CPD: Clay drainage and whole life value
Clay has been used to make drainage pipes for thousands of years. Now it is enjoying a renaissance, thanks to its sustainability credentials, performance qualities and abundance. Paul Wydell explains.
The construction industry has for some time recognised that focusing only on the initial capital cost of a structure will result in higher whole life costs for servicing, maintenance and refurbishment. Achieving the higher aims of “whole life value” has therefore become a key issue in both public and private sector construction procurement.
With consulting engineers, contractors and clients all looking for new ways to be even “greener” through product and installation specification, it is crucial that manufacturers continue to offer products, systems and processes that minimise the impact they have on the environment and offer increased whole life value.
Awarding contracts on the basis of lowest price tendered for construction works rarely makes financial sense – the long-term value over the life of the asset is a much more reliable indicator.
A 2005 National Audit Office report, Improving Public Services Through Better Construction, advocated that proper attention be given to sustainability and life cycle costing. Nevertheless, there remains a lack of clarity on whole life costings and what constitutes best value. Whole life value is about more than just cost alone: it covers wider aspects such as quality, time, and the environmental, social and economic impacts.
Waste water systems may be hidden underground, but out of sight should not mean out of mind in terms of sustainability. Clay sewerage pipes have been found in Ephesus, Turkey, that date back 2,500 years and this form of drainage continues to experience a strong resurgence in popularity. As well as being one of the oldest and most popular construction materials, clay is also regarded as one of the most sustainable.
Faced with stringent building regulations (Approved Document H (Drainage and Waste Disposal)) and environmental standards (ISO 14001: 2004), enhanced requirements for sewers, as detailed in Sewers for Adoption (Seventh Edition For England and Wales) and the Mandatory Build Standard for Wales, and ever-increasing demands on performance, clay is often a sustainable choice for specifiers.
Natural clay resources will allow people to continue building for generations to come. Although not renewable in the purest sense, the main reason clay is so sustainable is that it is a natural product, offers a long life cycle and re-uses energy in its manufacture.
To further reduce environmental impact, the process adopted by Hepworth Clay at its six quarrying sites in South and West Yorkshire also uses an onsite reservoir fed by moorland rainwater for all production and all other non-potable water uses.
The clay pipes’ thinner body walls are lighter but retain a range of enhanced product characteristics
The rain and groundwater surface run-off from clay quarries are intercepted and directed to primary settlement and secondary polishing ponds to preserve local watercourse quality. The same principle applies to rainwater run-off from the manufacturing site in Hazlehead, near Sheffield, which is collected and directed to primary settlement and secondary polishing ponds.
Clay is an abundant and naturally occurring raw material. At Hepworth Clay, the SuperSleve clay drainage range of vitrified clay pipes, bends, junctions and fittings is manufactured from Halifax Hard Bed coal measures and shale deposits near Penistone on the edge of the Yorkshire Pennines. These shale clay deposits date from the carboniferous geological period and are between 280 million and 345 million years old.
Suitable for use in sewers, commercial and industrial construction, highways and general building works, the Hepworth SuperSleve system consists of plain-ended or socketed pipe fittings, with additional push fit flexible couplings, and is available in 100 mm, 150 mm, 225 mm and 300 mm diameters.
The base raw material Halifax Hard Bed clay is rich in a natural iron content. The firing process produces a thin wall product with a high crushing strength, which is unique to the SuperSleve process. For example, a 150 mm diameter SuperSleve pipe has a crushing strength of 40 kN/m, which is roughly equivalent to the weight of three medium-sized family cars for each metre of pipe length.
The selection, suitability and consistency of the raw material and its subsequent blending is the key to providing a stable base for subsequent processes to build upon. The qualities of the raw clays are extensively tested and analysed from borehole samples in the quarries, as well as incoming quarried materials.
These samples are all analytically processed and tested in laboratories at Hazlehead. Once delivered to the plant the clays are crushed to less than 75 mm particle size and fed into a blending plant, where 40 horizontal layers each of 100 mm thickness produce a total mass of 9,000 tonnes of feedstock (blended raw clay material) in pre-determined proportions ready for production.
Nurturing nature after quarry life
One of the key sustainability issues with quarries is what happens when they have finished being excavated. For a company that owns and works six quarries in South and West Yorkshire, Hepworth Clay works with mineral planner and landscape architect Colvin & Moggridge to ensure the sites restore previously agricultural land to natural habitats and re-establish biodiversity in the area.
The company also provides resources to protect owned woodlands and wetlands for flora and fauna to flourish, and ensure aftercare programmes are in place to provide long-term development.
The blended clays are dried and pan ground to less than 1.6 mm particle size before being processed through a vortex-principle calcination plant. At temperatures of around 650 degrees Celsius, the clay particles are constantly in motion in the swirling vortex atmosphere and so every piece is evenly treated in a consistent way to drive off impurities, thereby reducing subsequent kiln firing times.
Ball milling reduces the particle size even further, to 250 micrometres, although 90% of the material is less than 20 micrometres. This unique process and fine particle size increases the finished product strength to more than double
that of the standard 1.6 mm particle-sized clay product.
The powdered material is pneumatically conveyed in steel transfer ducts to the pipe extrusion machines.
The powdered clays are thoroughly mixed with water and any entrained air is removed by strong vacuum immediately before extrusion at exactly 18% moisture content. Higher moisture content would make extrusion easier, but the pipes would then be too soft, and would subsequently deform. The residual moisture would also be too high going into the kiln and they would disintegrate by rapid moisture expansion.
Lower moisture content would not sufficiently hydrate the clays to make the extrusion mix uniform. The pipes would be extruded with some parts correctly mixed and some parts too dry, and would then disintegrate in the kiln.
All the water used in production is drawn from the Hazlehead plant’s onsite reservoir, which collects and stores rainwater from the surrounding moorland.
Unfired offcuts and trimmings that are generated post-extrusion are returned to raw material stockpiles for reintroduction with no loss of quality. Fired pipes from production testing can themselves be crushed and re-used in manufacture. Specifiers can be assured that 15% of material in every new pipe or fitting is from previously fired product.
The furnaces or kilns that produces SuperSleve pipes are called roller kilns. They are designed on a counterflow principle. The main advantage of this design is that it reduces the specific fuel consumption by ensuring the heat input at the high-temperature part of the kiln is recovered and recirculated through the cooler product in the pre-heat sections. This results in greater energy efficiency in comparison with other ceramic sector kilns.
During firing at a temperature of more than 1,150 degrees Celsius, a fundamental change takes place to turn a granular material into a vitrified finished body by removing most of the impurities and the chemically combined water from the crystalline structure of the clay minerals. This creates a product that is strong and impervious to water.
The SuperSleve unique “black core” is a trademark characteristic of the product. High natural iron content in the Halifax Hard Bed raw material is used in the firing process to produce red-burning haematite, tri-iron tetroxide and black-burning iron oxide. This results in a multi-layered appearance but a single material body, in which the inner black body provides enhanced strength and an almost impermeable membrane.
The 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 dimension control straightness, mechanical strength, in crushing and beam, chemical resistance and resistance to water jetting.
Strength in numbers
For structural performance, clay is in a league of its own. Other materials rely upon high-strength granular fill around the pipework to prevent them from becoming deformed. If the bedding is insufficient and subsequent deformation is determined, the system could fail and require remediation or renewal.
On the other hand, clay pipes carry most of the traffic load themselves by supplying up to 90% of the strength required. As a result of this compressive strength, they require far less aggregate as a bedding material, which reduces costs and the environmental impact during the construction process. At the same time there is no associated reduction in quality or increase in risk.
The installation of clay pipe systems can also involve the use of excavated material and recycled aggregate to minimise waste to landfill. This enables a low-impact installation that will leave a sustainable legacy.
Long-term performance and value for money are crucial elements for every construction site, and striking the right balance between cost effectiveness, longevity and aesthetic appeal can prove difficult. With a lifetime expectancy of more than a century (as estimated by BS EN 295-1:2013 – Vitrified Clay Pipe Systems for Drains and Sewers) clay drainage offers inherent strength
at a low lifetime cost, making it the “fit and forget” choice for all future drainage specifications.
The durability of clay pipes is as much an advantage during its service life as it is for installation. With a lifetime jetting guarantee, Hepworth pipes are able to withstand jetting pressure of 7,500 psi, at 20 gallons per minute held static for five minutes, allowing blockages to be cleared with reduced risk and in less time.
Paul Wydell is group product manager at Wavin UK
Hepworth Clay is the overarching brand of Hepworth SuperSleve, Hepworth HouseDrain and Hepworth Terracotta ranges. It has manufacturing facilities at Hazlehead near Sheffield and Forest Works near Burton Upon Trent.
Wavin UK was established in 1962 and is the UK’s leading supplier of water management, plumbing, heating and drainage systems for the building, construction and utilities markets. Wavin has an extensive product portfolio under the OSMA, Hep2O and Hepworth brands, which are available from plumbing and builders merchants across the UK.