Installing Li-Lo Flat Underground Water Tanks

Download Installing Li-Lo Flat Underground Water Tanks PDFInstalling Li-Lo Flat Underground Water Tanks (PDF Version)

Installing Li-Lo Flat Underground Water Tanks

Underground water tanks are a great way to harvest rainwater without consuming your above ground property space. National Poly Industries’ Li-Lo underground poly tanks offer an innovative flat design only requiring shallow excavation. They are manufactured in our Australian facilities according to a patented and leading German-engineered design for optimum strength.

This article explores how to install our Li-Lo underground tanks. For complete installation and maintenance instructions, please refer to our full underground tank installation guide.

Li-Lo Underground Tank Overview

Underground Tank Overview Diagram

Construction Site

Under all circumstances, the following points must be clarified prior to installation:

  • The structural suitability of the ground according to DIN 18196
  • Maximum groundwater levels which occur and drainage capability of the subsoil
  • Types of load which occur, e.g. traffic loads

An expert ground report should be requested from the local planning authority to determine the physical characteristics of your subsoil.


To ensure that sufficient space is available for working, the base area of the trench must exceed the dimensions of the tank by greater than 100 mm on each side; the distance from solid constructions must be at least 1000 mm.

If the depth of the trench is > 1250 mm an embankment must be designed according to DIN 4124. The construction site must be horizontal and plane and must guarantee sufficient load-bearing capacity.

The depth of the trench must be dimensioned so that the maximum earth coverage (see point 2 – installation conditions) above the tank is not exceeded. To use the system throughout the entire year, it is necessary to install the tank and those parts of the system which conduct water in the frost-free area. The frost-free depth is usually approximately 600 mm – 800 mm; precise information in this regard can be obtained from the responsible authority.

A layer of compacted, round-grain gravel (grain size thickness approximately 100 – 150 mm) is applied as the foundation. Please note the following with surrounding surface and soil types:

  • Sloped Surfaces – if tank is positions in ground less than 5 metres of a slope, a statically calculated supporting wall must be erected to absorb the soil pressure. The wall must exceed the dimensions of the tank by at least 500mm in all directions, and must be located at least 1000mm away from the tank.
  • Water Impermeable Soils – if tanks will be immersed deeper into the groundwater than is shown in the adjacent figure, sufficient dissipation must be ensured. Dissipation of the drainage water (e.g. via an annular drainage system) is recommended in the case of cohesive, water-impermeable soils.
  • Surfaces Adjacent to Vehicle Use – if tank is installed adjacent to surfaces which are used by heavy vehicles, the minimum distance away from these surfaces should be at least the depth of the trench.

Connecting Multiple Tanks

Connecting Multiple Tanks

Two or more tanks are connected via the assembly surfaces by means of GARANTIA special seals and basic pipes. The apertures must be drilled to the corresponding size using only the GARANTIA special crown bit. It must be ensured that the distance between the tanks is at least 500 mm. The pipes must project at least 200 mm into the tanks.

Tank Insertion and Filling Hole

Insertion and Filling Hole

The tanks must be inserted, impact-free, into the prepared trench using suitable equipment.

To avoid deformities, the tank is filled one-third full with water before filling in the tank surrounding.

Afterwards the surrounding (round grain gravel of 100–150mm thickness) is then filled in layers of at most 300mm. 30 cm steps and is compacted.

Note: The individual layers as well as the medial support column must carefully well compacted with manual tampering to avoid damage to your tank. Mechanical compaction machines must not be used under any circumstances. The surrounding towards the trench must be at least 100 mm wide.


Routing Connections

Feed and Overflow Pipes

All feed and overflow pipes must be routed with a decline of at least 1% in the direction of flow (possible, subsequent settling must be taken into consideration in this case). If the tank overflow is connected to a public sewer, this must be protected against reflux by means of a lifting station (mixed sewer) or reflux seal (pure rainwater sewer) according to DIN 1986.

All suction, pressure and control lines must be routed in an empty pipe, which must be routed as straight as possible, without bending, to the tank with a decline. Necessary bends must be formed using 30° moulded sections.

Important: The empty pipe must be connected to an aperture above the max. water level.

National Poly Industries’ Li-Lo flat underground tanks make excavation and installation easy. Being made of strong polyethylene rather than concrete mean they are also relatively inexpensive. Contact National Poly Industries today to discuss your needs and receive a competitive customised quote.