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Workshop Outline

Overview of Tank Options

Construction of Pre-Stressed Concrete Tanks

Concrete Tank Market

Structural/Seismic Consideration

Ground Improvement Methods

Purpose / Introduction

Engeo Geotechnical

From a geotechnical standpoint, differential settlement causes the most damage to tanks

Not our project!

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Tank Settlement Definitions

Total Maximum Settlement Tilt

Average Settlement

Bottom-Edge Differential Settlement

Tank Differential Settlement Definitions

Bottom-Center Differential Settlement

Circumferential Differential Settlement

Not shown – roof support column differential

settlement

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Differential Settlement Profiles of Tank Bottom

Differential Settlement Profiles Depend on: Soil Conditions Foundation Type

Mat, Footings and Slab, and Ring Foundation w/ aggregate-base supported steel tank bottom

Foundation Loading Roof Support Details - interior

columns or dome Fluid Height Embedment Depth Backfill Details

Typical Differential Settlement Tolerances

Structure Type Allowable Δ/l

Differential inches per 10 feet

Reinforced concrete with insensitive finishes

0.002 to 0.003 ¼ to 3/8 inch

Circular Steel Tanks on flexible base with fixed top

0.008 1 inch

Circular Steel Tanks on flexible base with floating top

0.002 to 0.003 ¼ to 3/8 inch

Values of Angular Distortion (∆/l) which can be Tolerated without Cracking After Skempton and MacDonald, 1956 Polshin and Tokar, 1957 and NAVFAC DM 7, 1986

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American Concrete Institute Criteria

Should Not ExceedTotal settlement 6 inchesUniform tilting 3/8-inch in 10 feetDifferential settlement should not exceed

¼-inch in 10 feet of the foundation diameter

Maximum combined uniform and tilting settlement at the tank foundation perimeter

6 inches

Geotechnical Considerations

Immediate v. Long Term Consolidation Settlement

Clay sites - long term most critical

Compensation effects due to excavation

Unloading decreases settlement potential (greatest at center of tank, least a edge)

Influence of maximum past pressure σ’consolidation

Virgin compression where σ’ > σ’consolidation

Recompression where σ’ > σ’consolidation

Effects both settlement magnitude and rates

Settlement occurs due to effective stress changes

Negative due to excavation

Positive due to water loads, footing loads, backfill loads behind walls, adjacent grade changes

Virgin compression 10X greater magnitude than RecompressionRecompression 10X faster than Virgin Compression

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Case Study — Central Valley

130-ft Diameter Concrete Tank Buried 12 feet - Dome Roof

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Stress Below Center of Tank (psf)

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pth

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Initial Effective Stress Effective Stress After Excavation Effective Stress After Loading

Excavate to 12 feet

Settlement Analysis – 3D Model

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Settlement Profile –Distance from Center of Tank

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Distance (f t)

Distance vs. Consolidation Settlement

Query Line 3 (Stage 6 = 100 y)Consolidation Settlement at Depth = 12 ft

Reference Stage: None

Dome Roof

Total Settlement ~ 1”

Angular Distortion ~ 0.003

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Distance (f t)

Distance vs. Consolidation Settlement

Query Line 1 (Stage 6 = 100 y)Consolidation Settlement at Depth = 12 ft

Reference Stage: None

Flat Slab Roof (Interior Columns)

Total Avg. Settlement ~ 1 ¼”

Angular Distortion ~ 0.004

Foundation System Options

Shallow Footing (minimal settlement)

Structural Mat Foundation (settlement mitigation)- Must be rigid enough to moderate settlement

Structural Deep Foundation (settlement mitigation)Piers or piles – floor spans between

Ground Improvement (settlement mitigation)- Mitigation of settlement – allows for shallow footing foundation

Where settlement is significant, a deep foundation or ground improvement should be considered

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Deep Foundation vs. Ground Improvement

Both serve to mitigate significant settlement

Ground Improvement Advantages

Deep Foundation Advantages

Allows for use of conventional shallow footing foundation

supported on improved groundNot subject to code-required detailing, which can be costly

Lower cost than a deep foundation system

Not as specialized – easier to develop general specifications

Ground Improvement Systems

Rammed/Vibro Aggregate Piers

Vibro Replacement Stone Columns/Impact Aggregate Piers

Jet Grouting/Soil Mixing

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Rammed/Vibro Aggregate Piers (remove and replace)

Ground Improvement Systems

Vibro Replacement Stone Columns / Impact Aggregate Piers

(full displacement)

Ground Improvement Systems

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Jet Grouting / Soil MixingGround Improvement Systems

Case Study – Ground Improvement City of Davis East Area Tank

4MG Concrete Water Storage Tank Buried up to 10 feet Total Settlement = 5 to 6 inches at center

and approximately 3 inches at edge (exceeds criteria for differential settlement of ¼-inch in 10 feet)

Ground improvement selected for settlement mitigation

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Aggregate Pier Layout

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