Soils (done) Flashcards
(36 cards)
What is the Plastic Limit (PL) of soil?
The water content where a soil switches from a semisolid state to a plastic state. It will deform under pressure but hold shape
What are the two phases of soil that are available to handle an applied load?
Liquid phase (pore water)
Solid phase (soil skeleton)
What is the “soil skeleton”?
The solid particles that make up the soil (not the water, liquid in between particles)
What is the equation to calculate effective stress in soil?
(Effective stress, sigma’) = (total stress, sigma) - (pore water pressure, u)
Stress is carried by both the soil skeleton and the pore water
What is shear strength of soil? (definition)
The resistance to shear deformation along a failure surface
What is the equation for shear strength in soil?
shear strength = cohesion + friction
s = c + [sigma*tan(phi)]
phi = friction angle
** mostly concerned with the strength of the soil skeleton because the water level can change:
shear strength = effective cohesion + effective normal stress * tan(effective friction angle)
What are the 3 strength indices used to approximate shear strength?
- California Bearing Ration (CBR)
- ratio of soil strength to that of crushed limestone
- 0-100 scale - SPT Blow Count (N-Value)
- Standard Penetration Test conducted with drill rig
- N-value is # of hammer blows/foot - Modulus of Subgrade Reaction (k)
- measures penetration of a plate bearing directly on soil
- measured in psi/in
How does compacting soil impact is physical properties? (strength, compressibility, permeability)
Compacting:
- increases strength
- decreases compressibility
- decreases permeability
- reorients, factures, and deforms particles
- most economic way to improve soil
Explain the primary difference between the Standard and Modified Proctor tests
Both:
- lab compaction tests where you drop a hammer to compact the soil
Standard:
- Lower compactive effort (energy) imparted
- less layers, lighter hammer, lower drop height
Modified:
- Higher compactive effort (energy) imparted
- more layers, heavier hammer, higher drop height
What are 3 ways to determine relative compaction in the field?
- Sand cone test
- destructive - Balloon test
- destructive - Nuclear density gauge
- non destructive
- radiation source emits beams or particles
How do you use the Proctor test to determine if the soil meets compaction standards?
- In a lab find the gamma_d max (dry unit weight max) and w_optimum (optimum water content %)
- Measure gamma_d and w in the field after compaction
- Compare the lab and field values
What is the Liquid Limit of soil?
The point when soil will no longer hold it’s shape, flow freely under pressure
What physical properties of soil do humans have control/effect on?
- Moisture content
- Density
What physical properties of soil are driven by the formation process? (humans have no control over)
- Grain shape
- Grain size
- Gradation
- Plasticity
- Organic Matter
- Chemical properties
What are two forms of weathering that form soil?
- Mechanical (frost, thermal, erosion, abrasion)
- Chemical (dissolution, oxidation, hydrolysis)
What is the difference between residual and transported soil?
Residual - weathers in place
Transported - deposited away from the parent rock (via wind, water, gravity, glaciers)
What are the five grain shapes of soil?
- Angular
- Subangular
- Subrounded
- Rounded
- Platy shape
How does grain size and gradation affect soil density?
Low density
- rounded shape
- poorly graded
High Density
- varying shapes
- well graded
What shape of soil packs the most dense?
Angular
Rounded packs the least dense
What is the primary force that affects large particles & silt? Clay?
Large particles and silt: friction the primary force
Clay: electric chare
How do you find the Plasticity Index (PI)?
PI = LL - PL
Why does soil fail in shear first even though it’s loaded in compression?
Soil particles slide or roll past each other
What is cohesion in the shear strength equation?
- stickiness/cohesion between soil particles
- affected by soil mineralogy and attraction between charged clay particles
- the failure envelope starts at c on the y-axis because soil inherently has strength from it initial cohesion
What is friction in the shear strength equation affected by?
- a mount of interlock
- shape
- size/gradation
