Abstract：The growing modes of surface structural planes under compressive load and the function of the bolts are studied by experimental methods. In the experiment，the model material is gypsum. Structural planes are modeled by cracks，which are prefabricated by iron plates. Brass wires，aluminous wires and fuses are used to model the bolts with different stiffness. Strain gauges are applied to measure the stress intensity around the crack tips. Strain and pressure are measured by electrical resistance strain gauges. The experiment is accomplished by pressure machine. The results demonstrate that，with the increase of compressive load，there are two propagation modes at the tips of the surface structural planes：wing cracks and secondary cracks. With the increase of the inclination angle between the structural plane and the surface，the propagation of wing crack becomes more easily. The secondary cracks may be more popular for the case of the small inclination angles. The role of bolts is to improve the performance of the structural mass，weaken the stress intensity at the crack tips and increase the crack propagating loads. The bolts do not change the crack initial angle，but they can affect the internal extending path of the wing crack. This will increase the thickness of separated bodies and reduce its spreading speed. The experiment discovers the mechanism that the bolts reinforce the surface structural planes to some extent.
