Design
and building of
steel construction of the tank can not be done without taking into
account the
relation between upper steel construction and soil basement.
Inclusion
of coefficient of the bed Kb
in solution of contact problem in the joint shell - bottom is not sufficient.The
badly designed or
badly executed foundations can cause failure condition of the tank and
at least
it can lead to differential settlement. Welded spatial constructions
with thin
walls are flexible and they are very sensitive to the settlement.
1. Shape of the bottom
a) cone up bottom
This is more often used bottom shape. It allows water drains from
inside
of the tank to the situated near to the shell draw-off sump. The
atmospheric waters do not move
toward to the center of the tank and are not kept bellow the bottom.
b) cone
down bottom.
This type of bottom is applied when the tank often must be emptied /
filled.
Value of the slope is the same as the slope of the cone up bottom. It
assures
full drain of the water dropped inside in the tank and minimal contact
surface
between the water inside and the bottom. One of the weaknesses is the
possibility for keeping and storing the atmospheric water felt below
the
bottom. Outside water can move to the center and increase the speed of
bottom
corrosion.
c) flat
bottom without slope
It can be applied in the small tanks where the removing of the water
dropped inside into the tank is not required.
d) flat bottom with single slope.
Can be applied where the diameter D ≤ 30 m. This type of
bottom
requires sloped
foundation and the construction of the shell must assure its vertical
position.
2. Type of foundation construction
a) foundation from compacted soil (earth type foundation).
This is most often applied scheme of foundation construction because it
is cheapest and easiest for execution. It is made according to the
scheme shown
in standard API 650. It is used when the soil can bear the pressure of the
upper
steel
construction and when the anchorage is not necessary. Even when there
is small
leak moving out of the soil is possible. It can leads to
destruction of the tank. In this reason the diameter of the tank must
be bigger
than the diameter of the tank with not less then 1,8 m. Earth
type foundation does not allow good leveling of the bottom i.e. of
the shell of the tank. When they are used it is possible the uneven
settlement
which cause additional efforts in the tank's elements.
Earth type foundation
b) reinforced concrete ring which is not placed
under the shell.
The tanks which are subject of this research
are the above ground facilities. They are placed on 0,3 ÷1,2 m above the
soil. This level difference is remarkable in the fuel
oil tanks where pump always must be under the liquids (Δh ≥ 0,7m). If
the
classical earth type foundation reaches this height the facility must
occupy
remarkable surface on the site. The reinforcing of such different
leveled
surfaces bears a risk for landslide (when the earth is covered by grass
or
asphalt) or it is slow, expensive and work consuming process (when the
earth is
paved).
In order to avoid this inconvenience appears the
idea of small foundation ring between the ground and the bottom level
which
ring is a combination free sand pillow and reinforced concrete ring in
the
periphery. The proposed construction is similar to the API Std. 650,
but the
foundation ring is moved in the outside direction where it can not be
influenced of the load of the shell and the tank roof upon it. When
there is
soil settlement under the tank, the reinforced concrete ring does not
allow
full drain of the water so that this solution is unfortunate. It should not be applied to
the new
build tanks.
Reinforced concrete ringwall foundation which
is not placed under the shell
c) reinforced concrete ring wall foundation.
The trend in the tanks building shows that the
volumes of the facility increase. Spatial steel construction of the
tanks
stands more flexible. In this reason bigger attention must be paid to
the shell
settlement and to the prevention measures. The use of the rigid
reinforced
concrete ring increase around the world. When the tanks are bigger
the
dimensions of the rings are: largeness not less then 0,6 m and height
1,5 – 2,0 m.This type foundation construction allows very
good leveling of the periphery of the bottom and the shell which is
positioned
on it. The uneven settlement of the tank is limited. It is possible
anchors to
be put there.
Reinforced concrete ringwall foundation
d) reinforced concrete slab
They could be applied when the tanks are relatively
small because this type of foundations is very expensive.
Thick concrete slabs are more favorable for upper
steel structure. They do not allow the uneven settlement of the tank.
The reinforced
concrete slabs are very recommendable when the level of the underground
water is
high.
Reinforced concrete slab
The special foundation constructions as the foundation
in the weak soil must be executed according to the concrete projects
after the
technical and economical analysis made on the ground of results of the
geological and engineer research.
