1

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

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APPENDIX BRECOMMENDATIONS 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. Earth foundation (Fig. B-1) b. Earth foundation with a concrete ringwall (Fig. B-2) c. Earth foundation with crushed stone ringwall (Fig. B-3) d. Concrete slab foundation, plain (Fig. B-4) e. 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.)

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.

B.2.4 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.

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 subsoils 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 non-uniform 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

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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.

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 1General Classification of Resistivity

Resistivity Range (ohm-cm) Potential Corrosion Activity

10,000 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

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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.

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 Bitumen-sand layers under steel tank bottoms inhibit cathodic protection current.

2 Bitumen-sand layers can be affected by welding resulting in porosity or cracks in

welds and producing smoke discomfort to welders

3 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.

4. 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.

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 Oil-sand layers under steel tank bottoms inhibit cathodic protection current.

2 Oil-sand layers can be affected by welding resulting in porosity or cracks in welds

and producing smoke discomfort to welders

c. Clean washed sand

Note;

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,

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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. 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.

d. Crushed limestone or clamshells (refer API Recommended Practice 651) can be used as an

acceptable material under the tank bottom

B.5. 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

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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)

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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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

Figure B-1 Earth Foundation.

Slope 1:10

Tank Shell Refer to B.4.2.

Tank Bottom

Drainage Pipes

Ground Level

Suitable Backfill Material

Notes:

1. 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

2. Tank with a cone up bottom, recommended foundation height at tank shell is 450 mm (18 in) above the surrounding ground elevation.

3. 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 4. Recommend laying sand or sand bitumen mix over backfill and under tank bottom as a corrosion mitigation barrier API 650

Appendix B.4.3. 5. 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.

6. 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

7. 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

8. 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.

9. The foundation under the shell shall be level within tolerances allowed under. API 650 7.5.5.2 b 10. Drainage pipes API 650 Appendix B Fig B-5 11. Suitable backfill material API 650 Appendix B 2.5 12. For Cathodic Protection refer to API RP 651

13. For hydrostatic testing requirements refer to API 650 7.3.6

Slope 1: 1.5

Seal the edge around tank

Tank foundation shoulder

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directly, a 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.

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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 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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Concrete Spread Footing

Beveled 25 mm (1 in)

Tank Shell

Refer to B.4.2 Tank Bottom

Drainage Pipes

Slope grade away from concrete

for a distance of 2 m (6ft)

Suitable Backfill Material

Concrete Ringwall

Notes:

1. Concrete ringwall is to be designed by a person experienced in tank foundation design 2. See Appendix B.6.3. for requirements for concrete ringwalls 3. 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.

4. 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

5. Concrete ringwalls should not be built less than 300 mm (12 in) in width 6. Concrete ringwalls that exceed 300 mm (12 in.) in width should have steel rebar distributed on both faces. 7. 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.

8. The need for a spread footing should be evaluated when building concrete ring wall on soft ground. 9. When the ringwall width exceeds 460 mm (18 in) using a spread footing beneath the wall should be considered. 10. Tank anchors maybe required see API 5.12 11. 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.

12. 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

13. 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

14. 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.

15. 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.

16. Drainage pipes API 650 Appendix B Fig B-5 17. Suitable backfill material API 650 Appendix B 2.5 18. For Cathodic Protection refer to API RP 651

19. For hydrostatic testing requirements refer to API 650 7.3.6

Beveled 50 mm (2 in)

Figure B-2 Concrete Ringwall Foundation

Seal the edge around tank

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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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Slope 1:10

Tank Shell

Refer to B.4.2 Tank Bottom

Optional

Flexible Membrane Liner.

Leak Detection / Drainage Pipes.

Ground Level

Crushed Stone

Notes:

1. Geotextile fabric material to be laid around crushed rock ringwall 2. Top of crushed rock ring wall under tank shell to be minimum of 600 mm (24 in) wide 3. 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

4. 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

5. 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.

6. 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.

7. 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

8. Recommended foundation height above surrounding ground is a minimum of 450 mm (18 in) 9. The flexible membrane liner can be used to increase the inspection interval of the tank bottom under API 653 6.4.3 10. 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.

11. The foundation under the shell should be level within tolerance under API 650 5.5.5.2 b

12. Drainage pipes API 650 Appendix B Fig B-5

13. Suitable backfill material API 650 Appendix B 2.5

14. For Cathodic Protection refer to API RP 651

15. For hydrostatic testing requirements refer to API 650 7.3.6

600 mm (24 in)

Geotextile Fabric.

Suitable Backfill Material

Figure B-3 Crushed Stone or Gravel Ringwall Foundation

Seal the edge around tank

Tank foundation shoulder

Slope 1: 1.5

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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 (

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B.7 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

Beveled 25 mm (1 in)

Tank Shell

Refer to B.4.2 Tank Bottom

Slope grade away from concrete for a distance of

2 m (6ft)

Notes:

1. Concrete slab is to be designed by a person experienced in tank foundation design 2. 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.

3. Tank anchors maybe required see API 5.12. 4. 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

5. 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.

6. 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.

7. The tank bottom extension must be visible for inspection at all times.

8. For hydrostatic testing requirements refer to API 650 7.3.6

Figure B-4 Concrete Slab Foundation, Plain or Piled

Concrete Slab

Piles

Seal the edge around tank

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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.

Tank

Tank Foundation

1 pipe to center of tank

Pipes 3 m (10 ft) inside tank foundation

At least 8 pipes with a max spacing of

10 m (32 ft) around the tank foundation

Notes:

1. 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

2. Recommend 50 mm (2 in) diameter pipes 3. Each end of drainage pipes shall have screens installed 4. Underside of drainage pipes to be a minimum of 100 mm (4 in) above the finish grade surrounding the tank

foundation.

Fig B-5 Drainage Pipes