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Underground Tank Installation

Since there are a variety of underground water tanks on the market, instructions for installing an underground water tank will differ from supplier to supplier. The following tank installation instructions are for Clark Tanks’ Li-Lo model of underground tanks.

Underground Tank Installation

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 the subsoil.


To ensure that sufficient space is available for working, the base area of the trench must exceed the dimensions of the tank by > 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 max. 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 approx. 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 8/16, thickness approx. 100 – 150 mm) is applied as the foundation.

Slope & Embankment - Li-Lo Underground Rainwater Tank

Slope, embankment, etc.

On installation of the tank in the immediate vicinity (< 5 m) of a slope, earthen mound or 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 500 mm in all directions, and must be located at least 1000 mm away from the tank.

Groundwater and cohesive (water impermeable) soils (e.g. clay soil)

If it is anticipated that the tanks will be immersed deeper into the groundwater than is shown in the adjacent figure, sufficient dissipation must be ensured. (See table for max. immersion depth). Dissipation of the drainage water (e.g. via an annular drainage system) is recommended in the case of cohesive, water-impermeable soils.

Ground Water - Li-Lo Underground Rainwater Tank

Installation adjacent to surfaces used by vehicles

If the underground tanks are installed adjacent to surfaces which are used by heavy vehicles weighing over 3.5 ton, the minimum distance away from these surfaces is at least the depth of the trench.

Vehicle Surfaces - Li-Lo Underground Rainwater Tank

Connection of several tanks

Two or more tanks are connected via the assembly surfaces by means of GARANTIA special seals and basic pipes (to be provided at construction site).

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.

Multiple Tanks - Li-Lo Underground Rainwater Tank

Insertion and Filling

The tanks must be inserted, impact-free, into the prepared trench using suitable equipment. To avoid deformities, the tank is filled 1/3 with water before filling in the tank surrounding.

Afterwards the surrounding (roundgrain gravel, max. grain size 8/16) is then filled in layers of max. 30 cm steps and is compacted.

The individual layers as well as the medial support column must be wellcompacted (manuel tamper). Damage to the tank must be avoided during compaction. Mechanical compaction machines must not be used under any circumstances. The surrounding towards the trench must be at least 100 mm wide.

Insertion and Filling

Routing Connections

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.

Routing Connections - Li-Lo Underground Rainwater Tank