The University of Toledo Soil Mechanics Laboratory

Transcription

1 The University of Toledo Soil Mechanics Laboratory 1 Unconfined Compression Test Introduction Soil, like other materials that engineers use, must be relied upon to satisfy particular engineering purposes. Soil must have sufficient shear strength to resist shear stresses that develop during loading. Soil also must have adequate stiffness so that deformations that result from loading do not cause problems. Therefore, geotechnical engineers test soils to determine such engineering properties as the drained or undrained shear strength and the stress-strain behavior. Normally, undisturbed soil samples are obtained using subsurface exploration equipment. Prepared specimens are then tested in the laboratory using unconfined compression, triaxial compression or direct shear testing equipment. For this laboratory, samples of soil are to be prepared by hand compaction using the Harvard miniature device and tested in the unconfined compression test machine. The unconfined compression strength and soil stiffness are used in design calculations to determine if the soil has adequate strength and stiffness to support foundations, slopes and embankment fills. Apparatus 1. Mixing tub 2. Harvard miniature mold and compactor 3. Spatula 4. Sample ejector 5. Calipers 6. Balance 7. Unconfined compression machine 8. Water content tare and oven Procedure 1. Obtain enough soil passing the #4 sieve to fill the Harvard miniature mold. 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 Harvard miniature mold as uniformly as possible using 3 layers with 25 blows per layer. 4. Trim the ends of the specimen perpendicular to the sides using a spatula. 5. Eject the soil from the mold, weigh the sample and measure the sample height and diameter. 6. Place the sample in the unconfined compression machine and apply vertical loads with the deformation rate setting at 0500 (0.05 in/min or 1.27 mm/min). 7. Obtain readings from the load ring gage and the deformation gage at every 20 divisions on the deformation gage until the sample fails. 8. Sketch the failed specimen. Use the failed test specimen to obtain the water content. 1 ASTM D a Unconfined Compression - 1

2 Calculations The calculations for stress and strain are completed using the equations and Table 1. The applied stress is computed using the load ring calibration shown below and the cross-sectional area A, corrected for sample strain. The stress-strain curve is graphed using the figure below. P A' (1) A' A 1 (2) L L o (3) L = (Deform. Gage Reading Initial Reading) x inch/div (4) Load Ring Calibration 500 Load (lb) = 0.80 x Dial Divisions Load (lbs.) Dial Divisions Results Use Table 1 to compute stress and strain and Figure 1 to plot the stress-strain curve. Conclusions Determine the unconfined compression strength Determine the undrained shear strength. Would you consider the soil specimen tested soft, medium, stiff or hard? Estimate a value for the modulus of elasticity for the soil. Unconfined Compression - 2

3 Table 1 - Unconfined Compression Test Unconfined Compression Test Group Date Soil Description: Sample Mass: (g) Sample Length: (cm) (in) Sample Diameter: (cm) (in) Sample Area: (cm 2 ) (in 2 ) Load Ring Calibration: 0.80 (lb./division) Deformation Gage Calibration: (inch/division) Deform. Sample Load Gage Length Sample Ring Corrected Reading Change Strain Reading Load Area Stress (Div.) (inch) (%) (Div.) (lbf) (in 2 ) (lbf/ in 2 ) Mass of Tare + Wet Soil (g) Mass of Tare + Dry Soil (g) Mass of Tare (g) Mass of Dry Soil (g) Mass of Water (g) Water Content, w (%) Initial Soil Volume (cm 3 ) Initial Wet Density, (g/cm 3 ) Initial Dry Density, d (g/cm 3 ) Sketch of Failed Specimen Unconfined Compression - 3

4 Axial Stress (lbf/in 2 ) Axial Strain (%) Figure 1 Stress-strain for Unconfined Compression Test Unconfined Compression - 4

5 Collar Apparatus Compactor Sample Ejector Mold Picture 1 Harvard Miniature Apparatus Deformation Gage Load Ring and Load Gage Load Machine Picture 2 Unconfined Compression Machine Unconfined Compression - 5