FP McCann’s Northamptonshire housing development box culverts may be the largest ever to be manufactured in Britain
A box culvert installed at a Permissions Homes development at Weldon Park, Corby, may be the largest ever to be manufactured and installed in Britain.
The 28-metre-long culvert solution is made of 20 box culvert sections, each spans 6000mm and 3000mm high. The culvert channels an existing watercourse through the site. Specialist civil engineering groundworks contractors, Paul John Construction, needed a culverting solution large enough to channel the watercourse and with sufficient structural capability to carry an access road into the development. Precast concrete box culverts offered the ideal solution. The 20 large sections were manufactured at FP McCann’s Byley factory, weighing 22 tonnes each. The units were installed over a pre-prepared concrete bed base using a specialist crane and jointed with bitumen rubber sealing strips. Commenting on the culvert installation, Selwyn Bryers (Director for Paul John Construction) praised the box culvert team at FP McCann for their help in designing the offsite precast solution. He added that the units were installed within a very brief period “…demonstrating the flexibility that concrete box culverts can offer in such civil engineering project”.
In addition to the giant box culvert, the site required another six separate smaller culverts consisting of 102 box culverts units (2100mm x 1000mm). One of these required an inbuilt mammal ledge.
With a section area of 18m 2 , BPDA believes that the units used in this project are likely to be the largest monolithic box culvert units ever to be manufactured in Britain (at least in the last 2 decades). For more information on that case study, please contact FP McCann for more information: https://www.precastdrainage.co.uk/members/fp-mccann
BPDA has published the results of a comparison study into the whole life carbon impact of concrete and equivalent plastic pipes, highlighting that concrete is the low carbon choice.
This year UK Concrete, the group representing the UK concrete industry will develop and adopt a new carbon roadmap to deliver a ‘net negative’ industry by 2050. The announcement means that the concrete industry is not only committing to meet the UK target of ‘Net Zero’ by 2050 but is also committing to removing more carbon from the atmosphere than the industry emits each year.
The concrete and cement industry has a strong track record, having already delivered a 53% reduction in absolute carbon emissions since 1990 and is decarbonising, as a whole, faster than the UK economy. At a British Precast level, since the launch of the Sustainability Strategy in 2007, manufacturing carbon emissions for the sector dropped by 43%.
One of the many steps on a roadmap to a future low carbon economy is selecting the right materials to build much needed infrastructure. The British Precast Drainage Association (BPDA) is publishing the results of a comparison study into the whole life carbon impact of concrete and equivalent plastic pipes, to highlight that concrete is the low carbon choice. The study draws on a sensitivity analysis carried out by external consultancy Circular Ecology, which allowed British Precast to model the impact of both concrete and plastic pipeline systems.
The findings show that at the majority of pipe diameters, when plastic pipe ring stiffness and resin sources are evaluated, installed concrete pipes have lower embodied carbon (measured in GWP = Global Warming Potential) than plastic alternatives. For large diameter pipes the difference is the most marked with the GWP impact as much as 47% higher than that of a concrete equivalent (assuming equal service lives).
When the full pipeline lifecycle in taken into consideration the BPDA believe that the GWP impact of a plastic pipeline could be more than double the impact of a concrete pipe due to concrete’s longer service life.
The RICS (Royal Institute of Chartered Surveyors) has long believed that it is crucial that specifiers make decisions based on the whole life carbon of construction products, rather than cradle to gate or partial studies which are essentially the ‘tip of the iceberg’. Looking at the full picture on emissions is a key theme of the document.
The BPDA adopted a ‘whole life’ principle similar to that advocated in PAS2080 right at the start of the project. Data in the study is based, where possible, on Environmental Product Declarations, like the BPDA DN600 example published in 2017. The study covers not just the creation and transportation of the pipe itself, but also the excavation and transportation of the required bedding material. Further considerations include the demonstrable 120+ years’ service life of concrete pipes compared to just 50+ years for many plastic pipes, a variable which is vital if projections of an 800-year service life requirement for UK water assets bare out. End-of-life scenarios are also considered including the potential need to incinerate the plastic pipe waste at the pipeline decommissioning stage, while concrete pipes can be dug up and reused elsewhere or broken up and recycled, allowing for carbonation to take place removing CO2 from the atmosphere.
When all these facets across the full life of the products are considered, the difference in global warming potential impact is stark. The graph below shows the relative impacts of comparable DN2100 pipes where resin is imported from outside the EU – here concrete pipelines have less than half the carbon impact of plastic pipes.
As a specifier, your choice of precast concrete pipes provides the opportunity to save carbon across the whole life of a pipeline. Lower carbon through design, significant extension of the working life and concrete carbonation at the end of life.
For more information on why precast concrete is the low carbon option download the summary results here
The last three months have been significantly difficult for our families, communities, and country as a whole. Several precast drainage manufacturing sites had to reduce operations and numbers of staff present on-site during the first weeks of this pandemic. All members of the British Precast Drainage Association (BPDA) are following Government advice by implementing social distancing and other precautions on site to mitigate hazards associated with Covid-19. All while maintaining supply of precast concrete products to the market.
As the lockdown eases, and infection rates drop across the country, we are expecting a slow return to normality across the industry and our members are committed to meet that expected market demand while ensuring a safe environment for their members of staff, contractors, hauliers, supply chain partners and neighbouring communities. These are challenging times, and precast drainage manufacturers understand their vital role in supporting a key sector which keeps our drainage and sewerage infrastructure functioning. Precast concrete manufacturers are putting lots of effort into supplying the market and ensuring that our products are manufactured and used safely and responsibly. As further advice from Government becomes available, our members will continue to adjust methods and practices to maintain such standards.
The precast concrete drainage sector remains ready to meet the industry supply requirements of the UK market, as we have been for well over 160 years.
The new code for sewers adoption, which comes into effect in April 2020, includes certain SuDS assets for the first time. British Precast sustainability & product manager Hafiz Elhag explores the implications along with the issues affecting selection of attenuation solutions.
A historic change to the Design & Construction Guidance (DCG) for the adoption of foul and surface water sewers in England comes into force in April 2020. It introduces for the first time a mechanism for water companies to take ownership and responsibility for sustainable drainage systems (SuDS).
Many conventional sewer systems in the UK and other parts of the world are no longer able to cope with increasing volumes of rainfall and increasingly frequent flooding incidents due to climate change. In 2014 SuDS was introduced as one of the main measures to control and mitigate flooding through the government’s National Planning Policy Framework (NPPF).
SuDS offer a range of alternative techniques to conventional piped sewerage in managing stormwater. These methods are designed to be more sympathetic to the natural landscape and conventionally include using ponds, swales, wetlands and natural waterways to control, manage and store surface water runoff.
The new DCG replaces Sewers for Adoption 7 and will offer clearer rules and requirements on where responsibility lies for the long-term maintenance of new SuDS. It is produced by industry trade association Water UK and was approved by Ofwat on 25 October 2019.
Water UK says water companies have taken the initiative on SuDS responsibility in the absence of a governmental plan. This has involved reinterpreting laws on sewers going back to the Victorian era.
The basic criteria that need to be met for a sewer to be “adoptable” can be found in Water UK’s Sewers for Adoption in England report. They include those assets that convey and returns flows to a sewer, surface water body or groundwater and those that have an effective point of discharge into a water body or onto land.
It is also worth noting that attenuation tanks are included in the new guidance for the first time, but underground tanks holding water without discharge will not qualify. Ultimately, it is for the water and sewerage company to apply these criteria to assets that are being offered for adoption.
Not only do SuDS ease pressure on drainage networks, they also improve the urban water cycle by enhancing the quality of surface water. Additional benefits include enhancement of the landscape, the environment, biodiversity and quality of life in urban communities.
Alongside natural sustainable drainage systems, manufactured proprietary technologies can be employed within a sustainable water management train to manage surface water runoff. Like the more natural techniques, manufactured SuDS can be used to intercept, collect, treat or store stormwater and direct them to nearby waterways, depending on specific site requirements.
In heavily built up urban developments, it may not be possible afford land area for natural SuDS solutions such as ponds. For many projects, stormwater attenuation is carried out using proprietary SuDS, most likely an underground tank system comprising a lightweight or precast concrete tank, box culvert, or piping system.
A SuDS survey of engineers, developers and others by the Institution of Civil Engineers and Wavin last year indicated that attenuation was the preferred SuDS solution of 70%aof those responding in England and 63% in Wales. Given that this is the first code that enables the adoption of SuDS, it is worth considering five issues affecting developers’ choice of attenuation solutions.
Top five issues affecting attenuation choice
As with any other project, the cost of building, running and maintaining a SuDS attenuation solution can be the main factor in determining which system to go for. Residential developments and housing estates in England have a significantly long lifespan.
According to the English Housing Survey (2014-2015), well over 75% of the 23.4 million homes in England are over 40 years old, over 56% are over 55 years old and more than a fifth of occupied houses are over 100 years old. It is therefore reasonable to assume that entire neighbourhoods and housing estates will have a >100 years lifespan.
SuDS will need to meet this life expectancy as they will need to perform effectively over the lifetime of a development. For SuDS not expected to last 100 years or more, the cost of replacement, renovation or major refurbishment will need to be incorporated into the whole-life cost assessment.
Members of BPDA are aware of these expectations and of the need for any underground attenuation solution, such as pipe-based, box culvert based or modular tank systems, to have an expected working life that matches the lifetime of the development - 100 years or more.
Landscape design and site requirements are the main factors determining particular attenuation systems. Ponds are usually preferred by planners, but they take up significant land area which may not be available in heavily built-up areas.
In such cases, underground tank systems may be the only choice and in places where even underground storage space is tight, developers may prefer systems that make the most out of available space underground with the lowest cover depth possible.
Constraints associated with construction can always pose engineering and execution challenges. Unsuitable ground conditions, such as elevated water tables, may make it impossible for some types of lightweight attenuation systems to be used.
The nature of soil can also pose risk for some types of underground attenuation systems, unless they are treated effectively to resist corrosion. The availability of space for lifting and installation machinery can be a challenge, but as all installations require excavators, these same excavators can usually be used in the installation of any engineered storage products requiring mechanical lifting.
Weather can also pose a challenge as some systems cannot be installed with stormwater inside the excavated area due to risk of floatation. Climate change could mean such events occur more frequently. Some engineered systems may also be adversely affected by bursts of bright sunlight and hot weather during installation, due to the high degradation rate of polypropylene components when exposed to UV light.
Manufacturers of precast engineered SuDS have solutions for these challenges that would not affect the effectiveness of the system or lead times.
Developers and adoption authorities need to have confidence in the structural integrity of an engineered attenuation SuDS system. The new DCG code offers detailed guidance on the standards to be used in constructing underground tank systems.
For example, concrete pipelines design will need to comply to BS 9295, British Standards’ guide to the structural design of buried pipelines. Other concrete structures will need to comply to relevant Eurocodes standards.
The adoption of SuDS will face further challenges as new product types, such as geocellular boxes and precast concrete tank systems, are brought under its remit. The absence of an established track record of deployment, along with limited experience of working with certain types of systems, poses a risk that designers may deviate from the established, universally agreed standards.
Where standards like BS 9295 or BS 5911 for concrete pipes and ancillary concrete products or CIRIA’s C737 report - Structural and Geotechnical Design of Modular Geocellular Drainage Systems - are not used, designers need to employ established engineering principles and justify why European and national standards should not apply.
SuDS provide a valuable contribution to sustainable development planning and execution, offering an environmentally sensitive approach to stormwater and surface water management. In this capacity, they should also have a lower impact and environmental footprint over their lifetime. This would include lower operational and capital carbon emissions, lower embodied water, transparency and responsible sourcing throughout the supply chain.
There is still a strong need for the sustainable drainage systems industry to improve transparency and provide better quality data so that the benefits of both natural and engineered SuDS become more visible. The differences between engineered SuDS and natural techniques can also be judged sustainably through a complete whole lifecycle assessment approach.
The price of the pipes is not the only consideration when costing a drainage installation project, as a video launched by the BPDA explains. Drainage: the compelling case for concrete is a straightforward guide to assessing the whole cost of drainage projects.
Bedding design and infill material are the main variables affecting cost and they differ depending on the types of pipes used in a given project. As the video reveals, the savings go beyond the price of the aggregate itself - less granular bedding means less imported aggregate and less soil taken to landfill, resulting in far fewer lorry movements.
Flexible plastic pipes have very little inherent strength and are usually installed with a full surround of granular material known as Bedding Class S. This transfers the majority of imposed loads, such as those coming from ground-level traffic, into the surrounding bedding.
Pipes made from rigid materials such as precast concrete do not deform, using their inherent strength to withstand loads and stresses. Typically 60-90% of the design strength is built into concrete pipes during manufacture, so bedding classes requiring at least half the quantity of granular material – such as B and N - can be used.
The BPDA drainage video points specifiers to BPDA’s free Concrete Pipe Calculator app, which combines a structural design calculator and material cost calculator and can run whole-cost scenarios for pipes of various materials and dimensions. Visit https://www.precastdrainage.co.uk/calculator-app for more information about the app.