SuDS Attenuation Tanks & CIRIA C680 Compliance, StormGrid Geocellular System

The geocellular attenuation tank that ships with the third-party CIRIA C680 load-test data your LLFA or SAB submission actually asks for, not a marketing badge.

A SuDS attenuation tank is a modular, geocellular underground tank that holds back storm rainfall and releases it at a controlled rate into the drainage network, cutting flood risk downstream while keeping a development within its permitted discharge. The compliance question that decides your design isn’t “how strong is the crate” — it’s whether the tank’s structural evidence, sizing and maintenance plan will pass drainage approval and manage flooding as the planners require.

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StormGrid Geocellular System
95% Void ratio
600 kN/m² CIRIA C680-method tested†
C737 / EN 17152-1 Current design basis
50-yr Design life*
SG-E20–H60 200–600 kN/m² range
330 m³ Per 40HQ container
Geocellular attenuation tank layout

Why SuDS Compliance, Not Tank Strength, Decides Your Attenuation Design

An attenuation tank is one component inside a wider Sustainable Drainage System, and UK planning treats surface water as a strategy, not a product purchase. Building Regulations Approved Document H sets a drainage hierarchy, infiltration first, watercourse next, sewer last, and an attenuation tank is the answer when infiltration isn’t viable on clay, high-water-table or contaminated ground (Approved Document H).

Take a constrained urban plot on impermeable clay with a 1-in-100-year storm to hold and a capped discharge rate: a lined geocellular tank sized to that discharge is often the only compliant way to unlock the development, because a soakaway won’t infiltrate and there’s no surface room for a pond. That’s a failed planning condition avoided, the difference between a scheme that proceeds and one that stalls at drainage sign-off.

On constrained sites where space is limited, a below-ground tank is frequently the only drainage strategy that fits, which is why modern SuDS design leans on geocellular storage for surface water management. The decision belongs in the drainage design early, not bolted on once the layout is fixed.

Attenuation vs Soakaway, the same crate, a different wrap

The single most common confusion in UK drainage is “are attenuation and soakaway crates the same thing?” They use the same geocellular cell. Wrap it in permeable geotextile and it infiltrates as a soakaway; line it with an impermeable geomembrane and add a flow-control chamber and it becomes an attenuation tank that discharges at a set rate, or a sealed rainwater harvesting store for non-potable reuse. StormGrid is the same module in all three roles, the wrap and outlet change, not the box. For a trafficked infiltration build instead, see our heavy-duty D400 soakaway crate.

Here’s the honest limit an engineer needs stated up front: a geocellular attenuation tank controls water quantity. It doesn’t, on its own, deliver the water-quality treatment, biodiversity or amenity benefits that surface SuDS features provide, which is why it belongs within a treatment train, not instead of one (susdrain).

Where the tank sits in a SuDS treatment train

Sustainable urban drainage systems manage a storm event as close to source as possible, green roofs, swales and ponds take surface water management and stormwater runoff first, and a high-void-ratio geocellular tank provides stormwater storage where those surface features can’t fit. In that SuDS strategy the tank’s role is peak-flow control: it holds excess stormwater and a flow control device meters the permitted flow rate, cutting the risk of flooding downstream during heavy rainfall. Local authorities acting as the LLFA in England, or a SAB in Wales, sign off that the stormwater management approach holds the site to its discharge limit.

That reframes the buying decision, and it carries real risk. A drainage strategy that ignores the hierarchy, or specifies a tank it can’t evidence, gets bounced at LLFA or SAB review, and a resubmission is one of the most expensive delays on a development. A tank that clears approval, sizes correctly for your discharge rate and stays maintainable is worth more than a headline tonne rating, because the evidence for those three things is exactly what most product pages leave out.

The SuDS Compliance Decoder, CIRIA C753, C737 & C680 Explained Honestly

Most “CIRIA C680” pages quietly skip the fact that the standard was superseded, and an engineer who cites a withdrawn 2008 document in a submission invites a query that delays approval. Getting the stack right is what turns a drainage submission from a query into an approval, because an LLFA reviewer checks that the structural design names the current C737 method. Here’s the honest map of which document does what in current UK practice.

Reference
Role in a compliant attenuation tank
Status
CIRIA C753 — The SuDS Manual (2015)
Principal design guide: planning, sizing, treatment train, operation & maintenance
Current
CIRIA C737 (2016/18)
Structural & geotechnical design of modular geocellular systems — strength, deformation, installation
Current — supersedes C680
CIRIA C680 (2008)
Earlier structural guidance for geocellular tanks; the vertical-load test method our third-party report uses
Superseded by C737
BS EN 17152-1
Product standard for PP / PVC-U stormwater boxes (infiltration, attenuation, storage)
Current
Eurocode EN 1990 / EN 1991-2 / EN 1997-1
Structural basis, traffic loads, geotechnical design method
Current
Building Regs Approved Document H
Drainage hierarchy; soakaway siting distances
Current
Flood & Water Management Act 2010, Sch. 3
Statutory SuDS approval (SAB). Live in Wales since 2019; England still planning-led (LLFA / NPPF)
Wales live / England pending

So when a supplier stamps a crate “CIRIA C680 compliant”, read it carefully. C680 is a 2008 structural document that C737 has replaced; the honest statement is that a unit has been vertical-load tested to the CIRIA C680-2008 method, with structural design carried out to the current C737 / EN 17152-1 / Eurocode framework (CIRIA guidance via susdrain).

England vs Wales, know which approval route you are in

In Wales, Schedule 3 of the Flood and Water Management Act 2010 has made SuDS approval by a SuDS Approving Body (SAB) mandatory since January 2019. In England, Schedule 3 isn’t yet in force, surface water drainage is handled through planning policy (NPPF) and Lead Local Flood Authority (LLFA) consultation, with Section 104 adoption by a water company for qualifying assets (Flood and Water Management Act 2010).

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Our CIRIA C680 Test Evidence, Intertek ISO 17025 Load Data

The UK’s most-read attenuation guides tell engineers the first question to ask any manufacturer is: “can you produce the CIRIA C680 test certificate?” — and warn that a supplier who can’t is a red flag. Most can’t. StormGrid can, so this is the certificate itself, not a claim about it.

An ISO 17025-accredited Intertek laboratory vertical-load tested a StormGrid unit to the CIRIA C680-2008 method (report 190702149GZU-001, issued 12 Aug 2024). The tested sample was a single 1000×500×500 mm module, loaded on the full surface via a rigid platen, including a 40 °C conditioned run that probes thermoplastic creep, the very failure mode long-term geocellular design worries about.

StormGrid is the geocellular tank documented to that method: a 600 kN/m² front-surface result to the CIRIA C680-2008 test, a 95% void ratio, and structural design to CIRIA C737, figures an engineer can drop into a submission and cite, not a claim to take on trust.

StormGrid Geocellular Tank Engineering Detail

CIRIA C680 Compliance Evidence Table

Test condition Face — front surface Face — section surface Verdict
23 °C ambient (20 mm/min) 600 kN/m²
(300 kN, no break > rig limit)		 186 kN/m² PASS
40 °C conditioned (10 mm/min) 600 kN/m² 179 kN/m² PASS

Method: CIRIA C680-2008 vertical load test, Intertek report 190702149GZU-001 (ISO 17025). The 300 kN front-surface load did not break the sample — it exceeded the test rig’s 300 kN cell, so the true ultimate is higher than the figure shown. Evidence is for the tested sample and method; it is not a substitute for site-specific structural design to C737.

Two things make this data unusually useful. The front-surface result held at 600 kN/m² even after 40 °C conditioning, so heat didn’t degrade the load path in the test, and because the rig topped out before the sample failed, the 600 kN/m² figure is a floor, not a ceiling. Unlike most suppliers, who quote a tonne rating with no report behind it, StormGrid hands the engineer the Intertek number and the load curves to build from.

“When a drainage engineer asks us for evidence, we don’t send a brochure line, we send the Intertek report number and let them read the load curves. The C680-2008 method is superseded for design by C737, and we say so on the certificate; the test still tells you exactly how the unit behaved under a full-surface load at 23 and 40 degrees.”

— Storm Manage Engineering Team

What the report gives your submission

On a car-park or HGV scheme rated to SG-H60 (600 kN/m²), the drainage engineer drops the Intertek report number and the 23 and 40 °C load curves straight into the structural appendix, the evidence an LLFA or SAB reviewer wants, sourced not asserted. That’s the difference between a first-time approval and a query that costs a fortnight, on a buried asset nobody can re-inspect once it’s backfilled.

Testing conducted by Intertek to ISO 17025 (ISO/IEC 17025). The report evidences vertical-load performance of the tested sample; whole-tank structural adequacy for a specific project is established by design to CIRIA C737 and the Eurocodes.

StormGrid Load-Class Range, Choose by Duty, Not by a Headline Number

The tested sample above is one unit; the product range is selected by application. StormGrid is graded by design load class, with 400 mm and 500 mm heights as configuration options within each grade, so you fix the site duty first, then the depth. It’s part of the wider modular attenuation tank range.

The figures below are the design nominal load classes from the StormGrid specification, distinct from the single tested sample, heavier grades use 100% virgin polypropylene, lighter grades recycled PP, which is standard practice and acceptable where structural performance is met.

Grade
Nominal load
Metric
Height options
Material
Typical duty
SG-E20 Eco Light
20 t/m²
200 kN/m²
H400 / H500
Recycled PP
Landscape, non-trafficked
SG-L30 Light Standard
30 t/m²
300 kN/m²
H400 / H500
Recycled PP
Pedestrian, low-load public space
SG-M40 Medium Duty
40 t/m²
400 kN/m²
H400 / H500
Recycled PP
Commercial sites, light parking
SG-H60 Heavy Duty
60 t/m²
600 kN/m²
H400 / H500
100% Virgin PP
Heavy parking, roads, industrial
SG-Custom
Project-specific
By design
Custom
By design
Special load / OEM-ODM

Storage per module: H400 = 190 L, H500 = 237.5 L. Load classes are design values from the product specification; the required cover depth for any grade is engineered to the vehicle load and ground conditions, not read from this table.

Why the buried number differs from the surface number

A crate’s design load class describes the buried unit; a car park or road surface is separately rated for its cover/grating. Storm Manage states them as two figures, the StormGrid grade and the surface class your civils drawing needs, so a “40 t/m²” grade is never confused with a vehicle-mass rating. The grade sets the crate; the cover build-up and surround carry the wheels, with the depth designed to the imposed loading in the national SuDS design standards (gov.uk, National Standards for SuDS).

How to Compare Geocellular Attenuation Tanks, Ask for the Test Data

Comparing crates on a headline tonne figure is how buyers get burned, a bigger number can hide a weaker unit tested on a different basis, and the crate is buried before anyone finds out. We won’t print invented load numbers for Wavin, Polypipe or ACO, reputable brands whose full test data is theirs to publish, not ours to guess. The one dimension worth comparing on is verifiable: does the manufacturer publish a third-party structural test, and does it name the method?

Geocellular Attenuation Tank Test Data Comparison
What to check Why it decides approval StormGrid
Published third-party load test Approving bodies want evidence, not a datasheet claim Intertek ISO 17025, report no. cited
Test method named A number with no method is unbuildable CIRIA C680-2008 vertical load
Current design framework stated C680 alone is superseded Designed to C737 / EN 17152-1
Void ratio Drives excavation volume 95%
Per-grade load classes Match crate to duty, not to a headline 200 / 300 / 400 / 600 kN/m²
BBA certificate A recognised UK third-party mark some LLFAs prefer Not held — evidence is Intertek + CE (EN 1852) + SGS‡

Get this comparison wrong and it is expensive: a crate that fails a structural check after it has been specified can cost weeks of programme and thousands of pounds in redesign on a car park or HGV scheme, long after the order shipped. Note the last row is deliberately honest, unlike a merchant listing that reprints a cover-class label, StormGrid ties every figure to a named test. StormGrid does not hold a BBA certificate; a buyer who requires BBA specifically should know that up front, and weigh our third-party Intertek load report, CE marking to EN 1852 and SGS compression testing against it (ISO 9001 factory systems apply).

The four questions that separate evidence from marketing

Whatever you specify, ask every manufacturer the same four: (1) can you send the third-party test report and its number? (2) which method was it tested to? (3) which current standard is the structural design to? (4) what are the per-grade load classes, not a single headline tonne? A supplier who answers all four without hesitation is one you can put in front of an LLFA.

Building a spec comparison for approval?

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Designing a Compliant Attenuation Tank, Sizing, Cover, Flow Control & Maintenance

A load report proves the unit; it doesn’t prove the scheme. Passing drainage approval means the tank is sized, sited and maintained to the design standards, and this is where the honest engineering lives. For the full build-up sequence, see our attenuation tank design and installation guide.

Sizing is a calculation, not a rule of thumb

You’ll see quick rules, for example, on non-clay ground one cubic metre (1,000 litres) of crate volume roughly serves 50 m² of roof or road area. Treat that as a first-pass sanity check only for the volume of water you must store: real sizing runs from a percolation test, greenfield runoff rates and a hydraulic calculation with a climate-change allowance, exactly as a UK civil-engineering forum thread put it — “ask a good geotechnical engineer to do infiltration testing and check the water table.”

Good drainage design models the peak flows from your design storm event against local rainfall patterns and ground conditions, then sets the storage volume and the release rate together. Attenuation systems that ignore that interaction either flood in the storm they were built for or waste capital on storage nobody needs.

Attenuation Tank Siting and Structure

Siting rules from Approved Document H

  • 01
    Soakaways kept a minimum distance from buildings and boundaries; water table must not reach the base of the unit at any time of year
  • 02
    Impermeable clay or high water table → infiltration ruled out → lined attenuation with controlled discharge
  • 03
    A flow control device (orifice plate or vortex regulator) meters outflow to the permitted rate into the watercourse or drainage network
  • 04
    Geotextile wrap for infiltration; geomembrane liner for attenuation and rainwater reuse
  • 05
    Cover depth follows the same logic as load class: it’s engineered to the grade and the wheel loads, not fixed by any single number. Pedestrian areas need less; parking and HGV routes need materially more, confirmed by the C737 design method for your crate and vehicle.

The honest bit: design life is not the same as service life

A 50-year design life is a specification, not a guarantee, as one UK trade feature put it, without a complete design-to-maintenance thread “the design life may be wishful thinking rather than an assured outcome.” What actually shortens a tank’s service life is rarely crate strength.

Where geocellular tanks really fail, and how to prevent it

CIRIA C737 records that geocellular failures are relatively rare, and that most trace to poor installation, temporary works or misread ground and groundwater, very few to inadequate long-term strength. The practical risk is siltation: without upstream silt capture, storage capacity drops over time as sediment fills the void, and CIRIA C753 (clause 21.5.3) advises sizing in additional storage volume for a hard-to-clean crate tank to allow for that loss. That’s why StormGrid schemes pair the tank with an inspection well base and a maintainable inlet, the storage is only as durable as its silt management (susdrain).

Geocellular structure is an actively engineered field, see, for example, published stormwater-crate patents on Google Patents. StormGrid uses a vertical-cell orientation with side plates, top covers and interlocking clips; the structural rating always traces to the test report, not the geometry alone.

Delivered Attenuation Projects, Compliance in the Field

Fixed-geometry attenuation punishes an awkward site. On rock-shallow or irregular ground a one-piece concrete or GRP tank forces deep excavation or wasted storage, and the fix is a late redesign that can add weeks to a programme and thousands in abortive groundworks, the worst kind of cost, because it lands after the dig has started and the flooding risk is already live.

StormGrid sidesteps that. On the Palm Desert golf-course scheme in California, a 99,000 ft³ (≈2,800 m³) storage duty fit a shallow, wide modular layout the site’s rock depth ruled out for a concrete tank; in Brampton and Calgary, Canada, large underground modular banks served community and highway-adjacent development where the footprint was tight and the discharge rate capped.

Unlike a fixed-geometry structure, the modular cell holds its 95% void ratio while bending around services and boundaries, which is how these overseas SuDS and stormwater projects, across Europe, North America, the Middle East and Australia, hit their storage duty without a bespoke design. Each was sized to the permitted discharge rate the drainage authority set for the site, the way UK flood-risk planning requires (gov.uk, flood risk and coastal change).

StormGrid modular stormwater attenuation system configuration

Palm Desert golf course, California

a 99,000 ft³ (≈2,800 m³) modular stormwater tank, sized around the site’s storage duty rather than a standard footprint

Brampton (Hwy 50) & Calgary, Canada

large underground modular attenuation for community and highway-adjacent development

South Al-Mutlaa, Kuwait

residential geocellular tank system under a Vision-2030-era stormwater scheme

Fontana & Murrieta, California

distributed and adjustable modular tanks fitted to constrained residential plots

Cost / TCO structure (Silver, indicative market ranges)

£200–350 / m³ installed is a commonly cited indicative UK range for geocellular storage, figures vary widely by project scale, ground and year, of which crate hardware is a minority; excavation, membrane, surround and labour dominate.

The efficiency lever is the void ratio: at 95%, a geocellular tank needs roughly 60% less excavation than a 30–35% void aggregate soakaway for the same storage, the saving lands in dig, haulage and disposal, not just the crate line.

Have a storage volume and site to price?

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StormGrid Procurement, Container Loading and Delivery Site

Procurement, Lead Time, Container Loading & OEM / ODM

Buying StormGrid manufacturer-direct changes the hardware line, not the groundworks, so plan the whole cost, and the logistics that come with an export supply chain.

  • MOQ
    from a single module up to 100,000+ m³ of storage
  • Container loading
    a 40HQ carries up to 330 m³ of installed storage volume with optimised stacking
  • Lead time
    typically 15–30 days after order confirmation
  • Terms
    FOB Shenzhen / Yantian / Shekou preferred; FOB Qingdao / Ningbo by arrangement
  • OEM / ODM
    logo, colour, load class and mould development for distributors and channel partners

Importing attenuation crates, the paperwork checklist

The genuine trade concern with overseas supply is verifiability, unknown batches, inconsistent quality, shipping slippage. Counter it with documents, not price: ask for the Intertek vertical-load report tied to the batch, ISO 9001 / 14001 system certificates, CE marking to EN 1852, a stamped module layout, and a written lead time in the contract. A credible manufacturer produces all five. For unit pricing on your volume, request a project quotation.

Cost and lead-time figures are indicative; request a costed quotation based on your storage volume, load class and delivery terms. FOB is an Incoterms cost-and-risk boundary, your delivered cost depends on freight, insurance and handling to site, which HMRC treats as part of the customs value (gov.uk, delivery costs in the customs value).

ENGINEERING & COMPLIANCE TOOLS

TOOL 01

Stormwater Attenuation Tank Calculator

LAUNCH CALCULATOR
TOOL 02

CIRIA C680 / SuDS Compliance Evidence Checklist

ACCESS CHECKLIST
TOOL 03

StormGrid Load-Class Selector

OPEN SELECTOR

SuDS Attenuation Tank & CIRIA C680 FAQ

Read that phrase carefully. CIRIA C680 (2008) was superseded by CIRIA C737 (2016) for the structural design of modular geocellular systems. The accurate statement is that a unit has been vertical-load tested to the CIRIA C680-2008 method, with structural design to the current C737 and EN 17152-1 framework, which is exactly how StormGrid is documented.

Ask for the third-party test report and its number, the method it was tested to, and the per-grade load classes, not a single headline tonne figure. StormGrid provides an ISO 17025 Intertek vertical-load report (600 kN/m² front surface at both 23 and 40 °C) on request. A supplier who cannot produce a named-method report is the red flag the UK guides warn about.

Both use the same geocellular crate. A soakaway is wrapped in permeable geotextile and lets water infiltrate the surrounding ground; an attenuation tank is lined with an impermeable geomembrane and releases stored water at a controlled rate into a watercourse or sewer. On clay or high-water-table sites where infiltration isn’t possible, you use the lined attenuation configuration.

Yes, with the right load grade, cover depth and pre-treatment. Match the StormGrid grade (e.g. SG-H60 at 600 kN/m²) to the wheel load, engineer the cover depth to the C737 method, and fit upstream silt capture, because the tank stores water but doesn’t treat it. Trafficked areas with oil-spillage risk also need an interceptor upstream.

A load report alone doesn’t, approval also needs the discharge rate, storage volume with a climate-change allowance, drainage-hierarchy justification, and a maintenance plan. In Wales this goes to a SuDS Approving Body under Schedule 3 of the Flood and Water Management Act 2010 (legislation.gov.uk); in England it’s handled through planning and LLFA consultation. StormGrid supplies the structural evidence layer of that submission; your drainage engineer builds the hydraulic and maintenance case around it.

The design life is 50 years, but service life depends on maintenance, not just the plastic. Siltation is the main cause of lost capacity, so CIRIA C753 advises allowing extra storage volume for hard-to-clean crates and fitting upstream silt capture plus an inspection access. Built and maintained to spec, StormGrid is designed to last the life of the development it serves.

Get a Compliant Attenuation Tank Spec’d for Your Site

Send your discharge rate, storage volume and load class, and our engineering team returns a module layout, the Intertek CIRIA C680 test report and an FOB quote, with the load grade and surface class stated separately, the way your LLFA submission needs them.

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