Rubber dams are a new type of hydraulic structure that emerged in the late 1950s with the development of the polymer synthetic materials industry. They are dams anchored to a concrete gate base slab, using high-strength synthetic fibers as the load-bearing skeleton and synthetic rubber as the bonding material and protective layer, and are formed by inflating with water or air. Currently, rubber dams are widely used both domestically and internationally in agricultural irrigation, hydropower generation, river impoundment, and urban river landscape water storage projects.

Based on the different inflating media, rubber dams can be divided into three types: air-filled, water-filled, and water-air hybrid. Water-air hybrid rubber dams have water at the bottom and air at the top, thus requiring both inflation and deflation equipment, making the project more complex and rarely used except in projects with special requirements. In the first 20 years after rubber dams were introduced to mainland China, more than 30 rubber dams were built, using both air-filled and water-filled dam bags. Among them, Huangtuliang Reservoir and Dongshanzui Reservoir successively installed rubber dams on their spillways. Since they were built on the basis of existing projects and lacked the conditions for large-scale foundation construction, inflatable dam bags were used. The rest were water-filled rubber dams. The following section will compare and analyze the two widely used schemes of water-filled and inflatable rubber dams in the project.

1. Dam Bladder Material

The "Technical Specification for Rubber Dams" [SL227-98] stipulates that the internal pressure ratio of water-filled rubber dams should be 1.25, and that of air-filled rubber dams should be 0.75. However, in practical applications, to ensure that the dam bag of air-filled rubber dams does not collapse during overflow at the dam crest, the internal pressure ratio of air-filled rubber dams is generally selected between 0.85 and 1.0.

For the same dam height, the radial strength of air-filled dam bags needs to be increased by approximately 20% to 40% compared to water-filled dam bags, and this increase is greater as the dam height increases.

According to the specification, the safety factor of air-filled dam bags should not be less than 8.0, and higher than the 6.0 of water-filled dam bags.

For dam heights within 5m, the thickness of air-filled dam bags should be increased by 2 to 6 mm compared to water-filled dam bags, an increase of approximately 25% to 35%. Furthermore, in practical applications, to ensure sufficient airtightness of the dam bag, the surface and inner rubber layers of air-filled rubber dams are usually also appropriately thickened.

2. Dam Bladder Area and Cost

The outline of a water-filled rubber dam bag consists of four parts: the upstream curved section of the dam face, the downstream curved section of the dam face, the upstream ground-hugging section, and the downstream ground-hugging section. The outline of an inflatable rubber dam bag typically consists of only three parts: the upstream curved section of the dam face, the downstream curved section, and the ground-hugging section.

• Under the same dam height, the length of the water-filled rubber dam bag along the water flow direction, the area per unit length (m), and the length of the dam bag touching the ground are all 14% to 20% greater than those of the inflatable rubber dam bag.
• Under the same dam height, the area per unit length of the air-filled rubber dam bag is smaller than that of the water-filled rubber dam. However, due to higher safety performance requirements, the inflatable rubber dam bag needs to be thicker than the water-filled rubber dam bag, resulting in a higher total material consumption (the cost of an air-filled rubber dam bag is about 5% higher than that of a water-filled rubber dam bag under the same dam height).

3. Filling and Exhausting System

The selection of inflation and drainage equipment needs to be determined based on a comprehensive calculation of the dam bag volume, design internal pressure ratio, and inflation/collapse time. For water-filled dams, the pump flow rate and head must also be considered.

The selection of inflation and drainage pipelines generally follows a reasonable gas/water flow velocity, and the pipeline diameter is determined based on the dam bag volume and inflation/collapse time. The diameter of the inflation and drainage pipelines for air-filled rubber dams is usually half that of those for water-filled rubber dams.

Comparing the total price of inflation and drainage equipment and pipelines, air-filled rubber dams are typically about 40% cheaper than water-filled rubber dams.

4. Corresponding Civil Engineering

The civil engineering for rubber dams includes the base slab, side abutments (or bank walls), central abutments, paving, apron, guide walls, and pump house (or fan house), etc.

Base: The ground-contact length of the dam bag in an inflatable rubber dam is shorter than that of a water-filled rubber dam of the same height. However, since the dam bag portion on the base plate has no water weight, the width of the base plate needs to be increased to ensure anti-sliding stability. Therefore, the base plate width of an inflatable rubber dam is usually greater than that of a water-filled rubber dam.

Rubber Dam Equipment Room:

• The pump house of a water-filled rubber dam is a semi-underground structure, with its base plate typically about 2m lower than the dam's base plate. It also requires clean water from the pumping source, necessitating the design of water intake pipelines, water supply wells, and other facilities.
• The blower room of a water-filled rubber dam can be arranged according to the terrain on both banks and is generally an above-ground structure.

To ensure the normal operation of the pumps, the engineering work for the pump house, water intake pipelines, and water supply wells of a water-filled rubber dam is often much larger than that of the blower room of an inflatable rubber dam.

5. Landscape Environmental Effects

Rubber dams are widely used in many urban river management projects due to their aesthetically pleasing appearance. The visual effects of inflatable rubber dams differ depending on the inflation medium used.

• Air-filled rubber dams, due to the lighter mass of the gas inside, require greater internal pressure to inflate, resulting in a roughly circular outer shape that often conveys a sense of tension.
• Water-filled rubber dams, on the other hand, are approximately elliptical in shape due to the natural weight of the water and upstream water pressure, exhibiting a streamlined appearance and a more comfortable feel.

Therefore, clients typically choose water-filled rubber dams when a project has aesthetic requirements.