Structural and base materials · Aggregates & Groundworks
Geocellular Attenuation & Soakaway Crate
This entry describes the crate as a way of making and keeping a void underground, and sets out the two quite different things that void can be asked to do depending on how it is wrapped.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What attenuation crate is
A geocellular crate is a moulded polymer box, largely empty, strong enough to carry the ground and whatever is above it while leaving most of its own volume as open space. Crates are stacked and connected into a block of the required shape, and the block is then buried. The point of the product is the void: almost all of the module's volume can hold water, where a pit filled with stone would give over most of its volume to the stone itself.
What the void does depends entirely on how the block is wrapped, and that is the decision the reader most often meets. Wrapped in a permeable geotextile, water entering the block is held briefly and then passes out into the surrounding ground — an infiltration arrangement, where the ground's own ability to accept water is what makes it work. Wrapped in an impermeable membrane, the block holds water and releases it at a controlled rate through an outlet — an attenuation arrangement, where nothing is expected to soak away at all. The same crate does both; the wrap and the outlet decide which.
Around the block there is normally granular fill, and the crate is not laid onto raw ground. A formation is prepared, a surround is placed and compacted, and the geotextile or membrane is lapped and sealed as the product requires. Inlets, outlets, any flow control, silt traps and inspection arrangements are connected into the block, and where they enter is fixed by the design rather than adjusted on site.
The hardest thing about the product is the least visible. Once buried, the block cannot be dug up to look at. Whatever inspection is possible has to have been built in — access ports, inspection channels, a means of jetting silt out — and it has to be built in at the time of construction, because it cannot be added afterwards. Silt carried in from the catchment is what fills the void up over time, and what stops it arriving is the treatment upstream rather than anything in the crate itself.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Attenuation crate
- Soakaway crate
- Geocellular storage unit
- Stormwater storage module
- Modular soakaway box
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Soakaways beneath driveways, parking areas, gardens and yards, where surface water is to be returned to the ground.
- Attenuation storage beneath hardstanding, where run-off is held and released at a controlled rate.
- Storage under permeable paving construction, where the surface admits water and the void beneath holds it.
- Storage beneath landscaped and planted areas where nothing above ground is wanted.
- Collection of roof water where it cannot be discharged directly to a sewer or a watercourse.
- Replacement or extension of an existing stone-filled soakaway where more storage is needed in the same footprint.
Consideration
Where it is commonly considered
- Where storage is needed below ground and there is no room for it anywhere else on the site.
- Where a stone-filled pit would have to be impractically large for the volume required.
- Where infiltration testing has been carried out and the ground's behaviour is actually known.
- Where the drainage design, including any flow control, has been prepared by qualified professionals.
- Where access for inspection and cleaning can be designed into the block from the start.
- Where the loading over the block — traffic, parking, landscape — has been established before anything is ordered.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where no infiltration testing has been done and the ground is simply assumed to accept water.
- Where the groundwater level, or its seasonal range, is unknown.
- Where the block would sit close to a building, a boundary, a tree or a slope and that relationship has not been assessed.
- Where the loading above has not been established and a crate is being selected without it.
- Where the site has contaminated ground or a source protection consideration that nobody has raised.
- Where no silt management upstream and no means of inspection have been designed into the arrangement.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Moisture behaviour
- The wrap decides what the void does: permeable to let water out into the ground, impermeable to hold it and release it through an outlet. Choosing the wrong one does not produce a lesser version of the right one, it produces a different thing entirely.
- Substrate dependency
- The formation beneath and the granular surround around the block are part of the construction. Both are prepared and compacted to the designer's requirements, and the ground's ability to accept water is established by testing rather than by inspection.
- Documentation
- Documentation covers the module, the loading arrangement it belongs to, the geotextile or membrane specified with it, the connections available and the inspection provisions the manufacturer offers.
- Maintenance
- Silt is what fills the void over the years, and it arrives from the catchment. The treatment upstream — silt traps, catchpits, filters — and the means of jetting or cleaning the block are the whole of the maintenance question.
- Repairability
- A buried block cannot be reached without excavation, and in most cases without taking up whatever is above it. That is the defining practical limitation and it is accepted knowingly at the time of design.
- Installation dependency
- The formation, the surround, the lapping and sealing of the wrap and the making of connections are where the construction succeeds or fails. A torn membrane or an unsealed lap is not visible once the excavation is backfilled.
- Thermal behaviour
- The block sits below ground and is affected by frost and by seasonal groundwater movement in ways that depend on depth and on the site, which are matters for the drainage design rather than for the product.
Exposure
Exposure and environmental conditions
- What is the ground actually like here, and has infiltration been tested rather than assumed?
- Where does the groundwater sit, and how much does it move through the year?
- What will pass over the ground above the block — vehicles, parking, planting, nothing?
- What is upstream of the block, and what will the water arriving carry with it?
- Is the block near a building, a boundary, a slope or trees, and what does that mean?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The formation beneath and the compacted granular surround are designed and constructed as part of the arrangement, not as backfill.
- The geotextile or membrane is chosen for what the block is meant to do, and its laps and seals are made as the product requires.
- Inlets, outlets, flow control and any silt trap are connected into the block at positions fixed in the design.
- Inspection ports, channels and jetting access have to be built in at the time of construction.
- Cover depth and the construction above the block are part of the loading arrangement rather than separate from it.
- Where the block sits under a paved area, the paving construction and the storage beneath are designed together.
Appearance
Appearance and finish considerations
- Nothing of the block is visible once it is complete, so appearance is entirely a question of what is laid over it.
- Inspection covers and access chambers are the only visible elements, and where they land matters in a finished landscape.
- Levels over the block are set by the drainage design, which can constrain the finished surface above it.
- Any settlement of the ground above shows as a depression in the surface, so the surround and its compaction are an appearance matter as well as a structural one.
- Planting over the block is limited by cover depth and by root considerations, which belong to the landscape design.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- What was the infiltration testing, and what did it find?
- What loading has the block been selected for, and who established it?
- How was the geotextile or membrane lapped and sealed, and was that recorded?
- What silt treatment is there upstream, and what reaches the block?
- How deep is the cover over the block, and what is built above it?
Upkeep
Maintenance and repair considerations
- How is the block inspected, and what access was built in for that?
- How would silt be removed from the units, and by whom?
- How often is anybody expected to look at this, and who holds that responsibility?
- What would have to be excavated to reach the block itself?
- Has the position and extent of the block been recorded on a drawing that will survive?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- The excavation, the formation and the compacted surround are prepared before any module goes in.
- Modules are assembled and connected as the manufacturer describes, because the block's behaviour under load depends on how it is built.
- The wrap is laid, lapped and sealed without damage, and a tear made during backfilling is invisible afterwards.
- Backfilling and compaction around a hollow block is done in a way that does not distort it, which constrains the plant used.
- Connections are made before backfilling, since nothing can be added once the excavation is closed.
- Groundwater and running sand in the excavation are site conditions that affect the method and are dealt with by the contractor and designer.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- Product documentation for the module, including the loading arrangement it belongs to.
- The geotextile or membrane specified with it, and the manufacturer's requirements for laps and seals.
- The drainage design showing the block's size, position, levels, connections and any flow control.
- The infiltration testing report on which the arrangement has been designed.
- A record of the inspection and cleaning provisions built in, and a drawing showing where the block is.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Has infiltration been tested here, and what did the results show?
- Is this block infiltrating into the ground or attenuating and discharging, and how is it wrapped for that?
- What loading is it designed for, and what will pass over the ground above it?
- Where is the groundwater, and how does it move through the year?
- What silt treatment is upstream of the block?
- How will this be inspected and cleaned once it is buried, and what access is being built in?
- How close is this to buildings, boundaries, trees or slopes, and who assessed that?
- What consents or approvals does a drainage arrangement of this kind need here?
- Where will the record of the block's position and extent be kept?
Blind spots
Commonly overlooked points
- Once it is buried, the block cannot be looked at — whatever inspection is possible has to be designed in before backfilling.
- Permeable and impermeable wraps make the same crate do opposite things, and the wrap is not an accessory decision.
- Silt, not water, is what eventually fills the void, and what controls silt sits upstream rather than in the crate.
- A crate is not a substitute for knowing whether the ground will accept water; infiltration testing is what establishes that.
- The granular surround is part of the construction, not backfill, and how it is compacted around a hollow block matters.
- The name is close to that of a cellular confinement product and the two are often confused; one is deliberately empty and the other is deliberately filled.
What this page does not do
- This page states no storage volume, no load class, no cover depth and no infiltration rate: all of those are drainage-engineering determinations made from site testing by qualified professionals.
- Nothing here indicates that a soakaway or attenuation arrangement is appropriate for any site. Whether water may be discharged to the ground at all is a matter for the relevant authority and for site investigation.
- A buried storage arrangement affects ground conditions around it, including near buildings, boundaries and trees, and those effects are assessed by qualified professionals.
- Local requirements govern surface water drainage in a given place, including consents and adoption, and they are confirmed with the relevant authority.
Related entries
Common comparisons
Entries met at the same decision but which behave differently, so the comparison is about the difference rather than a swap.
Related entries
Often used alongside
Entries commonly encountered together with this one. Co-occurrence only — never a claim that any combination is compatible or suitable.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Aggregates & Groundworks Materials
Reference entries for the unbound and geosynthetic materials that make up the ground beneath a building — crushed rock, fills, bedding, drainage media, geotextiles and the cellular products laid with them.
Browse all aggregates & groundworks entries →