The rubber revolution10 September 1998
Installations of rubber dams are increasing throughout the world. Andrew Thompson* discusses the evolution of the rubber dam and one of its most recent developments
The origins of the rubber dam can be traced back to Norman Imbertson of the Los Angeles Department of Water and Power in the US. In 1956, he thought of the idea of using a sealed rubber body attached to a foundation which could be raised or lowered by the addition or removal of water. The Imbertson inflatable gate, as this structure became known, evolved in to a flexible membrane filled with air or water which acted as a weir.
The Japanese were the first to show enthusiasm for this concept and in 1978 Japanese company bridgestone installed its first rubber dam. Since then it has supplied more than 800 inflatable rubber weirs for hydro power, irrigation, flood control and water supply projects throughout Europe, Asia and the Americas.
The standard construction of the rubber dam consists of an envelope of reinforced rubber securely clamped to a concrete foundation. Installation is extremely simple and usually only a single anchor is required on the upstream edge. The rubber envelope is filled with air to raise the upstream water level and deflated to bring the level down again.
After differing experiences with water-filled dams, Bridgestone decided to use the air-filled version. This has various benefits over its hydro counterpart and include:
•Shorter inflation and deflation times.
•Simple and cheap construction — as the air is fed into and discharged from the dam on one side there is a simplified pipework system. Air is lighter than water and so the foundation for the air-filled dam does not have to be as strong, therefore this makes it cheaper and easier to construct.
•Simple and cheap maintenance — the inflation system is uncomplicated and hardly requires any maintenance.
A unique feature of the Bridgestone air-filled rubber dam is an integral fin which reduces oscillation of the dam even in high overflow conditions. All gates have the potential to oscillate when overflowing due to the formation of a low pressure nappe. Instability of the nappe causes vibration which has been known to wear the rubber dam fabric where it comes into contact with the concrete foundation. Consequently, Bridgestone has designed the fin to allow aeration of the nappe, preventing low pressures.
The Bridgestone rubber dam may be installed in lengths up to 200m (depending on height) for a single span and can be up to 6m high. It is ideal for both new build and refurbishment projects to raise the upstream water level in any environmental conditions.
The rubber membrane (reinforced with layers of fabric) is made by a process of high pressure vulcanisation. The thick outer rubber cover is an ethylene propylene diene monomer (EPDM)-rich rubber compound. EPDM, which has been developed by Dupont, has been manufactured to give superior resistance to the ageing effects of ultra-violet rays, ozone and heat.
After 21 years of successful experience with inflatable weirs Bridgestone is confident to predict a service life in excess of 30 years for the rubber membrane.
The thicker cover provides a high level of protection against puncture or abrasion.
Since the inner pressure is low (approximately equal to the head of water) the structure is soft and will absorb the impact from floating debris without damage. It is not possible for the dam to explode like a balloon in the case of puncture due to the strength of the reinforced layers. A simple control system, which constantly monitors the inner pressure of the rubber dam, may also be provided to keep the dam in the raised position in the unlikely event that it is punctured.
One of the most recent developments for the Bridgestone rubber dam will take place in the Netherlands, where such an inflatable rubber structure will be used for flood protection for the first time.
Rijkwaterstaat (Department of Public Works) issued an invitation to tender for the design and construction of an inflatable barrier. This was to be constructed between the Ketelmeer and the Zwarte water to the northeast of Amsterdam. Known as the Waterkering Ramspol (Ramspol Water Barrier) the dam is being built to protect West Overijssel against high water from the Ijsselmeer during storm conditions.
On 1 October 1996 six contractors submitted their technical proposals and commercial bids. Hollandsche Betenen Waterbouw bv (HBW) was the eventual winner and was awarded the contract in April 1997. HBW had already signed a pre-bid agreement with Bridgestone Industrial, a UK subsidiary of the Japanese Bridgestone Corporation, and one year later HBW and Bridgestone are close to concluding the design of the barrier. It is expected to be fully operational during 1999.
The Ramspol Barrier is a development of Bridgestone’s inflatable rubber weir technology but includes many innovative features. It will consist of three spans; each 80m long and 13m wide at the foundation. In its raised position the crest will be 8.35m high; 0.3m above the high water level. These dimensions will make it the largest rubber dam in the world.
Each rubber bladder will be secured to a concrete foundation with dual anchors and the inflation medium will be a combination of air and water. This method of inflation minimises the width of the foundation and satisfies the performance criteria to achieve full closure within a maximum of 60min. To allow free passage of shipping the barrier must deflate below the foundation level when required. Bridgestone and HBW have co-operated to develop a foundation layout and rubber sheet geometry to ensure complete deflation. Operation of the complete system has been proven by hydraulic testing on a 1:25 scale model.
The dam is designed on the basis of a one in 10,000-year storm as the most extreme condition, with an expected lifetime of at least 100 years for non-replaceable components. The rubber bladder itself is expected to last more than 25 years.
By scale model testing and finite element modelling, HBW and Bridgestone have been able to calculate and verify the dynamic behaviour and stress distribution in the rubber body and clamping system. In the meantime HBW has completed the detailed design of the foundation and commenced construction work. The current situation on site is that HBW is ready to pour the concrete for the final foundation.
Function and performance are critical requirements but aesthetic and environmental factors must also be taken into consideration. By selecting an inflatable barrier the environmental and visual impact is minimal when in the deflated condition. HBW architects have also designed the control house to blend in with the profile of the barrier itself.
|The rubber dam versus the conventional dam|
|•Quicker and cheaper installation Rubber dams only require a simple foundation with a 10-15cm recess: steel-gate dams must have a 50-80cm recess. This saves time and money during installation. A longer rubber dam can be installed without piers: for steel-gate structures intermediate piers are generally needed every 15-30m. Rubber dams can also be installed in rivers with virtually any side slope angle, eliminating modifications to the bank up- and downstream of the dam. Steel-gate dams cannot be installed unless slopes are vertical. •Less maintenance Described as virtually maintenance-free, only the electrical equipment and blower require maintaining on a rubber dam. •Simple and flexible operation The rubber dam’s inflation/deflation mechanism is of a simple design with minimal moving parts, reducing the unanticipated release of water due to mechanical malfunction. In rigid dam structures downstream dirt and sedimentation can prevent the release of water. The flexible structure of the rubber dam virtually eliminates this, allowing the dam to be deflated. The flexible nature of the rubber dam also helps to minimise problems that could arise if the dam foundation subsides unevenly. •Extreme weather conditions The fin structure provides a flat structure upon deflation which prevents damage from floating debris.|