Resonant Column

and

Torsional Cyclic Shear

System

Combined Resonant Column (RC) & Torsional Cyclic Shear (TCS) Test apparatus

to determinate with saturated soil :

• Shear Modulus • Damping Modulus versus Shear Strain

The base pedestal is fixed (the same as a standard triaxial) but the

specimen top cap is free to rotate.

Ideal for Research Conforming to ASTM D 4015

A rotational force (torque) is applied to the specimen top by electro-magnetic system which applies the stress or strain loading in

frequency up to 250 Hz.

The Aim

Typically small and medium strain levels

High accuracy testing systems, suitable for that

levels of strains

The WF-Resonant Column allows the investigation of stress-strain behavior in

the small shear strains level field

This bottom half graph shows the range of strain encountered from machines or natural causes. The top half shows test systems that can perform these range of strains.

10 - 4 - 3 10 - 2 10 - 1 10 1 10

Earthquake

Ocean Wave Loading

Machine Foundations

Ca

use

s o

f V

ibra

tio

ns

10 - 4 - 3 10 - 2 10 - 1 10 1 10

Small Strain Triaxial

RC-Resonant Column

TCS-Torsional Cyclic Shear

Cyclic Simple Shear

Dynatriax - Cyclic Triaxial Bender Element

Dyn

am

ic S

yste

m R

an

ge

s

(% Strain)

The Aim

before throughout

Stress conditions of soil sample during earthquake

Soil response to cyclic vibrations

The Aim

Secant shear modulus

Damping ratio

Secant shear modulus

The Aim

Strain level and mechanical

behaviour

Small strain level behaviour

Medium strain level behaviour

Big strain level behaviour

The Aim

Strain-dependent shear modulus and damping ratio

G0 or Gmax

The Aim

Layer 1

Layer 2

Layer 3

Local Seismic Response of a real soil

Change of D and G against depth, due to different

density of the soil layers and to different

geostatical stress levels

The Aim

Typical range of G/Go curves against shear

strain for gravels, sands and clays

The Aim

Range of strain

Dynamic tests

Conventional triaxial tests

Large strains

Micro strains

Small

strains

Soil strains on site

Local measurement of strains

The Aim

The test procedure includes a series of measurements of the

resonance frequency against the increasing levels of shear strains,

in order to define the diagram ( – G).

For each level of strain, once the resonance frequency has been

measured, the damping ratio is also calculated, in order to define

the diagram ( – D).

The Aim

The System

The Cell

• double coaxial perspex cell, • electromagnetic system: 8 coils encircling 4 magnets connected to the sample upper end, • measuring system (axial transducer, proxy transducers, pressure transducers, volume change system)

Internal lexan cell wall

magnet

coils

Axial transducer

specimen

External perspex cell wall

Proxy transducers support

The Cell

Double cell

Electromagnetic system: fixed part

Magnets supporting frame and top cap: moving part

Proxy transducers motion system

• Electromagnetic drive system connects to the specimen top cap • Double cell system

The Cell

• The electromagnetic drive consists of eight coils mounted on a drive plate with

four magnets positioned on the specimen top cap assembly. When a sinusoidal

current is applied to the coils, it pulls the magnets in one direction and reverses

the direction as the sine wave changes from positive to negative. The actual

rotational movement of the top cap is determined by the stiffness of the specimen

being tested.

• The double cell is to allow us to have water in the inner cell up to the top cap with

a layer of silicon oil on top of the water. The outer cell confining pressure is air.

The water in the inner cell is to prevent air diffusion through the specimen

membrane and the silicon oil is to prevent air entering the water.

How it works

Electromagnetic system fixed to the inner cell top

Magnets supporting frame and top cap: free to rotate

How it works

• The picture shows the electromagnetic

drive system which is attached to the

top of the inner cell.

• The picture shows the top cap with the

four magnets. This is attached to the

specimen with a membrane and o rings,

the same as a standard triaxial set up.

This assembly is free to rotate.

How it works

Double cell

• The inner cell containing the specimen is filled with water with a silicon oil top to prevent air diffusion through the membrane. • The outer cell pressure is air which acts on the water producing equal pressure to the inner & outer cell. • We use a double cell to separate the air and water when applying cell pressure. The electromagnetic drive system can only run in air. If we used air around the specimen we can have air diffusion through the membrane. This happens in long term tests, so we use de-aired water as in our standard triaxial tests.

How it works

• Two proximity transducers are mounted on the electro- magnetic drive system to monitor the rotation of the top cap assembly. • Proximity transducers are non contact transducers which do not interfere with the rotation of the top cap. Therefore they have no influence on the recorded data.

The Measurements

The Control Box

Power Main switch

GND Ground

Accel Accelerometer

Axial Connection to LVDT for measurement of axial compression of the specimen

Aux 1 Auxiliary input for further appplications

Prox Connection to the couple of the proximity transducers

Cell, Pore e Back pressure

Serie of 3 connectors for the relevant pressure transducers

Volume Connection to the volume change transducers or differential pressure

Motion Connection to the motor drivers of the proximity transducers

Aux2 Auxiliary input for further appplications

Coils Uscita per il collegamento delle bobine del motore di coppia.

USB Connection to PC

Each cable is fitted with a specific connector for easy installation of the transducers

inside the cell body, near the sample.

The Control Box

The test is performed on a cylindrical sample (50 mm dia, 70 mm available on request), either undisturbed or remoulded

The RC system software has the following stages: 1. Saturation 2. Isotropic Consolidation 3. Resonant Frequency 4. Torsional shear

As in all standard triaxial tests, we start by saturating the specimen and applying the in-situ effective stress. Then we choose to determine the resonant frequency or the torsional shear strength.

Performing the Test

Performing the test:

Same as in the triaxial test

Same as in the triaxial test

An excitation current is applied to the electromagnetic drive system, to generate a constant torque to the top end of the soil sample. The frequency of this current is increased until the fundamental resonance frequency of the system is achieved.

Resonance frequency and relevant acceleration are measured.

From these data the G modulus is calculated

The damping ratio D is also measured during the “free vibration decay” procedure.

Further measurements are performed during torsional tests, where higher levels of excitation current and torque are applied.

Consolidation

Saturation

Measurements

Performing the Test

The dynamic behavior of soils is represented by the Shear modulus G, the Damping ratio D and the Shear Strain

G shear modulus and D damping ratio, are of key importance to determine the

mechanical behaviour of soils under small strain cyclic loading conditions

Performing the Test

The excitation Voltage is fixed and the frequency increased in automatic

increments or steps.

The system records the shear strain and calculates the Fundamental

Resonant Frequency corresponding to the maximum shear strain.

Resonant Frequency

Frequency, f (Hz)

Sh

ea

r s

train

, (

%)

2

SVG

F

LfV r

S

2

rf

ffD

2

12

Sh

ea

r s

train

, (

%)

fr Fundamental Resonant Frequency

f1 & f2 are the band width frequencies at which the amplitude 0.707 times the amplitude of the fundamental resonant frequency fr

Stokoe et al. 1999

Resonant Frequency

Torsional Shear The test (undrained conditions):

1. Saturation

2. Isotropic consolidation

3. The frequency of the cyclic Torsional shear (sinusoidal, <2 Hz) is constant while

amplitude is increased.

1. The system records the Torsional stress & strain values for each amplitude and

displays Hysteresis cycle from witch G and D are determined.

is measured through proximity transducers the

shear strength is evaluated through the applied torque

Resonant Frequency

From the frequency sweep graph the fundamental resonant

frequency and Modulus of damping can be determined.

In the resonant column test the half power bandwidth method can

be used to measure the material damping

Resonant Frequency

The bandwidth is the frequency difference between the upper and

lower frequencies for which the power has dropped to half of its

maximum, the frequencies F1 and F2 at which the amplitude is 0.707

times the amplitude at the resonance frequency Fr.

Graph showing consolidation curve

Saturation and Consolidation

Torsion Shear Test at 0.1Hz, Amplitude 1 Volt

Torsional Shear