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Hepworth | Clay sewer pipes offers strength and economy

Clay, one of the oldest and most sustainable pipe materials, remains a popular choice for drainage specialists. Hepworth Clay product manager.

Clay is one of the oldest materials for drainage pipes, with clay sewerage pipes dating back 2,500 years, being unearthed in Ephesus in Turkey. In recent years, it has seen something of a resurgence in popularity. This is not only due to its value for money but also its sustainability with a drive in the construction industry to reduce the carbon footprint of projects and new guidance and standards aimed at reducing environmental impacts. 

Performance

For structural performance, clay is in a league of its own. Its inherent strength makes it highly resistant to static and dynamic loadings, meaning it requires less granular bedding. It can be installed both shallow (600mm) and deep (10m) with the same bedding construction allowing for simplicity of design, specification and onsite construction. 

Hepworth’s SuperSleve manufacturing process results in pipes with thinner body walls that are lighter in weight than traditional clay pipes but have the enhanced product characteristics of dimensional straightness, mechanical strength, in crushing and beam, chemical resistance and resistance to high pressure water jetting.

With a lifetime jetting guarantee, Hepworth clay pipes are able to withstand jetting pressure of 517 bar, at 90 litres per minute held static for five minutes. 

This is significantly above the most stringent requirement of any UK water company for adoptable sewers at 280 bar. The need for clearing blockages with reduced risk and in minimal time has become increasingly important in the age of the “fatberg” with hardened fat, oil and wet-wipes continuing to pose a huge problem for water companies despite public awareness campaigns.

The inert nature of clay means that it is impervious to chemical or physical attack so it is unaffected by effluent that is approved for discharge into a public sewer. Clay pipe is also resistant to practically all chemicals and compounds that may be found in the ground, making it the perfect choice when specifying drainage for contaminated brownfield sites.

Whole life value

Initial capital is no longer the main measure for cost evaluation in the construction industry. 

Whole life value has become an increasingly important measure when awarding contracts not just for the cost implications but also the overall environmental performance of the installation. 

Clay has a life expectancy of more than a century (as determined by “BS EN 295-1:2013 – Vitrified Clay Pipe Systems for Drains and Sewers”), making it the “fit and forget” choice for all future drainage specifications. 

The UK’s glacial sewer replacement programme rate approaches 800 years, making it important for clay pipe to deliver longevity. The installed risk profile and service costs of clay are low, with no renovation or repairs anticipated when correctly installed. 

The joints utilise advanced technology which passes the angular deflection and shear resistance tests satisfying BS EN 295-1:2013 and ensuring that the 100 year-plus life expectancy applies to the entire system and not just the pipe.

Sustainable manufacture

Clay is a plentiful and 100% natural raw material that is sourced by Hepworth from local quarries to minimise raw material transportation. Careful blending of the clays takes advantage of their distinct minerology prior to heat treatment in a calciner, a globally unique process for clay pipe production. Filtered moorland rainwater hydrates the clays during pipe extrusion. The drying process uses recovered heat from energy efficient fast firing roller kilns. The loop is closed by the total end-of-life recyclability of clay pipes and up to 15% of any offcuts, trimmings or products used in testing are ground down and reintroduced to the manufacturing process with no loss of performance.

Hepworth Clay also minimises the local environmental impact of its manufacturing processes by restoring natural habitats in and around quarries once their resources have been utilised,  and planting trees as part of its carbon offsetting programme.

Installation

Pipes made from other materials rely on high quality aggregates to limit  deformation, which could lead to non-adoption or require remediation. Clay pipes supply 90% of the strength required for the total design load, so require far less aggregate as bedding material without a reduction in quality or increase in risk. Excavated material and recycled aggregate can also be used as a bedding material, reducing costs and the environmental impact during the construction process. The savings can be significant when a full depth of primary aggregate can actually be more expensive than the pipe. 

Safety in strength 

Due to the high inherent strength of clay pipes, higher safety factors are achieved in practice than those stated in “BS 9295 Guide to the structural design of buried pipelines”.

The diagram above shows the high level of support provided by two bedding classes. A design example of a 300mm diameter SuperSleve pipe in a “wide trench condition” at 6m deep plus a safety factor of 1.25 - produces a total external design load of 86kN/m. The bedding factor calculation result is 1.2. 

The total external design load divided by pipe strength of 72kN/m equals the minimum bedding factor.

The additional margin of safety above the minimum design requirement with a class F bedding factor of 1.9 (50mm regulating bed of aggregate underneath the pipe) is +58% or with a class S bedding factor of 2.5 (full granular surround) is +108%.

This increase provides a significant safety margin for current and future unforeseen site conditions that may have a reducing effect upon the level of support provided to the pipeline. This leads to a safe installation for the long term, protecting the specifier and installer.

The next revision of “BS 9295 Guide to the Structural Design of Buried Pipelines” is currently under revision and awaiting publication. 

It will replace “BS EN 1295-1:2019 Structural Design of Buried Pipelines Under Various Conditions of Loading” as the UK national design and guidance method. Among many other updates, it will give comprehensive guidance on the use of recycled aggregates in practice allowing contractors to minimise their environmental impact with the use of recycled aggregate when installing clay pipes. 

Hepworth Clay can advise on the optimum specification of a recycled bedding material as a replacement for primary aggregate confirming how this sustainable and cost-efficient solution can be deployed.

When all of these credentials are combined it is easy to see why clay drainage pipe is such a popular option for consulting engineers, contractors and clients. It ticks all of the boxes in terms of performance, sustainability and whole life value. For most construction materials, sustainability is a pipe dream; for Hepworth clay it is a natural and actual reality.

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