How modular geocellular soakaways work, when they suit a site, how they are sized, and the standards behind them — a practical primer for engineers, consultants and developers in the Kingdom.
A geocellular soakaway is an underground structure assembled from interlocking modular plastic cells, wrapped in a permeable geotextile, that stores stormwater and releases it into the surrounding soil by infiltration. It does the same job as a traditional gravel-filled soakaway pit, but a void ratio above 95 percent means it holds far more water in the same excavation — so the dig is smaller, faster and easier to design around services. The cells are selected to carry whatever sits above them, from landscaping to trafficked pavement.
A soakaway only works where the ground accepts water. Two site facts decide it: the soil's infiltration rate, measured by an on-site percolation (infiltration) test, and the groundwater level, which must sit below the base of the system. Where infiltration is good and groundwater is deep, a soakaway is usually the most efficient and lowest-cost option. Where soils drain poorly or groundwater is shallow, the design shifts to attenuation — storing stormwater and releasing it at a controlled rate to the network. The same geocellular units serve both roles; only the outlet arrangement changes.
Sizing is a balance between water coming in and water soaking out. The inputs are:
A complete design checks several storm durations to find the worst case, confirms the system empties within an acceptable time, and applies appropriate safety factors. The recognised method references are BRE Digest 365 (percolation testing and soakaway sizing) and CIRIA C753 (infiltration system design). The NGS Design Studio runs this analysis automatically and produces a sized design with an engineering summary.
Once the storage volume is set, the units are checked structurally for the loads above them — soil cover, surcharge and any vehicle loading — to CIRIA C737, the reference for the structural and geotechnical design of modular geocellular systems. The correct load class and cover depth depend on the application, from non-trafficked landscape areas through to highway loading; NGS confirms the cover and load class for each project against the product's tested capacity. This is also where the choice between the standard-depth and shallow cell families is made.
NGS supplies two AquaCell geocellular families for soakaway and attenuation use in the Kingdom:
| Family | Load / depth | Void ratio | Design life | Reference | Applications |
|---|---|---|---|---|---|
| AquaCell Geo-Cellular | 30T / 45T / 60T load classes (higher on request); 500 mm cell depth | ≥ 95% | 50 years | CIRIA C737 | Infiltration & attenuation |
| AquaCell EXO | Shallow / premium cells, 85 / 150 / 200 / 250 mm depths | ≥ 95% | 50 years | BS 7533-13 | Infiltration, attenuation & passive irrigation |
Specification figures are indicative; project load class, cover depth and configuration are confirmed by NGS engineering for each site. Datasheets are available on the Technical Documents page.
Geocellular soakaway design draws on Saudi municipal approvals together with established design references. The full index — SBC, MOMAH, Balady, CIRIA and BRE, each with a link to its official source — is on the Saudi stormwater & soakaway standards page. NGS does not host or distribute these copyrighted documents.
What is a geocellular soakaway?
An underground structure of high-void modular cells that stores stormwater and lets it infiltrate into the ground. It replaces gravel-filled soakaways, holding far more water in the same volume (void ratio above 95 percent) while carrying the loads above.
When is a soakaway suitable instead of an attenuation tank?
Where the ground absorbs water at an adequate rate (confirmed by a percolation test) and groundwater sits below the system. Where soils drain poorly or groundwater is high, attenuation with controlled release is used instead.
How is a geocellular soakaway sized?
By balancing inflow against infiltration: contributing area, design rainfall for the chosen return period, and the measured soil infiltration rate determine the storage volume the cells must hold. BRE Digest 365 and CIRIA C753 are the common method references.
How long does a geocellular soakaway last?
The NGS AquaCell range is engineered for a 50-year design life. Longevity depends on correct structural selection, geotextile and silt management, and maintenance access.