In mining area, the ground surface level is lowered and curved after the claps of cover-layers of coal-seams evacuated regions. The curvature variation of ground surface causes the variation of contact-surface relation between the structure base and the ground foundation. For this research, the position of maximum curvature variation point is predicted. At this position, for the potentially worst case, the contact-length between structure base and ground foundation is calculated. Then, the internal bending moment within the structure base caused by the ground foundation curvature variation is formulated by taking the contact-length as variable. The results show that the base bending moment is a function of fourth power of contact-length. Based on this result, the critical value of contact-length in mining area is established. Hence, the critical contact-length concept can be taken as a measure of curvature variation in mining area.