Research On The Failure Mechanism Of Apron Slab Of Arch Ring In Inverted Arch Plunge Pool Under Action Of High Water Head And Large Discharge

Plunge pool is one of the major measures for flood discharge and energy dissipation and preventing erosion to the body of high arch dam. The Inverted arch plunge pool is a new form of energy dissipation and erosion preventions for dam body. It is an optimized plunge pool and enjoys such advantages as using fewer anchorages, smaller cross sections, strong overloading and high stability. The stability of the apron slabs on the arch ring of the inverted arch plunge pool decides whether it can be safely applied under the action of high water head and large discharge. Consistent attention has been paid to the damage mechanism of the apron slabs on the arch ring by designers at home and abroad. On the basis of previous studies and in light of the inverted arch plunge pool under the action of surface hole nappe in Xiluodu arch dam, the hydraulic model and hydroelastic model in the partial arch ring were designed in consideration of gravity similitude law and elasticity similitude law. Under the condition of high water head and large discharge and that all the seal installations between the apron slabs and the anchoring on the slabs were all out of function, the damage mechanism of the apron slabs on the arch ring was researched on and theoretically analyzed in the whole flow field in the following three aspects:the law of changes in the dynamic water pressures on the surface and between the slot of apron slabs; the characteristics of vibrating displacement of apron slabs on the arch ring; the law of changes in the axial force on the apron slabs on the arch ring. The major research contents and findings are as follows:(1) RNG k-e turbulent flow model was adopted to make three dimensional numerical value simulation on the seven surface holes of the inverted arch pool and flat plunge pool. The comparison shows that there are lateral swirls as well as longitudinal swirls which are caused by incident water jet and the water jet forced moving to the left and right banks. Both swirls are forced to move to the upper reach and lower reach in a spiral way. The flow rolled upside at the end of the lateral swirl is changed into vertical swirl when it is backtracking. The major flow of the wall jet in the inverted arch pool mainly moves around the center of the overflow. Its velocity decreases and decreases slowly along its way when flowing laterally to the two banks while the velocity of the wall jet in the flat plunge pool is evenly distributed laterally and decreases fast along the way. In the flat plunge pool, the high speed jet current dashes to the side wall and the corner of the slabs, formed an obvious pressure peak value, and remains there even after the water level is lowered, therefore poses a threat to the slabs and the side wall. The flow pattern and the force on the slabs perform well in the water jet entrance area in the inverted plunge pool while the flow patter and force on the slabs perform well in the upper and lower reaches in the flat plunge pool.(2) A detailed research on the changing law of time and space in the dynamic water pressures of apron slabs on the arch ring was conducted in the whole flow field. The law of changes in the dynamic water pressures on the surface and between the slots of apron slabs on the arch ring along the flowing path and along the arch ring and the feature of time domain and frequency domain in the fluctuating pressure were found. The time average pressures on the surface and between the slots of apron slabs on the arch ring were analyzed. The forming mechanism of uplift force was dealt with and the reasons for the failure of stability in apron slabs were discussed. The possible ways for the movement of apron slabs in the arch ring were put forward. In consideration of the design of the soleplate in the inverted arch plunge pool, the respective average of the instant maximum pressure and time average pressure on the surface and between the slot of apron slabs on the arch ring along the flow path and along the arch ring was normalized. The ways of predicting the average value in the instant maximum pressure and time average pressure on the surface and between the slots of apron slabs on the arch ring were put forward. The time-space correlative properties of the dynamic water pressures of apron slabs on the arch ring shows that the fluctuating pressure remains almost unchanged and transmits vertically along the slot and it tends to transmit from the two ends to the arch crown along the arch ring.(3) The theoretical analysis was made on the vibrations of single apron slab on the arch ring and of multi apron slabs on the arch ring. The maximum displacement formula was put forward for the first time under different experimental conditions. From the formula it can be concluded that the radial displacement of apron slabs is controlled by the slot width of the apron slabs; if the arch abutment remains stable after the apron slabs are locked, the apron slabs will not be moved out of their seats; however, if any of the apron slab loses the geometric condition of forming arch, the arch ring is doomed to lose its stability. Hence, the control on the slot width, especially the slot width between the apron slabs of the arch ring, plays a crucial role in the local stability of the inverted arch plunge pool.(4) Experimental research was conducted on the vibrating displacement characteristics of single and group of apron slabs of arch ring in the inverted arch plunge pool. The characteristic area of the vibrating slabs was put forward and the respective area definition was given. The vibrating characteristics along the flow in each vibrating area were analyzed. The research finding shows that in the strong impact vibrating area, the slabs remain in a state of free violent vibrating when the number of slabs is low. With the increase of the number of slabs, the instant partial arch or long-term partial arch is formed; in the upper and lower reach, all the slabs form an long term complete arch, its head part forms an instant partial or long term partial arch. The most dangerous area is located at the boundary area between the nappe impacted area and the upper and lower stream wall jet area, namely, in the strong impact vibrating area and the head part of the vibrating area in the upper and lower reaches, that is, when-2.0=x/bm=1.5.(5) The transfer mechanism of the slabs'axial force is analyzed in theory. The mechanics equilibrium equation of apron slabs of the arch ring in the state of being locked is established. The relation between the slabs'axial force and upper lifting force and weight component together with the calculation formula is deducted. The changing law of the slab's axial force along the flow path and along the horizontal direction as well as the relationship between the slab's compressive strength and its displacement is obtained. The experiment results show that the slab's radial displacement increases with the increase of the number of vibrating slabs; if the contact area between the slabs decreases, the slabs are likely to be destroyed; hence, relative measures should be taken to avoid this in designing and constructing the inverted arch plunge pool.(6) The dynamic water pressures on the surface and between the slot of apron slabs on the arch ring and the frequency features of the slab's vibration and axial force is researched. The results show that the major frequency of slab's fluctuating pressure and the slab's vibration and the changes in slab's axial force ranges from OHz to 1Hz; the fluctuation energy mainly shows low frequency with low frequency narrow band distribution, close to large eddy dominant frequency in the jet entrainment and hydraulic jump in the inverted arch plunge pool. Although the eigenvalue of the slab's vibration velocity and acceleration is large, the destruction due to resonance is unlikely to occur. In the process of slab's moving up to being locked, the collision and running-in with neighboring slab will not lead to fatigue and damage of the concrete material at the end of the slab.(7)A comprehensive analysis of the above findings reveals the failure mechanism of apron slab of the arch ring in the inverted arch plunge pool. The research shows that the accumulative effect of the slot width between slabs results in slab's flying out of its seat which in turn brings about the instability and destruction of the arch ring; the mutual extrusion between slabs leads to the damage or partial fissure or partial breakage in the side of the slab due to the focus of stress; the increase of the axial force between the slabs, when transferred to the end of the arch, makes the arch seat fail to stand it and being damaged seriously or a whole slide may occur.