With the development of water conservancy construction over the past 30 years, rubber dams, as a novel dam type, have seen increasing use due to their unique advantages in creating waterfront landscapes and improving riverside microclimates. Rubber dams are thin and flexible, and overflows can easily induce vibration in the dam bag due to the instability of the water flow. This vibration is more complex than that of conventional structures. Dam bag vibration can easily lead to wear, delamination, and tearing, and in severe cases, damage the entire dam. Therefore, studying the causes and influencing factors of vibration and proposing corresponding vibration reduction measures has become a significant research topic in the rubber dam field. Drawing on extensive experience in rubber dam construction and operation at various locations, and drawing on recent domestic and international research results on vibration reduction, we briefly analyze the vibration mechanism, influencing factors, and vibration reduction measures of rubber dam bags.

Dam Bag Vibration Mechanism

Vibration in hydraulic structures is a direct or potential destructive factor for hydraulic structures. The causes of hydraulic structure vibration are complex and can be categorized by nature as forced vibration and self-excited vibration. The former is the dynamic response of an elastic structure under external disturbances, while the latter is sometimes caused by the unstable free vibration of a hydraulic structure under certain conditions. The vibration of a rubber dam bag is primarily forced vibration under external forces (such as water pulsation). A rubber dam bag is a flexible, thin shell filled with water or gas. Its shape and degree of freedom vary greatly under various external forces, making it sensitive to water pulsation and prone to vibration. This is especially true during overflows at the dam crest, where the elastomeric dam bag is more susceptible to vibration under the influence of fluctuating water pressure. Fluctuating water pressure is random, so too is the vibration of the dam bag. If the frequency of the forced vibration by external forces approaches the natural frequency of the dam body underwater, the vibration of the dam body will be exacerbated, and resonance may even occur. Resonance can severely damage the dam bag. When the internal stress generated by resonance exceeds the allowable stress of the dam material, the bag will fail. Chronic vibration of the rubber dam bag during operation can lead to wear, delamination, and tearing, and in severe cases, even damage the entire dam body, impacting the safety and effectiveness of the project.

Factors Affecting Dam Bag Vibration

Dam bag vibration induced by water instability during rubber dam overflow is more complex than vibration in general structures. Rubber dam vibration is influenced by numerous factors, including the dam crest overflow flow, downstream water depth, anchoring method, inflation height and medium, hydraulic layout, dam span, internal pressure ratio, wave load, flow pattern, climatic conditions, and the performance of the dam bag itself. For operational rubber dams, dam bag vibration is primarily influenced by the dam flow, downstream water level, dam span, bag anchoring and bank connection method, inflation medium, and bag construction and installation.

The Impact of Dam Crest Overflow on Dam Bag Vibration

When the dam crest overflow flow is low, although the vibration frequency is high, the low kinetic energy and pulsating pressure of the water flow minimize the impact on the dam bag vibration, resulting in a low amplitude and minimal threat to the rubber dam. As the overflow flow increases, the kinetic energy of the water increases, and the vibration intensity also increases.

Impact of Downstream Water Level on Dam Bag Vibration

When the downstream water level is very low, a remote hydraulic jump or no hydraulic jump forms downstream of the dam. The kinetic energy of the water propagates downstream, with little impact on the upstream dam bag. At this point, the vibration of the dam bag is primarily caused by the pulsating pressure of the water flowing through the dam. When the downstream water level rises to a certain value, the impact on the dam bag vibration increases, especially for dual-anchor water-filled dams. This is because a critical hydraulic jump forms immediately downstream of the dam, generating high pulsating pressure that easily affects the vibration of the dam bag. As the downstream water level increases further, the upstream and downstream water level difference decreases, reducing the kinetic energy of the water flow and gradually decreasing the vibration intensity. However, as the downstream water level decreases, the vibration frequency decreases, potentially leading to resonance in single-anchor water-filled dams.

Influence of Dam Span on Dam Bag Vibration

For a given dam height and crest overflow depth, the larger the dam bag span, the greater the corresponding vibration intensity. This is primarily due to the pressure differential causing the water within the dam bag to flow. When the dam bag span is large, the next flow of water follows before one current reaches its end, creating waves. These waves are continuously strengthened by the pressure differential, ultimately forming resonance, causing the dam bag to rise and fall and vibrate.

Impact of Dam Bag Anchorage and Bank Connection Type on Dam Bag Vibration

Double anchorage provides better vibration suppression than single anchorage. Double anchorage provides added restraint on the downstream side of the dam bag, minimizing vibration as water flows through it. The downstream tailwater also has minimal impact on the dam bag's support, reducing the threat of wear and damage. Single anchorage, on the other hand, allows the dam bag greater freedom of movement, making it more susceptible to flapping vibration. The dam bag can be connected to the banks or piers in two ways: inclined walls and vertical walls. Inclined wall connections, where the dam bag is anchored to the bank walls, result in less vibration. Vertical wall connections generally use a plug-type structure, which provides greater freedom but also results in more severe vibration.

Impact of the Inflating Medium on Dam Bag Vibration

The inflating medium in a rubber dam bag can be air, water, or a combination of air and water. For rubber dams of the same size, the cross-sectional perimeter of an air-filled rubber dam bag is smaller than that of a water-filled rubber dam. Furthermore, the density of air is lower than that of water, making air-filled rubber dams more lightweight than water-filled ones. To prevent air leakage, air-filled rubber dams typically use a single anchor, which gives the bag greater freedom of movement. Therefore, under the same conditions, air-filled rubber dams are more susceptible to vibration damage than water-filled rubber dams.

Impact of Non-Standard Dam Bag Installation on Dam Bag Vibration

A single-span rubber dam bag typically weighs around 20t, which can easily lead to installation discrepancies with design. One end is too tight, resulting in a smaller cross-section and a slightly lower dam crest elevation. Consequently, when the dam crest overflows, the varying depths of the overflowing water create varying pressures on the crest. Since rubber dams are flexible, this pressure is transferred to the water within the dam bag. Consequently, the pressure at both ends varies, and repeated pressure transfer within the dam bag causes the dam bag to fluctuate and vibrate.

Appropriate Vibration Reduction and Prevention Measures

To address the causes of vibration in the rubber dam bag, appropriate vibration reduction and prevention measures can be implemented from several aspects, including engineering design, construction, and management.

Engineering Design

Rubber dam design encompasses the dam bag, civil engineering, anchoring structure, and control system. Civil engineering primarily includes the foundation slab, upstream and downstream anti-seepage and anti-scour facilities, side and wing walls, and slope protection. During layout, the foundation slab should be appropriately elevated above the riverbed, as far as possible above the normal downstream tailwater level, without compromising the river's flood discharge capacity. This creates a low fixed dam. The dam is backed by a steep slope, allowing water to rapidly drain into the stilling basin after passing through the dam, improving flow conditions and protecting the dam bag from downstream tailwater impacts. The foundation slab and both end walls, all contact areas with the dam bag, must be smooth to minimize vibration wear on the bag. The dam site should be located in a river section with smooth flow patterns and relatively stable riverbed slopes. This not only avoids wavy hydraulic jumps and zigzag currents, preventing harmful scouring and siltation, but also ensures smooth water flow over the dam, reducing vibration and wear on the dam bag. For different rubber dam types, it is important to use anchoring methods that minimize dam bag vibration. Double anchoring saves dam bag material compared to single anchoring and also minimizes bag vibration. Therefore, double anchoring should be used whenever possible, with the distance between the two anchor lines increased to create a semicircular cross-section of the dam bag to increase the vibration threshold. When a dam collapses and releases floodwater, excess water within the dam is often pushed upstream toward the downstream anchor line, forming a small dam similar to a pipe head. This water blockage and vibration of the dam bag occur. To address this issue, a perforated rubber hose is laid at the fold line inside the dam bag. One end of the hose is connected to a drain pipe, and the other end is connected to the so-called pipe head. This small amount of water trapped within the dam bag is diverted back toward the drain pipe, causing the dam bag to collapse.

Construction

Whether using bolted pressure plate anchoring or wedge compression anchoring, accurate layout is crucial, requiring a dimensional accuracy of ±2-3mm. During the second phase of concrete construction for the wedge compression anchoring slot, the slot top and bottom lines must be straight, and the slot walls must be smooth and flat. The wedges must be precast in the prefabricated plant, requiring a solid, dense, smooth surface, and high precision. Bolted pressure plate anchoring requires precise bolt positioning, and the backing and pressure plates must be precisely manufactured. There must be no warping, deformation, chipping, or roughness. During construction and installation of the dam bag, accurate positioning is crucial, minimizing deviations. Design requirements must be met as closely as possible, with tolerances within the allowable range.

Project Operation and Management

Operational management is crucial to ensuring the full effectiveness of the rubber dam project. First, control the overflow from the dam crest, followed by adjusting the downstream water level. If the downstream water level is low, it should be raised to at least half the dam height to minimize vibration hazards. Furthermore, varying the internal water pressure within the dam bag can alter the natural frequency of the dam body to avoid the dominant frequency of water pulsation and prevent dam bag resonance. During flood season, care should be taken to collapse the dam bags to facilitate flood and sediment drainage and reduce vibration and wear.

Summary

The factors that cause vibration in rubber dam bags are complex. To reduce vibration, we must first have a clear understanding of the vibration mechanism of the dam bags. During project design, we must fully study the actual project conditions and take measures to minimize vibration whenever possible. Second, during installation, the dam bags must be accurately positioned and constructed in accordance with design requirements to avoid significant deviations. Third, after completion, strict management must be implemented, with a set of reasonable and feasible operating procedures established to standardize operations and ensure the safe and normal operation of the rubber dam.