Freestanding filter beds can be subject to high circumferential stresses resulting from daily and seasonal temperature fluctuations. The passive pressures, which result from radial movements of the wall during cooling, are especially critical.

Traditional brick, cast-in-situ concrete and stiffly joined structures have high levels of failure associated with the development of excessive circumferential stresses. The passive pressures at wall/filter medium interface increase with time in a ratcheting manner during successive cycles of expansion and contraction, causing an escalation of circumferential stresses induced in the wall. The failure normally occurs suddenly, although it may take years to develop and even with thorough and regular visual inspection it is difficult to predict. As CIRA Report 58 states, “The failure by ring tension is characterised by vertical cracks which progressively increase in width, but many years may lapse before the wall collapses”.

In designing for long-term serviceability it is essential to understand the nature of the stress escalation phenomenon. Charcon Specialist Products research has identified the weakness inherent in the stiff and stiffly connected panel structures and the potential problem of low cycle fatigues of highly stressed, stiffly connected panel structures when subjected to thermal stress fluctuations.

Charcon Specialist Products has long recognised the dangers of insufficient flexibility and their beds incorporate a specially designed and patented panel to panel connection assembly. This assembly combines the unique features of high strength to enhance structural integrity and high flexibility to prevent the development of excessive circumferential stresses due to temperature variations. These features make the assembly particularly durable and lead to longer life. An important aspect of this assembly system is that passive pressures are reduced to acceptable levels and do not play a critical role in the structural performance of Charcon Specialist Products filter beds and walls.

Charcon Specialist Products has extensive experience in the design and construction of filter bed retaining walls of ring tension type. Many structures have been in service for over 30 years and are expected to continue satisfactorily for years to come.

Patently Reliable

Charcon Specialist Products’ patented system incorporates neoprene washers which prevent damage, arising from thermal movement, overcoming the problem of failure caused by passive resistance.

Used for many years by water companies and authorities nationwide, the Charcon Specialist Products’ system not only offers total reliability but proven value and design flexibility in the construction of new, and the repair of, existing filter bed walls.

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