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The vibration of slabs is determined by the dynamic load and reaction of the slab-soil system. In the case of random effects of a turbulent flow, for example, on the slab of a spillway apron, the fluctuations of the load should be given as a spectral density function and the reaction of the system is given as a transfer function [1]. The spectral density function of the load can be obtained by calculation or from the data of hydraulic investigations [2]. Calculation of the transfer function is hampered primarily by the indeterminacy of selecting the model of the foundation. The available experimental data and calculation methods pertain mainly to relatively small machine foundations [11]. The available data of experiments with large masses [3] are insufficient to accept confidently the known recommendations for calculating the slabs of a spillway apron or hydroelectric station building,which generally have an area of contact with the soil measuring hundreds of square meters.In this article we will present data of full-scale experiments to determine the parameters of free vertical vibrations of rigid square slabs of varying size, including apron blocks. An analysis of the occurrence of vibrations shows that the vertical vibrations of rigid slabs on saturated sand can be regarded approximately as vibrations of a system with one degree of freedom. In this case the deformability of the foundation can be assigned by the coefficient of uniform elastic compression obtained from experiments with static loads. The mass of the system is determined by the sum of the mass of the slab and the "apparent mass" of the saturated soil, equal to the mass of the soil and water in a layer of thickness 0.65L beneath the base of the square slab with edge dimension L. The relative damping factor of the system increases slightly with slab size (from 0.34 to 0.44). The data obtained am compared with the available published observations [3].The experiments were carried out in the foundation pit of the Kanev hydroelectric station under construction. In the experiments we recorded the vibrations of the slabs under the effect of a fixed impulse caused by a falling weight and the displacements of individual slabs under the effect of brief (3-4 rain) static central loads. The static load was created by a set of reinforced-concrete weights. To measure the vibrations we used three VDTs2N-M seismic-type vibrographs and a measuring device with an OT-24 electromechanical oscillograph. The static displacements of the slab were measured by three dial gauges with a 1-~ scale division.The investigations were carried out on one isolated block of the apron which was concreted gradually up to the final size, which slightly exceeded the design height. Thus, we obtained four rigid square slabs within the block (Table 1). The slabs were tested after the modulus of elasticity of the concrete reached about 100,000 kg/cm z. Keyways, notches, and bypasses for reinforcement were made where the next slabs were to be concreted to provide continuity of the slab.The s...

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