Dynamic Triaxial Test

Experimental Setup:

1. Cyclic Triaxial loading frame
   
2. Load cell
   
3. LVDT
   
4. Triaxial cell
   
5. Soil specimen
   
6. DAQ & computer system
   
7. Cell Pressure control
   
8. Pore pressure control
   
9. Volume change device

Testing procedure (ASTM D 3999-91):

• The saturated porous stone disc of diameter same as the sample is placed on top of the pedestal of triaxial testing machine and the circular filter paper of same size is placed over the disc. Specimen is placed on top of the filter paper. The filter paper with porous stone is placed on the top of the specimen to allow two-way drainage.
• The latex membrane is stretched in the membrane stretcher and placed on the soil specimen. O rings are placed at top and bottom of platens of the soil specimen to prevent the cell water entering into the specimen.
• The triaxial cell is placed over the base and tightened with the screws. The cell is then filled with water and a small confining pressure of about 10 kPa is applied to hold the specimen in place.
• The soil specimen needs to be completely saturated before isotropic consolidation phase.
• Saturation process consists of three steps: i) CO₂ saturation, ii) Water saturation, iii) Back pressure application.
• CO₂ is applied continuously for minimum 3-6 min from bottom of the specimen and then allow it to go out of the specimen from the top. CO₂ replaces the air in void space of the specimen which gets easily dissolved in the water present within the specimen. The CO₂ flushing process is repeated 4-5 times to ensure better saturation.
• Water saturation is done by supplying water from bottom of the specimen and allow it to go out of the specimen from the top to do proper water flushing of the specimen. The water used for flushing needs to be distilled & de-aired water.
• The force saturation is performed by applying cell pressure and the back pressure at constant increments with constant difference between these two pressures. The sample is allowed to saturate for some time (10-20 min) after each increment of cell pressure and the back pressure. This increase should be followed by a check for saturation value (B), also known as skempton’s pore pressure parameter. It is important to note that cell pressure always be higher than back pressure. The sample is said to be fully saturated if the B value is greater than 0.95 can be acquired.
• Isotropic consolidation stage is started by applying confining pressure. During the Consolidation stage, drainage valve is kept open and the volume change is measured until no change in volume is observed (when primary consolidation is over).
• In Cyclic triaxial test, no drainage is allowed during shearing stage and pore pressure is measured throughout the test using the pore pressure transducer. The test can be performed using stress controlled mode (constant displacement) or stress controlled mode (constant load) by applying uniform sinusoidal displacement or load amplitude respectively
• The Cyclic Triaxial machine is set in motion at an appropriate frequency (range = 0.1 to 2 Hz) and at chosen amplitude (displacemen/load). Data acquisition system (DAQ) is attached with the computer & various transducers of cyclic triaxial system, which records the data with the help of cyclic triaxial software. The experiment is stopped usually at 30 displacement/load cycles (if needed may go for higher cycles).

Dynamic Triaxial Test

Observation Sheet for Strain controlled Cyclic Triaxial test (Constant Displacement):

Weight of Sample:___________________ In-situ Density: _______________________
Initial Water Content:_________________ Strain rate:______________________________
Diameter:________________________ Area(A₀):________________________
Height:_________________________ Volume:__________________________
Cell Pressure:__________________________Back Pressure: ______________________
Confining Pressure (σ3):_______________ Saturation value (B):_____________________
Frequency of cyclic loading (Hz)___________________________
Amplitude of cyclic loading (mm)_____________________________

Observation Sheet for Stress controlled Cyclic Triaxial test (Constant Load):

Weight of Sample:___________________ In-situ Density: _______________________
Initial Water Content:_________________ Strain rate:______________________________
Diameter:________________________ Area(A₀):________________________
Height:_________________________ Volume:__________________________
Cell Pressure:__________________________Back Pressure: ______________________
Confining Pressure (σ3):_______________ Saturation value (B):_____________________
Frequency of cyclic loading (Hz)___________________________
Amplitude of cyclic loading (mm)_____________________________

Calculations:

• Axial strain: ε_a = (Δ_H/H₀)x100
  
• Excess pore pressure (Δu) = Pore pressure (u) – Initial pore pressure (u₀)
  
• Deviator stress (σ_d) during cyclic loading:
  σ_d = Load/A₀ (1 Kg/cm²= 100kPa)
  
• Calculations of Dynamic Parameters of soil (G, D):
  Hysteresis loop (Deviator stress versus axial strain curve) is used to find the Shear Modulus (G) and Damping ratio (D) of soil.

E = Dynamic Young Modulus (slope of the centre line of Hysteresis loop)
D = A_L/ (4πAT) (needs to be calculated from Hysteresis loop)
G = E/2(1+ν) (ν is poisons ratio )
(ν = 0.5 as Cyclic triaxial tests is commonly run at CU type triaxial conditions)

Dynamic Triaxial Test

Graphs:

Input data:
Axial strain versus Time curve
For Strain controlled cyclic triaxial test (Constant Displacement test)
Deviator stress versus Time curve
For Stress controlled cyclic triaxial test (Constant Load test)

Output data:
Deviator stress versus Time curve
For Strain controlled cyclic triaxial test (Constant Displacement test)
Axial strain versus Time curve
For Stress controlled cyclic triaxial test (Constant Load test)

Analysis of the cyclic triaxial data:
Deviator stress versus Axial strain curve (Hysteresis loop)
Excess pore pressure versus Time curve

Example:

A cyclic triaxial test was performed on Sabarmati soil (86% sand & 14% silt). The in-situ density and in-situ water content was 1.9 gm/cc and 20% respectively. The shear strength parameters was c=0 & Φ = 26 deg. The strain controlled cyclic triaxial tests were performed on specimens at 0.5 Hz frequency and 1 mm displacement amplitude. The size of the specimen was 50 mm diameter and 100 mm height. The specimen was isotropically consolidated at confining pressure of 300 kPa after acquiring saturation value (B) of 0.97.

Input curve: Axial strain versus Time curve

Output curve: Deviator stress versus Time curve

Analysis of cyclic triaxial test data
i. Deviator stress versus Axial strain (Hysteresis loop)

ii. Excess pore pressure versus Time curve
(pore pressure ratio = excess pore pressure/confining pressure;
At 5^th cycle, pore pressure ratio is 1, which indicates that soil has liquefied.)

Results:

The first five cycles are usually considered for calculating shear modulus and damping ratio:
G = 3649 kPa (average value of the first five cycles)
D = % (average value of the first five cycles)

Liquefaction analysis of soil was also studied. At 5^th cycle, excess pore pressure became equal to the confining pressure (300 kPa). the pore pressure ratio (Δu/σ₃) became 1.0 at 5^th cycle, which means the effective stress of soil became zero and there is no strength in soil; thus soil became liquefied. The soil showed liquefaction in 5 cycles for cyclic loading of 1 mm displacement amplitude at 0.5 Hz frequency.

General Remarks

• Saturation value B must be acquired to be more than 0.95 before starting the CRS test.
• The loading applied to a specimen during cyclic triaxial is constant in both amplitude and frequency and is most often applied in the form of a uniform, repeating sine wave.
• The alignment of top and bottom platens is critical to avoid non-uniform stress states within the specimen. Since axial loading in cyclic triaxial test is in extension as well as in compression, the loading rod should be rigidly connected to the top platen.

Dynamic Triaxial Test

Theory:

Cyclic triaxial tests are commonly used to measure the cyclic strength or liquefaction resistance of soils. The stresses applied to an element in filed are quite different than the way they are applied in a cyclic triaxial test. The primary difference between the field conditions and those on an inclined plane within the specimen is and increase or decrease in the confining pressure equal to one-half the deviator stress. To match the stress conditions experienced in the field, it is important to decrease or increase the cell pressure by one-half the deviator stress as the deviator stress is respectively increased or decreased.

The maximum cyclic axial stress than can be applied to a saturated specimen is controlled by the stress conditions at the end of confining stress application and the pore water pressure generated during testing. For an isotrpically consolidated specimen tested in cyclic compression, the maximum cyclic axial stress than can be applied to the specimen is equal to the effective confining pressure. Since soils are not capable of taking tension, cyclic axial stresses greater than this value tend to lift the top platen from the soil specimen. The pore water pressure increases during the test performed on isotropically consolidated specimen, the effective confining pressure is reduced, contributing to the tendency of the specimen to necking during the extension portion of the loading cycle. A 90 degree change in the direction of the major principal stress occurs during the two halves of the loading cycle on isotroipcally consolidated specimens and at certain levels of cyclic stress application on anisotropically consolidated specimens.

The cyclic load can also be defined in the form of “Single Amplitude” loading and “Double Amplitude” loading. Single amplitude axial strain is defined as the total strain that occurs during half cycle of loading, either in compression or extension. Double amplitude axial strain is defined as the total strain that occurs between any two adjacent peak compressive and extensive strains, as shown below.