UU Triaxial Test - Fullerton

1 Civil & Environmental Engineering Department EGCE 324L (Soil Mechanics Laboratory) Spring 2008 Instructor: Binod Tiwari, PhD Date: 4/28/2008 1UU Triaxial Test Concept of Shear Strength Please refer the same materials you got in Unconfined Compression test. a. Triaxial Shear Test Figure 1 Triaxial compression testing device Civil & Environmental Engineering Department EGCE 324L (Soil Mechanics Laboratory) Spring 2008 Instructor: Binod Tiwari, PhD Date: 4/28/2008 2 Triaxial test is more reliable because we can measure both drained and undrained shear strength. Generally diameter (3 tall) or diameter (6 tall) specimen is used. Specimen is encased by a thin rubber membrane and set into a plastic cylindrical chamber. Cell pressure is applied in the chamber (which represents 3 ) by pressurizing the cell fluid (generally water). Vertical stress is increased by loading the specimen (by raising the platen in strain controlled test and by adding loads directly in stress controlled test, but strain controlled test is more common) until shear failure occurs.

2 Total vertical stress, which is 1 is equal to the sum of 3 and deviator stress ( d). Measurement of d, axial deformation, pore pressure, and sample volume change are recorded. Depending on the nature of loading and drainage condition, Triaxial tests are conducted in three different ways. UU Triaxial test CU Triaxial test CD Triaxial test In this lab, we will conduct UU Triaxial test. Civil & Environmental Engineering Department EGCE 324L (Soil Mechanics Laboratory) Spring 2008 Instructor: Binod Tiwari, PhD Date: 4/28/2008 3 Unconsolidated Undrained Triaxial Test (UU Triaxial Test) As drainage is not permitted and consolidation is not necessary, this test is very quick, and also referred as Q-test. As drainage is not permitted, u increases right after the application of 3 as well as after the application of d. As Uc = B. 3 and Ud = A. d Total u = B.

3 3 +A. d u = B. 3 +A. ( 1 - 3) This test is common in clayey soils. Application UU Triaxial test gives shear strength of soil at different confining stresses. Shear strength is important in all types of geotechnical designs and analyses. Equipment Strain controlled Triaxial load frame Triaxial cell assembly Cell pressure supply panel Scale Balance sensitive to g Moisture cans Oven Procedure (Follow the specific guideline provided in a separate sheet) Measure diameter, length, and initial mass of the specimen. Measure the thickness of the rubber membrane. Set a soil specimen in a Triaxial chamber. Increase the cell pressure to a desired value (70 kPa for the first case and 140 kPa in the second case). Shear the specimen at the rate of 1%/min or mm/min (for 70 mm sample height). In automated device, the software calculates it automatically based on the soil type. Record L, and d in every 10 seconds (computer does it automatically).

4 Continue the test until the deviator stress shows ultimate value or 20% axial strain. After completion of the test, release the cell pressure to 0, vent the pressure and bring the cell down by bring the lower platen down, drain the cell, and clean the porous stone and the assembly. Sketch the mode of failure. Measure the weight of the soil specimen again, and put the specimen into the oven. Measure the weight again after 24 hours. Repeat the test for the second specimen too (140 kPa of cell pressure and third specimen 210 kPa of cell pressure). Civil & Environmental Engineering Department EGCE 324L (Soil Mechanics Laboratory) Spring 2008 Instructor: Binod Tiwari, PhD Date: 4/28/2008 4 Calculations Calculate axial strain. = LL L = Vertical deformation of the specimen. Calculate vertical load on the specimen. You will get it directly from the force transducers. Calculate corrected area of the specimen (Ac) =10 AAc A0 = Initial cross-sectional area x D2/4 Calculate the stress on the specimen.

5 CALoad= Plot d versus axial strain separately for three tests . Plot d vs a for three tests in the same plot. Plot Mohr circle based on 1 and 3 at failure. They should give the same d value. Add one Mohr circle for unconfined compression test too (That you did last week). Make a straight line, which is tangent to all Mohr s circles. This gives cu with a horizontal line, u = 0. Therefore this test is called = 0 test. 2duc = Calculate the moisture content of the specimen after the test. Calculate the initial void ratio of the specimen (Use the equations provided in the earlier classes). Figure 2 Total stress Mohr circle and failure envelope obtained from UU Triaxial test Civil & Environmental Engineering Department EGCE 324L (Soil Mechanics Laboratory) Spring 2008 Instructor: Binod Tiwari, PhD Date: 4/28/2008 5UU Triaxial Test Laboratory Data Sheet I. GENERAL INFORMATION Tested by: Date tested: Lab partners/organization: Client: CSUF Project: Soils Lab Boring no.

6 : N/A Recovery depth: N/A Recovery date: Recovery method: N/A Soil description: II. TEST DETAILS Initial specimen diameter, Do: Initial specimen area, Ao: Initial specimen length, Lo: Initial specimen volume, Vo: Moist mass of specimen after test, M: Dry mass of specimen, Ms: Moisture content, w: Total unit weight, : Dry unit weight, d: Degree of saturation, S: Membrane type: Standard Rubber Membrane Axial strain rate, 1/ t: Deformation indicator: LVDT Force indicator: LVDT Cell pressure, 3: Specimen preparation method: Hand Compaction Notes, observations, and deviations from ASTM D2850 test standard: III. MEASUREMENTS AND CALCULATIONS Axial Deformation ( L) Axial Load (P) Axial Strain ( 1) Corrected Area (A) Deviator Stress ( ) EQUATIONS: a = L/Lo A = Ao/(1- a) = P/A 1f = 3 + f 3: f: 1f: Civil & Environmental Engineering Department EGCE 324L (Soil Mechanics Laboratory) Spring 2008 Instructor: Binod Tiwari, PhD Date: 4/28/2008 6 I.

7 GENERAL INFORMATION Tested by: Date tested: Lab partners/organization: Client: CSUF Project: Soils Lab Boring no.: N/A Recovery depth: N/A Recovery date: Recovery method: N/A Soil description: II. TEST DETAILS Initial specimen diameter, Do: Initial specimen area, Ao: Initial specimen length, Lo: Initial specimen volume, Vo: Moist mass of specimen after test, M: Dry mass of specimen, Ms: Moisture content, w: Total unit weight, : Dry unit weight, d: Degree of saturation, S: Membrane type: Standard Rubber Membrane Axial strain rate, 1/ t: Deformation indicator: LVDT Force indicator: LVDT Cell pressure, 3: Specimen preparation method: Hand Compaction Notes, observations, and deviations from ASTM D2850 test standard: III. MEASUREMENTS AND CALCULATIONS Axial Deformation ( L) Axial Load (P) Axial Strain ( 1) Corrected Area (A) Deviator Stress ( ) EQUATIONS: a = L/Lo A = Ao/(1- a) = P/A 1f = 3 + f 3: f: 1f: Civil & Environmental Engineering Department EGCE 324L (Soil Mechanics Laboratory) Spring 2008 Instructor: Binod Tiwari, PhD Date: 4/28/2008 7 USER S GUIDELINE FOR THE ELE Triaxial DEVICE (UU Triaxial TEST) A.

8 POWER AND MAIN SUPPLY SETTINGS (This step is already done) 1. Press the Power Display power button to the ON position. After Stabilization (approx. 15 min.), push the Tare button to zero the display. 2. Turn on the laboratory vacuum supply. The associated amount of vacuum available to your system will be displayed on the Vacuum Supply Gauge . 3. Turn on the laboratory compressed air supply. Viewing the Pressure Supply gauge, adjust the Master Regulator until the desired maximum supply pressure is reached. The displayed pressure should be about 10 psi more than the required cell pressure. Do Not exceed 150 psi (1034 KPa) pressure. 4. Turn on the laboratory water supply. B. FILLING OF THE DE-AIRED WATER TANK SYSTEM (This step is already done) 1. Turn the De-Airing Water Control valve to the Fill position. 2. When the tank water level is about 1 from the top, turn the De-Airing Water Control valve to the vent position (Very Slowly to allow water to drain).

9 C. DE-AIRING THE WATER TANK (This step is already done) 1. Turn the De-Airing Water Control valve to the Vacuum position. 2. Apply vacuum for 10-15 minutes and, at the same time, gently shake the tank occasionally to enhance the removal of air from water. 3. Turn the De-Airing Water Control valve to the vent position. D. FILLING THE BURETTE CHANNELS (This step is already done) 1. Set all five valves on the test cell to the closed position. 2. Set the De-Airing Water Control valve to the Pressure position. 3. Set the Burette/Annulus Input Control valve to the vent position. 4. Set the Annulus Control Switch to the open position (Normal). 5. Slowly turn the Burette/Annulus Flow Control valve to the Fill position. When the water reaches the desired level, turn the Burette/Annulus Flow Control valve to the Cell Operate position. Do not overfill. Water should not be allowed to flow into the pressure tube at the top. 6. Repeat the above steps until two burette channels being used are filled to the desired level.

10 Note: If the water level in the De-Aired Water Tank System drops to about 1 from the bottom, repeat the filling and de-airing procedures described above. Civil & Environmental Engineering Department EGCE 324L (Soil Mechanics Laboratory) Spring 2008 Instructor: Binod Tiwari, PhD Date: 4/28/2008 8E. DE-AIRING THE BURETTE CHANNELS (This step is already done) 1. Set the Burette/Annulus Input Control valve for each channel to the vacuum position. Under normal operating conditions, the de-airing process should be completed in about 5-10 minutes. 2. After completion, set all Input Control valves back to the vent position. F PREPARATION OF THE SAMPLE 1. Trim the sample to be tested using a Miter Box. 2. Measure the height and diameter of the sample at various locations to get an average value. 3. Measure the weight of the trimmed sample before test. 4. Wrap the sample in a plastic sheeting to prevent any moisture loss. 5. Use the trimmings to get the Moisture Content of the sample before test.