6.2 Trench
To ensure that sufficient space is available for working, the base area of the trench must exceed the
dimensions of the tank by 20”/500 mm on each side; the distance from nearby foundations or any
other built structure must be at least 40”/1000 mm.
The trench embankment must be designed so that slippage or collapse of the embankment wall is
prevented. The foundation soil has to be level and must guarantee sufficient load-bearing capability.
The depth of the trench must be dimensioned so that the max. earth coverage (see section 2 –instal-
lation conditions) above the tank is not exceeded. In order to be able to use the system year-round,
the tank and those parts of the system that conduct water must be installed in the frost-free zone of
the soil profile. Normally, the frost-free depth is between 24” – 32”(60-80 cm), but you should obtain
the information that is specific to your area from local agencies.
A 6” – 8” (15-20 cm) layer of compacted, pea gravel (grain size 8/16) should be used as a foundation.
If the soil has inadequate load-bearing capabilities, a
reinforced concrete slab (approx. 4” thick) should be
poured to ensure stability.
6.2.1 Slope, embankment, etc.
On installation of the tank in the immediate vicinity (< 16 ft/5
m) of a hillside, mound or slope, a statically calculated
supporting wall has to be constructed in order to accept the
soil pressure. The wall must exceed the dimensions of the
tank by at least 20”/500 mm in all directions, and must be
located at least 40”/1000 mm away from the tank.
6.2.2 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, additional provisions should be made to
guarantee sufficient drainage (See table for max.
immersion depth).
Installation of a drainage pipe is recommended in the
case of cohesive, water-impermeable soils.
6.2.3 Installation adjacent to surfaces used by
vehicles
If the underground tanks are installed adjacent to
surfaces that are used by vehicles heavier than
passenger cars, the minimum distance away from these
surfaces is at least the depth of the trench.