Soil Mechanics Flashcards
Soils come from solid rocks
- Formed from the rocks weathering
2 soil types
- Residual = at site of weathering
- Transported soils
Soil description contains
- Q1 = Disturbed or undisturbed?
- Q2 = Fine or coarse (cohesive/granular)
- Q3 = (If fine) is it: soft, firm, stiff or very stiff
- Q3 = (If coarse) is it: loose, medium dense, dense or very dense
- Q4 = Colours
- Q5 = Structure (only undisturbed soils)
- Q6 = Main soil type (clay, sand, gravel, silt)
- Q7 = Additional constituents
Soil classification
- Separation into groups of characteristics and behaviours
- Main factor is mechanical properties (permeability, stiffness and strength)
Corse grained soil
- Gravel and sand
- If well or poorly graded = sieve analysis
- If silty of clayey = Atterberg’s limits
Fine grained soil
- Inorganic and organic silt and clay
- For both low and high plasticity = Atterberg’s limits
Soil grading
Well graded = range of sizes
Gap graded = missing particle sizes
Difference between coarse and fine grained soil
- Grain shape
- Grain size
- Consistency
Consistency of soil
- Either: solid, semisolid, plastic or liquid
- Cohesion/adhesion between particles (soil resistance)
- Largely depends on moisture
Atterberg Limits
- Liquid State
- Plastic State
- Semi Solid State
- Solid State
Field Compaction
- Smooth wheel roller
- Pneumatic rubber tired rollers
- Can be used on sandy terrain
- Sheepsfoot rollers (clayey soils)
- Vibratory rollers (granular soils)
Soil Permeability
- The measure of capcaity to allow water flow through pore spaces between solid particles
- Measured by applying hydraulic pressure gradient and measuring water velocity
Factors Affecting Permeability
- Soil Type
- Particle size distribution
- Particle shape/texture
- Density
- Void ratio
- Saturation
- Soil fabric
- Type of flow
- Temperature
Permeabilty Affectors - Soil type and particle size distribution
- Coarse more permeable than fine because of distribution
- Smaller particles = smaller voids + increased resistance
Permability Affectors - Particle shape and texture
- Rough edges provide more friction to flow due to twisted paths
Permability Affectors - Void ratio
- Placement and compaction affects ratio
- More voids = more flow
Permability Affectors - Density
- Loose soil = more permeability
- Compaction reduces the flow
Permability Affectors - Degree of saturation
- How much water gets in voids is important
- When testing permeability an effort is made to reduce air bubbles to allow full saturation
Permability Affectors - Soil fabric
- Soils are homogenous and are made of different layers and types of soil
- Therefore where and the angle water hits affects permeability
Importance of permeability
- Influences rate of settlement under load
- Affect stability of slopes
- Use as seepage rate
Darcy’s Law
- Velocity of flow through soil = directly proportional to hydraulic gradient
Mohr-Coulomb Failure Criterion (1900)
- Theory: material fails because of combination of normal and shear stress not from max of one alone
Igneous Rocks
- Granite
- Basalt
Sedimentary Rocks
- Sandstone
- Shale
- Limestone
Metamorphic Rocks
- Slate
- Marble
Cohesion
Same molecules sticking
Adhesion
Different molecules sticking
