Subcontractors have been urged to review their selection criteria when fitting geocellular flood alleviation products and refrain from a ‘one product fits all’ culture when constructing for different applications.
Wavin Plastics, one of the UK’s leading suppliers of water management systems, says that some geocellular products on the market are being used in unsuitable applications - typically where lower load-bearing capacity products are fitted in trafficked areas. Wavin are keen to stress that the issue lies not with the geocellular products, but with the lack of guidance offered to subcontractors and designers around the correct use and installation of these systems. The company firmly believes that the correct information must be made available for the subcontractor to make a decision regarding product type and applications.
This lack of clarity has led to ‘system failures’ in recent years, usually involving lower load-bearing capacity products being used incorrectly below car parks. The Environmental Protection Group (EPG) recommend that a combination of the following three factors should be taken into consideration when designing systems to be used under trafficked locations, to ensure that the correct product is selected:
- Proven load bearing performance of the product, including the deflections and creep under load.
- Expected site loadings.
- Depth of cover over the units - for landscaped/non-trafficked applications, typically a unit with a minimum vertical strength of 17.5 tonnes /m2 should be suitable to support the design loads quoted in relevant standards.
With a growing environmental need to deal with flood alleviation, the popularity of geocellular flood alleviation structures has increased dramatically in recent years due to their lightweight, easy to install and high capacity characteristics. There are now more than 15 different products available on the market, many of which originate from Continental Europe. Load-bearing capacities for the different products available typically range from 90 tonnes/m2 down to less than 10 tonnes/m2.
Michelle Fleming at Wavin commented: “A ‘one product fits all culture’ has developed alongside the phenomenal growth in the use of modular box systems. Decisions on the application of these structures cannot be left solely in the hands of the subcontractor. Manufacturers and specifiers have a responsibility. The applications suitable for each type of product must be made much clearer.”
She continued: “As an industry we need to be more transparent about the load-bearing capacities and deflection characteristics of different systems, encouraging subcontractors to question the structural suitability for different applications. As with any buried structure, multiple factors need to be taken into account to guarantee structural integrity, particularly the relationship between deflection characteristics, lateral and vertical strength. However, assuming the unit has adequate deflection characteristics, typically, under a car park with occasional lorry traffic and a cover depth of at least 0.6m, a product with a minimum vertical strength of 40 tonnes/m2 is required.