Designing an underground stormwater storage system is a unique step in the overall construction process for each site. The size of the inlets and the conveyance pipes are determined by flow rate, elevation, and slope. This information is entered and calculated by stormwater management software to determine the drainage calculation. But many of the measures build upon one another or vary based on different criteria, so it can be a pretty complex process. That’s why we’re going to take a look at the top 10 factors that go into designing and installing one of these systems.
The final design application of any underground system impacts the key criteria of design and installation. Infiltration basins rely on the hydrologic capacity of the soil and the overall footprint to ensure the runoff can infiltrate in a timely manner. This could mean elevations cannot be changed, compaction may be limited, and overall footprint must remain the same when trying to convert from one product to another. Detention and reuse systems are more focused on capacity and rate control, so the above criteria for an infiltration system may be less important, but considerations like an impermeable liner may be necessary. Even when using an underground concrete system, a liner may be required as concrete is porous.
Sizing is the next step in the design process. In principle, we’re talking about how much water the system can hold. Stormwater routing software requires different inputs to determine the discharge rate of the influent. Depending on the height, shape, and incremental void space, each system can vary on the results needed to meet regulatory approval.
In large open areas, footprint is not a strong consideration because the space is readily available, but for sites with tight constraints, footprint can be critical and require a flexible storage solution. System footprint also varies by application. The footprint for detention and reuse systems can sometimes impact the stage storage and discharge rate used to size the system. In detention uses, it’s important to fit as much storage in a small footprint as possible, even when more space is available. For infiltration, the footprint can play a huge role in the function of the system and regulatory approvals. The footprint is designed to provide the required infiltration rate, dewatering time, and loading ration. Therefore, reducing it could mean the system no longer meets the design specifications or regulations.
Inverts are usually set by the engineer to meet several factors. The inverts may be based on infiltration testing, limiting zones, connection height, minimum cover, or other factors. Problems may arise when a contractor or manufacturer attempts to change the product after the design is in place, which could cause a change in elevation, thus disrupting the alignment of connections and drainage efficiency. This could have many negative repercussions that would require a full system redesign.
Cover material and depth play a very important role in the load rating of the product. Manufacturers calculate and test the cover differently, but the cover requirements are always the same. The more cover depth you have, the more load distribution before reaching the product itself. And on the opposite end, less covers means less distribution, which results in a higher load on the product. This means that changing the heights and inverts could impact structural performance.
Base preparation can impact many aspects of installation and performance. If the surface is not graded and prepared properly, the installation can become more difficult as the product will not align or has been placed incorrectly. From there, base preparation can have other impacts, such as soil strength and infiltration rate. If the soil is not capable of supporting the load, long-term settlement can occur, while over-compaction of the base could result in a decline in the native soil infiltration rate and a system not functioning as designed.
Another commonly overlooked aspect of a project is the construction equipment; this includes not only the equipment used to install the system itself but the equipment used to pave the parking lot above or construct an adjacent building. It is important to remember that in the last few stages of the project it is not typical that the site has reached its final grade just yet. Therefore, some construction equipment, especially crane outriggers, can exceed AASHTO standards. If not properly understood and calculated before use, higher loads on a system can cause them to reach their yield point, resulting in failure.
Not only is backfill material selection critical to the long-term structural performance of the system, but the placement of the backfill is equally important. The backfill placement must follow the manufacturer’s requirements and be placed in uniform lifts. If this is not done properly, it can lead to product movement, system racking, or other damages.
Finally, proper material compaction is vital to prevent movement and deflection of backfill and structure during operation. If the backfill material is placed incorrectly, it could allow for additional shifting of the backfill, which can increase stress on the product or cause surface deformation.
Manufacturers use many different materials ─ from polyvinyl chloride to polyethylene and polypropylene ─ when manufacturing stormwater products. In addition to the different types of material, manufacturers may also use different qualities of material. These can range from narrow to wide specification virgin material or post-manufacture, or even post-consumer recycled material. Narrow specification virgin material is the most controlled, while post-consumer recycled material is the least controlled. This material variability impacts the performance and longevity of the system.
Total Installation Cost
Underground stormwater storage systems can be a large part of a construction project’s overall cost. Therefore, it’s an area where owners and contractors are always looking to keep costs down. There is a general misconception that product costs can be compared evenly across the board. The problem is that comparing boxes to arches or arches to concrete is like comparing three distinctively different animals. The only way to truly understand the cost of installing an underground system is to sum all the components: product, fabric, backfill, excavation, labor, etc. and determining a total cost. For example, some products require more product material but much less excavation and backfill stone, while others may have extremely low product cost but very high installation costs. Looking at single line item will not result in the most cost-effective price tag.
Brentwood’s StormTank team has designed, or redesigned, thousands of stormwater management systems. And we continue to aid engineers by getting involved early in the design process. This enables us to weigh all important considerations and provide the most cost-effective system design options for the engineer to take back to the owner. We’re happy to look over your plans or do a complete, from-scratch design, so be sure to reach out and use our stormwater experts as a resource!