• 11th Jan 2010
  • 17 comments

CPD articles

CPD: Don't let excess water be a drain on your site

With flooding predicted to become a growing problem, the government is acting to reduce the impact of surface water runoff, explains Colin Nessfield

More than two thirds of the 57,000 homes affected by the 2007 summer floods were flooded not by swollen rivers, but by surface water runoff or overloaded drainage systems. The government estimates that 80,000 properties are currently at very high risk from surface water flooding, which causes, on average, £270m of damage every year.

The continuing growth in urbanisation, combined with more extreme weather events linked to climate change, will only exacerbate the problem. Clearly, a sustainable approach to all surface drainage is needed, which is why draft legislation currently before Parliament will, in effect, make sustainable drainage systems (SUDS) mandatory by 2011.

As a versatile SUDS technique, concrete block permeable paving (CBPP) is set to become the norm for hard surfaces on all types of development, whether that be new-build or refurbishment. Construction managers therefore need to understand the principles behind SUDS, as well as the specific requirements for permeable paving.

SUDS law 

SUDS is a design philosophy which generally uses a combination of techniques in a sequence, known as a management train. SUDS manages surface water by attenuation and filtration with the aim of replicating, as closely as possible, the natural drainage from a site before development.

There are three established pillars of

SUDS:

Quantity – management of rainwater and avoidance of flooding

Quality – removal and treatment of diffuse pollution from runoff amenity – improvement of the external environment.

Amenity – improvement of the external environment.

Government planning policy guidance already requires SUDS to be used on all developments wherever possible. This requirement is supported by the Building Regulations as well as other guidance, including the Code for Sustainable Homes and the Department for Transport’s manual for Streets. But there has been some confusion over where responsibility lies and where SUDS should be adopted, limiting take-up by developers: an issue the government is now determined to address with the forthcoming Flood and Water management Bill.

Assuming it is adopted as drafted, the Bill will apply to surface water drainage from all new buildings and roads in England and Wales (with similar legislation also likely in Scotland). it takes on board many of the proposals of the Pitt review into the 2007 flooding events.

In general terms, the Bill proposes SUDS on all new developments and roads “where practicable”. current arrangements for approval, construction and maintenance of conventional piped drainage will be replaced with similar procedures for SuDS. unitary and county authorities will be handed new responsibilities as “SUDS approving bodies” (SaBs) and they will be required to “adopt” all SUDS schemes, (with the exception of SUDS schemes on properties that are currently exempted from adoption).

New national standards for the construction and maintenance of SUDS will be developed by 2011. Developers will then be required to demonstrate compliance with these standards and pay a financial bond to SaBs, as they do under the current regime for conventional piped drainage. The right to connect to public surface water sewers will no longer be automatic, but conditional on meeting the national standards. this measure anticipates that the standards will require on-site treatment and temporary storage with SUDS before water can pass off-site into sewers.

Concrete block permeable paving

Many SUDS techniques are based on ‘soft’ landscaping features such as swales or ponds, but these are often not suited to urban environments. in contrast, CBPP is a versatile SUDS technique based on a simple concept, whereby a pavement surface also acts as a drainage system. it has been in use worldwide for decades. CBPP allows water to pass through the surface, between each block, and into the underlying permeable sub-base where it is stored and released slowly, either into the ground, to the next SUDS management stage or to a drainage system. at the same time, many pollutants are substantially removed and treated within the cBPP itself. unlike conventional road constructions, the permeable sub-base aggregate is specifically designed to accommodate water and comprises coarse graded aggregate. there are also a number of systems on the market that replace the permeable sub-base with lattice plastic cellular units that are connected to form a raft structure.

Typically, CBPP offers the same performance as conventional precast concrete paving, including slip and skid resistance, durability and strength. Various shapes, styles, finishes and colours are available allowing real design freedom. the difference with cBPP is that enlarged joints filled with an angular aggregate, rather than sand, allow water to continue passing through over the long-term. in addition to new-build projects, cBPP can also be retrofitted into existing schemes.

CBPP systems

Systems A, B and C (see box opposite) were initially identified by interpave, the trade association representing the major precast concrete paving manufacturers in the UK, and these designations have now been adopted in British Standards, The SUDS Manual (ciria 2007) and elsewhere. there is no difference between the surface appearance of the different systems, but each has unique characteristics that make it suitable for particular site conditions.

CBPP practicalities

Permeable pavements must be designed both to support traffic and to manage surface water. therefore there are two sets of calculations required, and the final thickness of the permeable sub-base will be based on the greater of the two calculations.

Unlike impermeable paving, the surface of CBPP can be completely flat, as water passes straight into the gaps between blocks, avoiding ponding. This means that CBPP surfaces are independent of cross-falls, channels, gulleys and other impediments. the maximum gradient of the pavement surface itself should be about 5% (1 in 20). But if the ground itself slopes more steeply, various techniques can be used in the sub-surface structure to accommodate this. experience has shown that thoughtful handling of services is key to the long term success of cBPP projects. it is not necessary to design all paved areas as permeable: CBPP can cope with runoff from nearby impermeable surfaces, including roofs. With careful layout, services and utilities can be located within conventional impermeable areas, service corridors or verges, negating the need to excavate below or risk of disturbing the CBPP.

All three systems described above for infiltrating and non-infiltrating CBPP may be used close to buildings, as they allow dispersed infiltration similar to natural vegetation. However, if a concentrated outflow (such as a roof drainage outlet) is used within the cBPP, this should be placed at a sufficient distance to ensure the stability of the building is not affected.

Construction of CBPP

Comprehensive guidance on the specification and construction of complete permeable pavements is available from interpave. the concrete block layer should be built in accordance with BS 7533: Part 3: 2005, and mechanised block-laying techniques can be used for greater efficiency. it is important to understand that the coarse-graded aggregate permeable sub-base materials differ from those typically used in conventional impermeable pavements. as they lack fine materials, there is potential for segregation during the transportation and construction process. if this occurs remedial action must be taken. the nature and grading of the permeable sub-base will vary between different sources and it is often best to undertake site trials to determine the appropriate construction methodology.

In the case of more heavily-trafficked permeable pavements, hydraulically bound coarse graded aggregate is included to strengthen and stiffen the pavement. the permeable sub-base should be laid in 100-150mm layers and compacted to ensure that the maximum density is achieved for the particular material type and grading within a tolerance of +20mm to –15mm of the design. crushing the individual particles or reducing the void ratio below the design value must be avoided. the materials are relatively selfcompacting and heavy compaction is not usually required.

Recycled material can be used where a source is conveniently available but care should be taken that this is of consistent quality, has the appropriate grading and is free of unacceptable materials, such as organic matter or steel scrap.

Protecting CBPP

It is essential to prevent contaminants such as soil, mud and cement from entering any part of the sub-base or the paving surface, both during and after construction, so that the pavement remains permeable throughout its design life. Simple measures such as keeping muddy construction equipment away from the area, installing silt fences, staged excavation and temporary drainage swales which divert run-off away from the area should be considered.

Permeable pavement construction materials must be kept clean during the construction phase, which can be a challenge when roads need to be installed early for site access. one solution is to use a protective dense bitumen macadam (DBM) layer during site works, which is subsequently left in place and then punched through to allow drainage just before completion.

Colin Nessfield is secretary of Interpave, which represents the major precast concrete paving manufacturers in the UK, working closely with organisations such as Defra, the Environment Agency, the Scottish Environmental Protection Agency, CIRIA and SUDSnet in driving forward sustainable drainage solutions. Its manufacturing members continue to develop innovative concrete block permeable paving products and systems.

Guidance on all aspects of permeable paving and precast concrete paving can be downloaded free from www.paving.org.uk

Formpave's Aquaflow paving used in a residential setting

See permeable paving options here
See permeable paving options here

CPD TEST PAPER

Sustainable drainage and permeable paving

Completed CPD questionnaires can be sent by fax to 020 7490 4957, or by post to: CM CPD, Atom Publishing, 45/47 Clerkenwell Green, London EC1R 0EB. Your questionnaire for the January edition should arrive by 1 April. 

DOWNLOAD THE CPD TEST PAPER HERE
DOWNLOAD THE CPD TEST PAPER HERE
  • 20th Jan 2010, at 04:15 PM
  • Patrick Lynch

Why can't the CPD be done online anymore? Seems to be a step backwards.

  • 22nd Jan 2010, at 12:02 PM
  • Neal Appleby

What was wrong with doing CPD's online?

  • 22nd Jan 2010, at 01:50 PM
  • ELAINE KNUTT, EDITOR, CONSTRUCTION MANAGER

We've had a few readers asking about why it's no longer possible to answer an online CPD questionnaire. Unfortunately, we can't offer this facility at the moment, but we will keep the situation under review, depending on the volume of CPD responses by post and fax.

  • 26th Jan 2010, at 02:38 PM
  • Ivor Brown

Construction Manager is supposed to be a magazine for the times we live in. To stop taking CPD responses online is a move back into last century.

  • 26th Jan 2010, at 08:27 PM
  • Steven Baldwinson

Why can't we do CPD online anymore, it was so much easier.

What's a fax?

  • 27th Jan 2010, at 09:39 AM
  • Andy White

What century are we in? Yes thought so, the 21st century.

The reason I ask this, is it seems so stupid to stop offering the CPD online, as previously stated it was so much easier and was also environmentally friendly.

Please get it back online soon, as I will not waste paper, therefore I will be slipping behind with my own CPD.

Come on CIOB, get in the 21st century!

  • 28th Jan 2010, at 12:20 PM
  • H. Day

Please get back on line and in tune with your readers.

  • 29th Jan 2010, at 10:42 AM
  • B E Lister

In common with many of your other readers I will not be participating in CPD if it is not online.

  • 29th Jan 2010, at 10:53 PM
  • B. Potts

Great new website......shame CM are still asking their users to use fax machines! One step forward and two back!

  • 31st Jan 2010, at 03:17 PM
  • Paul McCann

Whilst I agree with the comments of the other respondents I will continue completing the CPD test papers as they are a easy way to attain some CPD hours, and in addition the articles are in the main useful and informative. However as I work from home mostly and I don’t have a fax I have to go to the trouble of completing the test paper, scanning in the document and then sending it via my laptop, in this case the old ways are sometimes the best!

  • 31st Jan 2010, at 03:52 PM
  • B Rose

The CIOB is failing big time by not offering online submission, no wonder many of our colleagues don't join as they consider it out of date. Please rejoin the 21st C not the 18th!

  • 5th Feb 2010, at 06:13 AM
  • Hani El-Falahi

Apart from reinstating the online submission system, I hope CM would consider updating members' CPD records automatically on the CIOB website each time correct answers were given. The old system of sending tokens by post was never accurate, many monthly CM CPDs were filled in with no tokens rec'd for them.

thanks

  • 8th Feb 2010, at 09:57 AM
  • Mark Stracey

This is definately a backward step; you have got to reinstate the online CPD. I also agree with the issue of posting out tokens as I still have not received several from last year.

  • 26th Feb 2010, at 10:53 AM
  • John Gaddass

I think not having the CPD online just re-enforces the view that our industry is behind the times. As CM likes to think it is at the leading edge this is embarrassing.

  • 14th Apr 2010, at 09:36 AM
  • Roy Winder

I agree with the comments above the online CPD is much more efficient and cost effective.

I have been incapacited due to a major operation which I am presently recovering from so I trust when I submit my paper for the above this will be accepted?

  • 25th May 2010, at 02:29 PM
  • Tom

Hi, Water affects different materials in a variety of ways. Concrete and brick are very resistant to water damage, while timber can be resistant to short-term wetting but decays if it remains wet for a long period. Dense materials retain little water but dry slowly, whereas porous materials can retain large amounts of water, but can often be dried quickly. Thanks, Tom

  • 16th Oct 2010, at 07:25 PM
  • Nicholas Paul Brookes

As a retired member I agree with many others that using the post or fax is a retrograde step. Practically none of my business is done by fax now. In fact I haven't even bought a new print cartridge for it this year it gets used so little. What was the problem with on-line??

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