soil mechanics – dynamic systems combined resonant column (rc) & torsional cyclic shear (tcs)...

Soil Mechanics – Dynamic systems

Combined Resonant Column (RC) & Torsional Cyclic Shear (TCS) Test apparatus to determinate with saturated soil :

• Shear Modulus

• Damping Modulus

versus Shear Strain

WF Resonant Column Apparatus



The base pedestal is fixed (the same as a standard triaxial) but the specimen top cap is free to rotate.

Ideal for ResearchConforming 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 aimThe 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


The aimThe aimThis 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 - 310 - 210 - 110 1 10

Earthquake

Ocean Wave Loading

Machine Foundations

Cau

ses

of

Vib

ratio

ns 10 - 4 - 310 - 210 - 110 1 10

Small Strain Triaxial

RC-Resonant Column

TCS-Torsional Cyclic Shear

Cyclic Simple Shear

Dynatriax - Cyclic TriaxialBender Element

Dyn

amic

Sys

tem

Ra

nges

(% Strain)


The aimThe aim

before throughout

Stress conditions of soil sample during earthquake


Soil response to cyclic vibrations

The aimThe aim


Secant shear modulus

Damping ratio

Secant shear modulus

The aimThe aim


Strain level and mechanical behaviour

Small strain level behaviour

Medium strain level behaviour

Big strain level behaviour

The aimThe aim


Strain-dependent shear modulus and damping ratio

G0 or Gmax

The aimThe 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 aimThe aim


Typical range of G/Go curves against shear strain for gravels, sands and clays

The aimThe aim


Range of strain

Dynamic tests

Conventional triaxial tests

Large strains

Micro strains

Small

strains

Soil strains on site

Local measurement of strains

The aimThe 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 SystemThe System


The CellThe 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 Parts

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 CellThe 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 does it work ?


Electromagnetic system fixed to the inner cell top

Magnets supporting frame and top cap: free to rotate

The CellThe Cell


• The top picture shows the electromagnetic drive system which is attached to the top of the inner cell.

• The bottom 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.

The CellThe Cell


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.

The CellThe Cell


• Two proximity transducers are mounted on the electromagnetic 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 switchGND GroundAccel AccelerometerAxial Connection to LVDT for measurement of axial compression of the specimenAux 1 Auxiliary input for further appplicationsProx Connection to the couple of the proximity transducersCell, Pore e Back pressure

Serie of 3 connectors for the relevant pressure transducersVolume Connection to the volume change transducers or differential pressureMotion Connection to the motor drivers of the proximity transducersAux2 Auxiliary input for further appplicationsCoils 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. Saturation2. Isotropic Consolidation3. Resonant Frequency4. 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 GShear modulus G, the Damping ratio DDamping ratio D and the Shear StrainShear 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)

2SVG

F

LfV rS

2

rf

ffD

2

12

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 shearThe 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

soil mechanics – dynamic systems combined resonant column (rc) & torsional cyclic shear (tcs)...

Documents

strain slide

aim slide

saturated soil

soil layers

level of strain

system slide

test systems

earthquake slide