precast-manholes

Posted by & filed under Pipes & Manholes.

Advances in production techniques for precast manholes offer multiple advantages for the construction sector. Innovations championed by BPDA

members mean manhole installation can be completed in about an hour, with a much higher success rate when compared with traditional techniques.

Precast manhole systems are easier to install, but they also improve onsite safety and raise the bar on quality and performance as well as lowering costs and reducing waste to landfill. To help contractors deliver best practice on-site, BPDA has now produced a handy guide to installation.

The Pocket Guide to Installing Concrete Manholes can be downloaded from the BPDA website as a PDF or is available as a printed document. It lists seven simple steps to successful precast manhole installation (with minimum 125mm wall thickness) and the full version can be found at https://www.precastdrainage.co.uk/uploads/downloads/manhole_guide_interactive.pdf

 

Design flexibility

Precast concrete manhole systems are suitable for a wide-range of pipe connections and can be retrofitted without complete replacement of the chamber. The systems comprise a precast concrete base unit with channel and benching and predetermined combinations of flexible and watertight inlet(s) and outlet. Base units and chamber rings are made with thick strong walls and lifting points, eliminating the need for a concrete or granular surround unless specifically required by the client. High-performance seals and extra-thick chamber walls ensure long-term watertightness and durability. The excavation is backfilled sooner than with traditional techniques, minimising the health and safety risks associated with open excavations, and there is less need for work in confined spaces, which also lessens risk to workers. By reducing project time, overall costs are also brought down.

Sourcing constituent parts from local suppliers and a rise in the use of recycled materials keeps embodied carbon impact to a minimum. Production techniques for precast manhole systems continue to advance and the use of modern logistics ensures excellent and consistent product quality and reliable service.

In summary the seven steps are:

  1. Safety

Safety must always be the first priority for any construction project and all site activities must be preceded by an appropriate risk assessment. Typical activities include vehicle offloading, movement of components, excavation, backfilling and the lifting and positioning of components.

  1. Preparation

Excavate a trench of appropriate dimensions to accommodate the manhole structure. The trench must allow sufficient working space outside the chamber for access and backfilling to the required specification, taking into account the ground conditions, depth of excavation and any other relevant factors. The heights of the manhole components supplied by the manufacturers are nominal, so it is beneficial to measure the units prior to installation in order to assist with obtaining the required height of the completed chamber.

  1. Installing the precast manhole base

Prior to lowering into the trench, the precast base unit may be pre-fitted with a lubricated outlet if required. A plastomeric sealing strip/elastomeric seal is used to form a waterproof joint between units. It may be fitted before lifting into position or after

each unit has been individually placed. Concrete to concrete contact between units must be avoided.

 

Place the base unit onto the prepared granular bed and mate the stub pipe with the installed outlet pipe. Check the base position for alignment, level and inverts. Note that precast bases have an inbuilt fall across the main channel and can be installed level.

  1. Fitting the chamber rings

Make sure that the joints are clean and free from foreign objects before fitting the next chamber ring unit. The plastomeric sealing strip/elastomeric seal should already be in place on the installed unit and ready to receive the next chamber ring unit. Repeat with further ring units until the chamber has been constructed to the required height. Ensure that the steps are correctly aligned.

  1. Fitting the cover slab

Place the cover slab directly on the last chamber ring with the access opening lined-up with the steps. Apply slight pressure onto the cover slab using suitable protection, such as timber, to seal the chamber.

  1. Backfilling

When using wide-wall precast concrete manhole chamber rings, the excavated soil can be returned as backfill unless an alternative arrangement is specified by the client. Compact the backfill soil as specified in the design.

  1. Operation and maintenance

Precast concrete manhole base systems are strong and durable and eliminate the risk of inconsistent quality from site-based operations. They are designed to remain watertight and maintain their structural integrity for over 120 years.

 

 

Fact Zone: Concrete Manholes

Up until the 2000s concrete manhole construction has required manhole bases to be constructed onsite from ready-mixed concrete. This required the channels, connections and benching to be constructed in confined spaces with works often carried out in wet and hostile conditions. Additional external contractors were required to supply and pour the concrete and the process for each installation would take several hours due to routine logistical and operational challenges. Furthermore, construction was not always successful. In 2011, a revision of Part 3 of BS 5911, the main standard for precast concrete manholes, introduced a new type of precast manhole system. This comprised a factory-made precast base with elastomeric or plastomeric seals on all joints and connections to ensure permanent watertightness.

 

 

 

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Posted by & filed under Pipes & Manholes.

A report into Sweden’s water and wastewater networks anticipates a long-life for the next generation of pipes. The authors of Sustainable water and wastewater pipe systems of the future, which has been published by the Swedish Water & Wastewater Association (SWWA), argue that the networks currently being installed should have an operational life of at least 100 years and that pipes laid from 2020 should have an operational lifetime of 100-150 years.

 

The report also argues that the current renewal rate needs to increase by 40% to maintain the current condition of the network. However, when renewal is carried out, it should be done to such a standard that the new pipes have an average operational lifetime of 100-150 years.

 

Concrete pipes account for 69% of the waste- and storm water pipelines in four of the main water utilities – NSVA (six southern municipalities), Kretslopp och vatten (Gothenburg), VA Syd (Malmö) and Höganäs. This is representative of the country as a whole, though some have been rehabilitated or replaced in the 10 years since this figure was calculated.

 

A survey in Malmö found that many concrete pipes are still operating successfully 100 years after installation. Unreinforced concrete pipes manufactured in Sweden today are generally 150-1000mm diameter, with the larger reinforced concrete pipes coming in at 400-3000mm. The main method for condition assessment of gravity sewer lines is CCTV, particularly for assessment of pre-stressed steel reinforcement pipes.

 

Concrete pipes laid in the 1940s generally have a shorter life of 50-100 years due to the shortage of cement during the Second World War and its substitution with finely ground limestone filler. They currently require replacement. It was during the 1960s and 1970s that most concrete pipes were installed and, given a pipe-life of 100 years, a major replacement programme will be required around 2050.

 

The various causes of deterioration of underground concrete pipes are already fairly well known. Sulphuric acid attacks from the hydrogen sulphide (H2S) in sewage can reduce the thickness of concrete, especially where the sewage flow slows down or becomes stationary.

 

Increasing durability

The durability of concrete pipes can be improved in a number of ways and cured-in-place pipe (CIPP) lining is common in Sweden. Resistance to acid and sulphate degradation can be improved by mixing a number of alternative binding agents into the concrete, particularly a ground granulated blast-furnace slag (GGBS) such as Alfarör, which uses a 15% slag mix and fly ash.

 

Some of the older pipes in Sweden are believed to have been made using 100% Portland cement. In the UK almost all concrete pipes are manufactured to exposure class DC-4, which includes 30% fly ash or GGBS to 70% Portland cement.

 

Other additives, such as limestone filler or polymers, can be effective, as can a number of surface treatment methods including internal centrifugal spraying.

 

Recent innovations include mixing bactericidal additives, such as a cationic polymer, into fresh concrete. The polymer is particularly effective in binding and rendering H2S bacteria, while avoiding harm to other bacteria. It has been used successfully in North America since 1996 and became available to the Swedish market in 2010.

 

In parts of the world where bacterially-induced H2S formation appears as a result of exposure to an optimum temperature of around 30°C, calcium aluminate cements such as Ciment Fondu Lafarge are used. They have greater chemical resistance than most Portland cement and are used for both the manufacture of concrete pipes and also as cement mortar insulation.

 

Further Research

The report’s authors recommend that more research is undertaken on how to prevent, control and forecast the degradation of concrete in ageing sewer pipe systems. Particular regard needs to be paid to developing innovative non-destructive methods for condition assessment and online surveying, which can provide valuable data to support maintenance planning and prevent unexpected disruption to operations.

 

New non-destructive techniques and sensors need to be developed, along with forecasting tools that can enable proactive maintenance and minimise leaks, bursts and network failure. For concrete sewerage this involves the generation of a model for pipe degradation based on the key factors of H2S, temperature, soil movement, reinforcement and corrosion.

 

Selected information translated from the SWWA’s report The Sustainable Water Management System of the Future (Framtidens hållbara VA-ledningssystem, 2018). Authors: Helena Mårtensson, Annika Malm, Bror Sederholm Jan-Henrik Sällström, Jan Trägårdh (original article published at the Summer issue of the BPDA Newsletter).

Manhole_Guide

Posted by & filed under Uncategorized.

BPDA has just published a new site guide on the installation of precast base manholes. The guide identifies seven basic steps needed for correct, easy and safe installation.

Precast base manholes have been in use in the UK for well over 10 years. In 2011, the main British standard for concrete manholes, BS 5911-3, was revised to allow such manhole solutions to be Kitemarked and more widely specified in the UK. The new system reduces installation time and cost significantly: what used to take several hours, involving sub-contractors and wet trades on site, now takes approximately one hour and would not involve more than a handful of operatives.

BPDA has put together an operative short site pocket guide to explain the installation process: From trench excavation and ground preparation, to the installation of different manhole units and final backfilling. The guide was set up in an ‘endorsement fold’ format to ease use on site.

A PDF copy of the new pocket guide can be found at the BPDA downloads page, or to request a hard copy please complete an Online Request Form.

Posted by & filed under Design.

BPDA has just published a newly revised version of the precast concrete drainage sector’s “complete technical design guide”. Since our last issue of the guide, a number of standards, design methodologies and guidance documents were published. Moreover, feedback and enquiries we received over the last two years made us think about how to expand the content to address new areas. Here is a summary of the new sections we added to the precast drainage technical guide:

 

Chemical ground condition and the concrete exposure class

We added some information about the design life of precast drainage products and how a 100+ years can be achieved by using a DC-4 ‘Design Chemical’ exposure class. Our Technical Guide never used to refer to this in the past as we thought, at the time, that standards and guidance documents such as BS 8500, BRE’s Special Digest SD-1 and ‘Sewers for Adoption’ should offer sufficient guidance on this. However, as concrete drainage products with lower exposure classes exist in the UK market, we felt it is important to explain how the exposure classes and chemical ground conditions affect the intended working life of concrete products. This new information can be found in the Guide’s ‘Forward’.

 

Pipelines’ hydraulic design

More information was added to this section as European Standard EN 16933-2 now supersedes EN 752 on hydraulic design of drains and sewers. Terms associated with SuDS and oversizing for surface water drainage were also added.

 

Pipelines structural design

All pipeline design load tables have been amended to account for loadings aligned to the Eurocodes. The cover depths included in these tables have also been expanded to include covers as low as 0.6m, and as deep as 10m. The ‘light road’ loading category has also been removed.

 

Box Culverts design

A new section was included on the structural design of box culverts in accordance to Eurocodes, which superseded the now withdrawn BS 5400 on bridge design a few years ago. There is also a more detailed section on the hydraulic design of box culverts with examples and guidance adapted from CIRIA’s Culvert Design and Operation Guide.

 

Installation of precast drainage products

More information on innovative pipeline lifting and installation solutions (such as the pipe lifter) was added. A section on the installation of box culverts was added too.

 

Future revisions?

It is understood that BS 9295 is undergoing a major revision which will affect options associated with the structural design of concrete pipeline systems. We do not think it will invalidate the content of this new version, but additional options associated with design (e.g. limit state design), trench width and bedding factors will need to be introduced.

Posted by & filed under Design.

Precast concrete box culverts have traditionally been used as channels to divert watercourses where new construction creates a barrier to the natural pathway of flow. They are strong, durable and are available in a wide range of sizes. This makes box culverts extremely versatile and has resulted in them being used for other applications including: surface water storage, attenuation tanks, pedestrian subways, access shafts, service tunnels, sea outfalls and crossings beneath roads.

 

Unlike structures manufactured from other materials such as steel, precast concrete box culverts do not require additional protective treatments to prolong service life or improve performance in normal conditions of use. They do not rust and the smooth internal finish of the concrete ensures efficient flow characteristics.

 

The most common loads applied to the design of a box culvert are:

  • The unit’s self-weight
  • Where a culvert is buried, the weight of fill above the culvert
  • Where the culvert is installed under a road, the weight of road surface material (in addition to that of the fill)
  • Vertical live loads, such as those from vehicular wheel loads or pedestrian traffic
  • Horizontal live loads from vehicles braking or accelerating
  • Where buried, horizontal earth pressure loading applied to the culvert’s side walls
  • There will be a ground bearing pressure on the bottom of the box culvert
  • There might also be water pressure from inside, where the culvert is carrying a river for example, or from hydrostatic pressure from outside where the culvert is in a high water table of flooded ground

Members of the British Precast Drainage Association (BPDA) design and manufacture box culverts specifically to meet particular applications and defined loading conditions. Box culverts which are to carry highway or railway loading are designed to the standards and specifications as stipulated by the client. Box culverts can be specially made to higher loading requirements. Some box culverts can take LM1/LM2 bridge loadings at 0.25m of cover.

 

BPDA has an extensive library of useful information and guidance publications for designers, installers and asset owners. They are available to download free of charge from the BPDA’s website. The site also holds a comprehensive and searchable FAQ section and an online enquiry facility; go to www.precastdrainage.co.uk

Posted by & filed under Pipes & Manholes.

In the UK, high pressure water jetting is widely used to clean and mostly to clear blocked sewer pipelines – often as an emergency operation. The European Standard BS EN14654-1:2014 Management and control of operational activities in drain and sewer systems outside buildings, Part1: Cleaning, states that “Maximum safe working pressures to avoid damage will vary according to the material of the pipe, condition of the pipe and type of nozzle”. Risk of damage to the pipeline can be reduced if the jetting nozzle is kept moving and the direction of the jet does not focus on the wall of the pipe. Damage in this way may be avoidable, but the risk of damage increases where there is an obstruction within the pipeline and the jetting nozzle cannot pass through the blockage

 

In 2005 the WRc published the Second Edition of the Sewer Jetting Code of Practice. This document provides guidance on good working practice when using high pressure water jetting equipment. The code establishes a maximum jetting pressure for pipeline materials, varying from 1500psi for brick sewers up to 5000psi for concrete pipelines – see Table 1 below.

 

Table 1: Water jetting pressure maximum limits for different pipe materials from the Water Jetting Code of Practice.

 

Material Concrete Clay Plastic Brick
Max pressure

(PSI)

5000 5000 2600 1500

 

 

It is generally accepted that smaller diameter pipes are most effectively cleaned with pressure as the source of energy within the pipes. In theory, if the code is closely followed, it should prevent damage to the fabric of drains and sewers from occurring, but the huge difference in maximum jetting pressures from 5000psi for concrete down to 1500psi for brick, can potentially lead to confusion. It would make sense if a maximum jetting pressure was applied consistently nationwide at a value known to result in effective cleaning and clearing of blockages. Sadly, the required pressure to shift some stubborn blockages exceeds the resilience of some materials and damage to the sewer is more likely unless the operation is carried out to the textbook and with extreme care.

 

For larger diameter pipelines there is an argument that flushing the blockage through using a large volume of water at lower pressure is an alternative to high pressure jetting. However, to generate a high flow rate, access to a drinking water hydrant or a large capacity water tanker will be required. This can be a more expensive, time consuming, wasteful and less environmentally friendly solution than using lower volumes of water jetted at higher pressure.  It is important that users understand the differences in the resilience of different pipeline materials to high pressure water jetting and the implications of using different cleaning and blockage clearing methods.

 

BPDA has an extensive library of useful information and guidance publications for designers, installers and asset owners. They are available to download free of charge from the BPDA’s website. The site also holds a comprehensive and searchable FAQ section and an online enquiry facility; go to www.precastdrainage.co.uk

Sustainable Drainage

Posted by & filed under Pipes & Manholes.

It has been estimated by DEFRA that significantly less than 1% of public sewers are being renovated or replaced each year. If that rate of replacement is continued, it will take approximately 800 years before a new pipeline laid today can be expected to be replaced. No product carries a certificate to say that it will last 800 years. Most will claim a Design Life of between 50 and 125 years. This does not mean that a pipe product will cease to work efficiently at the end of this time because its service life may be very different.

Design life is usually the period over which an asset’s depreciation is calculated. Design life should not be confused with service life, which is the length of time a component can be expected to perform before its performance falls below the original design requirements without requiring renovation or replacement. A major advantage of precast concrete drainage systems is that they have a proven long service life, typically in excess of 120 years. This 120 year “reference service life” is a requirement for infrastructure design and asset management assessment standards such as PAS 2080 Carbon Management in Infrastructure and Series 1700 of the Specification for Highway Works in England (NG 1704).

In the UK some soils are more aggressive to pipes than others. As a safeguard precast concrete is manufactured using Design Chemical Class 4 concrete to achieve a working life of 100 years in soils with an Aggressive Chemical Environment for Concrete class AC-4, without the need for additional protective measures.

However, where a sewer, drain or other component within the system is liable to carry: a rising main discharge; septic sewage; untreated or corrosive trade effluents; and in situations without adequate ventilation, then additional protective measures should be considered to achieve a design life of, say, 120 years.

BPDA has an extensive library of useful information and guidance publications for designers, installers and asset owners. They are available to download free of charge from the BPDA’s website. The site also holds a comprehensive and searchable FAQ section and an online enquiry facility; go to www.precastdrainage.co.uk

concrete drainage

Posted by & filed under Pipes & Manholes.

Minimum cover depths for concrete pipes vary depending on the vehicular traffic loading likely to be sustained by the pipeline. There are various industry specifications and Standards that state values for minimum cover depths based on these loading conditions.

Most loading conditions used for design either relate to patterns of wheel loading generated from main road traffic or to pipelines installed within fields where only occasional trafficking, for example from agricultural equipment may be expected. A third loading scenario is also sometimes used which is based on light road trafficking, for example traffic within a residential area, although heavier vehicles may have access to these locations and the main road loading condition is often preferred for design.

Cover depths of less than the minimum values published in these documents should only be used with the appropriate authority’s permission.

Sewers for Adoption, for example, requires the minimum depth of cover to the crown of gravity pipes without protection to be as follows:

  • In domestic gardens and pathways where there is no possibility of vehicular access, 0.35 m;
  • 0.5 m for domestic driveways, parking areas and yards with height restrictions to prevent entry by vehicles with a gross vehicle weight in excess of 7.5 tonnes;
  • Domestic driveways, parking areas and narrow streets without footways (e.g., mews developments) with limited access for vehicles with a gross vehicle weight in excess of 7.5 tonnes, 0.9 m;
  • Agricultural land and public open space, 0.9 m;
  • Other highways and parking areas with unrestricted access to vehicles with a gross vehicle weight in excess of 7.5 tonnes, 1.2 m.

It is common practice that pipes laid under main roads to have at least 1.2m of cover to avoid conflict with other services. This is also true for the grass verges at the side of the road and for light roads, which may on occasion need to carry main road traffic.

However, minimum cover depths could be reduced (with the appropriate authority’s permission) if appropriately bedded concrete pipes are used according to transport research organisation TRL simplified tables of external loads on buried pipelines. It says the inherent strength of standard Strength Class 120 concrete pipes enables the cover depth to be reduced to a minimum depth of 0.6m beneath a highway when installed in conjunction with a full granular bedding surround, Bedding Class S.

For concrete pipes laid in fields the BPDA recommends a minimum cover of 0.6m should be provided to prevent damage from agricultural operations.

  • Where concrete pipes are required to be laid in fields at cover depths of less than 0.6m BS9295 Annex A, A16 gives recommendations for protection.
  • The preferred solution is to use a reinforced concrete slab installed over the pipeline which extends to provide at least 300mm bearing each side of the trench. A layer of compressible material placed directly over the pipeline aids in the prevention of the slab loading directly onto the pipeline should settlement occur. Another method of protection at shallow cover depth is to use a concrete surround to the pipeline, Bedding Class A.

For pipelines under construction, where plant has to cross a pipeline, consideration should be given to providing dedicated crossing points which may consist of heavy steel plates bridging the trench to transfer vehicle loads away from the pipeline or additional cover material placed over the pipeline.

BPDA has an extensive library of useful information and guidance publications for designers, installers and asset owners. They are available to download free of charge from the BPDA’s website. The site also holds a comprehensive and searchable FAQ section and an online enquiry facility; go to www.precastdrainage.co.uk

Joseph Bazalgette

Posted by & filed under SuDS.

Joseph Bazalgette

Sir Joseph Bazalgette (1819 – 1891) was the chief engineer of London’s Metropolitan Board of Works. Born in London on the 28th March 1819, Bazalgette began his career as a railway engineer. During this role he gained considerable experience in land drainage and reclamation.

When the London Metropolitan Board of Works was established in 1856, Joseph Bazalgette was elected the first and only chief engineer. The board of works was the first organisation provided to supervise public works all over the city.

 

Mid-19th Century London

In the mid-19th century, London was struck by a cholera epidemic, which killed over 10,000 people. At the time it was thought that the disease was caused by foul air that filled the streets of London. Along with the frequent occurrence of cholera outbreaks, a hot summer brought with it the ‘Great Stink’, which overwhelmed the city.

 

The Solution

The condition of Mid-19th Century London was an incentive for parliament to give legislation to the Board of Works. The Legislation allowed them to begin improvements on the sewers and streets. It was expected that the new sewer system would eliminate the great stink, which would reduce the outbreaks of cholera.

Bazalgette’s solution was to create a sewer network for central London that extended 82 miles. The underground network consisted of main sewers to intercept sewage outflows as well as street sewers. These new sewers would replace the raw sewage flowing through the thoroughfares and streets of London.

Although the impression was that foul air caused cholera was wrong, it didn’t mean that the sewer system was set up to fail. Instead, the sewers eliminated the disease by removing the contamination carried in the water supply.

The system was opened by the Prince of Wales in 1865 and was fully completed 10 years later.

 

Sustainable drainage systems today

Concrete drainage systems have been the material of choice for over a century. They can offer the most environmentally friendly and competitive installed option today.  Sir Joseph Bazalgette’s sewers were the first sustainable drainage system to be built. Overall the system required 670,000m³ of concrete and it is still in use now. The inherent strength and durability of precast concrete drainage can help protect the system during construction and throughout its long lifetime of operation.

 

To find out more about sustainable drainage systems, visit the British Precast Drainage Association (BPDA) website: precastdrainage.co.uk.

concrete manhole

Posted by & filed under Pipes & Manholes.

When inspection or maintenance of buried wastewater drainage systems is required a manhole is specified. They are most often built at the point where one pipeline connects to another, where a pipeline changes size, direction or gradient and at a spacing that enables equipment to be used effectively. Here we detail some advice for specifying manhole systems.

  1. Consider a precast concrete solution

It can take up to 40 hours to construct a manhole using traditional techniques. Construction in-situ like this can also involve working in wet and difficult confined spaces. This is where off-site manufactured precast concrete manhole base systems offer some big advantages. They are safer, quicker and cheaper to install. In addition, they are watertight, of consistently high quality, create less waste on site and have a lower carbon footprint.

  1. Each manhole should be correctly constructed for each specific location

Due to the precast concrete base being factory manufactured to a specific configuration, when the components arrive on site they can quickly, simply and safely be placed in position. This negates the need for lengthy site-based operations.

The BPDA estimate that a contractor using a precast manhole base system could save up to 50 per cent on installation time. Construction costs can also be reduced by 15 to 30 per cent, particularly when manholes are installed without a concrete or granular surround.

  1. Manholes should be compliant with appropriate Standards

The technical requirements for reinforced and unreinforced manholes are described in the European Standard BS EN1917:2002 Concrete manholes and inspection chambers, unreinforced, steel fibre and reinforced and the British Standard BS5911 Part3: 2010+A1:2014 Specification for unreinforced and reinforced soakaways.

This British Standard is referenced in Approved Document H of the Building Regulations, which deals with drainage and waste disposal. This document details the rules which construction of drainage and waste disposal systems must comply.

The BS is also referenced in Sewers for Adoption and the partner water utility publications throughout the UK. These require developers and installers to build drainage systems to a minimum standard and quality for adoption by the relevant water company.

The advantage of using a BS-compliant and Kitemarked precast concrete solution supplied by a member of the BPDA is that users can be sure that the product will comply with all necessary technical requirements.

  1. Will you choose a round or square manhole chamber?

One of the biggest advantages of using a concrete manhole is that concrete is very strong in compression. A circular precast concrete manhole exploits this trait. Its circular shape ensures that ground and hydrostatic pressure is evenly distributed around the circular shaft. This enables circular precast manholes to be installed to a far greater depth than precast manholes of ‘equivalent strength’ with flat sides and corners.

  1. Consider the need for access

Whatever the shape of the manhole, if it includes a ladder or step irons for access then users need to be aware that the Health and Safety Executive’s confined spaces regulations. These recommend a 900mm clearance between the ladder/steps and the back of the shaft.

Ladders and steps usually protrude by at least 100mm, so for compliance users will need to consider a circular manhole with a diameter no less than 1,050mm.

Precast manhole systems offer great versatility and are designed to accommodate all standard pipe materials and sizes. Existing precast manholes can even be retrofitted with new connections from future development without the need to replace the entire manhole.

  1. Minimise the need for granular backfill

The robust design and wide chamber walls of the factory manufactured precast concrete manhole base system means that the use of granular or concrete backfill can be eliminated, unless it is specifically required by the client. The excavation can be backfilled sooner using the soil that was excavated, thus making the installation safer and faster whilst simultaneously reducing the cost of backfill and disposal of the excavated material.

  1. Save on carbon emissions

Factory manufactured precast concrete manhole base systems also offer a significant saving in embodied carbon for the installation. The BPDA estimates that carbon savings could be as much as 43 per cent per manhole compared with traditional in-situ construction. A notable part of this carbon saving is the avoidance of concrete or granular backfill and the use of excavated soil.

The BPDA estimates that 15,000 tonnes of CO2 equivalent of embodied carbon would be saved annually if all manholes manufactured by the Association’s members changed from in-situ construction to the circular precast base system.

  1. Consider durability

The precast manhole base system is manufactured under factory conditions by quality assured processes. This ensures the finish and quality is to a higher and to a more consistent standard than could have been achieved on site. The combination of durable precast concrete and quality controlled offsite manufacturing process will ensure that a precast manhole system has a long service life.

For more information about concrete manholes visit the British Precast Drainage Association website.