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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)
De
pth
(ft
)
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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Con
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atio
n S
ettl
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t (in
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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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Con
solid
atio
n S
ettl
emen
t (in
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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