Rain in the UAE doesn’t fall often, but when it does, it tends to fall hard, and fast. On flat sites with poor infiltration, these areas quickly become parking lots where the water has pooled. Precast concrete soakaways are there to absorb that one-off of water without anyone realizing anything and it’s no issue. Let’s take a real look at how these systems are being produced and installed and why they are more important in the UAE than perhaps they would be in other countries if it was only for the few days when it rains.
A soakaway is a below ground drainage structure, engineered to catch the surface water runoff and slowly release it into the surrounding soil through infiltration. A soakaway allows the ground to absorb the load of rainwater, over hours, not minutes, instead of sending it straight into the stormwater sewer.
The concept is the same as that of natural rainwater runoff on undeveloped land. Water falls to the ground, infiltrates and is replenished to the groundwater table. When that same land is paved with concrete, asphalt and roofing, the water goes nowhere except a drain. A soakaway artificially restores some of this natural capacity for infiltration.
When Choosing a Storage Solution for Your Concrete You Have the Option of a Precast Concrete Ring or a Plastic Crate
Usually soakaways will be manufactured in one of two main patterns, precast concrete ring systems or modular plastic crate systems. Precast concrete soakaway rings are made of high strength, robust concrete incorporating tongue and groove joints for rapid assembly in a soakaway chamber, which does not require a separate concrete surround to be poured after installation.
By contrast, plastic crate systems can achieve void ratios of 95 percent or higher, and can achieve 30 to 35 percent or more storage efficiency compared to a rubble-filled pit, making them desirable for large-volume attenuation projects. Precast concrete rings, however, offer a structural strength, resistance to ground movement and durability that plastic can not completely match. And in ground conditions where chemical exposure or heavy above-ground vehicle loading would stress a lighter plastic structure, precast concrete rings would not. In such cases, that structural strength becomes the lifeline of a project, especially for UAE sites that have varying subsoil conditions and load requirements.
The First Step Is a Site Assessment and Percolation Testing
The process begins with an understanding of the ground itself before any concrete is cast for a particular project or an engineer determines the size of the chamber. The percolation test is simply a hole dug, filled with water, and the rate at which the water drains out of the hole provides engineers a clue as to how the soil around the hole does absorb water.
This is a step that makes all the difference in the world below it. A soakaway is reduced in size for permeable, sandy soil which can absorb water rapidly. Clay soils are dense and absorb slowly, if at all, and may require a much bigger drainage system, or an alternate drainage solution. One of the most popular mistakes made when installing drainage is to skip this test and estimate the size of the soakaway.
Step 2: Mix Design for Structural and Chemical Durability
Precast concrete soakaway rings must be made with a mix designed to meet two requirements: structural strength to resist the forces of the earth and the chemical effects of soil. Over time, buried concrete faces a slow and constant threat from moisture and salt from groundwater, soil, and overall exposure.
Mix design should also consider the chemistry of the soils and groundwater of the region for concrete manufactured in the UAE because they may be significantly different, leading to different concentrations of salt or sulfates that can affect the performance of concrete in the underground environment.
Step 3: Reinforcement and Ring Design
Precast soakaway rings are generally reinforced to resist lateral earth load from the surrounding backfill, and any vertical load from above the top of the chamber. Ring size by diameter and wall thickness are sized according to the anticipated chamber depth and traffic loading for the site.
At this point, the tongue and groove joint profile is cast in each ring, which helps rings fit neatly together on site for a clean installation without the need for wet concrete joints or extra formwork to keep the rings tightly in place.
Step 4: Mold Preparation and Casting
The casting is done in a controlled manufacturing facility with precisely cut molds that are formed to the ring’s exact diameter, wall thickness and joint profile. Here, consistency is more important than might at first be thought, because the soakaway rings fit end to end when being installed and so any slight difference in dimensions between rings can lead to problems of misalignment when the complete soakaway is assembled on site.
Step 5: Curing
Cast rings are cured to the desired compressive strength under controlled conditions prior to leaving the plant. Controlled curing is useful in the UAE as the summers are very hot and cracking of the surface and loss of long-term strength could occur if the curing was hastened under uncontrolled ambient conditions.
Step 6: Quality Control
Manufacturers inspect each ring before it is sent away for dimensional accuracy, especially at the tongue and groove joint; a poorly formed joint will have an impact on the way the rings seal and stack during installation. Round off the process with strength testing and surface inspection, which will ensure that any faulty unit is not sent to a job site.
Step 7: Excavation and Bedding
Installation is done on site, starting with the excavation of a hole that is designed to match the diameter and depth of the soakaway, based directly on the results of the percolation test. A rule of thumb is to dig a soakaway at least 1 metre below the surface of the ground, but it varies greatly from site to site depending on the drainage system to be installed.
The first ring requires a stable base which is prepared usually by compacting a granular material. This fundamental level is so important as it sets the alignment for each subsequent ring on top of it.
Stacking and Assembly Step 8
The rings are placed in the trench one at a time, with each ring engaged with a tongue and groove connection to the ring underneath. The design does not require an additional concrete surround to install, making it a faster installation than systems which would require wet-pouring between sections.
Installers usually fill in around the outside of the completed ring stack with graded, permeable stone or granular material, forming a buffer zone between the ring stack and the surrounding natural soil to allow water to flow from the ring stack into the natural soil, and place a geotextile membrane layer between the stone and the natural soil to prevent soil particles from migrating into the system, diminishing its infiltration over time.
Step 9: Pipe Connection & Final Backfill
The surface runoff is brought in the soakaway chamber by the inlet pipe which is usually connected to the upper part of the ring assembly by a pre-formed inlet. After the connection and sealing, the surrounding excavation is backfilled in compacted layers to restore the surface above to its intended finishing such as landscaping or paving or to a trafficked surface.
In areas where there will be vehicular traffic on top of the soakaway, these two capacities, backfill compaction and ring loading capacity, must take that added load into consideration from the beginning – not as a post-facto after the first truck is driven across it.
Soakaways Are Employed in the UAE in Numerous Places
Precast concrete soakaways can be seen on sites where there is a need to control stormwater on site without the public drainage network becoming overwhelmed. That includes residential and commercial developments that have roof and paved area stormwater management, parking lots and logistics facilities with large impermeable surfaces, and landscaped areas where sustainable drainage design minimizes reliance on municipal stormwater infrastructure.
Precast concrete systems are favored for areas with higher demands for structural or chemical exposure, industrial areas, high-traffic pavement areas, and those where the soil chemical profile is aggressive due to the durability these systems provide when compared to less demanding systems.
When Speaking to a Soakaway Supplier What Should You Ask?
A good manufacturer is going to want to know what the results were of the percolation test performed on your site before sizing a system to your guess, not based on the results. Inquire about the concrete grade and reinforcement, how they seal their ring joints without extra concrete work, and if the product is rated to withstand the traffic and/or structural loading your site anticipates on top of the chamber.
Bringing It Together Precast Concrete Soakaways
An unglamorous but very important job is one that a precast concrete soakaway carries out – it removes the shock of storm water from the flow of the grid and returns it to where it belongs – the ground. Everything from percolation testing to mix design, from reinforced casting to careful on-site stacking, is every bit of it designed to produce a chamber that’s discreetly absorbing water for decades without a care in the world above. A place where it can take only a few minutes for a heavy downpour to overwhelm unprepared drainage, that’s a lot of quiet reliability.
