Water Storage & Rain Harvesting

Quick Guide: How to Install an Underground Water Storage Tank

[fa icon="book"] 4 Minute Read
[fa icon="calendar"] Mar 6, 2018 7:40:18 AM / by Matthew Firth


Installing an underground water storage tank is simple when you have the right tools for the job. In this article, you will learn a simple 6-step installation method that includes an explanation of the recommended materials and machinery to be used. Using the correct backfill material to bury a poly tank is particularly important as it will lengthen the life of the tank. Because these tanks are getting buried, you must also be extra cautious about where you are installing them.

Before You Start: It's important you read all warnings on the tank lid before you begin installation.

Backfill Material Instructions:

Using the right backfill material will minimize the possibility of the tank collapsing. This is an extremely critical part of installation and must be followed.

  • Recommended: The ideal soil is pea gravel. Sand or native soil can be used but it must be flowable, compactable and free from rock.
  • NOT Recommended: Soils that shrink or swell should not be used. 

Important: All material must be free of sharp objects that could cause damage to the tank.

6-Step Installation Guide for Underground Water Tanks:

  1. Excavate - Measure the length and width of the tank. Add 18” - 24” on both ends and sides of the tank and mark those dimensions on the ground. Excavate to a depth that will provide a minimum of 6” and a maximum of 48” of cover over the top of the tank. 


  2. Prepare Base - Remove any roots and rake the bed flat. The preferred tank bedding material is well packed pea gravel with minimums of 6” in soil terrain and 12” in rock terrain. Sand or native soil can be used but it must be flowable, compactable and free from rock. After measuring and filling in the pea gravel base, compact and level before setting the tank.

    Pro Tip: Ensure there are NO SHARP OBJECTS mixed with the pea gravel. This could puncture the tank without you knowing.


  3. Set the Tank - Place the tank in the middle of the hole. Begin pushing the tank back and forth to allow the pea gravel base to fill in the ribs along the bottom of the tank. (Read Backfill Material Instructions)

    Pro Tip: If you are lowering the tank in the hole using a crane or excavator, level the tank before removing the straps in case further excavation is needed.


  4. Install Connections - Install bulkhead fittings in tank. You MUST plumb with a flexible hose directly from the bulkhead fitting. This prevents any movement that could break the fitting and put extra stress on the tank. Extra fittings that are installed in this tank are the responsibility of the owner. 

    Pro Tip: Underground tanks must be vented to prevent them sucking in when water is being pumped out.


  5. Add Risers - Remove lid and gasket that is supplied with the tank. These components are used in step (d).
    a) Silicone around tank and screw on the first riser.
    b) Bolt the riser to the tank using the 2 supplied bolts and nuts.
    c) Repeat steps (a) and (b) with additional risers.
    d) Add lid and gasket to the top riser installed. 


  6. Backfill - First, begin filling in the center of the tank with pea gravel or recommended material and compact. Make sure all gaps under the tank are completely filled. Backfill with 12” layers and compact each layer before the next layer is added. ALWAYS COMPACT EVENLY AROUND THE ENTIRE TANK. Make sure to compact backfill under pipes.

    Pro Tip: Mound the soil over the top to provide positive drainage away from the tank.


Now you have a basic idea of how to install an underground water storage system, you're set to go! Regular checks of an undergound tank is recommended to ensure nothing out of the ordinary has happened. If you are using your underground tank for drinking or potable water, you should also make an effort to monitor the water quality to be sure no debris has entered through the tank lid. If you have any questions, comments or additional ideas regarding installation, post them below and I will help out where I can.

Topics: Installing a Water Storage Tank

Matthew Firth

Written by Matthew Firth

I really enjoy teaching people about topics that I am passionate about.

Get Monthly Helpful Tips

See the Original Algae Resistant Water Tanks


Recent Posts