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Soil mechanics

Resonant column

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Main features
  • Combined Resonant Column / Torsional Simple Shear device
  • Automatic detection of fundamental frequency
  • RC: damping ratio from half power bandwidth and from free vibration data
  • TSS: damping ratio from hysteresis loops
  • Internal floating frame for large angular and axial deformation
  • Confining pressure up to 1 MPa
  • Suitable for 50 mm dia. specimen (or 38 mm on request)
  • Integrated signal generator and oscilloscope 
  • PC with dedicated software included 
General description
General description
Resonant Column combines the features of both resonant column and torsional shear into a single unit, including a current-driven motor to apply torsional load to the sample, a series of transducers with signal conditioning, a cell and back pressure electropneumatic control system and a data logger.
In the Resonant Column test a cylindrical soil specimen is restrained at the bottom and dynamically excited at the top. The torsional force at the top is generated using an electrical motor constituting eight drive coils encircling four magnets attached to a drive plate. The generated frequency is up to 300 Hz. The fundamental mode of vibration is determined from the maximum amplitude of motion; from the Standards ASTM D4015 resonant frequency, shear wave velocity and shear wave modulus
are calculated using elasticity theory. The corresponding shear strain is evaluated from the motion amplitude. Material damping can be determined from the half-power bandwidth or from a free-vibration decay curve, which is generated by shutting off the driving power.
In the Torsional Shear test the soil specimen is deformed cyclically at a low frequency (a maximum of 10 Hz), whilst continuously monitoring torque and deformation.
From the torque-deformation curves, a relationship between average shear stress and average shear strain is obtained, which in turn provides the shear modulus and the damping ratio.

The system consists of the following components:
 
Triaxial cell
Aluminum cell with stainless steel columns and acrylic transparent cylinder with 170 mm int. dia. x 200 mm ext. dia., including channels for bottom drainage; internal floating frame for assembling the electrical motor that applies the torsional loads; this motor has four NeFeB 10 x 25 x 40 mm magnets and eight coils.
Test accessories for 50 mm (or 38 mm available on request) dia.specimens.
n°2 calibration bars kit + n°1 calibration weight.
 
Main control box, PC and software
Compact unit connected to PC contains all control, power supply and electrical and pneumatic
devices. This system contains also the air actuators (I/P converters) and the amplification equipment.

Sensors
The sensor kint containg: Axial LVDT transducer, volume change apparatus, three pressure transducers, two Eddy current ndisplacement sensors (with miniaturized driving system), MEMS
accelerator.

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Technical specifications

  • Maximum torque: 1.2 Nm
  • Maximum angular deformation: 10°
  • Maximum cell and back pressure: 1 MPa.
  • 8 channels signal conditioning unit
  • USB data acquisition and signal generation board
  • Two electro-pneumatic converters for cell and back pressure
  • Excitation frequency: Dynamic (RC) 1-300 Hz; Cyclic (TS) from 0 to 50 Hz maximum
  • Dimension: Control Box 51x45 x 35 cm (h x w x d); Cell 55 cm x 27 cm (h x diam.)
  • Weight: approx 50 kg

Ordering info

31-WF8500 
Combined resonant column/torsional shear device for the automatic determination of damping ratio from half power bandwith and free vibration decay method.                                            
 

Accessories

Accessories

28-WF4051/A
Membrane stretcher for 50 mm diameter samples.
 
28-WF4051/B
O-ring placing tool for 50 mm diameter samples.
 
28-WF4055
Rubber membranes for 50 mm diameter samples (pack of 10).
 
28-WF4056
O-ring for 50 mm diameter samples.
 
28-WF4051/E
Lateral filter drains (pack of 50).
 
31-WF4051/H1
Two-part split former for 50 mm diameter samples with vacuum attachment.
 
28-WF4051/G
Hand sampler complete of cutter, wooden dolly and receiver for 50 mm samples.


Air compresor 

28-WF2016/A 
Air compressor, 10 bar/145 psi maximum working pressure, output 10.2 cfm, 100 l receiver. 230 V, 50 Hz, 1 ph

28-WF2016/2 
Air filter/water trap for air compressor

De airnig water system 

28-WF4221/A 
De airing tank, 23 l cap.

28-WF2001 
Portable vacuum pump. 220-240 V, 50-60 Hz, 1 ph

28-WF2064 
Rubber tubes for vacuum (two pieces required)

86-D2005 
Air drying unit

86-D0819 
Silica gel dessicant with indicator

28-WF4225
Valve panel for use with de-airing tank


Additional Information

TEST STAGES

Saturation stage
During saturation stage a small amount of cell and back pressures are applied in steps, with a consequent dissolution of the air contained in the intergaranular spaces. A control system generate the cell and back pressures using air/water interfaces. Cell, back and pore pressures are measured by pressure transducers 1000kPa cap, 0.1 kPa accuracy. Volume change is measured using high sensitivity differential pressure transducer.

Consolidation stage
The sample is subjected to the same back pressure used during the last saturation step while the cell pressure depends on the effective stress required in the next steps. When pore water pressure and volume changes are completely dissipated the consolidation stage ends.
During this stage the axial strain is measured using a LVDT transducer ± 12.5 mm travel, 0,2% class.
RC and TSS tests are usually performed in undrained conditions, closing the drainage channels and measuring changes in the pore water pressure.

Resonant Column test (RC)
A signal generator supplies a sinusoidal voltage to the driving amplifier and a proportional current to the coils attached to the cell body. The magnetic field in the coils interacts with the magnets attached to the driving plate, that in turn conveys a torsional oscillation to the top of the specimen. As the frequency of the input signal varies, the dynamic response of the specimen results in a varying motion amplitude. The amplitude is captured either by an accelerometer attached to the driving plate and by proximity displacement transducers measuring the relative movement of the driving plate relative to the coils.
The frequency that maximizes the motion of the top of the specimen is associated to the first-mode resonance and is found applying an input signal with a frequency sweep. The secant shear modulus of the soil can be evaluated from the resonant frequency. The damping ratio can be evaluated either from the complete frequency response of the soil specimen (“half power band width”), or from a free-vibration decay curve that is generated by shutting off the driving power.
At a given consolidation effective stress, RC tests are repeated several times, increasing progressively the amplitude of the input voltage, thus obtaining the secant shear modulus and the damping ratio corresponding to increasing shear strain values.

Torsional Simple Shear test (TSS)
A sinusoidal current is applied to the coils in a quasistatic condition and the motion of the top of the specimen is monitored using the proximity displacement transducers. The input current (proportional to the shear stress) and the corresponding torsional rotation (proportional to the shear strain) are simultaneously recorded. The shear modulus of the soil is determined from the average slope of the stress-strain loops, while material damping is related to the area of the hysteresis loop.
At a given consolidation effective stress, TSS tests are repeated several times, increasing progressively the amplitude of the input voltage, thus obtaining the secant shear modulus and the damping ratio corresponding to increasing values of the shear strain

Documents

Partner's Documents

Instruction manual