Dynamic Load Test-PDA

Pile Dynamic Load Testing is a reliable and cost effective testing method used to determine the bearing capacity of the deep foundation elements. Temeltek offers Dynamic Load Testing Services for the assurance of the quality on on driven piles, drilled shafts, auger-cast piles, micropiles, helical piles, and other cast in place foundations. One of the major advantages of the Dynamic Load Testing Method that it can be conducted upon several foundations elements in a single day.

Temeltek’s Dynamic Load Testing Service provides information on resistance distribution (shaft resistance and end bearing) and evaluates the shape and integrity of the foundation element besides the bearing capacity of the foundation elements. Dynamic load testing (or dynamic loading) is a method to assess a pile’s bearing capacity by applying a dynamic load to the pile head (a falling mass) while recording acceleration and strain on the pile head.

Force and velocity are measured by the accelerometers and strain transducers as the falling weight hits deep foundation element. A force transducer may also be utilized instead of strain transducers in order to speed up the test and in certain cases more accurate force measurements may be yielded. Acquired data in real time is analyzed with Pile Driving Analyzer (PDA) dynamic testing system. . Data is further analyzed with the CAPWAP® software. Detailed reports for each Dynamic Load Testing project are prepared by Temeltek Engineers and these reports includes includes a simulated static load test in the form of a calculated load-set curve. Dynamic Load Tests are standardized by ASTM D4945-12 Standard Test Method for High-Strain Dynamic Testing of Piles. The test may also be configured to meet the requirements of the Rapid Load Test standard ASTM D7383.

• High Strain Dynamic TestingWhen a hammer or drop weight strikes the top of a foundation element, 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. 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 transmit data to a high strain dynamic testing system such as the 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

  Soil resistance forces along the shaft and at the toe cause wave reflections that travel and are felt at the top of the deep 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.

  The resulting estimated total soil resistance includes both static and viscous components. The static resistance is 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 related to soil grain size.

  The energy delivered to the pile is directly computed as the work done on the pile from the integral of force times incremental displacement (which is easily evaluated as force times velocity integrated over time. 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 indicates a pile with no defects.

  High Strain Dynamic Testing encompasses Dynamic Pile Monitoring and Dynamic Load Testing. Both are covered by ASTM D4945. Pile Driving Monitoring consists of performing real time evaluation of Case Method capacity, energy transfer, driving stresses and pile integrity for every blow. Dynamic Load Testing involves combining field measurements obtained with a high strain dynamic testing system such as the PDA with wave-equation based analytical procedures performed with a signal matching program such as CAPWAP® . Dynamic Load Testing predicts 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.

  Low Strain Dynamic Load Testing

  Wave propagation theory can also be applied to situations where a light impact is applied to a pile, resulting in a low strain. A compression wave will still travel down the pile when it is impacted by a small hand held hammer. Much like in High Strain Testing, this wave will travel at a constant speed c. Changes in pile impedance Z produce wave reflections.

  The application of the wave equation theory to waves caused by small impacts is the basis for Low Strain Dynamic Integrity Testing. This procedure is performed with a Pile Integrity Tester (PIT), a hand held hammer to generate an impact, and an accelerometer placed on top of the pile to be tested to measure the response to the hammer impact. Given a known stress wave speed, records of velocity (integrated from the accelerometer signals) at the pile head can be interpreted to reveal pile non-uniformities (changes in impedance). Interpretation is usually done in the time domain (Pulse echo, or Sonic echo) but data can also be evaluated by measuring the hammer force and analyzing in the frequency domain (Transient Dynamic Response). Pile length may also be determined. This non destructive testing method is usually applied to concrete piles, concrete filled pipe piles, drilled shafts, auger cast-in-place (continuous flight auger) piles, and sometimes timber piles. Usually the method is applied to piles not connected to a structure, but good results are often obtained for piles embedded in structures (such as cell-phone towers, transmission towers, and bridges). This method is covered under ASTM D5882.