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Soil Mechanics – Dynamic systems Combined Resonant Column (RC) & Torsional Cyclic Shear (TCS) Test apparatus to determinate with saturated soil : Shear.

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Presentation on theme: "Soil Mechanics – Dynamic systems Combined Resonant Column (RC) & Torsional Cyclic Shear (TCS) Test apparatus to determinate with saturated soil : Shear."— Presentation transcript:

1 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

2 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 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.

3 Soil Mechanics – Dynamic systems 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

4 Soil Mechanics – Dynamic systems The aim 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 Earthquake Ocean Wave Loading Machine Foundations Causes of Vibrations Small Strain Triaxial RC-Resonant Column TCS-Torsional Cyclic Shear Cyclic Simple Shear Dynatriax - Cyclic Triaxial Bender Element Dynamic System Ranges (% Strain)

5 Soil Mechanics – Dynamic systems The aim beforethroughout Stress conditions of soil sample during earthquake

6 Soil Mechanics – Dynamic systems Soil response to cyclic vibrations The aim

7 Soil Mechanics – Dynamic systems Secant shear modulus Damping ratio Secant shear modulus The aim

8 Soil Mechanics – Dynamic systems Strain level and mechanical behaviour Small strain level behaviour Medium strain level behaviour Big strain level behaviour The aim

9 Soil Mechanics – Dynamic systems Strain-dependent shear modulus and damping ratio G 0 or G max The aim

10 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 aim

11 Soil Mechanics – Dynamic systems Typical range of G/Go curves against shear strain  for gravels, sands and clays The aim

12 Soil Mechanics – Dynamic systems Range of strain Dynamic tests Conventional triaxial tests Large strains Micro strains Small strains Soil strains on site Local measurement of strains The aim

13 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

14 Soil Mechanics – Dynamic systems The System

15 Soil Mechanics – Dynamic systems 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

16 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

17 Soil Mechanics – Dynamic systems Electromagnetic drive system connects to the specimen top cap Double cell system The Cell

18 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 ?

19 Soil Mechanics – Dynamic systems Electromagnetic system fixed to the inner cell top Magnets supporting frame and top cap: free to rotate The Cell

20 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 Cell

21 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 Cell

22 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

23 Soil Mechanics – Dynamic systems The Control Box

24 Soil Mechanics – Dynamic systems PowerMain switch GNDGround AccelAccelerometer AxialConnection to LVDT for measurement of axial compression of the specimen Aux 1Auxiliary input for further appplications ProxConnection to the couple of the proximity transducers Cell, Pore e Back pressure Serie of 3 connectors for the relevant pressure transducers VolumeConnection to the volume change transducers or differential pressure MotionConnection to the motor drivers of the proximity transducers Aux2Auxiliary input for further appplications CoilsUscita per il collegamento delle bobine del motore di coppia. USBConnection 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

25 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. 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

26 Soil Mechanics – Dynamic systems 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 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. ConsolidationSaturationMeasurements Performing the test

27 Soil Mechanics – Dynamic systems Shear modulus G Damping ratio DShear Strain  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

28 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

29 Soil Mechanics – Dynamic systems Frequency, f (Hz) Shear strain,  (%) f r Fundamental Resonant Frequency f 1 & f 2 are the band width frequencies at which the amplitude times the amplitude of the fundamental resonant frequency f r Stokoe et al Resonant frequency

30 Soil Mechanics – Dynamic systems 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

31 Soil Mechanics – Dynamic systems Resonant frequency

32 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 times the amplitude at the resonance frequency Fr.

33 Soil Mechanics – Dynamic systems Graph showing consolidation curve Saturation and consolidation

34 Soil Mechanics – Dynamic systems Torsion Shear Test at 0.1Hz, Amplitude 1 Volt Torsional shear


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