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11.2 Pile capacity (static methods, dynamic methods, pile load tests)

4 min readLast Updated on August 16, 2024

Pile capacity is crucial for deep foundation design. Static methods calculate ultimate capacity by summing shaft and base resistance, considering soil-pile interaction. Dynamic methods use real-time measurements during installation to estimate capacity and assess driving conditions.

Load tests provide the most reliable capacity data. Static load tests apply incremental loads, while dynamic tests use pile motion analysis. Combining multiple methods improves accuracy and understanding of pile-soil interaction, leading to more efficient and reliable deep foundation designs.

Static methods for pile capacity

Calculating ultimate pile capacity

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  • Ultimate pile capacity calculated by summing shaft resistance and base resistance
  • Shaft resistance depends on soil-pile interface friction and adhesion
  • Base resistance relies on bearing capacity of soil beneath pile tip
  • Factors affecting pile capacity include
    • Pile length and diameter
    • Pile material (concrete, steel, timber)
    • Installation method (driven, bored, cast-in-place)
    • Soil stratification and properties

Methods for estimating shaft resistance

  • α-method used for cohesive soils
    • Utilizes undrained shear strength (Su) and empirical adhesion factor (α)
    • Shaft resistance calculated as Qs=αSuAsQ_s = α * Su * As
    • α varies with soil consistency and pile properties
  • β-method employed for cohesionless soils
    • Considers effective stress (σ'v) and friction angle (φ)
    • Shaft resistance calculated as Qs=Kσvtan(δ)AsQ_s = K * σ'v * tan(δ) * As
    • K represents lateral earth pressure coefficient
    • δ denotes interface friction angle between soil and pile
  • λ-method applicable to both soil types
    • Uses depth-dependent factor (λ) to account for stress changes
    • Shaft resistance calculated as Qs=λ(σv+2Su)AsQ_s = λ * (σ'v + 2Su) * As
    • λ decreases with depth due to arching effects

Base resistance calculation approaches

  • Bearing capacity method for cohesive soils
    • Ultimate base resistance given by Qb=NcSuAbQ_b = Nc * Su * Ab
    • Nc represents bearing capacity factor (typically 9)
    • Ab denotes pile base area
  • CPT-based method for cohesionless soils
    • Utilizes cone penetration test data
    • Base resistance estimated as Qb=qcAbQ_b = qc * Ab
    • qc represents average cone tip resistance near pile tip

Dynamic methods for pile capacity

Real-time capacity estimation during driving

  • Dynamic methods measure pile motion and force during installation
  • Provide immediate feedback on pile capacity and driving stresses
  • Key parameters measured include
    • Pile top acceleration
    • Strain in pile shaft
  • Advantages of dynamic methods
    • Rapid assessment of multiple piles
    • Cost-effective for large projects
    • Ability to detect installation problems (pile damage, soil variability)

Case Method for simplified dynamic analysis

  • Utilizes pile top measurements to estimate capacity
  • Considers soil damping and quake (elastic soil deformation)
  • Capacity estimated using formula Ru=(F1+F2)/2+(F1F2)/(2c)(1Jc)R_u = (F_1 + F_2)/2 + (F_1 - F_2)/(2c) * (1 - J_c)
    • F1, F2 represent force measurements at different times
    • c denotes wave speed in pile
    • Jc accounts for soil damping effects

Wave equation analysis (WEAP)

  • Simulates pile driving process numerically
  • Considers factors such as
    • Hammer energy and efficiency
    • Soil resistance distribution
    • Dynamic soil properties (quake and damping)
  • Provides estimates of
    • Pile capacity
    • Driving stresses
    • Hammer performance

Pile Driving Analyzer (PDA) equipment

  • Measures strain and acceleration at pile top during driving
  • Provides real-time data for capacity estimation
  • Allows assessment of
    • Pile integrity
    • Hammer performance
    • Driving system efficiency
  • Data used for further analysis (CAPWAP)

Interpreting pile load test results

Static load testing procedures

  • Most reliable method for determining actual pile capacity
  • Involves applying incremental loads to pile
  • Types of static load tests
    • Compression tests (most common)
    • Tension tests (for uplift capacity)
    • Lateral load tests
  • Load-settlement curves analyzed to determine ultimate capacity
  • Interpretation methods include
    • Davisson's criterion (offset limit method)
    • 10% diameter method (for friction piles)
    • Chin's method (for extrapolation)

Advanced load testing techniques

  • Osterberg cell (O-cell) tests
    • Apply bi-directional loads within pile
    • Separate measurement of shaft and base resistances
    • Advantages include higher test loads and reduced surface setup
  • Statnamic load testing
    • Applies rapid load to pile using fuel combustion
    • Cost-effective alternative to static tests for certain soils
    • Requires dynamic soil parameters for interpretation

Dynamic load testing methods

  • High-strain dynamic tests (HSDT)
    • Use PDA equipment during restrike or at end of driving
    • Provide estimates of static and dynamic pile capacity
    • Allow comparison with static load test results
  • Factors affecting dynamic test results
    • Time effects (setup or relaxation)
    • Soil type and drainage conditions
    • Driving system characteristics

Selecting pile capacity methods

Considerations for method selection

  • Soil type and variability (cohesive, cohesionless, layered)
  • Pile type and installation method (driven, bored, cast-in-place)
  • Project scale and importance
  • Required accuracy of capacity estimates
  • Budget and time constraints
  • Local experience and regulations

Combining multiple methods for comprehensive assessment

  • Static methods for preliminary design and capacity estimation
  • Dynamic methods for quality control during installation
  • Load tests for critical structures or unique soil conditions
  • Benefits of combined approach
    • Validation of design assumptions
    • Improved accuracy of capacity predictions
    • Enhanced understanding of pile-soil interaction

Developing site-specific correlations

  • Compare predicted capacities with measured values
  • Analyze trends based on soil type, pile characteristics, and installation methods
  • Develop correction factors or modified design parameters
  • Improve future designs and capacity estimates for similar conditions
  • Consider statistical analysis of load test results
    • Determine reliability of capacity predictions
    • Assess variability in soil conditions and pile performance
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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