Soil Mechanic Laboratory Assist Lec Lubna Mohammed Abd
Soil Mechanic Laboratory Assist. Lec. Lubna Mohammed Abd Environmental Engineering Department Al- Mustansiriyah University Shear Strength (Direct Shear Test) Sources: Soil Mechanics – Laboratory Manual, B. M. DAS (Chapter 15) Soil Properties, Testing, Measurement, and Evaluation, C. Liu, J. Evett
Class Outlines n Shear strength in soils n Introduction n Definitions n Direct shear test n Introduction n Procedure n Calculation n Results and Figures
Shear Strength n The strength of a material is the greatest stress it can sustain n The safety of any geotechnical structure is dependent on the strength of the soil n If the soil fails, the structure founded on it can collapse
Slope Failure in Soils Failure due to inadequate strength at shear interface
Shear Failure in Soils
Bearing Capacity Failure Civil Engineering - Texas Tech University
Transcosna Grain Elevator Canada (Oct. 18, 1913) Civil Engineering - Texas Tech University West side of foundation sank
Significance of Shear Strength n Engineers must understand the nature of shearing resistance in order to analyze soil stability problems such as; n Bearing capacity n Slope stability n Lateral earth pressure on earth-retaining structures n Pavement
Shear Strength in Soils n It is a measure of the soil resistance to deformation by continuous displacement of its individual soil particles n Shear strength in soils depends primarily on interactions between particles n Shear failure occurs when the stresses between the particles are such that they slide or roll past each other n The shear strength of a soil is its resistance to shearing stresses.
Shear Strength in Soils (cont. ) n Soil derives its shear strength from two sources: n Cohesion between particles (stress independent component) n n n Cementation between sand grains Electrostatic attraction between clay particles Frictional resistance between particles (stress dependent component)
Shear Strength of Soils; Cohesion n Cohesion (C), is a measure of the forces that cement particles of soils n n Dry sand with no cementation Dry sand with some cementation Soft clay Stiff clay
Shear Strength of Soils; Internal Friction n Internal Friction angle (f), is the measure of the shear strength of soils due to friction
Mohr-Coulomb Failure Criteria n This theory states that a material fails because of a critical combination of normal stress and shear stress, and not from their either maximum normal or shear stress alone. n The relationship between normal stress and shear is given as
Mohr-Coulomb Failure Criterion Shear Strength, S = C Normal Stress, n = = g h
General State of Stress σ1 σ3 τf σn σ1 major principle stress σ3 Minor principle stress Confining stress
State of Stresses in Soils Consider the following situation: - A normal stress is applied vertically and held constant - A shear stress is then applied until failure Normal stress σn Shear stress σ3 σ3 σ1
Determination of Shear Strength Parameters n The shear strength parameters of a soil are determined in the lab primarily with two types of tests; n Direct Shear Test Normal stress σ n Triaxial Shear Test n 1 Shear stress σ3 Soil 3
Direct Shear Test n Direct shear test is Quick and Inexpensive n Shortcoming is that it fails the soil on a designated plane which may not be the weakest one n Used to determine the shear strength of both cohesive as well as non-cohesive soils n ASTM D 3080
Direct Shear Test (cont. ) n The test equipment consists of a metal box in which the soil specimen is placed n The box is split horizontally into two halves Shear stress σ n Vertical force (normal stress) is applied through a metal platen n Shear force is applied by moving one half of the box relative to the other to cause failure in the soil specimen Normal stress σn 3 Soil
Direct Shear Test
Direct Shear Test
Direct Shear Test
Shear stress Direct Shear Test Data Peak Strength Residual Strength
Direct Shear Test Data Volume change DH
Direct Shear Test (Procedure) 1. Measure inner side or diameter of shear box and find the area 2. Make sure top and bottom halves of shear box are in contact and fixed together. 3. Weigh out 150 g of sand. 4. Place the soil in three layers in the mold using the funnel. Compact the soil with 20 blows per layer. 5. Place cover on top of sand 6. Place shear box in machine. 7. Apply normal force. The weights to use for the three runs are 2 kg, 4 kg, and 6 kg if the load is applied through a lever arm, or 10 kg, 20 kg, and 30 kg, if the load is applied directly. Note: Lever arm loading ratio 1: 10 (2 kg weight = 20 kg)
Direct Shear Test (Procedure) Start the motor with selected speed (0. 1 in/min) so that the rate of shearing is at a selected constant rate 9. Take the horizontal displacement gauge, vertical displacement gage and shear load gage readings. Record the readings on the data sheet. 10. Continue taking readings until the horizontal shear load peaks and then falls, or the horizontal displacement reaches 15% of the diameter. 8. Civil Engineering - Texas Tech University
Calculations 1. Determine the dry unit weight, gd 2. Calculate the void ratio, e 3. Calculate the normal stress & shear stress
Figures Peak Stress Shear stress, s s 3 s 2 s 1 N 3 = 30 kg N 2 = 20 kg N 1 = 10 kg Horizontal displacement, DH
Shear Stress, s (psf) Figures (cont) ( 2, s 2) ( 3, s 3) ( 1, s 1) C Normal Stress , psf
Vertical displacement Figures (cont) Horizontal displacement
- Slides: 31