L10 - Soils and Soil Hazards

what are pedologists?

soil scientists

what do pedologists, geologists, and engineers consider when dealing with soil?

pedologists: soil's ability to support plant life (nutrients, organics, hydrologic properties)

geologists: origin, transport, and deposition of soil, reflection geologic processes

engineers: properties applicable to engineering (ex: strength, compressibility)

how do pedologists classify soil?

in horizons (O, A, B, C horizons)

explain the contents of the O, A, B, and C soil horizons of pedologists, going from shallowest to deepest

O: humus just below surface (organics)

A: zone of leaching - has been leached of most soluble minerals, rich in clay and insoluble minerals

B: zone of accumulation - abundant in minerals dissolved from A horizon that reprecipitate lower in soil, little organic matter. At the base of the B horizon, there are larger weathered fragments of bedrock that become more abundant

C: slightly weathered parents material - fractured and weathered bedrock

picture of soil horizons

how do geologists view soil? how do they classify soil?

1. they look at origin, depositional environment, and transportation agent

2. classify as residual soils ("in situ" from weathering of bedrock, aka no transportation involved) and transported soils (soils that accumulate after being eroded)

residual soils

soils that are a product of weathering of bedrock. there is no transportation involved (in-situ). one of the ways that geologists view soil

transported soils

soils that accumulate after being eroded. one of the ways that geologists view soil

how do engineers view soil?

as anything that can be excavated with a shovel

how is soil considered by engineers?

1. particle size (cobble, gravel, sand, silt, clay)

2. particle size distribution/gradation: (well-graded, poorly-graded, or gap-graded soils)

3. density: related to compressibility, aka its tendency to compact under an applied load.

4. Shear strength: determines the ability of soil to support load of a structure, or to remain stable on a slope

what particle size distribution do engineers not like for soil?

gap-graded soils

what do the factors of soil apply to with engineers?

water content of soil

how is soil classified according to engineers, going from shallowest to deepest? how does this contrast with pedologists?

1. topsoil (includes humus) - holds moisture and prevents erosion

2. subsoil (includes A & B horizons) - small rock material, lots of clay

3. bedrock - where above soil is created or deposited into

what is the unified soil classification system for size?

soils are classified by 1 size

G - gravel

S - sand

M - silt

C - clay

O - organic

grading in the unified soil classification system

G - gravel, S - sand: W/P - well/poorly graded

M - silt, C - clay: H/L - high/low plasticity

O - organic - silt, clay, or Pt

is grading the same as sorting?

no, grading is the opposite of sorting

how does the unified soil classification system work?

soils are classified by size (GSMCO) and grading (WPHL) by a letter

what is the difference between sand and clay/silt?

clay and silt pass the 200 mark in the gradation sieve test

what is the plasticity of soil? why is it important?

soil's ability to undergo deformation without cracking

important to consider when building on silt/clay

what are soil hazards?

- soil liquefaction (earthquake hazards)

- subsidence due to dewatering (groundwater)

A: expansive (swelling soils

B: soft soils

C: collapsible soils

D: sensitive soils (quick clays)

what are expansive soils? where do they happen? why are they a problem?

1. soil that contains a high proportion of swelling clays (minerals that get water molecules in b/w silicate sheets, causing swelling)

2. happens in upper part of soil (because they undergo repeated cycles of wetting/drying)

3. they cause expensive structural damage (ex: roller coaster roads, cracks in buildings)

how are expansive soils a concern in tunnel construction?

tunnel construction is prone to chemical weathering by hydration

1. tunnels pass through dry materials

2. tunneling process exposes rock to atmosphere (which contains water in air)

3. water uptake swells rock due to anhydrite to gypsum chemical conversion, causing the volume to increase

why use pre-loads in deltas and floodplains?

**

what hazards might be associated with deltas and floodplains?

due to the deposition of fine-grained material from other regions, the soil is soft. When this soft soil has a load applied to it, it's an unstable foundation and the soil can shift

what is the problem with soft soils?

1. loading can induce settlement (volume reduction) and/or "failure" of soft soils. they have to be preloaded

what is preloading

**

what are collapsible soils? what is the issue with them?

soils deposited with very open structure between soil grains

2. addition of water reduces volume

what is hydrocompaction?

tendency for loose soils in arid (dry) environments to compact under load when wetted

what is a collapsing soil?

loose and/or weakly cemented soil

prone to large settlements under a load and/or as a result of cementing agent deterioration when water is added

why are some regions of US more affected by hydrocompaction than others?

arid regions of US don't concern themselves with water initially - so the soil may not be hydrocompacted when initially built

what are quick clays? where do they happen?

1. fine-grained soil deposited in marine conditions

2. where cationic strength of pore water was high

what happens to quick clays when seawater is replaced by fresh water?

chemical alteration to quick clays due to the fact that there's no salt in freshwater - chemical structure is changed, clay loses strength

results in extremely sensitive material, which can cause landslides that occur at shallow slopes (1-2 degrees)

what is the sensitivity of fine-grained soils?

a ratio: (undisturbed peak strength)/(disturbed strength)

how does the chemical structure of the clay change?

Goes from dispersed to not dispersed (sheets, which slide and shift)