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Conditions for Rigorous Bender Element Test in Triaxial Cell

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Title: Conditions for Rigorous Bender Element Test in Triaxial Cell


1
Conditions for Rigorous Bender Element Test in
Triaxial Cell
  • Introduction
  • Technical requirements
  • Boundary conditions
  • Interpretation of the received signal
  • Conclusions

2
Introduction
  • Non-destructive, straightforward, and flexible
    method to determine Gmax for a range of soils
  • Difficult to interpret, subjective,
    not-resilient, giving inconsistent results
  • The aim is to highlight the conditions needed to
    perform a rigorous bender element test in
    triaxial cell
  • These are divided into
  • Technical requirements
  • Boundary conditions

3
Technical requirements
  • Pulse generator, function generator, oscilloscope
    with screen
  • Shielded and grounded cables
  • Properly connected and encased bender elements
  • Leak free connections
  • Noise free environment

4
Technical requirements
  • Design details
  • Leaks
  • Wear and tear

5
Boundary conditions
  • Spatial boundary conditions
  • Alignments of the bender elements
  • Reflections of the waves from the sample edges
  • Near field effect (relative distance between the
    transmitter and receiver)
  • Contact between the benders and the soil
  • Poor coupling
  • Overshooting (response of benders at high
    frequencies)

6
Boundary conditions
  • Alignments of the bender elements
  • Reflections of the waves from the sample edges
  • Benders on sides of the triaxial samples have
    different boundary conditions
  • Stress concentrations in the end zones of the
    specimen

7
Boundary conditions
  • Near field effect (relative distance between the
    transmitter and the receiver)

8
Boundary conditions
  • The propagation of body waves through triaxial
    sample
  • After Jovicic, Coop na Simic(1996)
  • After Sanches-Salinero, Roset and Stokoe (1986)

9
Boundary conditions
  • Good coupling between the benders and the soil

10
Boundary conditions
  • Overshooting (response of the benders at high
    frequencies)

11
The interpretation of the received signal
  • The signal should be clear, unfiltered but devoid
    of noise (technical requirements fulfilled)
  • The measurement should be taken in the time
    domain at frequencies for which the near field
    effect is negligible
  • The signal should bear no or little signs of poor
    coupling between the soil and the element (no
    overshooting)
  • The measurement is best taken directly of the
    screen

12
Conclusions
  • The bender element tests in triaxial cell is
    difficult but not impossible to standardise
  • The interpretation of the arrival time still
    requires an educated judgment of the user
  • The quality of the conditions for the rigorous
    bender element tests typically changes during the
    course of triaxial tests
  • The automatic interpretation of the arrival time
    using numerical processing (frequency domain)
    needs to consider changes in boundary conditions
    during the triaxial tests
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