Raising of problem. At the problem solving about determination of deflected mode (DM) of build constructions by the finite element method (FEM) on accuracy of solving substantial influence is rendered by the sizes of effective area of foundation. It is suggested to develop the criteria of determining the size of effective area. Presently at the calculation of vertical fallouts of earth dams with the trapeziform section (fig. 1), is assumed that the epure of contact pressures has a rectangular form [2, 6]. Thus actual epure of contact pressures on the sole of dam has form of trapezoid (fig. 1). Thus, there is a disparity between actual and accepted in the normative documents in the contact pressures on the sole of earth dams. Purpose. At writing of this article we were pursue a purpose to calculate the value of damping coefficient of vertical dead stress on the depth of foundation, trapeziform loading determined and to foundation attached. About it has been already written not a bit in scientific literature [2; 5; 6; 7; 13]. In our view, for determination of vertical fallouts of foundation of earth dams it is necessary to use the formula of D-1 DBN [7], corrected in it the damping coefficient of vertical stress on a depth, conditioned of dam weight, that is to calculate a trapezoidal form of environmental stress (fig. 1). Conclusion. The damping coefficients of vertical stress calculated by us on a depth (tablas. 1) allow more exactly to determine their values, than coefficients, presented in normative documents [7]. This is caused by more complete, than it takes a place in normative documents, in the light of configuration of the environmental stress.