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EGB373 (completed)

Soil shear strength Part I

QuestionAnswer
The soil fail when the... applied shear stresses are equal or exceed the shear strength of the soil
Bearing capacity, lateral earth pressure, and slope stability are some examples of common geotechnical applications that depend on the... shear strength of soil
Applications 1) Slope Stability 2) Bearing Capacity 3) Retaining Walls Others… • Tunnel Linings • Roadway Base • Excavations
Soils generally fail in... shear
Shear failures occur in soils when the... shear stresses are larger enough to make the soil particles roll and slide past each other
At failure, shear stress along the failure surface (mobilized shear stresses) reaches... the shear strength
Shear strength “The ability to resist sliding along internal surfaces within a soil mass” (Murthy, 1992)
Shear strength is Measured by two parameters: c' (cohesion) and ϕ' (internal friction angle)
Higher the values ( c' and ϕ') higher the shear strength
Mohr-Coulomb Failure Criterion (2) (in terms of total stresses) - 𝜏_f is the maximum shear stress the soil can take without... failure, under normal stress of σ
Mohr-Coulomb Failure Criterion (3) (in terms of effective stresses) - 𝜏_f is the maximum shear stress that soil can takes without failure, under normal... effective stress of σ'
Mohr-Coulomb Failure Criterion (4) - Shear strength consists of two components: cohesive and frictional
Factors affecting ɸ’ 1) Mineralogy (e.g., quartz vs. mica) 2) Grain shape (e.g., angular vs. rounded) 3) Grain size distribution (e.g., well-graded vs. poorly-graded) 4) Void ratio, density, porosity (e.g., compacted vs. loose) 5) Organic material (very weak)
Angle of repose - When bulk granular materials are poured onto a horizontal surface, a conical pile will form. The internal angle between the surface of the pile and the horizontal surface is known as the... angle of repose Angle of repose is related to friction between soils which is influenced by the geometry of the particles (Shape, angularity and texture)
In geotechnical engineering, angle of repose refers to the... maximum angle at which an object can rest on an inclined plane such as slopes without sliding down
Cohesion - Factors affecting c’: 1) Cementation (e.g., CaCO3) 2) Electrostatic attraction – van der Waals forces appreciable for small, high-surface area materials 3) Negative pore pressure (unsaturated soils) 4) Interlocking
Determination of shear strength parameters of soils (c, ɸ or c’, ɸ’) (1) Laboratory tests on Undisturbed soil specimens taken from the ground
Determination of shear strength parameters of soils (c, ɸ or c’, ɸ’) (2) Field tests
Most common laboratory tests to determine the shear strength parameters are, 1) Direct shear test 2) Triaxial shear test
Field tests 1. Vane shear test 2. Torvane 3. Pocket penetrometer 4. Fall cone 5. Pressuremeter 6. Static cone penetrometer 7. Standard penetration test
Simulating field conditions in the laboratory Representative soil sample taken from the site Step 1 - Set the specimen in the apparatus and apply the initial stress condition Step 2 - Apply the corresponding field stress conditions
In many foundation design problems and retaining wall problems, it is required to determine the... angle of internal friction between soil and the structural material (concrete, steel or wood)
Direct Shear Test - pros and cons • Normally Performed on Granular Soils to Find Friction Angle • Simple to perform • Particle Sizes Limited by Shear Box Size • Residual Friction Angle Can be Determined at Large Strain Values
Direct Shear Test - pros and cons (2) • Cohesive Soils Require Area Correction • Shear Plane Predetermined, shear stress not uniform •Drainage cannot be controlled
Advantages of direct shear apparatus Due to the smaller thickness of the sample, rapid drainage can be achieved Can be used to determine interface strength parameters Clay samples can be oriented along the plane of weakness or an identified failure plane
Disadvantages of direct shear apparatus Failure occurs along a predetermined failure plane Area of the sliding surface changes as the test progresses Non-uniform distribution of shear stress along the failure surface
Created by: Asher - S
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