The University of Toledo Soil Mechanics Laboratory

Transcription

1 The University of Toledo Soil Mechanics Laboratory 1 Triaxial Compression Test Introduction Soil is tested in axial compression or direct shear to determine shear strength parameters and axial stiffness for use in geotechnical design. Undisturbed soil samples, or test specimens prepared from disturbed samples, are tested using unconfined compression, triaxial compression or direct shear tests. The unconfined compression test has limitations though since the soil is tested under zero confining stress and the moisture condition of test specimens cannot be varied. Triaxial compression cells are designed so confining (cell) pressure can be applied to the soil and so that water and water pressure can be introduced into the specimens to saturate the specimens and to vary the pore water pressures. Thus it is possible to vary the total and effective stresses within soil specimens to simulate actual subsurface conditions and to determine the shear strength parameters (cohesion intercept and angle of internal friction) required for design. Triaxial compression test procedures can be varied in order to simulate different loading conditions by controlling the flow of water into or out of the test specimen (drainage). Valves leading to the specimen ends are used to control the flow. If cell pressure is applied to a specimen and drainage is prevented before loading (valves closed), then the soil condition is unconsolidated. If time is allowed for drainage after the cell pressure is applied and before the loads are applied (valves open), then the soil condition is consolidated. If the test specimen is loaded to failure in a short period of time and drainage is not permitted (valves closed), then the loading condition is undrained. If the specimen is loaded slowly and drainage is allowed (valves open) then the loading condition is drained. Three types of triaxial compression tests are summarized in Table 1. For this laboratory, three identical test specimens are prepared with Proctor compaction equipment and tested at different cell pressures using the triaxial compression apparatus. The results are used to plot stress-strain curves, to plot Mohr s circles and to determine the cohesion and angle of internal friction. Table 1 Triaxial Compression Tests Type of Test Consolidation Loading Condition Unconsolidated, Undrained (UU) No Undrained Consolidated, Undrained (CU) Yes Undrained Consolidated, Drained (CD) Yes Drained 1 ASTM Triaxial Compression - 1

2 Apparatus 1. Mixing tub 2. Proctor compaction equipment 3. Sample ejector 4. Specimen molds (3), Dimensions: D = 3.56 cm (1.4 in.); H 2 D. 5. Calipers 6. Balance 7. Triaxial cell 8. Pressure manifold 9. Compression machine 10. Water content tares and oven Procedure A. Preparation (Before the laboratory) 1. Obtain enough moist soil passing the #4 sieve for a Proctor sample, 2.3 kg (5.0 lbs.) 2. Slowly add water to the soil and mix until the soil will not crumble when squeezed in the hand but not so much water that the soil becomes sticky. 3. Compact the soil in Proctor mold following the standard Proctor procedure. Obtain density and moisture information from the Proctor sample. 4. Use the soil sample extruder with the specimen mold attachment to extrude the soil from the Proctor mold into three compression molds. 5. Trim the ends of the specimen perpendicular to the ends using a spatula. 6. Eject the soil from the mold, weigh the sample and measure the sample height and diameter for each test specimen. 7. Use a membrane stretcher to place a latex membrane on the test specimens. 8. Assemble the test specimens in triaxial cells and prepare for the triaxial compression tests. Preparation can include saturation and consolidation. B. Laboratory (For unconsolidated, undrained test) 1. Place the triaxial cell with the test specimen in the unconfined compression frame. 2. Apply cell pressure (σ 3 ) to the test specimen making sure that the valves to the top and bottom of the test specimens are closed. Recommended pressures are 101.3, 202.6, and kpa (14.7, 29.4 and 58.8 psi). 3. Apply vertical loads with deformation rate setting 26.1 (0.05 in/min or 1.27 mm/min). 4. Obtain readings from the load ring gage and the deformation gage at every 20 divisions on the deformation gage until the sample fails. 5. Sketch the failed specimen. Triaxial Compression - 2

3 Calculations Stress and strain are computed as defined in Equations 1 through 4, however the stress required for triaxial compression is the deviator stress. The applied stress is computed using the load ring calibration shown below and the cross-sectional area A, corrected for sample strain. σ = A' P A' A = 1 ε L ε = (3) L o L = (Deformation Gage Reading Initial Reading) x inch/div (4) (1) (2) 4000 Load Ring Calibration Load (N) = 4.83 N/Div x Dial Divisions Load (N) Dial Divisions Deviator stress is the difference between the vertical stress (σ 1 ) and the horizontal stress (σ 3 ), where the horizontal stress is equal to the applied cell pressure. For the triaxial compression test, the load that is computed from the initial load ring reading after the cell pressure is applied is due to the fluid pressure acting on the loading piston. The net load (P net ) is computed by subtracting the initial load (P init ) from load (P). This load corresponds to the vertical load that is applied in excess of the load imparted to the piston from the cell pressure. Deviator stress is computed by dividing the net load by the corrected area. σ 1 and σ 3 are the maximum and minimum principal stresses. The stress-strain curve is plotted in Figure 2 using deviator stress and axial strain. Mohr s circle are plotted for each test in Figure 3 using the maximum and minimum principal stresses at failure in order to determine the cohesion intercept and angle of internal friction. P net = P - P init (5) Deviator Stress = σ 1 - σ 3 = P net / A (6) Triaxial Compression - 3

4 Results Use Table 1 to compute stress and strain and Figure 2 to plot the stress-strain curves. Determine the modulus of elasticity for each test. Use Figure 3 to plot the Mohr s circle and to obtain the cohesion intercept the angle of internal friction. Conclusions How does the modulus of elasticity vary with confining pressure? Are the cohesion intercept and the angle of internal friction representative for this soil? Triaxial Compression - 4

5 Table 2 - Triaxial Compression Test Triaxial Compression Test Group Date Soil Description: Sample Mass: (g) Sample Length: (cm) Sample Diameter: 3.56 (cm) Sample Area: 9.93 (cm 2 ) 9.93 (10-4 ) (m 2 ) Load Ring Calibration: 4.83 (N/division) Deformation Gage Calibration: (inch/division) Cell Pressure: (kpa) Load at Zero Strain, P init : (N) Deform. Sample Sample Load Net Corrected Deviator Gage Length Strain, Ring Load, Load, Area, Stress, Reading Change ε Reading P P net A σ 1 σ 3 (Div.) (cm) (%) (Div.) (N) (N) (m 2 ) (kpa) Mass of Tare (g) Tare + Wet Soil (g) Tare + Dry Soi (g) Water Content (%) Wet Density (kg/m 3 ) Dry Density (kg/m 3 ) Sketch of Failed Specimen Triaxial Compression - 5

6 Table 3 Triaxial Compression Test Summary Table Sample No. 1 Sample No. 2 Sample No. 3 σ 3 = σ 3 = σ 3 = Deviator Deviator Deviator Stress, Strain Stress, Strain Stress, Strain σ 1 σ 3 ε σ 1 σ 3 ε σ 1 σ 3 ε (kpa) (%) (kpa) (%) (kpa) (%) Triaxial Compression - 6

7 Axial Stress (lbf/in 2 ) Axial Strain (%) Figure 2 Stress-Strain Curves, Triaxial Compression Test 300 Shear Stress (kpa) Normal Stress (kpa) Figure 3 Mohr s Circles Triaxial Compression - 7

8 Picture 1 Triaxial Compression Cell and Compression Machine Picture 2 Triaxial Pressure Manifold Triaxial Compression - 8