Soil Mechanics

Question 1

Soils come from solid rocks

Answer 1

• Formed from the rocks weathering

Question 2

2 soil types

Answer 2

• Residual = at site of weathering
• Transported soils

Question 3

Soil description contains

Answer 3

• 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

Question 4

Soil classification

Answer 4

• Separation into groups of characteristics and behaviours
• Main factor is mechanical properties (permeability, stiffness and strength)

Question 5

Corse grained soil

Answer 5

• Gravel and sand
• If well or poorly graded = sieve analysis
• If silty of clayey = Atterberg’s limits

Question 6

Fine grained soil

Answer 6

• Inorganic and organic silt and clay
• For both low and high plasticity = Atterberg’s limits

Question 7

Soil grading

Answer 7

Well graded = range of sizes
Gap graded = missing particle sizes

Question 8

Difference between coarse and fine grained soil

Answer 8

• Grain shape
• Grain size
• Consistency

Question 9

Consistency of soil

Answer 9

• Either: solid, semisolid, plastic or liquid
• Cohesion/adhesion between particles (soil resistance)
• Largely depends on moisture

Question 10

Atterberg Limits

Answer 10

1. Liquid State
2. Plastic State
3. Semi Solid State
4. Solid State

Question 11

Field Compaction

Answer 11

• Smooth wheel roller
• Pneumatic rubber tired rollers
• Can be used on sandy terrain
• Sheepsfoot rollers (clayey soils)
• Vibratory rollers (granular soils)

Question 12

Soil Permeability

Answer 12

• The measure of capcaity to allow water flow through pore spaces between solid particles
• Measured by applying hydraulic pressure gradient and measuring water velocity

Question 13

Factors Affecting Permeability

Answer 13

• Soil Type
• Particle size distribution
• Particle shape/texture
• Density
• Void ratio
• Saturation
• Soil fabric
• Type of flow
• Temperature

Question 14

Permeabilty Affectors - Soil type and particle size distribution

Answer 14

• Coarse more permeable than fine because of distribution
• Smaller particles = smaller voids + increased resistance

Question 15

Permability Affectors - Particle shape and texture

Answer 15

• Rough edges provide more friction to flow due to twisted paths

Question 16

Permability Affectors - Void ratio

Answer 16

• Placement and compaction affects ratio
• More voids = more flow

Question 17

Permability Affectors - Density

Answer 17

• Loose soil = more permeability
• Compaction reduces the flow

Question 18

Permability Affectors - Degree of saturation

Answer 18

• How much water gets in voids is important
• When testing permeability an effort is made to reduce air bubbles to allow full saturation

Question 19

Permability Affectors - Soil fabric

Answer 19

• Soils are homogenous and are made of different layers and types of soil
• Therefore where and the angle water hits affects permeability

Question 20

Importance of permeability

Answer 20

• Influences rate of settlement under load
• Affect stability of slopes
• Use as seepage rate

Question 21

Darcy’s Law

Answer 21

• Velocity of flow through soil = directly proportional to hydraulic gradient

Question 22

Mohr-Coulomb Failure Criterion (1900)

Answer 22

• Theory: material fails because of combination of normal and shear stress not from max of one alone

Question 23

Igneous Rocks

Answer 23

• Granite
• Basalt

Question 24

Sedimentary Rocks

Answer 24

• Sandstone
• Shale
• Limestone

Question 25

Metamorphic Rocks

Answer 25

• Slate
• Marble

Question 26

Cohesion

Answer 26

Same molecules sticking

Question 27

Adhesion

Answer 27

Different molecules sticking