Presentation on theme: "WF Resonant Column Apparatus"— Presentation transcript:
1 WF Resonant Column Apparatus Combined Resonant Column (RC) & Torsional Cyclic Shear (TCS) Test apparatus to determinate with saturated soil :Shear ModulusDamping Modulusversus Shear Strain
2 WF Resonant Column Apparatus The base pedestal is fixed (the same as a standard triaxial) but the specimen top cap is free to rotate.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.Ideal for ResearchConforming to ASTM D 4015
3 The aim Typically small and medium strain levels The WF-Resonant Column allows the investigation of stress-strain behavior in the small shear strains level fieldTypically small and medium strain levelsHigh accuracy testing systems, suitable for that levels of strains
4 The aim (% Strain) Dynamic System Ranges 10 10 10 10 1 10 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 strainsDynatriax - Cyclic TriaxialBender ElementCyclic Simple ShearTCS-Torsional Cyclic ShearDynamic System RangesRC-Resonant ColumnSmall Strain Triaxial10- 4- 310- 210- 110110Machine FoundationsCauses of VibrationsOcean Wave LoadingEarthquake10- 4- 310- 210- 110110(% Strain)
5 Stress conditions of soil sample during earthquake The aimbeforethroughoutStress conditions of soil sample during earthquake
6 Soil response to cyclic vibrations The aimSoil response to cyclic vibrations
7 The aimSecant shear modulusSecant shear modulusDamping ratio
8 Strain level and mechanical The aimStrain level and mechanicalbehaviourSmall strain level behaviourMedium strain level behaviourBig strain level behaviour
9 Strain-dependent shear modulus and damping ratio The aimStrain-dependent shear modulus and damping ratioG0 or Gmax
10 Local Seismic Response of a real soil The aimLocal Seismic Response of a real soilChange of D and G against depth, due to different density g of the soil layers and to different geostatical stress levelsLayer 1Layer 2Layer 3
11 The aimTypical range of G/Go curves against shear strain g for gravels, sands and clays
12 The aim Range of strain Soil strains on site Micro strains Dynamic testsConventional triaxial testsLarge strainsMicro strainsSmall strainsSoil strains on siteLocal measurement of strains
13 WF Resonant Column Apparatus 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 – 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 (g – D).
15 The Cell double coaxial perspex cell, electromagnetic system: External perspex cell walldouble coaxial perspex cell,electromagnetic system:8 coils encircling 4 magnetsconnected to the sample upperend,measuring system (axialtransducer, proxy transducers,pressure transducers, volumechange system)Axial transducerProxy transducers supportcoilsmagnetspecimenInternal lexan cell wall
16 The Cell Parts Double cell Electromagnetic system: fixed part Magnets supporting frame and top cap: moving partDouble cellProxy transducers motion system
17 The Cell Electromagnetic drive system connects to the specimen top cap Double cell system
18 How does it work ?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.
19 The Cell Electromagnetic system fixed to the inner cell top Magnets supporting frame and top cap: free to rotate
20 The CellThe 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.
21 The Cell The inner cell containing the specimen is filled with water with asilicon oil top to prevent air diffusionthrough the membrane.The outer cell pressure is air whichacts on the water producingequal pressure to the inner & outer cell.We use a double cell to separate theair and water when applying cellpressure. The electromagnetic drivesystem can only run in air. If we usedair around the specimen we can haveair diffusion through the membrane.This happens in long term tests, so we usede-aired water as in our standardtriaxial tests.Double cell
22 The Measurements Two proximity transducers are mounted on the electro- magnetic drive system tomonitor the rotation of thetop cap assembly.Proximity transducers arenon contact transducerswhich do not interfere withthe rotation of the top cap.Therefore they have noinfluence on the recorded data.
24 The Control BoxPower 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.
25 The test is performed on a cylindrical sample Performing the testThe test is performed on a cylindrical sample(50 mm dia, 70 mm available on request), either undisturbed or remouldedThe RC system software has the following stages:1. Saturation2. Isotropic Consolidation3. Resonant Frequency4. Torsional shearAs 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.
26 Performing the test Performing the 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.SaturationConsolidationMeasurements26
27 Performing the testThe dynamic behavior of soils is represented by the Shear modulus G, the Damping ratio D and the Shear Strain gG shear modulus and D damping ratio, are of key importance to determine the mechanical behaviour of soils under small strain cyclic loading conditions
28 Resonant frequencyThe 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.
29 Resonant frequency fr Fundamental Resonant Frequency f1 & f2 are the band width frequencies at which the amplitude times the amplitude of the fundamental resonant frequency frStokoe et al. 1999Frequency, f (Hz)Shear strain, g (%)Shear strain, g (%)(0.707 x fr) is the average area enclosed by the sine wave.G is the shear modulus or stiffness of the material being testedD is the damping modulus, the rate of which energy decays in the specimen. This can be achieved by free vibration where the power is switched off and the vibrations reduce to zero. Or as shown above by the half power method of 0.707fr to determine points f1 & f2
30 Torsional shear The test (undrained conditions): Saturation Isotropic consolidationThe frequency of the cyclic Torsional shear (sinusoidal, <2 Hz) is constant whileamplitude is increased.The system records the Torsional stress & strain values for each amplitude anddisplays Hysteresis cycle from witch G and D are determined.g is measured through proximity transducers the shear strength t is evaluated through the applied torque
31 Resonant frequencyFrom 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 dampingThe 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 times the amplitude at the resonance frequency Fr.
32 Resonant frequencyFrom 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 dampingThe 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 times the amplitude at the resonance frequency Fr.Consolidation of the specimen prior to either Resonant or Torsional tests
33 Saturation and consolidation Consolidation of the specimen prior to either Resonant or Torsional testsGraph showing consolidation curve33
34 Torsional shearTorsion Shear Test at 0.1Hz, Amplitude 1 Volt