Installing an Underground Tank Without a Water Table: A 4-Step Quick Guide

Installing a Tank in the Absence of an Aquifer: If you need to install and backfill a tank where there is no aquifer, here is a clear, step-by-step procedure organized into four phases.

Below is a practical guide designed to be read “on-site”: what to do, in what order, and with the specified minimum dimensions (clearance from the perimeter, subfloor thicknesses, 50% backfill, and finishing of the top 50 cm).

Installing a Tank Without a Water Table: Where to Start

Before beginning the installation of the tank without a water table, the operating procedure calls for creating a perimeter trench at least 50 cm wide, or following the specifications set forth in any plans related to the specific construction site.

This step is necessary to properly prepare the work area for excavation and all subsequent operations: laying, leveling, backfilling, and reburying.

Perimeter clearance and excavation layout

The first step is to define the excavation, taking into account:

• A minimum clearance of 50 cm around the perimeter, unless otherwise specified in the design.
• Excavation depth calculated to allow for the construction of the subbase with the specified thicknesses.

At this stage, it’s best to think in terms of a “sequence”: excavation is not an end in itself, but must allow for the proper construction of the subbase and subsequent backfilling, without improvisation.

Excavation Depth and Subgrade: Minimum Thickness Requirements

The depth of the excavation must be calculated to allow for the construction of at least:

• 30 cm of stabilized material
• 20 cm of sand

These two layers serve as the operational reference for the installation subbase. In practical terms, the goal is to prepare a suitable and properly leveled subbase before installing the tank.

Phase 1: Excavation to specified dimensions and preparation of the subbase

In Phase 1, two related activities are performed:

1. Execution of the excavation with a fixed cross-section.
2. Preparation of the subfloor.

In practice, after completing the excavation, the following materials are spread across the bottom:

• at least 30 cm of stabilized material
• 20 cm of sand

Everything must be properly leveled (this marks the completion of Phase 1). In other words: first, prepare the “work surface” (stabilized base + sand), then move on to handling and installing the tank.

Step 2: Position the tank and fill it halfway

Once the subfloor has been prepared, we move on to the actual installation:

• Using an appropriate lifting device, place the tank in its intended position.
• Proceed with the tank leveling operations.

After leveling, the recommended sequence is as follows:

1. Fill the tank to 50% capacity.
2. Begin backfilling the excavation with fine sand until it is 50% full.

Phase 2 focuses on a simple concept: work proceeds “in parallel” on the reservoir and the excavation, bringing both to 50% completion according to the procedure described.

Step 3: Complete filling the tank and backfill with fine sand

In Phase 3, the recommendation is:

• Finish filling the tank.
• Continue filling with fine sand (backfilling).

This phase also includes liaison activities:

• Connect all the necessary pipes.

The idea is to follow a systematic sequence: first, complete the work on the tank (filling it), then make the necessary connections, and finally backfill with fine sand.

Step 4: Completing the backfilling and finishing the last 50 cm

Phase 4 involves completing the backfilling of the tank without a water table and finishing the surface.

The operational guideline is:

• Complete filling the excavation with fine sand (Step 3).
• The last 50 cm can be filled with soil suitable for the desired surface layer (Step 4).

This procedure allows you to clearly distinguish between two materials or criteria: first, fine sand to complete the excavation, then the top 50 cm of cohesive soil consistent with the planned surface treatment.

Quick Operational Summary (Checklist)

For a quick read, here’s the complete sequence:

1. Clear perimeter space of at least 50 cm (or as specified in the construction plans).
2. Excavation to a depth appropriate for the subgrade: 30 cm of stabilized material + 20 cm of sand.
3. Phase 1: The subgrade has been stabilized and the sand has been leveled.
4. Install the tank using the appropriate equipment and level it.
5. Step 2: Fill the tank 50% and backfill with 50% fine sand.
6. Step 3: Complete filling the tank + continue backfilling with fine sand + connect the necessary pipes.
7. Step 4: Fill the excavation with fine sand and cover the top 50 cm with suitable soil.

Conclusion

By following these four steps, installing a tank without a water table becomes a well-organized process: excavation and subgrade preparation, installation and leveling, progressive backfilling (to 50% and then completion), backfilling with fine sand, and finishing the top 50 cm.