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

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.

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

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)

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

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

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

Simple shear deformation of soil Shear stress vs. shear strain Peak Type II – dense sands and overconsolidated clays Type II – A soils Critical state Type I – loose sands, normally consolidated and lightly overconsolidated clays Type 1 soil - Show gradual increase in shear stresses as the shear strain increases until an approximately constant shear stress which we call the critical state shear stress is attained. Type II soil – show a rapid increase in shear stress reaching a peak value at low shear strains compared to type I soils. And then show a decrease in shear stress with increasing shear strain (strain soften) ultimately attaining a critical state shear stress.

Simple shear deformation of soil Volumetric strain vs. shear strain Void ratio vs. shear strain Type I – loose sands, normally consolidated and lightly overconsolidated clays Type I – loose sands, normally consolidated and lightly overconsolidated clays Compression Critical void ratio Type 1 soils – compress that is they become denser until a constant void ratio which we will call the critical void ratio is reached Type II soil – compress initially (attributed to particle adjustment) and then expand, that is they become looser until a critical void ratio is attained. The critical state shear stress is reached for all soils when no further volume change occurs under continued shearing. We will use the term critical state to define the stress state reached by a soil when no further change in shear stress and volume occurs under continuous shearing at a constant effective stress. Type II – dense sands and overconsolidated clays Expansion Type II – dense sands and overconsolidated clays

Effect of increasing the normal effective stress Shear stress vs. shear strain Volumetric strain vs. shear strain What is the effect of increasing the normal effective stress For type I – the amount of compression and the magnitude of the critical state shear stress will increase For type II – the peak shear stress tends to disappear the critical shear stress increases and the change in volume expansion decreases

Effect of increasing the normal effective stress Shear stress vs. normal effective stress Void ratio vs. shear strain If we were to plot the peak shear stress and the critical state shear stress for each constant normal effective stress for type 1 and 11 soils. We would get: The critical void ratio is dependent on the magnitude of the normal effective stress

Effect of overconsolidation ratio Shear stress vs. shear strain Volumetric strain vs. shear strain Two over consolidated soils with different overconsolidation ratios but the same mineralogical composition would exhibit different peak shear stresses and volume expansion. The higher overconsolidated soil gives a higher peak shear strength and greater volume expansion. Higher overconsolidated soil gives a higher peak shear strength and greater volume expansion

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