363923636-Shear-Strength-of-Soil-2-1.pptx

363923636-Shear-Strength-of-Soil-2-1.pptx

1. EAG 345 – GEOTECHNICAL ANALYSIS By: Dr Mohd Ashraf Mohamad Ismail (ii) Typical response of soils to shearing forces

2. Shear strength Shear strength of a soil is its resistance to shearing force When a soil is sheared the particles move relative to each other. If the soil is not constrained, then it can either compress or expand in volume as shown in the figure below .

3. Responses of soils to shearing forces Behavior of two group of soils when subjected to shearing forces: Type I: Loose sands, normally consolidated and lightly over consolidated clays (OCR ≤2) ≥ Type II: Dense sands and heavily over consolidated clays (OCR >2) Uncemented soils; very weak interparticle bonds and comprises most soils Cemented soils; strong interparticle bonds through ion exchange or substitution

4. Revision: Consolidation The container is completely filled with water, and the hole is closed. (Fully saturated soil) A load is applied onto the cover, while the hole is still unopened. At this stage, only the water resists the applied load. (Development of excess pore water pressure) As soon as the hole is opened, water starts to drain out through the hole and the spring shortens. (Drainage of excess pore water pressure) After some time, the drainage of water no longer occurs. Now, the spring alone resists the applied load. (Full dissipation of excess pore water pressure. End of consolidation)

5. Revision: Overconsolidated Ratio OCR = The over consolidation ratio or OCR is defined as the highest stress experienced divided by the current stress. = Current vertical stress or overburden effective stress = past maximum vertical stress or preconsolidation stress OCR = 1, the soil is normally consolidated soil

6. Simple shear deformation of Type I soil Type I: Loose sands, normally consolidated and lightly over consolidated clays (OCR ≤2) Constant vertical effective stress Shearing under constant volume Shear stress vs. shear strain Volumetric strain vs. shear strain Void ratio vs. shear strain + ve

7. Simple shear deformation of Type I soil Type II: Dense sands and heavily over consolidated clays (OCR >2) Constant vertical effective stress Shearing under constant volume Shear stress vs. shear strain Volumetric strain vs. shear strain Void ratio vs. shear strain - ve

8. Simple shear deformation of soil Shear stress vs. shear strain Critical state Type I – loose sands, normally consolidated and lightly overconsolidated clays Peak Type II – dense sands and overconsolidated clays Type II – A soils

9. Simple shear deformation of soil Volumetric strain vs. shear strain Type I – loose sands, normally consolidated and lightly overconsolidated clays Type II – dense sands and overconsolidated clays Compression Expansion Type I – loose sands, normally consolidated and lightly overconsolidated clays Type II – dense sands and overconsolidated clays Critical void ratio Void ratio vs. shear strain

10. Effect of increasing the normal effective stress Shear stress vs. shear strain Volumetric strain vs. shear strain

11. Effect of increasing the normal effective stress Shear stress vs . normal effective stress Void ratio vs. shear strain The critical void ratio is dependent on the magnitude of the normal effective stress

12. Effect of overconsolidation ratio Volumetrric strain vs . shear strain Shear stress vs . shear strain Higher overconsolidated soil gives a higher peak shear strength and greater volume expansion

13. Homework Read Soil mechanics and foundations (Muni Budhu) page 223 – 227 to understand this topic clearly Refer the multimedia resources provided with this book chapter 7; sections 7.3