Previous experiences in existing engineering are often referenced in designing a new engineering. Restricted by the limited data of free fields of rocks under chemical explosions

a unified equation has not been suggested in designing codes for protective structures. A weighted least squares fitting method is suggested as an effective method to predict the free field in rocks. The weighted factors were defined according to the similarities of the engineering

including mass of explosive

rock quality and wave impedance. According to several empirical equations and the suggested method

a modified equation for free fields in rocks is built and discussed.