ppt resonant column last version.ppt
TRANSCRIPT
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
Soil Mechanics – Dynamic systems
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.
Soil Mechanics – Dynamic systems
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
Soil Mechanics – Dynamic systems
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 V
ibra
tions 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
Ran
ges
(% Strain)
Soil Mechanics – Dynamic systems
The aimThe aim
before throughout
Stress conditions of soil sample during earthquake
Soil Mechanics – Dynamic systems
Soil response to cyclic vibrations
The aimThe aim
Soil Mechanics – Dynamic systems
Secant shear modulus
Damping ratio
Secant shear modulus
The aimThe aim
Soil Mechanics – Dynamic systems
Strain level and mechanical behaviour
Small strain level behaviour
Medium strain level behaviour
Big strain level behaviour
The aimThe aim
Soil Mechanics – Dynamic systems
Strain-dependent shear modulus and damping ratio
G0 or Gmax
The aimThe aim
Soil Mechanics – Dynamic systems
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
Soil Mechanics – Dynamic systems
Typical range of G/Go curves against shear strain for gravels, sands and clays
The aimThe aim
Soil Mechanics – Dynamic systems
Range of strain
Dynamic tests
Conventional triaxial tests
Large strains
Micro strainsSmall
strains
Soil strains on site
Local measurement of strains
The aimThe aim
Soil Mechanics – Dynamic systems
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).
WF Resonant Column Apparatus
Soil Mechanics – Dynamic systems
The SystemThe System
Soil Mechanics – Dynamic systems
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
Soil Mechanics – Dynamic systems
The Cell Parts
Double cell
Electromagnetic system: fixed part
Magnets supporting frame and top cap: moving part
Proxy transducers motion system
Soil Mechanics – Dynamic systems
• Electromagnetic drive system connects to the specimen top cap
• Double cell system
The CellThe Cell
Soil Mechanics – Dynamic systems
• 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 ?
Soil Mechanics – Dynamic systems
Electromagnetic system fixed to the inner cell top
Magnets supporting frame and top cap: free to rotate
The CellThe Cell
Soil Mechanics – Dynamic systems
• 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
Soil Mechanics – Dynamic systems
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
Soil Mechanics – Dynamic systems
• 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
Soil Mechanics – Dynamic systems
The Control Box
Soil Mechanics – Dynamic systems
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
Soil Mechanics – Dynamic systems
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
Soil Mechanics – Dynamic systems
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 calculatedThe 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
Soil Mechanics – Dynamic systems
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
Soil Mechanics – Dynamic systems
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
Soil Mechanics – Dynamic systems
Frequency, f (Hz)
2SVG
FLfV r
S
2
rfffD
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
Soil Mechanics – Dynamic systems
Torsional shearThe test (undrained conditions):1. Saturation2. Isotropic consolidation3. 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
Soil Mechanics – Dynamic systems
Resonant frequency
Soil Mechanics – Dynamic systems
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.
Soil Mechanics – Dynamic systems
Graph showing consolidation curve
Saturation and consolidation
Soil Mechanics – Dynamic systems
Torsion Shear Test at 0.1Hz, Amplitude 1 Volt
Torsional shear