Response of an Embedded Block Impacted by High-Velocity Jets

High-velocity plunging jets, issuing from ood release structures of dams, may result in scouring of the rocky riverbed and even endanger the foundation of the dams. Assessment of the scour extent is essential to ensure the safety of the dam and appurtenant structures as well as to guarantee the stability of its abutments. The existing near-prototype scaled experimental facility developed at the Laboratory of Hydraulic Constructions (LCH) of the Ecole Polytechnique Fédérale de Lausanne (EPFL) has been modified to study the complex interaction between pressures fluctuations acting inside a cylindrical plunge pool and inside a full interconnected 3-dimensional fissure. The present facility allows to simulate near-prototype jets in terms of velocity, turbulence and aeration. A movable highly instrumented block, simulating an "artificial rock block" founded in a fissured rock mass with one degree of freedom (along the vertical axis), has been inserted in the existing facility. The block and the measured box (new set-up), simulating the fissured rock mass, represent a sophisticated installation allowing to perform several measurements simultaneously. The block, having a cubic shape of 200 mm side and a density similar to in-situ rocks, is equipped with pressure, displacement and acceleration transducers. It is embedded in an artificially created surrounding rock mass equipped as well by pressure transducers. Between the block and the measurement box a 3-dimensional fissure of 1 mm thickness has also been created. The plunge pool and the new experimental set-up have been impacted by high-velocity jets to generate different loading conditions (core, transition or developed jets impacts). The purpose of the research project is to study the behavior of a single rock block separated from its surroundings by a 3-dimensional fissure and impacted by a high-velocity impinging water jet subjected to a natural aeration. Pressure fluctuations (pressure field) and block responses (displacements and accelerations) are recorded simultaneously for several jet impacts positions on the block upper face (at the plunge pool bottom level), for different water depths (Y/D ratio between 0 and 9.7) and near prototype jet velocities (2.5 - 27.0 m/s). The influence of the jet solicitations (symmetrical or asymmetrical jet impacts related to the block center) have been analyzed for several parameters: pressure field surrounding the block, dynamic block impulsion, natural and passive air entrainment, fissure geometries, block degree of freedom and block rotations in the fissured rock mass. The main conclusions coming from these analyses display interesting results. The pressure field acting on the block upper face follows the distribution found in literature (exponential distribution) whereas the pressures acting inside the 3-dimensional fissure are quite constant. The extreme pressures (positive and negative values) are attenuated inside the fissure. No transient phenomena have been observed inside the fissure. For the first time, the computation of the dynamic block impulsion has shown the relevance of the added mass and its different behavior whether the block is loaded by a symmetrical or an asymmetrical jet impact. When a body immerged in a fluid is subjected to accelerations, the surrounding fluid must accelerate as well. The inertia of the entrained fluid is the added mass. It influences the amplitudes of the vertical displacements. The added mass values obtained experimentally using the LCH facility are different from the literature values determined for different test conditions (a body moving in a quiet fluid and laterally confined). The highly instrumented block is strongly confined in the measurement box: it is surrounded on five of its six faces by the measurement box with a distance of 1 mm and it is directly loaded by high-velocity jets on its free surface. These conditions may explain this difference between observed and literature values. Theoretical block uplift shows good similitude with the measured uplift when the added mass is integrated in the computation of the dynamic block impulsion. Only the amplitude of the vertical displacements could not be always simulated exactly by the theoretical uplift, but the vertical fluctuations could be well reproduced. The maximum uplift was observed for a jet impacting on a corner of the block (∼160 mm for a block side of 200 mm). The air entrainment generated by a suction-based passive aeration system, together with the natural jet aeration, seems influence the extreme pressures (maximum and minimum) but not the block responses (displacements) related to a jet only naturally aerated. The influence of the air entrainment needed a more accurate investigation. To define the geometry of the fissure surrounding the block and to limit the block degrees of freedom to one, two type of lateral guides, fixed on the block lateral faces, have been tested. If the block is loaded symmetrically no differences have been observed, whereas for an asymmetrical jet impact for the same jet impact but on another block side some differences in the block responses (displacements) have been observed. The degree of freedom of the block influences strongly the pressure field generated inside the 3-dimensional fissure. When the block is fixed inside the measurement box, the pressures increase the hydrodynamic loading generating the propagation of the fissure networks in the rock mass. This pressure increase may reach some Bars of difference between the block free to move or fixed. The largest differences have been observed for a jet impacting on a corner of the block.