Rock Stress Measurement by Hydraulic Fracturing with Hemispherical-ended Borehole

关于大型岩石洞室结构稳定性预测的最新发展表明，需要在现场进行系统的应力测量，例如，系统测量洞室周围的应力分布。在确定岩体内某一点处的状态应力的水力压裂中，需要通过向钻孔中施加液压来测量钻孔表面的破裂现象，该破裂现象是场应力的函数。具有半球形端部钻孔的水力压裂与常规水力压裂非常相似，除了施加液压的位置，即在半球形形成的钻孔底面处的压力。该方法可以在单孔内测量岩体的三维应力状态。首先，提出了无限大岩体中球形孔洞的水力劈裂法。然后用理论确定的腔体表面的应力系数对断裂现象进行了分析。阐明了该方法可确定岩石应力的两个大小和三个方向的主应力。其次，提出了半球形端部钻孔水力压裂，并利用有限元法确定了应力系数，用于分析孔底表面的压裂现象。最后，分析了在一般三维岩石应力作用下，半球形钻孔表面的破裂现象，指出其破裂受差应力、主应力方向、岩体抗拉强度等因素的控制，从而得出半球形钻孔水力劈裂可以确定岩石应力的结论。

Recent development in relation to prediction of structural stability of large rock cavern has indicated the need for systematic stress measurements in the field, e.g. systematic measurement of the stress distributions around caverns. In the hydraulic fracturing to determine the state stress at a certain point within a rock mass, the fracture phenomena of a borehole surface, which is a function of the field stress, needs to be measured by applying hydraulic pressure into the borehole. Hydraulic fracturing with hemispherical-ended borehole is very similar to conventional hydraulic fracturing except for the location of applying hydraulic pressure, namely the pressure at the borehole bottom surface which is formed hemispherically. This method has a possibility to measure the three dimensional stress state in rock mass in a single borehole.Firstly, hydraulic fracturing of spherical cavity in infinite rock mass is suggested. Then the fracture phenomena are analyzed by the stress coefficients determined theoretically on the surface of the cavity. It is clarified that two magnitudes and three directions of the principal stresses of rock stress are determined by this method. Secondly hydraulic fracturing with hemispherical-ended borehole is suggested and the stress coefficients are determined by FEM for analyzing fracture phenomena at borehole bottom surface. Finally, analyzing the fracture phenomena on the hemispherical-ended borehole surface under general three dimensional rock stresses, it is made clear that those are dominated by the deferential stress, the directions of principal stresses, the tensile strength of rock mass and so on. It is concluded that the rock stress can be determined by the hydraulic fracturing with hemispherical-ended borehole.