Soil Flashcards
Soil vs. Rock similarities
both are natural aggregate of mineral grains
Soil vs Rock differences
- soil aggregates can be separated by mechanical means (agitation in water)
- Rock aggregate minerals are connected by stronger forces
Soils are products of….?
rock weathering
gravel =
sand =
silt =
clay =
- gravel = small rocks
with a mix of minerals. - sand = small grains of a SINGLE mineral
- silt = same but grains not visible to naked eye
- clay = tiny plate-like minerals
why is soil called a 3-phase material?
solid - soil grains
liquid - usually water
gas - usually air
soil is a frictional material. Does it have tensile strength?
NO
does soil exhibit linear plastic behaviour?
NO, non-linear
Soil is anisotropic and heterogenous, what does that mean?
huge range of properties. Every soil is different, different in each location and huge variety
geotechnical challenges of soil
- settlement
- strength (bearing capacity and slope stability
- seepage
- liquefaction
residual vs transported soil
residual is a product of rock weathering and remains in place, transported is taken to other locations
where would you find clay minerals with good drainage and soils with good engineering properties?
hilly and mountainous areas.
flat low-lying areas produce what type of soils?
poor drainage with little or no vertical seepage. Bad engineering properties
Aeolian, fluvial and glacial soils
aeolian transported by wind, fluvial rivers/streams, glacial glaciers
volcanic soils
- layers represent seperate volcanic events
- high water content
- ash soils tend to drain quickly (no puddles)
- often sensitive
sorting process of transported soils, leads to more __________ deposits (especially fluvial and aeolian)
homogeneous deposits
residual soils formation
formed by growth and decay of plants (peat, mosses) or accumulation of fragments of inorganic skeletons or shells
Auckland clays are a common example of?
residual soil
traditional soil mechanics is based on the properties of which soil type?
transported
would angular sand grains or round sand grains be stronger?
angular, because they can interlock
what dominates the behaviour of clay ?
water interaction
why is clay so problematic?
small amounts can dominate overall behaviour of soil. Clay has low strength, swells, is highly compressible, and has low hydraulic conductivity
is clay hydrophilic or hydrophobic?
hydrophilic
2 clay mineral building blocks
- Silicon tetrahedron
2. Alumina Octahedron
3 common clay minerals in increasing activity and decreasing order of stacking
- Kaolinite
- Illite
- Bentonite
Kaolinite structure
- 1:1 silica:octahedron
- sheets are stacked forming aggregates
- hexagonal flakes
- common in sedimentary and residual soils
- pottery clay
- low activity
Illite structure
- 2:1 silica:octahedral
- octahedral sandwiched between 2 silica sheets
- potassium between layers
- most common in engineering
Bentonite structure
- 2:1 silica:octahedral
- octahedral sandwiched between 2 silica sheets
- water between layers
how do volcanic clays differ?
not organised in sheets and have unique behaviour
________ identified by greasy feel when soil worked between thumb and forefinger. ______ can be weathering product of this.
Allophane, Halloysite
soil fabric structure
Fabric + composition + inter particle forces
describe what is meant by soil fabric
the arrangement of particles. Elementary mineral units tend to settle individually in quiet water. Sediments become more anisotropic. Platy/ elongated particles tend to rotate into horizontal positions.
cohesionless soils
forces between particles are due to friction
cohesive soils
forces arise from electric repulsion through absorbed water layer between clay particles
first step in soil classification. What do you use?
determine grain size.
using a sieve if coarse, or for fine grains use hydrometer analysis
sedimentation analysis / hydrometer analysis, uses which law?
Stoke’s Law
void ratio
ratio of volume of voids : volume of solid particles
relative density
measure of how densely the grains are packed in a coarse soil
clay goes from solid - semi solid - plastic - liquid, as what increases?
water content
Atterberg limits concern what?
soil plasticity
liquid limit
water content at the boundary of liquid - plastic
Casagrande Apparatus. The moisture content at which a _ mm-wide groove in a soil pat will close for a distance of 13 mm when dropped __ times in a standard brass cup falling _ cm each time at a rate of 2 drops/sec in a standard liquid limit device
Casagrande Apparatus. The moisture content at which a 2 mm-wide groove in a soil pat will close for a distance of 13 mm when dropped 25 times in a standard brass cup falling 1 cm each time at a rate of 2 drops/sec in a standard liquid limit device
The liquid limit for sedimentary soils is not sensitive to wetting and drying. TRUE/FALSE
TRUE
soils containing ________ are very sensitive to cycles of wetting and drying
allophane
liquid limit cone penetrometer. The liquid limit of the soil is the moisture content at which an __ g, __ degree cone sinks exactly __ mm into a cup of remoulded soil in a 5s period.
liquid limit cone penetrometer. The liquid limit of the soil is the moisture content at which an 80 g, 30 degree cone sinks exactly 20 mm into a cup of remoulded soil in a 5s period.
Plastic limit
moisture content at which a thread os soil just begins to crack and crumble when rolled to a diameter of 3mm
Plasticity index =
= liquid limit - plastic limit.
range in which soil behaves plastically
liquidity index is a useful indicator of what?
state of soil.
LI > than 1 = sensitive soil.
w > LL = soft / low strength soil
w ~ PL = strong soil
USCS symbols for: GRAVEL: SAND: SILT: CLAY: ORGANIC: PEAT:
USCS symbols for: GRAVEL: G SAND: S SILT: M CLAY: C ORGANIC: O PEAT: Pt
what would SC, SW and MH stand for?
SC = clayey sand SW = graded sand MH = elastic silt
On the Plasticity chart, The A-line seperates?
The U-line indicates?
A- line seperates clay from silt, and organics from inorganics.
U-line indicates upper bound for general soils.
