
Combining formula (18), formula (19) and formula (20), we can obtain
We can use the same way to get
4. Composite Stress Intensity Factor and Fracture Angle for Unstable Rock
At first, we suppose that composite stress intensity factor is
Because the horizontal seismic force (PL) and vertical seismic force (PV) can not be considered at the same time. So we can add a coefficient C1 and C2 in front of them respectively. It builds the following functions
where the coordinates of the center of gravity of the unstable rock is; The maximum value of pore water pressure of control fissure is P0; Sum of forces in the axial direction of is; Sum of forces in the axial direction of is. So its inference is
If D1 and D2 are constants, according to Westergaard’s stress function, we will get
Combining formula (7), formula (21), formula (22), formula (23), formula (27), we will calculate
According to the maximum circumferential stress theory , we will get fracture angle
We put trigonometric function into formula (29), and it will get
When it is negative (“−”), fracture angle is greater than 180˚. Obviously, it is not in conformity with the actual situation. So fracture angle is
Considering gravity, fracture water pressure and seismic force, this paper constructs unstable rock mechanics model, which is very common in the rock engineer. What’s more, we derive composite stress intensity factor of the type I - II by fracture mechanics. And according to the maximum circumferential stress, we calculate theory Fracture angle by trigonometric function. In short, the results have certain theoretical guiding significance and economic value for prevention of disaster and engineering safety assessment.
 Nara, Y., Oe, Y., Murata, S., et al. (2015) Estimation of Long-Term Strength of Rock Based on Subcritical Crack Growth. Engineering Ge-ology for Society and Territory, Volume 2. Springer International Publishing, 2157-2160.
 Zygouri, V. and Koukouvelas, I.K. (2015) Evolution of Rock Falls in the Northern Part of the Peloponnese, Greece. IOP Conference Series: Earth and Environmental Science, IOP Pub-lishing, 26, Article ID: 012043.
 Chen, H.K., Zhou, Y.T. and Wang, Z. (2014) Study on Damage Characteristics of Unstable Rocks under Excitation Effect. Applied Mechanics and Materials, 459, 575-581. http://dx.doi.org/10.4028/www.scientific.net/AMM.459.575
 Johari, A., Momeni, M. and Javadi, A.A. (2015) An Analytical Solution for Reliability Assessment of Pseudo-Static Stability of Rock Slopes Using Jointly Distributed Random Variables Method. Iranian Journal of Science and Technology Transactions of Civil Engineering, 39, 351-363.
 Li, Y., Zhou, H., Zhu, W., et al. (2015) Numerical Study on Crack Propagation in Brittle Jointed Rock Mass Influenced by Fracture Water Pressure. Materials, 8, 3364-3376. http://dx.doi.org/10.3390/ma8063364
 Liang, L., Xiong, J. and Liu, X. (2015) Experimental Study on Crack Propagation in Shale For-mations Considering Hydration and Wettability. Journal of Natural Gas Science and Engineering, 23, 492-499. http://dx.doi.org/10.1016/j.jngse.2015.02.032
 Erdogan, F. and Sih, G.C. (1963) On the Crack Extension in Plates under Plane Loading and Transverse Shear. Journal of Basic Engineering, 85, 519-525. http://dx.doi.org/10.1115/1.3656897