5.2.3.3. A reinforced concrete slab may be required under the
tank as a foundation in the excavation where the bottom is
unstable.
5.3. GEOTEXTILE
5.3.1. The tank owner or the owner’s technical representative is
responsible for determining whether a geotextile or an alternate
filtering technique is appropriate for a specific installation.
Geotextile allows the passage of water, but prevents the
migration and mixing of in situ soil and the select backfill
material. Geotextile helps preserve the integrity of the select
backfill envelope, which surrounds and supports the tank.
5.3.2. Xerxes recommends that geotextile be used when the
tank is installed in:
• areas with frequently changing groundwater conditions or
areas subject to tidal fluctuations,
• unstable soils such
as cited in SECTION 5.2.3,
• water conditions with silty in situ soil.
5.3.3. For further information concerning geotextile specifica-
tions and installation procedures, consult the geotextile
supplier’s installation guidelines or instructions.
5.3.4. Polyethylene film is not considered an effective geotextile
material. It may tear or degrade while in service.
6. ANCHORING TANKS
Xerxes recommends that all tanks be anchored in installa-
tions in which the tank could be exposed to water. Xerxes
recommends that every site be thoroughly evaluated for the
potential of a rise in the local water table or of trapped
water. Failure to anchor a tank under these conditions may
damage the tank or surrounding property.
6.1. DEADMAN ANCHORS
6.1.1. A deadman is typically a reinforced concrete beam.
6.1.2. The length of the deadman is typically equal to the length
of the tank.
6.1.3. A deadman may be fabricated in multiple sections as long
as the total length is not decreased and each section contains at
least two balanced anchor points.
6.1.4. The width and thickness of the deadmen depend on the
tank diameter, water-table height, number of attached collar
risers and burial depth.
6.1.5. Deadmen should be designed according to the American
Concrete Institute (ACI) code.
6.1.6.
Refer to TABLE 6-1
for typical deadmen dimensions given
the situation of an empty tank with a burial depth of 3 feet, with
groundwater to grade and with one attached collar riser.
13
5.2.2.2. If more than one tank is to be installed in the same hole,
allow for at least 18 inches between the tanks.
(See FIGURE 5-4.
)
FIGURE 5-4
5.2.3. UNSTABLE IN SITU SOIL CONDITIONS
Xerxes recommends that the tank owner seek the advice of
a local foundation professional engineer if the in situ soil is
extremely soft or inherently unstable (for example, peat,
quicksand, muck, landfill, very soft or highly expansive
clay, underground stream, etc.).
5.2.3.1. If the soil has less than 750 lbs./sq. ft. cohesion as
calculated from an unconfined compression test; or in soils
having an ultimate bearing capacity of less than 3,500 lbs./sq.
ft.; or where soil will not maintain a vertical wall, the excavation
must allow a minimum space equal to half the diameter of the
tank between the excavation wall and both the side and the endcap
of the tank, to enhance lateral resistance.
(See FIGURE 5-5.)
5.2.3.2. The spacing between adjacent tanks is to be at least 18
inches.
(See FIGURE 5-5.)
FIGURE 5-5
1/2
Tank
Dia.
18"
1/2 Tank Dia.
1/2 Tank Dia.
Unstable Soil
18”
18”
18”
18”
18”
Stable Soil
