What is a Geocellular Storage

Geocellular storage is an underground stormwater management system constructed from modular plastic cells that create a large volume of space for temporarily storing rainwater. The system receives runoff from roofs, roads, car parks and other impermeable surfaces, holding water within the interconnected cells before it is released at a controlled rate or allowed to infiltrate into the surrounding ground. Geocellular storage is widely used as part of Sustainable Drainage Systems (SuDS) in the UK, particularly on developments where conventional surface drainage features cannot provide sufficient storage.

The main advantage of geocellular storage is its high void ratio, which allows substantial volumes of water to be stored within a relatively compact underground structure. Many proprietary systems provide approximately 90% to 95% or more usable void space, depending on their design. This makes them considerably more space-efficient for water storage than conventional stone-filled drainage structures.

Geocellular systems can be installed beneath landscaped areas, car parks and other suitable surfaces, provided the units and installation are designed for the expected loads. Their performance depends on storage capacity, groundwater conditions, structural design, maintenance arrangements and the ability of the receiving system to accept discharged water.

How Geocellular Stormwater Storage Works

A geocellular storage system consists of interconnected plastic modules assembled into an underground storage structure. The modules contain an arrangement of internal columns, walls or supports that provides structural strength while leaving most of the internal volume available for water. The assembled structure is surrounded by appropriate geotextile or geomembrane materials according to its intended hydraulic function.

During rainfall, surface water enters the drainage network through gullies, channel drains, rainwater pipes or other collection points. It is conveyed towards the geocellular structure, usually after passing through suitable silt removal or treatment components. Water then occupies the voids within the modules, temporarily reducing the volume discharged into the downstream drainage system.

In an attenuation system, the stored water leaves through an outlet fitted with a flow control device. The device limits the discharge rate, allowing the structure to fill during periods when incoming runoff exceeds the permitted outflow. As rainfall reduces, the stored water continues to discharge until the structure has drained down.

In an infiltration system, water passes through the permeable outer layers and enters the surrounding soil. The rate of infiltration depends on ground permeability, groundwater levels, available infiltration area and the hydraulic conditions within the system. Unlike attenuation, this arrangement may not require a conventional piped outlet for normal operation, although appropriate exceedance arrangements may still be necessary.

A typical geocellular installation contains the following elements:

• Modular storage cells forming the underground water storage volume.

• An inlet pipe or distribution arrangement delivering surface water into the structure.

• Geotextile or geomembrane layers selected for infiltration or watertight storage.

• Suitable bedding and surrounding fill to provide structural support.

• A silt trap or upstream treatment arrangement to reduce sediment entering the cells.

• Inspection and maintenance access compatible with the selected system.

• A controlled outlet for attenuation systems or a suitable infiltration interface for infiltration systems.

• An overflow or exceedance arrangement where required by the drainage design.

These components must operate as a complete system. Large storage capacity alone will not prevent flooding if the inlet is obstructed, the outlet cannot discharge or the structure receives more water than its design allows.

Attenuation and Infiltration: Two Different Geocellular Applications

Geocellular modules can be used for stormwater attenuation, infiltration or a combination of both. Although the same general cell technology may be used, the surrounding membranes and discharge arrangements differ significantly. Selecting the wrong construction can prevent the system from performing its intended function.

An attenuation tank is normally wrapped in an impermeable geomembrane to prevent stored water escaping into the surrounding ground. A protective geotextile may be installed around the membrane to reduce the risk of puncture during construction. Water remains within the storage structure until it is released through the designed outlet.

An infiltration system uses permeable geotextile rather than a completely impermeable enclosure. This allows stored water to pass into suitable surrounding soils while helping prevent soil particles from migrating into the storage structure. Its effectiveness relies on verified infiltration conditions rather than simply the amount of storage provided.

A combined system may incorporate infiltration while also providing an additional controlled discharge route. Such arrangements require specific hydraulic design because the relative contribution of infiltration and piped discharge changes with rainfall, groundwater conditions and soil saturation.

Ground investigation is particularly important before specifying infiltration. Clay-rich or poorly permeable ground may not accept water rapidly enough to empty the storage structure between rainfall events. High groundwater levels can also reduce infiltration performance and may make the proposed arrangement unsuitable.

In England, infiltration testing for SuDS design is commonly undertaken using methods associated with BRE Digest 365, where appropriate. The test results help establish how quickly water can enter the ground and whether sufficient infiltration capacity is available. Local drainage requirements and the relevant guidance must also be considered.

Storage Capacity, Cell Dimensions and Hydraulic Design

Geocellular storage capacity is determined by the overall volume of the assembled structure and the effective void ratio of the selected modules. Because internal supports occupy some space, the available water storage volume is slightly smaller than the external dimensions suggest. Manufacturer data should be used when calculating the actual capacity.

For a simple rectangular installation, the approximate storage volume can be calculated as:

Storage volume (m3) = Length (m) x Width (m) x Height (m) x Void ratio

For example, consider a geocellular structure measuring 20 m long, 5 m wide and 0.6 m high. Its external volume is 60 cubic metres. If the effective void ratio is 95%, the nominal water storage volume is approximately 57 cubic metres, before allowing for any further design-specific deductions.

This calculation establishes the approximate capacity of the selected cells, not the volume required for a particular development. The necessary storage must be determined through a hydraulic assessment of the contributing catchment, rainfall conditions, infiltration or discharge rates and applicable design criteria.

The relationship between inflow, outflow and storage is central to attenuation design. Storage is required when incoming runoff exceeds the rate at which water can leave the system. The accumulated difference determines how much water must be held before it can be discharged.

Important design inputs include:

1. The impermeable and permeable areas contributing runoff.

2. Design rainfall events, including the applicable allowances for future climate conditions.

3. Existing site runoff characteristics and any required discharge limits.

4. The maximum permitted outflow rate.

5. Available underground space and constraints from existing services.

6. Groundwater level, soil conditions and possible infiltration capacity.

7. Structural loads from overlying soil, traffic and other imposed loads.

8. The time required for the system to empty sufficiently before subsequent rainfall.

An outlet flow control device may use an orifice, vortex control or another suitable arrangement. Its discharge characteristics influence both the maximum stored volume and the rate at which the tank empties. The selected device must therefore be included in the hydraulic calculations rather than treated as an independent accessory.

The time needed for a storage structure to drain down is particularly important where rainfall events occur close together. A system that remains substantially full when another storm begins has less available capacity for additional runoff. Design assessments should account for this behaviour using the relevant rainfall and drainage modelling approach.

The nominal storage volume also does not establish the maximum safe operating water level. Pipe connections, available head, overflow levels and upstream drainage levels can limit how the storage is used. These factors must be considered when determining the effective storage available to the system.

Underground Installation and Structural Requirements

Geocellular storage structures are frequently installed where underground space is available but surface space is needed for other purposes. Common locations include landscaped areas, access roads and car parks. The installation depth and construction method depend on the cell product, surrounding ground and loads expected during both construction and normal use.

The modular units must resist vertical loading from the soil cover and any traffic or other loads above the structure. They must also withstand lateral ground pressures and other forces affecting the installation. Because products have different structural properties, the permitted cover depths and loading arrangements must be established from the manufacturer’s technical information and a suitable structural assessment.

The surrounding material plays an important role in transferring loads and supporting the cells. Unsuitable backfill, poor compaction or uneven bedding can create local concentrations of stress. Installation instructions should specify the required bedding, sidefill, compaction procedures and restrictions on construction equipment.

Groundwater conditions introduce additional design considerations. An impermeable underground tank may experience buoyancy forces when groundwater rises around it, particularly when the tank is empty or only partly filled. The structure and surrounding construction must provide sufficient resistance to uplift under the relevant design conditions.

For infiltration systems, the interface with the surrounding soil must remain permeable. Incorrect backfill selection or damage to the geotextile can reduce infiltration performance and allow soil migration into the storage structure.

The proximity of buildings is another important consideration. Excavation beside foundations can affect ground support, while infiltration near buildings may influence ground moisture conditions. Appropriate separation distances and structural precautions must be determined for the site rather than relying on a single universal distance.

Geocellular installations also require coordination with underground utilities. The available excavation depth may be restricted by existing sewers, water mains, electrical services or other infrastructure. These constraints can affect the shape, location and number of storage structures needed.

Sediment, Maintenance and Long-Term Performance

Sediment accumulation is one of the principal operational concerns for geocellular storage. Surface water can transport silt, sand, leaves and other debris into the drainage system. If these materials enter the storage cells, they can settle within the structure and gradually reduce available volume or obstruct water movement.

Unlike a conventional accessible tank, many geocellular systems contain internal supports and narrow passages that cannot be reached easily with standard cleaning equipment. Some proprietary systems incorporate inspection channels or dedicated maintenance routes, but these facilities are not present in every design. Maintenance access must therefore be considered before the system is installed.

Pre-treatment is particularly important. Silt traps, catchpits and suitable upstream treatment features can intercept sediment before it reaches the storage modules. The appropriate treatment arrangement depends on the contributing catchment and the expected pollution load.

Runoff from a relatively clean roof may require different treatment from runoff collected across a heavily trafficked car park. Where there is a risk of contaminated runoff, the proposed infiltration arrangement must be assessed carefully because pollutants could enter the soil and potentially affect groundwater.

Routine inspection should focus on the components that can be accessed and maintained. These normally include inlet chambers, sediment traps, outlet controls, inspection points and overflow arrangements. The frequency of inspection should reflect site conditions, rainfall exposure and the maintenance requirements of the installed system.

A recurring increase in upstream water levels may indicate a restriction at the inlet or outlet, although other hydraulic conditions can produce similar symptoms. Sediment accumulation within the storage structure is also possible, particularly where pre-treatment has been inadequate. Investigation should establish the cause before attempting high-pressure cleaning or other interventions that may not be suitable for the cell construction.

The use of geocellular storage should also be considered in the context of the wider SuDS network. Where space and site conditions allow, source control features such as permeable paving, rain gardens and swales may reduce the volume and pollutant load reaching the underground tank. This can improve the overall drainage arrangement and reduce reliance on a single buried storage structure.

Geocellular storage is especially useful where substantial stormwater attenuation is required within limited space. Its long-term value depends on accurately calculated storage capacity, verified ground conditions, suitable structural design and accessible upstream treatment and flow control components. A system designed around these requirements can manage peak runoff effectively while preserving the intended use of the ground above it.