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Dynamic Pile Load Testing
James A. Baigés, PE September 3, 2015
Pile Design Outline Ø Project Scope Ø Subsoil Exploration Program-
Ø Site Geology, Aerial Photo interpretation – stereo pairs
Ø SPT, CPT, DMT, Test Pits Ø Lab – Consolidation Tests, Triaxial, DS,
classification tests Ø Soil Characterization – general subsoil
profile
Pile Design Outline Ø Engineering Analyses Ø Deep Foundation – driven piles, drilled shafts, auger
cast piles (Ensoft-APile, Driven, Coyle, etc.)
Ø Design Pile Capacity
Ø Axial (compression, tension) Ø Lateral loading (LPILE) Ø Pile group interaction Ø Wave Equation (GRLWEAP) – hammer
selection, driveability, driving stresses eval., production driving.
Load transfer in an axially loaded pile.
Pile Testing Program
Ø Objectives – to confirm design loads Ø Pile Load Test Types
Ø Static (compression and tension) Ø Dynamic Ø Others (Statnamic and O-cell – for drilled shafts )
Comparison – Static vs Dynamic STATIC
Advantages -more familiar -failure criterion well known
(Davisson) Disadvantages Expensive Reduced number of tests Setup very time consuming Req. to provide instrumentation to
obtain load distribution along pile.
DYNAMIC Advantageous Reduce costs – more testing Fast – setup is simple Load distribution (SF, EB) Disadvantages Some don’t acknowledge
method – lack of confidence Req. experienced personnel FS req. usually greater than 2
Dynamic Foundation Testing Attempts to determine pile capacity using dynamic
analysis date back to the 19th century. At that time, a dynamic formula that considered the
energy of the pile driving hammer and the set of the pile was developed to find bearing capacity.
Dynamic formulae are still used today, in spite of their
inaccuracies and of the fact that they cannot predict stresses during driving.
Wave Equation Analysis • In the 1950’s, E.A. Smith of the
Raymond Pile Driving Company developed a numerical analysis method to predict the capacity versus blow count relationship and investigate pile driving stresses.
• The model mathematically
represents the hammer and all its accessories (ram, cap, cap block), as well as the pile, as a series of lumped masses and springs in a one-dimensional analysis.
• The soil response for each pile segment is modeled as viscoelastic-plastic.
High Strain Dynamic Testing
• When a hammer or drop weight strikes the top of a foundation, a compressive stress wave travels down its shaft at a speed c, which is a function of the elastic modulus E and mass density.
• The impact induces a force F and a particle velocity v at the top of the foundation.
• The force is computed by multiplying the measured signals from a pair of strain transducers attached near the top of the pile by the pile area and modules.
High Strain Dynamic Testing • The velocity measurement is obtained by
integrating signals from a pair of accelerometers also attached near the top of the pile. Strain transducers and accelerometers are connected to a Pile Driving Analyzer® (PDA), for signal processing and results.
• As long as the wave travels in one direction, force and velocity are proportional: F = Zv, where: Z = EA/c is the pile impedance E is the pile material modulus of elasticity A is the cross sectional area of the pile c is the material wave speed at which the
wave front travels
High Strain Dynamic Testing • Soil resistance forces along the shaft and at the toe cause
wave reflections that travel and are felt at the top of the foundation.
• The times at which these reflections arrive at the pile top are related to their location along the shaft. The measured force and velocity near the pile top thus provide necessary and sufficient information to estimate soil resistance and its distribution.
• Total soil resistance computed by the PDA includes both
static and viscous components. The static resistance can be obtained by subtracting the dynamic component from the total soil resistance.
• The dynamic component is computed as the product of the pile velocity times a soil parameter called the Damping Factor. The damping factor is an input to the PDA and is related to soil grain size.
High Strain Dynamic Testing
• The energy delivered to the pile is directly computed as the work done on the pile from the integral of force times incremental displacement ( ∫Fdu ) which is easily evaluated as force times velocity integrated over time ( ∫Fvdt ).
• Maximum compression stresses at the pile top come
directly from the measurements. The measurements also allow direct computation of the compression stress at the pile toe and the tension stresses along the shaft.
• Pile integrity can be evaluated by inspecting the measurements for early tension returns (caused by pile damage) prior to the reflection from the pile toe; lack of such reflections assures a pile with no defects.
High Strain Dynamic Testing
• High Strain Dynamic Testing encompasses Dynamic Pile Monitoring and Dynamic Load Testing. Both are covered by ASTM D4945.
• Pile Driving Monitoring consists of using a PDA to perform real time
evaluation of Case Method capacity, energy transfer, driving stresses and pile integrity for every blow.
• Dynamic Load Testing involves another technique that evolved from
Smith’s approach of modeling the wave propagation theory of pile driving, the Case Pile Wave Analysis Program (CAPWAP®).
• CAPWAP combines field measurements (obtained with the PDA) and wave-equation type analytical procedures to predict soil behavior including static-load capacity, soil resistance distribution, pile soil load transfer characteristics, soil damping and quake values, and pile load versus movement plots (e.g. a simulated static load test). CAPWAP analysis is made on the PDA data after the test is complete.
High Strain Dynamic Testing PDA Setup
Strain and Accelerometers on an HP 14x73 Steel Pile
Pile Testing at Isidoro Garcia Stadium, Mayagüez, PR
PDA
CAse Wave Analysis Program (CAPWAP)
Field documentation
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