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