Cavitation studies in dam-water systems subjected to dynamic excitations are very few. No observations of cavitation in an actual water reservoir under earthquake-induced ground motions have been reported. Most experimental work on cavitation has been directed to high speed flow problems such as around moving turbine blades or through pipe bends. In only one experiment, that by Niwa and Clough [6], has cavitation been observed in the water reservoir of a dam under excitation provided by a shaking table. The dam model, a 27 in high single monolith of Koyna Dam (prototype height 338 ft), was separated from the water by a plastic membrane. Since air could access the dam-membrane inter- face, separation (i.e., cavitation) occurred whenever the absolute water pressure reduced to the atmospheric pressure. Ideally, the membrane (necessary to pre-
vent water leakage and contact with the plaster dam) would be omitted and the vapor and atmospheric pressures reduced by the length scale, but these are near impossible to accomplish. Note that the unscaled atmospheric pressure acting on the water in the experiment confined possible cavitation to separation between the dam and membrane; i.e., no cavitation in the water could occur.
For strong excitations, these separations extended over the top third of the dam face and were followed by compressive pressures upon impact. Although it was stated that the impact action definitely increased these pressures, a care- ful examination of the pressure time histories does not indicate that this effect was great. Some of the large pressures were not preceded by cavitation, and many cavitation closures did not produce large pressures. The peak pressure amplitude generated in the cavitation region was about the same as that which occurred in the lower part of the reservoir; this distribution is not untypical of linear response.
Several two-dimensional analytical studies have been carried out to inves- tigate the consequences of water cavitation. Clough and Chu-Han [2] employed the case 1 mechanism with the cavity confined to a separation at the dam face.
The pressure formulation (dynamic pressure as the independent variable) was used for water assumed incompressible, and all degrees of freedom away from the dam were condensed out (possible because of the incompressibility assump- tion). For a 160 m high gravity dam subjected to the 1940 S00E El Centro ground motion ·scaled amplitude-wise by two ( maximum horizontal acceleration of 0.65 g), impacts of the water against the dam following separation increased the tensile stresses by 20-40 % in the upper part of the dam. It was also pointed out that since water compressibility can significantly affect the water pressure response, it should be included in cavitation studies.
Another analytical study by Zienkiewicz, Paul and Hinton [7] employed a displacement potential formulation for compressible water [5] for two- dimensional gravity dams. A bilinear pressure-volume strain relation [1] was used for the water to include cavitation (case 2 mechanism). The formulation
led to coupled matrix equations for the dam and water which were solved iter- atively at each time step in a staggered scheme; linearization of the equations could not be performed as the fluid nonlinearity entered into a mass-like term.
No energy absorption was employed in the water except for an upstream trans- mitting boundary. The model was capable of capturing cavitation anywhere in the reservoir; in fact, isolated regions of cavitation occurred at considerable distances from the dam, a behavior not elaborated on in the reference. Anal- yses of Koyna, Pine Flat and Bhakra Dams (338, 400 and 610 ft heights, re- spectively) were made for the 1967 Koyna and 1940 El Centro ground motions scaled amplitude-wise by 1.5 (maximum horizontal accelerations of 0.94 and 0.49 g, respectively). Effects of the cavitation on the dam response were mod- erate and, in general, tended to reduce the peak responses somewhat. However, time histories of the water pressure responses were very different depending on whether or not cavitation was included. Peak compressive pressures, attained after closure of the cavitated regions, increased considerably. Possible drawbacks of the displacement potential formulation are the dependence of the pressures on the second time derivative of the potential function, which may be inaccurate, and the necessity to use ground displacements to define the earthquake motion, which seldom can be determined accurately.
Recently, Fenves and Vargas-Loli [3] have proposed a mixed displacement- pressure formulation for the water that leads to a symmetric matrix equation for the dam-water system. They used the bilinear model mentioned above. A close examination of their water element reveals it to be essentially identical to that employed here, in spite of different derivations. An analysis of Pine Flat Dam used the S69E component of the 1952 Taft Lincoln School Tunnel ground motion scaled to a peak acceleration of 1 g. A stiffness-proportional damping was added to the water to give 0.1 % damping in the fundamental pressure mode of the water. The envelope of cavitation spanned the top half of the reservoir near the dam and extended over the top few layers of elements to the end of the mesh 1200 ft away from the dam where a transmitting condition was applied.
Effects of cavitation on the dam response were less than in [7], and still tended to reduce the peak amplitudes. Cavitation again strongly affected the water pressures.
In both [7] and
[3],
closure of a region of cavitation resulted in a sharp spike of compressive pressure followed by high frequency oscillations. The de- gree to which this behavior depended on the numerical discretization was not examined, although damping added in [3] was intended to stabilize the high frequency response. This behavior can be troublesome if large enough, leading to additional cycles of cavitation and spread of a cavitated zone. Some of the studies presented in this chapter investigate the mesh dependence of the solution when cavitation occurs.In aH the foregoing analytical investigations, the dam was assumed to be- have linearly. However, under the high levels of excitations required to trigger water cavitation, the dam can experience high tensile stresses that exceed the plain concrete strength. Consequently, concrete cracking should be expected.
As will be seen in Chapter 4, the behavior of cracked dams alters the hydro- dynamic pressures on the upper part of the dam where most of the cavitation takes place. Hence, in a complete cavitation study, the dam should be treated
as the situation warrants.