AGENDA ITEM
650-554
TITLE:
Bottom Underside Corrosion Mitigation (prevention)
Date:
1st June, 2008
Handled By: Alan Watson
A.R. Watson USA
4016 E Maryland St.
Bellingham, WA 98226
Cell phone: 251-751-7732
Fax:
360-752-1779
E-Mail:
arwatson@msn.com
Purpose:
To revisit Appendix “B” Recommendations for Design and Construction of
Foundations for Aboveground Oil Storage Tanks with bottom underside corrosion
mitigation (prevention) in mind
Source:
EEMUA Publication No 183: 1999 “Guide for the Prevention of Bottom Leakage
from Vertical, Cylindrical, Steel Storage Tanks”
Alan Watson 25 years experience with lifting aboveground storage tanks and has seen
what has caused bottom underside corrosion from faulty foundations
Input from aboveground storage tanks terminal operators.
Industry Impact: Clarifications to existing text with an end result of extending the tank bottom
life from underside corrosion.
Acknowledgements: I wish to thank the Engineering Equipment and Materials Users Association
and the British Standards Institution for allowing various diagrams from their
respective publications to be reproduced in this document
Edit Legend:
Red is additions
Blue are deletions
1
APPENDIX B—RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION OF
FOUNDATIONS FOR ABOVEGROUND OIL STORAGE TANKS
B.1
Scope
B.1.1 This appendix provides important considerations for the design and construction of foundations for
aboveground storage tanks with flat bottoms. Recommendations are offered to outline good standard industry
practice, to point out some pre-cautions that should be considered in the design and construction of storage tank
foundations
B.1.2 Since there are a wide variety of surface, subsurface and climatic conditions, it is not practical to establish
design data to cover all situations. However it is common practice to build tanks on the following foundation types:
a.
b.
c.
d.
e.
Earth foundation (Fig. B-1)
Earth foundation with a concrete ringwall (Fig. B-2)
Earth foundation with crushed stone ringwall (Fig. B-3)
Concrete slab foundation, plain (Fig. B-4)
Concrete slab foundation, with piles (Fig. B-4)
The allowable soil loading and the exact type of subsurface construction to be used must be decided for each
individual case after careful consideration. The same rules and precautions shall be used in selecting foundation sites
as would be applicable in designing and constructing foundations for other structures of comparable magnitude.
B.2
Subsurface Investigation and Construction
B.2.1 At any tank site, the subsurface conditions must be known to estimate establish the soil bearing capacity
and settlement that will be experienced. This informat in information is generally obtained from soil borings, load
tests, sampling, laboratory testing and analysis by an experienced geotechnical engineer familiar with the history of
similar structures in the vicinity. The subgrade must be capable of supporting the load of the tank and its contents.
The total settlement must not strain connecting piping or produce gauging inaccuracies, and the settlement should
not continue to a point at which the tank bottom is below the surrounding ground surface. The estimated settlement
shall be within the acceptable tolerances for the tank shell and bottom.
B.2.2 The methods and extent of the soil investigation should be based on factors as structure geometry and
loading, allowable structure settlements, types of soil strata, uniformity of the strata and uniformity of the properties
of the strata, adjacent structures, topographical features that could affect the design or constructability of the
anticipate structure and the general geotechnical knowledge of the area.
Typically, a sufficient numbers of borings (and/or Cone Penetration Tests) to cover the area in question should be
made. An area where previous information is available and the strata are uniform would require less investigation
than a site where the subsurface is unknown or highly variable. Typically, several borings are made under the
location where the future tank shell will be erected and one in the center of the tank
The investigation should extend to a depth such that the vertical loads imposed on the soil could neither precipitate a
local or general failure nor be a source of significant settlement. This would generally be equal to the depth where
the increase in vertical stress due to structure load is less than 10% of the effective overburden stress.
B.2.3 When actual experience with similar tanks and foundations at a particular site is not available, the
following ranges for factors of safety should be considered for use in the foundation design criteria for determining
the allowable soil bearing pressures. (The owner or geotechnical engineer responsible for the project may use factors
of safety outside these ranges.)
B.2.4
a.
From 2.0 to 3.0 against ultimate bearing failure for normal operating conditions.
b.
From 1.5 to 2.25 against ultimate bearing failure during hydrostatic testing.
c.
From 1.5 to 2.25 against ultimate bearing failure for operating conditions plus the
maximum effect of wind or seismic loads.
Some of the many conditions that require special engineering consideration are as follows:
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a.
Sites on hillsides, where part of a tank may be on undisturbed ground or rock and part
may be on fill or another construction type or where the depth of required fill is variable.
b.
Sites on swampy or filled ground, where layers of muck or compressible vegetation are at
or below the surface or where unstable or corrosive materials may have been deposited as
fill.
2
c.
Sites underlain by soils, such as layers of plastic clay or organic clays that may support
heavy loads temporarily but settle excessively over long periods of time.
d.
Sites adjacent to water courses or deep excavations, where the lateral stability of the
ground is questionable.
e.
Sites immediately adjacent to heavy structures that distribute some of their load to the
subsoil under the tank sites. There by reducing Loads from nearby structures reduce the
subsoil’s capacity to carry additional tank and foundation loads without excessive
settlement.
f.
Sites where tanks may be exposed to flood waters, possibly resulting in uplift,
displacement, or scour.
g.
Sites in regions of high seismicity that may be susceptible to liquefaction.
h.
Sites with thin layers of soft clay soils that are directly beneath the tank bottom and that
can cause lateral ground stability problems.
i.
Sites where tanks, buildings or existing foundations have been removed resulting in nonuniform bearing capacity under the new structure.
j
Sites where the rerouting of existing aboveground and underground water courses can
impact the tank support.
B.2.5 If the subgrade is inadequate to carry the load of the filled tank without excessive settlement, one or more
of the following general methods can be considered to improve the support conditions:
a.
Removing the objectionable material and replacing it with suitable, compacted material.
b.
Compacting the soft material by preloading the area with an overburden of soil. Strip
Wick or sand drains may be used in conjunction with this method.
c.
Stabilizing the soft material by chemical methods or injection of cement grout.
d.
Transferring the load to a more stable material underneath the subgrade by driving piles
or constructing foundation piers. This involves constructing a reinforced concrete slab on
the piles to distribute the load from the tank bottom.
e.
Constructing a slab foundation that will distribute the load over a sufficiently large area
of the soft material so that the load intensity will be within allowable limits and excessive
settlement will not occur.
f.
Use of vibro-compaction, vibro-replacement, deep dynamic compaction, stone columns
and other soil improvement methods.
g.
Slow and controlled filling of the tank during hydrostatic testing. When this method is
used, the integrity of the tank may be compromised by excessive settlements of the shell
or bottom. For this reason, the settlements of the tank shall be closely monitored. In the
event of settlements beyond established ranges, the test may have to be stopped and the
tank re-leveled. See API 650 7.3.6
B.2.6 The fill material used to replace muck or other objectionable material or to build up the grade to a suitable
height shall be adequate for the support of the tank and product after the material has been compacted. It is very
important that the fill material be , well graded granular and free of vegetation, organic matter, cinders, lumps of
clay, rocks, paper, plastic, wood, welding electrodes etc. and any material that will cause corrosion of the tank
bottom. The grade and type of fill material shall be capable of being compacted with standard industry compaction
techniques to a density sufficient to provide appropriate bearing capacity and acceptable settlements. The placement
of the fill material shall be in accordance with the project specifications prepared by a qualified geotechnical
engineer or a person experienced in tank foundation design. If large particle sizes are used, differential aeration
corrosion (pitting) may occur at locations where the large particles or debris contact the steel tank bottom. In this
case, cathodic protection may not be effective in preventing pitting.
B.2.7 Consider the chemistry of native soil and fill material when designing the foundation. Osmosis and/or
periodic flooding can cause corrosive material such as chloride salts and sulfates to migrate up to the tank bottom
and increase the risk of corrosion.
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B.2.8 Soil resistivity provides valuable information about the corrosivity of the material used under and around a
tank. A general resistivity classification is given in Table 1. There are several techniques for measuring soil
resistivity. A common method is described in ASTM G 57.
Table 1—General Classification of Resistivity
Resistivity Range (ohm-cm)
<500
500-1,000
1,000-2,000
2,000-10,000
>10,000
Potential Corrosion Activity
Very Corrosive
Corrosive
Moderately corrosive
Mildly corrosive
Progressively less corrosive
B.2.9
The resistivity of the pad material may be higher than the existing surrounding soil. Corrosive soil
beneath the higher resistivity pad material may contaminate the pad fill by capillary action. Thus, resistivity of
surrounding soil may be used to determine the probability of corrosion on the tank bottom. The results of soil
resistivity surveys can be used to determine the need for cathodic protection. However, other properties of the soil
should also be considered.
B.2.10
In coastal areas, salt will be washed off the roof and down the sides of the tank by rain and may flow
beneath the tank causing accelerated corrosion. This also can occur in areas where fertilizers or chemicals may be in
the atmosphere either from spraying or industrial operations. The tank pad also can become contaminated by
wicking action that can draw contaminants such as chlorides up from the water table. Cathodic protection is usually
necessary for corrosion prevention in these situations. Where possible, it is recommended to seal the gap between
the tank bottom edge projection and its foundation to prevent moisture from going under the tank bottom.
B.3
Tank Foundation Elevations Grades
B.3.1. Unless otherwise specified by the owner, the finished tank grade shall be crowned from its outer periphery
to its center at a slope of 1 inch in 10 ft (cone up). The crown will partly compensate for settlement which is likely to
be greater at the center. It will also facilitate cleaning and the removal of water and sludge through openings in the
shell or from sumps situated near the shell. Because crowning will affect the lengths of roof-supporting columns, it
is essential that the tank Manufacturer be fully informed of this feature sufficiently in advance. (For an alternative to
this paragraph. see B.3.2.)
B.3.2. As an alternative to B.3.1 the tank bottom may be sloped downward toward a sump (cone down). The tank
manufacturers must be advised as required in B.3.1
B.3.3. In wet climates the grade or elevation surface on which of the lowest point of a tank bottom will rest should
be constructed at least preferably 0.3 m 450 mm (1 ft) (18 in) or more above the surrounding ground surface dike
yard. This will provide suitable drainage, help keep the tank bottom dry and compensate for some small settlement
that is likely to occur. If a large settlement is expected, the tank bottom elevation shall be raised so that the final
elevation above grade will be a minimum of 150mm (6 in) 450 mm (18 in.) after settlement.
B.3.4. In dry climates the grade or surface on which the lowest point of a tank bottom will rest can be reduced to
300 mm (12 in) above the surrounding dike yard.
B.3.5. In lieu of elevating the tank foundation, consideration can be given to providing an adequate sump volume
in the dike yard for the worst case storm or snowmelt.
B.4.
Foundation Material
B.4.1 There are several different materials that can be used for the grade or surface on which the tank bottom will
rest. To minimize future corrosion problems and maximize the effect of corrosion prevention systems such as
cathodic protection, the material in contact with the tank bottom should be clean well graded, granular material. fine
and uniform. Gravel or large particles shall be avoided. Clean washed sand 75nn – 100mm (3 in – 4 in) deep is
recommended as a final layer because it can readily shaped to the bottom contour of the tank bottom to provide
maximum soil contact are and will protect the tank bottom from coming into contact with large particles and debris.
Large foreign objects or point contact by gravel or rocks could cause corrosion cells that will cause pitting and
premature tank bottom failure. Precautions shall be taken during installation of the grade material and erection of the
tank to prevent contamination of the grade material.
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B.4.2. If cathodic protection is to be installed (see API Recommended Practice 651) the clean washed sand layer
should be increased to a minimum of 150 mm (6 in).
B.4.2.1. API Recommended Practice 651 Allows tank bottoms to be placed directly on concrete when proper
drainage is provided.
B.4.3.
The following materials:
a.
Bitumen-sand (cold patch asphalt) mix 50 mm (2 in) thick laid on top of the foundation under the tank
steel bottom
Note;
1
2
Bitumen-sand layers under steel tank bottoms inhibit cathodic protection current.
Bitumen-sand layers can be affected by welding resulting in porosity or cracks in welds
and producing smoke discomfort to welders
In very hot climates or heated tanks the bitumen-sand layer may leach out from under the
tank shell causing the tank shell to become unsupported in places.
Bitumen-sand must be laid a minimum of 50 mm (2 in) thick to prevent the bitumen-sand
from cracking and allowing corrosion to occur.
3
4.
b.
Oil-sand mixture consists of approximately 90 liters (18 gals.) of heavy base petroleum oil per cu
meter (per cu yd). The sand has the correct amount of oil when it can be formed into a ball without
dripping. Sand should be coated but not running with excess oil.
Note;
1
2
c.
Oil-sand layers under steel tank bottoms inhibit cathodic protection current.
Oil-sand layers can be affected by welding resulting in porosity or cracks in welds and
producing smoke discomfort to welders
Clean washed sand
Note;
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1.
Measuring pH indicates the hydrogen ion content of a soil. Corrosion of steel is fairly
independent of pH when it is in the range of 5.0 to 8.0. The rate of corrosion increases
appreciably when pH is < 5.0 and decreases when pH is > 8.0. pH is determined in
accordance with ASTM G 51 or equivalent. There is currently no industry consensus on
an acceptable range for pH levels; therefore the tank owner/operator should specify the
acceptable pH level. There are practical and possible economic limitations in achieving
minimum levels of pH content
2.
Chlorides will affect the resistivity of soil and act as a depolarizing agent, increasing the
current requirement for cathodic protection of steel. Pitting corrosion on steel can begin
at chloride levels as low as 10 ppm, depending on the tank location. Chloride content
may be determined in accordance with ASTM D 512 or equivalent. There is currently no
industry consensus on an acceptable range for chloride levels; therefore the tank
owner/operator should specify the acceptable chloride level. There are practical and
possible economic limitations in achieving minimum levels of chloride content.
3.
Sulfate levels >200 ppm frequently indicate high concentrations of organic matter.
Sulfate content may be determined in accordance with ASTM D 516 or equivalent. There
is currently no industry consensus on an acceptable range for sulfate levels, therefore the
tank owner/operator should specify the acceptable sulfate level. There are practical and
possible economic limitations in achieving minimum levels of sulfate content.
4.
Sulfide levels > 0.10 ppm may indicate that sulfates have been reduced by bacteria.
Sulfide content may be determined in accordance with EPA 0376.1 or equivalent. There
is currently no industry consensus on an acceptable range for sulfide levels; therefore the
tank owner/operator should specify the acceptable sulfide level. There are practical and
possible economic limitations in achieving minimum levels of sulfide content.
5
5.
d.
B.5.
Random testing of the sand should be conducted to determine if the electrical resistivity
and chemical properties are at acceptable levels. Sand samples used to determine the
properties of the material should be taken from the actual material that is to be used
during construction. If test results do not meet owner/operator specified levels, then
additional steps such as rewashing and/or adding Portland cement or lime, or securing
another source of sand, may need to be taken.
Crushed limestone or clamshells (refer API Recommended Practice 651) can be used as an acceptable
material under the tank bottom
Foundation Construction.
B.5.1. During construction, the movement of equipment and materials across the prepared grade will mar damage
the graded surface. After the tank foundation pad has been completed by the foundation contractor, equipment and
workers should not be allowed to be on the tank pad unless bottom plates have been laid down first. This prevents
contamination of the tank pad with clay and other foreign materials from equipment tires, tracks and worker's
boots/shoes. These irregularities should be corrected If contaminates are introduced, they should be removed before
bottom plates are placed for welding
B.5.2. The bottom side of each steel plate to be used for tank bottom construction should be inspected
immediately before placement onto the pad to ensure that any contaminating debris that is adhered to it (e.g., mud)
is removed and that the plate surface is clean.
B.5.3. Adequate provisions, such as making size gradients in sub- layers progressively smaller from bottom to top,
or the use of synthetic membrane or geotextile fabric liners should be made to prevent the fine material from
leaching down into the larger material, thus negating the effect of using the fine material as a final layer. This is
particularly important for the top of a crushed rock ringwall.
B.5.3. Provision should be made to prevent fine material from leaching downward into larger material, thus
undoing the use of fine material as the top layer. This is particularly important for the top of a crushed rock ringwall.
To prevent this occurrence, a synthetic membrane or geotextile fabric liner may be used to separate layers , or the
grain size of sub-layers can be made progressive larger from the lower level upward.
B.5.4. Suitable compaction methods should be employed on each layer to achieve the desired density. The
maximum placement of backfill should be performed in thin lifts suitable for the material or as directed by the
recommendations of the soils report.
CAUTION: Compaction by water flooding is not recommended because the water used to flood the sand pad may
cause contamination and deterioration of the original chemical properties of the sand material. A municipal potable
water supply may not be acceptable because of chlorination that may produce high chloride levels.
Note;
For more information on tank bottom corrosion and corrosion prevention that relates to the foundation of a tank, see
API RP 651. “Cathodic Protection of Aboveground Petroleum Storage Tanks”
B.6.
Typical Foundation Types
B.6.1.1 Introduction
Many satisfactory foundation designs are possible when sound engineering judgment is used in their development.
Three designs are referred to in this appendix on the basis of their satisfactory long term performance. Several
design examples are given below to illustrate various options and advantages or disadvantages of each type and are
not meant to exclude other designs from being used.
B.6.1.2 Small Tank Foundations
For small tanks, foundations can consist of compacted crushed stone, screenings, fine gravel, clean sand or similar
material placed directly on virgin soil. Any unstable material must be removed and any replacement material must
be thoroughly compacted. Two recommended designs that include ringwalls are illustrated in Figures B-1 and B-2
and described in B.4.2 and B.4.3.
B.6.2.
Earth Foundations without a Ringwall (Fig B-1)
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B. 6.2.1 When an engineering evaluation of subsurface conditions that is based on experience and/or exploratory
work has shown that the subgrade has adequate bearing capacity and that tank settlements will be acceptable,
satisfactory foundations may be constructed from earth materials. The performance requirements for earth
foundations are identical to those for more extensive foundations. Specifically, an earth foundation should
accomplish the following:
a.
Provide a stable plane for the support of the tank.
b.
Limit overall settlement of the tank grade to values compatible with the allowances used in the
design of the connecting piping.
c.
Provide adequate drainage.
d.
Not settle excessively at the perimeter due to the weight of the shell wall.
Note. If the bearing strength of the soil will allow, the load under the tank foundation (including, roof, shell and
contained liquid) should be approximately equal to the load under the center of a full tank.
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Tank Shell
Refer to B.4.2.
Seal the edge around tank
Tank Bottom
Tank foundation shoulder
Slope 1: 1.5
Slope 1:10
Suitable Backfill Material
Drainage Pipes
Ground Level
Figure B-1 Earth Foundation.
Notes:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Tank shoulder 1 m (3 ft) wide for tanks under 15 m (50 ft) diameter and 1.5 m (5 ft) wide for tanks over 15 m (50 ft)
diameter
Tank with a cone up bottom, recommended foundation height at tank shell is 450 mm (18 in) above the surrounding ground
elevation.
For tank with a cone down bottom, recommended foundation height is equal to the greater of 450 mm (18 in), or the
dimension from the bottom of the tank shell to the bottom of the tank sump, plus the slope of the drainage pipe from the
center of the tank to terminate outside the foundation, plus 100 mm (4 in). The bottom of the drainage pipe shall be 100 mm
(4 in) above the surrounding grade
Recommend laying sand or sand bitumen mix over backfill and under tank bottom as a corrosion mitigation barrier API 650
Appendix B.4.3.
Protect the tank shoulder from erosion by providing a 25 mm (1 in.) hot sand bitumen mix, chip seal, or other suitable
method. The protection layer is to cover the tank foundation shoulder from grade to tank bottom edge. Seal the protection
layer to the tank bottom as a moisture prevention barrier. Do not lay material over the tank bottom extension.
A release prevention barrier (RPB) (if specified) such as a flexible impermeable membrane liner will block groundwater
from migrating upward into the foundation and serve to contain and channel leaks for detection. The liner material should
be compatible with and impermeable to the stored substance, and be protected from the backfill by a geotextile fabric. The
liner should slope from the center of the tank towards the tank edge. See API 650 Appendix “I” “Under Tank Leak
Detection and Subgrade Protection”
A release prevention barrier (RPB) such as a flexible impermeable membrane liner can be used to increase the inspection
interval of the tank bottom API 653 6.4.3
The outside edge of the tank bottom should be sealed to prevent moisture from going under the tank. Assure that sealing
material at the bottom junction does not extend above top of tank bottom extension to form a water trap outside of the tank
shell. After tank hydro-testing inspect this area and remove any sealing material or bitumen mix which extrudes out from
beneath tank to form such a water trap. The tank bottom extension must be visible for inspection at all times.
The foundation under the shell shall be level within tolerances allowed under. API 650 7.5.5.2 b
Drainage pipes API 650 Appendix B Fig B-5
Suitable backfill material API 650 Appendix B 2.5
For Cathodic Protection refer to API RP 651
For hydrostatic testing requirements refer to API 650 7.3.6
B.6.3. Concrete Ringwall Foundation (Fig B-2)
B.6.3.1 Large tanks, tanks with heavy or tall shells and/ or self-supported roofs impose a substantial load on the
foundation under the shell. This is particularly important with regard to radial shell distortion at the tank top in
floating-roof tanks. When there is some doubt whether a foundation will be able to carry the shell load directly, a
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concrete ringwall foundation should be used. As an alternative to the concrete ringwall noted in this section, a
crushed stone ringwall may be used. A foundation with a concrete ringwall has the following advantages:
a.
It provides better distribution of the concentrated load of the shell to produce a more nearly
uniform soil loading under the tank.
b.
It provides a level, solid starting plane for construction of the shell.
c.
It provides a better means of leveling the tank grade, and it is capable of preserving its contour
during construction.
d.
It retains the fill under the tank bottom and prevents loss of material as a result of erosion.
e.
It contributes to limiting moisture under the tank if adequately sealed
A disadvantage of concrete ringwalls is that they may not smoothly conform to differential edge
settlements. This disadvantage may lead to high bending stresses in the bottom plates adjacent to the
ringwall.
Note: If the backfill directly on the inside of the ringwall has not been installed and compacted correctly,
settlement at that location can lead to high bending stresses in the bottom plate adjacent to the ringwall.
B.6.3.2 When a concrete ringwall is designed, it shall be proportioned so that the allowable soil bearing is not
exceeded. The ringwall shall not be less than 300mm (12 in.) thick. The centerline diameter of the ringwall should
equal the nominal diameter of the tank: however, the ringwall centerline may vary if required to facilitate the
placement of anchor bolts or to satisfy soil bearing limits for seismic loads or excessive uplift forces. The depth of
the wall will depend on local conditions, but the depth must be sufficient to place the bottom of the ringwall below
the anticipated frost penetration and within the specified bearing strata. As a minimum, the bottom of the ringwall, if
founded on soil, shall be located 0.6 m (2 ft) below the lowest adjacent finish grade. Tank foundations must be
constructed within the tolerances specified in API 650. 7.5.5. Recesses shall be provided in the wall for flush- type
cleanouts, draw off sumps and any other appurtenances that require recesses.
B.6.3.3 A ringwall should be reinforced against temperature changes and shrinkage and reinforced to resist the
lateral pressure of the confined fill with its surcharge from product loads. ACI 318 is recommended for design stress
values, material specifications, and rebars development and cover. It is recommended that other relevant
publications of ACI be utilized as applicable. The following items concerning a ringwall shall be considered:
a.
The ringwall shall be reinforced to resist the direct hoop tension resulting from the lateral earth
pressure on the ringwalls inside face. Unless substantiated by proper geotechnical analysis, the
lateral earth pressure shall be assumed to be at least 50% of the vertical pressure due to fluid and
soil weight. If a granular backfill is used, a lateral earth pressure coefficient of 30% may be used.
b.
The ringwall shall be reinforced to resist the bending moment resulting from the uniform moment
load. The uniform moment load shall account for the eccentricities of the applied shell and
pressure loads relative to the centroid of the resulting soil pressure. The pressure load is due to the
fluid pressure on the horizontal projection of the ringwall inside the shell.
c.
The ringwall shall be reinforced to resist the bending and torsion moments resulting from lateral,
wind, or seismic loads applied eccentrically to it. A rational analysis, which includes the effect of
the foundation stiffness, shall be used to determine these moments and soil pressure distributions.
d.
The total hoop steel area required to resist the loads noted above shall not be less than the area
required for temperature changes and shrinkage. The hoop steel area required for temperature
changes and shrinkage is 0.0025 times the vertical cross-sectional area of the ringwall or the
minimum reinforcement for walls called for in ACI 318, Chapter 14.
e.
For ringwalls, the vertical steel area required for temperature changes and shrinkage is 0.0015
times the horizontal cross-sectional area of the ringwall or the minimum reinforcement for walls
called for in ACI 318, Chapter 14. Additional vertical steel may be required for uplift or tensional
resistance. If the ring foundation is wider than its depth, the design shall consider its behavior as
an annular slab with flexure in the radial direction. Temperature and shrinkage reinforcement shall
meet the ACI 318 provisions for slabs. (See ACI 318, Chapter 7.)
f.
When the ringwall width exceeds 460 mm (18 in.), using a footing beneath the wall should be
considered. Footings may also be useful for resistance to uplift forces.
g.
Structural backfill within and adjacent to concrete ring- walls and around items such as vaults,
under tank piping, and sumps requires close field control to maintain settlement tolerances.
Backfill should be granular material compacted to the density and compacting as specified in the
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9
foundation construction specifications. For other backfill materials, sufficient tests shall he
conducted to verify that the material has adequate strength and will undergo minimal settlement.
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10
Tank Shell
Refer to B.4.2
Seal the edge around tank
Tank Bottom
Beveled 25 mm (1 in)
Drainage Pipes
Beveled 50 mm (2 in)
Concrete Ringwall
Suitable Backfill Material
Slope grade away from concrete
for a distance of 2 m (6ft)
Concrete Spread Footing
Figure B-2 Concrete Ringwall Foundation
Notes:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
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Concrete ringwall is to be designed by a person experienced in tank foundation design
See Appendix B.6.3. for requirements for concrete ringwalls
The top of the concrete ringwall should be smooth and level. The concrete strength shall be at least 20 MPa (3000 lbs/sqin)
after 28 days. Reinforcement splices must be staggered and should be lapped to develop full strength in the bond. If
staggering of laps is not possible, refer to ACI 318 for additional development requirements.
Where a concrete ringwall is provided under the shell, the top of the ringwall should be level within tolerances under API
650 5.5.5.2 a
Concrete ringwalls should not be built less than 300 mm (12 in) in width
Concrete ringwalls that exceed 300 mm (12 in.) in width should have steel rebar distributed on both faces.
After concrete ringwall has been built, remove any unsuitable material from inside of the ringwall and replace with
suitable thoroughly compacted fill material. Do not allow any water to run under the concrete ringwall.
The need for a spread footing should be evaluated when building concrete ring wall on soft ground.
When the ringwall width exceeds 460 mm (18 in) using a spread footing beneath the wall should be considered.
Tank anchors maybe required see API 5.12
50 mm (2 in) Sand Bitumen Mix or a minimum of 1/2in Asphalt Board laid on top of concrete ring wall will stop sand
from eroding out from under the tank bottom, and will allow the sketch plate / backing strips from touching the concrete
ring wall. This will prevent moisture from going under the tank bottom. API 651 allows tanks bottoms to be placed
directly on concrete when proper drainage is provided.
A release prevention barrier (RPB) such as a flexible impermeable membrane liner slopes from the center of the tank and
is attached to the inside of the concrete ring wall. The liner will block groundwater from migrating upward into the
foundation and serve to contain and channel leaks for detection. The liner material should be compatible with and
impermeable to the stored substance, and be protected from the backfill by a geotextile fabric. See API 650 Appendix “I”
“Under Tank Leak Detection and Subgrade Protection”
A release prevention barrier (RPB) such as a flexible impermeable membrane liner can be used to increase the inspection
interval of the tank bottom API 653 6.4.3
The outside edge of the tank bottom should be sealed to prevent moisture from going under the tank. Assure that sealing
material at the bottom junction does not extend above top of tank bottom chime to form a water trap outside of the tank
shell. After tank testing inspect this area and remove any sealing material or bitumen mix which extrudes out from beneath
tank to form such a water trap. The tank bottom extension must be visible for inspection at all times.
Recommend laying sand or sand bitumen mix over backfill and under tank bottom as a corrosion mitigation barrier API
650 Appendix B.3.2.1.
Drainage pipes API 650 Appendix B Fig B-5
Suitable backfill material API 650 Appendix B 2.5
For Cathodic Protection refer to API RP 651
For hydrostatic testing requirements refer to API 650 7.3.6
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B.6.4. Crushed Stone or Gravel Ringwall Foundation (Fig B-3)
B.6.4.1 A crushed stone or gravel ringwall will provide adequate support for high loads imposed by a shell. A
foundation with a crushed stone or gravel ringwall has the following advantages:
a.
It provides better distribution of the concentrated load of the shell to produce a more nearly
uniform soil loading under the tank.
b.
It provides a means of leveling the tank grade, and it is capable of preserving its contour during
construction.
c.
It retains the fill under the tank bottom and prevents loss of material as a result of erosion.
d.
It can more smoothly accommodate differential settlement because of its flexibility.
A disadvantage of the crushed stone or gravel ringwall is that it is more difficult to construct it to close tolerances
and achieve a flat, level plane for construction of the tank shell.
B.6.4.2. For crushed stone or gravel ringwalls, careful selection of design details is necessary to ensure satisfactory
performance. The type of foundation suggested is shown in Figure B-3. Significant details include the following:
a.
The 0.9 m (3 ft) shoulder and berm shall be protected from erosion by being constructed of
crushed stone or covered with a permanent paving material.
b.
Care shall be taken during construction to prepare and maintain a smooth, level surface for the
tank bottom plates.
c.
The tank grade external to the tank shall be constructed to provide adequate drainage away from
the tank foundation.
d.
The tank foundation must be true to the specified plane within the tolerances specified in API 650
7.5.5
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Tank Shell
Leak Detection / Drainage Pipes.
Refer to B.4.2
Tank Bottom
Seal the edge around tank
Tank foundation shoulder
Slope 1:10
Slope 1: 1.5
600 mm (24 in)
Crushed Stone
Ground Level
Suitable Backfill Material
Geotextile Fabric.
Optional
Flexible Membrane Liner.
Figure B-3 Crushed Stone or Gravel Ringwall Foundation
Notes:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
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Geotextile fabric material to be laid around crushed rock ringwall
Top of crushed rock ring wall under tank shell to be minimum of 600 mm (24 in) wide
Tank shoulder 1 m (3 ft) wide for tanks under 15 m (50 ft) diameter and 1.5 m (5 ft) wide for tanks over 15 m (50 ft)
diameter
For tank with a cone down bottom, recommended foundation height is equal to the greater of 450 mm (18 in), or the
dimension from the bottom of the tank shell to the bottom of the tank sump, plus the slope of the drainage pipe from the
center of the tank to terminate outside the foundation, plus 100 mm (4 in). The bottom of the drainage pipe should be 100
mm (4 in) above the surrounding grade
Recommend laying sand or sand bitumen mix over backfill and under tank bottom as a corrosion mitigation barrier API 650
Appendix B.3.2.1.
Protect the tank shoulder from erosion by providing a 25 mm (1 in.) hot sand bitumen mix, chip seal, or other suitable
method. The protection layer is to cover the tank foundation shoulder from grade to tank bottom edge. Seal the protection
layer to the tank bottom as a moisture prevention barrier. Do not lay material over the tank bottom extension.
A release prevention barrier (RPB) such as a flexible impermeable membrane liner slopes from the center of the tank and is
bolted to the inside of the concrete ring wall. The liner will block groundwater from migrating upward into the foundation
and serve to contain and channel leaks for detection. The liner material should be compatible with and impermeable to the
stored substance, and be protected from the backfill by a geotextile fabric material. See API 650 Appendix “I” “Under Tank
Leak Detection and Subgrade Protection”
Recommended foundation height above surrounding ground is a minimum of 450 mm (18 in)
The flexible membrane liner can be used to increase the inspection interval of the tank bottom under API 653 6.4.3
The outside edge of the tank bottom should be sealed to prevent moisture from going under the tank Assure that sealing
material at the bottom junction does not extend above top of tank bottom chime to form a water trap outside of the tank
shell. After tank testing inspect this area and remove any sealing material or bitumen mix which extrudes out from beneath
tank to form such a water trap. The tank bottom extension must be visible for inspection at all times.
The foundation under the shell should be level within tolerance under API 650 5.5.5.2 b
Drainage pipes API 650 Appendix B Fig B-5
Suitable backfill material API 650 Appendix B 2.5
For Cathodic Protection refer to API RP 651
For hydrostatic testing requirements refer to API 650 7.3.6
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B.6.4 Concrete Slab Foundations (Fig B-4)
B.6.4.1 When the soil bearing loads must be distributed over an area larger than the tank area or when it is
specified by the owner, a reinforced concrete slab shall be used. Often small diameter tanks (<30ft diameter are
placed on concrete slab foundations. A small concrete slab is often less expensive to construct than a equivalent
concrete ringwall. Piles beneath the slab may be required for proper tank support.
B.6.4.2 The structural design of the slab, whether on grade or on piles, shall properly account for all loads imposed
upon the slab by the tank. The reinforcement requirements and the design details of construction shall be in
accordance with ACI 318.
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Tank Shell
Seal the edge around tank
Refer to B.4.2
Tank Bottom
Beveled 25 mm (1 in)
Concrete Slab
Piles
Slope grade away from
concrete for a distance of
2 m (6ft)
Figure B-4 Concrete Slab Foundation, Plain or Piled
Notes:
1.
2.
3.
4.
5.
6.
7.
8.
B.7
Concrete slab is to be designed by a person experienced in tank foundation design
The top of the concrete slab should be smooth and level. The concrete strength should be at least 20 MPa (3000
lbs/sqin) after 28 days. Reinforcement splices must be staggered and should be lapped to develop full strength in
the bond. If staggering of laps is not possible, refer to ACI 318 for additional development requirements.
Tank anchors maybe required see API 5.12.
Where a concrete slab foundation is provided, the first 0.3 m (1 ft) of the foundation (or width of the annular ring),
measured from the outside of the tank radially towards the center, should comply with the concrete ringwall
requirement. The remainder of the foundation should be within level to tolerance under API 650 7.5.5.2.c
50 mm (2 in) Sand Bitumen Mix or a minimum of 1/2in Asphalt Board laid on top of concrete slab will allow the
sketch plate / backing strips from touching the concrete slab. API 651 allows tanks bottoms to be placed directly on
concrete when proper drainage is provided.
The outside edge of the tank bottom should be sealed to prevent moisture from going under the tank. Assure that
sealing material at the bottom junction does not extend above top of tank bottom chime to form a water trap outside
of the tank shell. After tank testing inspect this area and remove any sealing material or bitumen mix which
extrudes out from beneath tank to form such a water trap.
The tank bottom extension must be visible for inspection at all times.
For hydrostatic testing requirements refer to API 650 7.3.6
Drainage Pipes under Tank Foundations
B.7.1. Drainage pipes should be installed in the tank foundation to allow water to drain out of the foundation and
to prevent moisture from contacting the tank bottom
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Tank
Tank Foundation
At least 8 pipes with a max spacing of
10 m (32 ft) around the tank foundation
1 pipe to center of tank
Pipes 3 m (10 ft) inside tank foundation
Fig B-5 Drainage Pipes
Notes:
1.
2.
3.
4.
The drainage pipes should be made of non-corrosive material (e.g. PVC or fiberglass) and have screened
slots in the pipe to maximize the drainage of water from under the foundation
Recommend 50 mm (2 in) diameter pipes
Each end of drainage pipes shall have screens installed
Underside of drainage pipes to be a minimum of 100 mm (4 in) above the finish grade surrounding the tank
foundation.
B.8
Hydrostatic Testing
B.8.1.
API 650 7.3.6 provides for testing the tank foundations by water loading the tank.
B.9
Tank Foundations for Undertank Leak Detection
B.9.1 API 650 Appendix “I” provides designs and recommendations on the construction recommendations of
tank and foundation systems for the detection of leaks through the bottoms of storage tanks.
B.9.2 A foundation which incorporates a Release Prevention Barrier (RPB) such as a flexible, impermeable
membrane liner with leak detection piping, or a concrete slab, may be used under some conditions to increase the
internal inspection interval for the tank. See API 653.6.4.3
B.9.3 A flexible, impermeable membrane liner will block groundwater from migrating upward into the
foundation and serve to contain and channel leaks for detection. The liner material should be compatible with and
impermeable to the stored substance, and be protected from the backfill by a geotextile fabric. The liner should slope
downward from the center of the tank towards the tank edge.
B.10
Tank Anchorage
B.10.1 When a tank is required to be mechanically anchored, see API 650 5.12.
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