2 PRELIM - SOIL MECHANICS

Types of Shear Failure

General Shear Failure

Location Shear Failure

Punching Shear Failure

Shear Stresses

the maximum resistance of soil to shearing stresses

General Shear Failure

• Type of Failure: Provided that Dense Sand or Stiff Cohesive Soil Support the Footing.

• The Footing Moves Slightly Downward

• Sudden Failure with a Significant Bulging of the Ground Surface Alongside the Footing

• Sudden Collapse together with Tilting of the Footing

• The Load-Settlement Diagram is Equivalent to Stress-Strain for Solid Sand or Over-Consolidated Clay

• The Ultimate Load is Apparent on this Curve

Local Shear Failure

• Type of Failure: In moderately Compressible Soils or Loose Sand

• Large Deformation takes place under the footing

• Large Vertical Settlement Occurs Prior to Slight Bulging of the Ground Surface

• Tilting of Footing is Not Necessary

• 1st Major Non-Linearity in the Load-Settlement Curve (Open Circle)

• Ultimate Load is Not Clear

Punching Shear Failure

• Type of Failure: Very Loose Sands and Weak Clays

• Loaded Base Sinks into Soil like a Punch

• No Bulging of Ground Surface and No Tilting of Footing

• Large Vertical Strains with No Lateral Deformation

• After 1st yield, The Load-Settlement Curve will Steepen Slightly BUT remain flat

• Ultimate Load is not Clear

Soil Formation

• the process by which rocks are broken down into smaller particles through physical, chemical, biological weathering

• soil is formed affects it’s strength compressibility, and drainage characteristics, residual soils remain at their origin, while transported soils are carried by water, wind, or ice

Settlement of Soil

The Vertical Deformation of Soil due to Applied Loads; may be immediate or long-term and excessive settlement can lead to cracking and structural damage if not properly accounted for

Consolidation of Soil

The Gradual Reduction in Soil Volume cased by the expulsion of Pore Water under sustained load

Soil Stabilization

is the process of improving soil properties through mechanical or chemical methods

enhances strength and durability, allowing weak soils to be used in construction projects

Bearing Capacity of Soil

is the maximum load per unit area that soil can safely support without failure

it depends on soil strength, foundation depth, and soil conditions

accurate evaluation ensures safe foundation design

Properties of Soil

1. Index Properties

2. Physical Properties

3. Mechanical Properties

4. Hydraulic Properties

Index Properties

mainly for identification and classification of soils

Water Content (Moisture Content)

ratio of the weight of water to the weight of dry soil, expressed as a percentage

Specific Gravity

ratio of the density of soil solids to the density of water

Grain Size Distribution Parameters

Coefficient of Uniformity

Coefficient of Curvature

Atterberg Limits

Liquid Limit (LL) and Plastic Limit (PL)

Liquid Limit (LL)

Moisture Content at which soil changes from Plastic to Liquid State

Plastic Limit (PL)

Moisture Content at which soil changes from Semi-Solid to Plastic State

Plasticity Index

Bulk Unit Weight

Dry Unit Weight

Saturated Unit Weight

Void Ratio

Porosity

Degree of Saturation

Classification System

• Geological Soil

• Particle Size

• Unified Soil

• Asshto Soil

Geological Soil Classification

based on their origin and mode of formation

types:

residual soils - formed by in place weathering of rock

transported soils - moved by agentsL water, wind, ice, gravity

Particle Size Classification and Unified Soil Classification System (USCS)

Soils are Grouped According to Grain Size, determined by sieve and hydrometer analysis

Aastho Soil Classification System

Primarily used in highway and pavement engineering