The Foundation of a Long-Lasting Water System
When you invest in a large-scale water storage solution, the installation is just as important as the tank itself. A full 50,000-litre tank weighs approximately 50 tons. Without a stable base, that weight can cause the ground to shift, potentially damaging the tank liner or the steel structure.
For most Oasis Aluzinc tanks under 350,000 litres, a concrete slab isn’t strictly necessary. Instead, you can prepare a sand pad foundation. This is a cost-effective, durable, and highly effective base—provided it is done correctly.
Step 1: Site Selection and Excavation
The first step is to choose a site that is naturally flat and stable. Avoid areas with “disturbed” earth or loose fill-material, as these are prone to subsidence.
- Mark the area: Your foundation must be at least 2 metres wider than the diameter of the tank. This extra space provides a “buffer zone” to prevent erosion around the tank edges.
- Clear the debris: Excavate the area to a depth of 150mm. It is vital to remove all sharp rocks, roots, or organic matter that could eventually pierce the internal PVC liner.
Step 2: Ensuring a “Dead Level” Surface
One of the most critical parts of the process to prepare a sand pad foundation is leveling. A tank that is even slightly tilted puts uneven pressure on the steel bolts and the liner.
- Light Compaction: Once excavated, lightly compact the earth to a minimum of 50kPa.
- The Level Test: Use a long straight-edge and a spirit level (or a laser level for larger diameters) to ensure the site is flat. Aim for a tolerance of no more than 10mm across the entire diameter.
Step 3: Choosing and Placing the Sand
Not all sand is created equal. For a successful tank base, you should use screened pit sand or plaster sand. Do not use “river sand” or unscreened sand, as these often contain small pebbles or sharp shells that can damage the liner over time.
- Fill the excavation: Pour the sand to a depth of 150mm.
- Screed the surface: Spread the sand evenly across the site.
- Final Check: Ensure there are no “high spots” in the center. The pad should be perfectly flat to allow the liner to seat correctly once the water weight is added.
Step 4: Erosion Control and Drainage
In South Africa, heavy summer thunderstorms can quickly wash away an unprotected sand pad. To prevent this, you must protect the perimeter of your foundation.
- The Aggregate Ring: Once the tank is installed, place a bed of 19mm crushed rock aggregate around the entire circumference. This ring should be roughly 1 metre wide and 100mm thick.
- Diversion Trenches: Ensure that the surrounding landscape slopes away from the tank. If the tank is at the bottom of a slope, dig a small diversion trench to channel runoff water around the pad rather than through it.
Step 5: The “Anchor” Fill
A common mistake is leaving a newly installed tank empty. As part of the process to prepare a sand pad foundation, you must be ready to fill the tank to at least 10% of its volume immediately after assembly. This water acts as a weight to “seat” the liner into the sand and prevents the tank from shifting or being damaged by high winds while empty.
Need Help with Your Installation?
While many homeowners and farmers choose to prepare a sand pad foundation themselves, Oasis Tanks has a network of accredited installers across South Africa who can handle the entire process for you.